Mussels

Tweekleppiges (Bivalvia) is 'n klas seediere wat tot die orde van weekdiere (Mollusca) behoort. Hierdie klas sluit onder andere gapermossels, oesters en mossels in. Die tweekleppiges het 'n skulp wat uit twee dele bestaan wat met 'n buigbare ligament aanmekaar gehou word. 'n Paar spesies kan swem, maar die meeste anker hulself aan 'n substraat vas.

Kieue word vir asemhaling gebruik, maar kan ook met voeding help, omdat die meeste tweekleppiges filter-vreters is. Die geslagte is verskillend en bevrugting vind uitwendig plaas. Die diere word veral gekenmerk deur hul uitwendige skelet (skulpe), wat uit 2 beweegbare dele (kleppe) bestaan. Die klas diere is baie suksesvol en die verskillende spesies word in die seë op ʼn verskeidenheid dieptes, asook in varswater aangetref.

Hulle is uitstuitend waterlewend en die bekendste tweekleppiges is die mossels en oesters. Die tweekleppiges (klas Bivalvia) is aan slakke, seekatte en inkvisse verwant en almal is weekdiere (phylum Mollusca). Die meeste tweekleppiges se liggame word heeltemal deur hul skulpe bedek en 'n kopgedeelte kan nie onderskei word nie. Die voet is in baie gevalle tot 'n mesvormige struktuur gemodifiseer en dien as 'n soort graaf.

ʼn Radula (raspertong) ontbreek by die meeste spesies. Die dier is of heeltemal sessiel (sittend), of hulle kan net stadig beweeg, maar hulle kom nogtans wydverspreid voor. Hulle is egter nie soos slakke tot 'n bestaan op land aangepas nie. Ongeveer 15 000 spesies is bekend.

Die skulp

Die skulpe van die tweekleppiges bestaan uit 2 helftes, ʼn linker- en 'n regterklep, wat meestal spieëlbeelde van mekaar is. Die 2 helftes word deur ligamente aanmekaar geskarnier en dit word deur 2 spiere, die sluitspiere, geslote gehou. Wanneer die spiere ontspan, word die helftes deur die ligament oopgetrek.

Die 2 skulprande het tande wat inmekaarsluit om te verhoed dat die kleppe op mekaar rondskuif. Die manier waarop die tande inmekaarsluit (slot), asook die tande self, dien as 'n taksonomiese kenmerk en is belangrik vir die klassifikasie van die verskillende tweekleppiges.

Daar word 2 soorte slotte onderskei: die taksodonte en die heterodonte slot. Die skulpgroottes wissel baie; sommige varswaterspesies is slegs 1 mm groot, terwyl party seelewende spesies groter as 1 m kan word. Die kleppe groei saam met die diere en die oudste deel daarvan is die umbo, die gedeelte naby die ligamente waar die 2 kleppe aanmekaargehou word. Nuwe dele van die skulp groei hier aan, sodat 'n reeks konsentriese ringe om die umbo die aantal groeiseisoene aandui.

Sagte dele

Die materiaal waaruit die skulp gevorm word, word deur die mantel afgeskei. Dit is 'n dun vlies tussen die weekdier se liggaam en die skulp. Die mantel is aan die een sy uitgegroei tot 2 buise, die sifons, waarmee water van en na die liggaam gevoer word. Sommige spesies leef diep onder die substraatoppervlak en hulle het gevolglik besonder lang sifons.

Die spesies begrawe hulle met hul voet, 'n gespierde orgaan wat buite die skulp gestoot kan word en mesvormig is. Die voet kan ook gebruik word om mee te kruip, of in die geval van hartmossels, om mee te spring. Sessiele (sittende) spesies het geen voete nie maar hulle word met ʼn afskeidingstof van die bissusklier aan die substraat geheg. Die afskeidings verhard in die water tot bissusdrade. Die meeste tweekleppiges leef hoofsaaklik van fitoplankton (plant- plankton) wat hulle uit die water filtreer terwyl dit deur die liggaam stroom.

'n Instroomkanaal voer die water na die kieue, wat gemodifiseer is om die plankton uit die water te haal, en wat as 'n gaswisselingsorgaan dien. Die plankton word met slym uit die water gehaal en daarna deur trilhare na die mond gevoer. Nadat gaswisseling plaasgevind het, verlaat die water die liggaam deur die uitstroomopening. Ander tweekleppiges skraap voedsel van die substraat met behulp van tentakels, wat verlengings van die mondhoeke is.

Voedseldeeltjies klou aan die slymbedekte tentakels vas en word deur trilhare na die mond gebring, waar die voedseldeeltjies van die ander materiaal geskei word. Die senuweestelsel is relatief eenvoudig en geen ware brein word aangetref nie. Orie paar senuweeknope is aanwesig: die serebrale, pedale en die visserale ganglia. Die ganglia word deur 2 senuweekoorde en 'n aantal ander vesels met mekaar verbind, en die sintuie is swak ontwikkel.

Voortplanting

Die meeste tweekleppiges is eenslagtig, hoewel ʼn aantal hermafroditiese (tweeslagtige) spesies ook aangetref word, maar selfbevrugting kom nie dikwels voor nie. Sommige oesters kan hul geslagte tydens hul leeftyd verander. Eiers en spermselle word in die water vrygestel en bevrugting vind buite die diere se liggame plaas. 'n Vryswemmende larwe wat met trilhare bedek is (veligerlarwe), broei na 'n tydperk uit. Dit sak later na die bodem en gedaantewisseling vind plaas.

Varswater tweekleppiges se lewensiklus wyk ietwat van die patroon af. In sommige spesies is die vryswemmende larwestadium by 'n sittende, parasitiese bestaan aangepas. Die larwes staan as glogidia of haustoria bekend en hulle heg hulle 'n tyd lank aan visse se kieue totdat hulle (die larwes) vir gedaantewisseling gereed is. Daarna verlaat hulle die visse en sak na die bodem af, waar hulle tot volwasse tweekleppiges ontwikkel.

Betekenis vir die mens

Tweekleppiges is veral as 'n voedselbron belangrik, want mossels en oesters word in sommige lande op groot skaal geëet. Hartmossels word dikwels ook as asbakke gebruik, terwyl die doopvontskulp as doopvont kan dien. Sommige oesterspesies vorm pêrels. Die voorkant van baie mossels se skulprande is voorsien van skerp tande, wat hulle gebruik om gate in hout mee te boor. Die saagsels dien ook as voedsel. Paalwurms het veral vroeër skepe met houtrompe en houtpale by hawens erg beskadig.

Klassifikasie

Die klassifikasie van die klas Bivalvia kan verwarrend wees en daar bestaan 'n hele aantal klassifikasiestelsels. In die algemeenste stelsel wat tans gebruik word, word die klas in 3 subklasse ingedeel. Die subklas Protobranchia bevat die primitiefste tweekleppiges met eenvoudige kieue. Sommige van die diere, soos lede van die familie Nuculidae, neem nog voedsel met hul tentakels in.

Lede van die subklas Lamellibranchia (Polysyringia) het gevoude kieufilamente, wat vir gaswisseling en die filtrering van voedsel uit die water gebruik word. Die orde Anisomyaria bevat die mossels (genera Mytilus en Modiolus) en die oesters (genera Ostrea, Crassostrea, Spondylus, Pinctada, Anomia en Lima). Die ander families bevat onder meer die kammossels en die skeepsmossels.

Taksonomie

'n Japanse oester.

• Klasse: Bivalvia Linnaeus, 1758
  • Subklas Protobranchia Pelseneer, 1889
  • Subklas: Pteriomorphia Beurlen, 1944
  • Subklas: Palaeoheterodonta Newell, 1964
  • Subklas: Heterodonta Neumayr, 1884

Bronne

• Wêreldspektrum, 1982, ISBN 0908409680, volume 27, bl. 201 - 203
• The Reef Guide: Fishes, corals, nudibranchs & other invertebrates: East & South Coasts of Southern Africa. Dennis King & Valda Fraser. Struik Nature. 2014 ISBN 978-1-77584-018-3

Eksterne skakels

• Wikimedia Commons het meer media in die kategorie Tweekleppige.

Los bivalvos (Bivalvia, bi = dos; valvia = cáscara o placa), lamelibranquios (Lamellibranchia) o pelecípodos (Pelecypoda) son una clase del filu Mollusca con unes 13 000 especies, la mayoría marines. Presenten un cascu con dos valva llaterales, xeneralmente simétriques, xuníes por un gonciu y lligamentos. Diches cáscares cerrar por aición d'unu o dos músculos aductores.

Atópase-yos soterraos en fondos blandos (infauna), como habitantes fixos de superficies y estructures ríxides o llibres sobre los fondos epifauna. Delles especies furen el sustrato (roca o madera) y delles más son comensales o parásites.lolo

Carauterístiques

Escarecen de cabeza estremada y de tentáculos. Nun tienen mexelles nin rádula. La boca presenta palpos llabiales carnosos. Alienden por aciu unu o dos pares de branquies, xeneralmente llaminares.

Nes conches de los bivalvos reparar gran variedá de tamaños, formes, colores y dibuxos esculpíos na superficie. El tamañu fluctúa dende conches diminutes (2 mm) hasta especies que pueden algamar 15 dm de llargu y un pesu de 250 kg.^{[ensin referencies]} Ente los moluscos bivalvos más conocíos podemos nomar: ostra, amasuela, navaya, moxón, chancia de los barcos, coquina, etc.

Nel cantu anterior del mantu estremen trés pliegues: internu, mediu y esternu. La plega interna ye muscular, el mediu destaca pola so función sensorial y l'esternu ta rellacionáu cola secreción de la concha. La concha ta estremada en dos valva xuníes dorsalmente na charnela, que consiste nun lligamentu elásticu formáu por conquiolina y secretado pol mantu; nun ta bien calcificado, polo que permanez flexible y elásticu. El músculu o "pie" carauterísticu de los moluscos, nos bivalvos puede presentase modificáu o bien amenorgáu según el vezu de les distintes especies.

• Vista interna d'una cáscara.

• 1: Planu de simetría. 2: Llinies de crecedera. 3: Lligamentu. 4: Umbo.

Historia natural

Tolos representantes d'esta clase son acuáticos, tanto marinos como dulceacuícolas, y pueden atopase dende les llendes cimeres de la pleamar hasta les zones abisales. La proteición de les conches dexa que delles especies especializaes soporten les condiciones de la franxa costera intermareal.

Típicamente, les especies que viven sobre sustratos blandios como fangos y arenes, presenten un pie que-yos dexa escavar y tien forma d'hachu. Les especies sésiles caltiénense xuntaes al sustrato, bien seya por cimentación, como les orca, o por aciu la secreción d'una serie de filamentos que conformen el biso (moxones).

Aquellos bivalvos que viven so l'arena (suelu blandu), aliméntense penerando pequeñes cantidaes d'agua mientres les marees altes, d'onde estrayen el alimentu. Nun suelen viaxar llueñe, yá que los sos órganos tán afechos pa penerar l'agua al traviés de les cáscares de la so concha en cuenta de que'l bivalvu mesmu treslládese pa introducir agua nel so interior. Nes ocasiones en que se mueve, utiliza'l so pie, un músculu que-y dexa soterrar se nel sable.

Ecoloxía y Paleontoloxía

Los Bivalvia son moluscos esencialmente acuáticos, na so mayoría marinos, qu'habiten con preferencia la rexón bentónica sublitoral, anque tamién los atopa hasta les fondures abisales.El salín y la temperatura cuntar ente los factores que más inflúin na so distribución. Tienen gran valor nos analises paleoambientales por cuenta de la so estrecha rellación col tipu de sustrato, la bona presencia nel rexistru fósil dende'l Cámbricu, la so gran diversidá y bayura en variaos paleoambientes, y escelente representación nos medios acuáticos actuales. Estos moluscos viven nun ampliu rangu de temperatures de l'agua; polo xeneral el tamañu y espesura de la conchilla escayen al menguar la temperatura. Son más comunes n'agües bien osixenaes, anque delles especies comenencioses o con adaptaciones especiales pueden vivir n'ambientes probes n'osíxenu hasta dafechu anóxicos. Ye ampliu tamién el rangu de salín que soporten, dende agües duces continentales hasta mares hipersalinos, pero son muncho más diversos en condiciones de salín marín normal.

El salín y la temperatura tienen poca influyencia sobre los vezos de vida, sicasí la depredación condiciona notablemente dellos aspeutos xenerales de la morfoloxía funcional de los bivalvos, como la secreción del biso o la escavación, anque se conoz menos la so influyencia nel desarrollu de variantes morfolóxiques específiques pa protexese de depredadores (cementación, escayos sifonales). Los factores más importantes qu'afecten la manera de vida de los bivalvos son la disponibilidad d'alimentu, el tipu de sustrato y el movimientu de l'agua. La morfoloxía funcional de les valvasestá bien rellacionada col calter del sustrato. Tocantes a la turbulencia de l'agua, puede dicise que polo xeneral prefieren ambientes con movimientu de l'agua moderao; en condiciones aturbolinaes predominen los cementantes, nidificadores, furadores y escavadores rápidos.

L'estudiu de la diversidá morfolóxica de los bivalvos en rellación a distintes estratexes de manera de vida demostró qu'esisten morfoloxíes adaptatives recurrentes^[1], tipificadas caúna por conxuntos de calteres específicos. Munchos bivalvos (especialmente los infaunales) viven col so comisura empobinada en posición perpendicualr a la interfase ente'l sustrato y l'agua, pero esisten dalgunos (comunes ente los epifaunales, anque tamién los hai infaunales) que lo faen con una de les cáscares contra'l sustrato o de manera que la comisura queda aproximao paralela o oblicua a la interfase. Estos postreros bivalvos denominar pleurotéticos, y reconócense por ser inequivalvos en dalgún grau (cáscares derecha y izquierda con convexidá, espesura, ornamentación o entá coloración distintes), pueden tener cáscares torsionadas, que la so comisura nun se topa nun planu y, nel casu de los pleurotéticos epifaunales, son cuasi siempres monomiarios. Los pleurotéticos inclúin a tolos bivalvos cementantes, la mayoría de los sofitaos, munchos bisados y dellos escavadores.

Sicasí, estos vezos nun son exercíos con exclusividad, presentándose frecuentes combinaciones de los mesmos. Delles especies escavadores (especialmente árcidos) empleguen un débil biso p'aumentar la estabilidá nos estratos blandos. Otres especies que viven fixes pol biso, pertenecientes a los xéneros Isognomun y Barbatia, habiten introducíes a la manera d'una cuña ente colonies de corales o roques, igual que delles especies nidificadoras llibres. Munches formes nadadores, como los pectínidos, pasen bona parte de la so esistencia sofitaes no fondero o fixes pol biso. Petricola pholadiformis de normal vive como un escavador en sedimentos blandos o furador en sustratos duros. Na canal de Beagle, Hiatella solida afitar pol biso a los sustratos predresos y al cachiyuyo, en llatitúes menores ye furadora en sables duros o incrustante de bivalvos y gastrópodos, y en Brasil vive fixa sobre roques, estrelles de mar y briozoos o ente tubos de poliquetos^[2].

Estes variaciones en vezu pueden producise a lo llargo de la ontogenia, como se deduz dacuando de cambeos morfolóxicos rexistraos na conchilla.

La diversidá de formes que presenten les conchillas se correlaciona cola diversidá de hábitats ocupaos y la variedá de maneres de vida adoptaos, y asina grupos de bivalvos pocu rellacionaos filogenéticamente pueden desenvolver conchillas con calteres similares^[3].

Referencies

• Jonathan Elphick, Jen Green, Berbara Taylor, Richard Walker (2001). Gran Enciclopedia de los Animales. Santiago de Chile: La Tercera..

Enllaces esternos

• Wikimedia Commons acueye conteníu multimedia sobre Bivalvia.
• Bivalvia - Malacoloxía - Páxina de la Sociedá Española de Malacoloxía.

İkitaylılar (lat. Bivalvia, Lamellibranchia və ya Pelecypoda) — molyusklar tipinə aid heyvan sinfi.

Molyuskaların bu sinfinə daxil olan nümayəndələrində çanaq ikitaylıdır. Mantiya boşluğunda qəlsəmələr yerləşir. Baş reduksiyaya uğramışdır. Ürək üç kameralı olub, iki qulaqcıq və bir mədəcikdən ibarətdir. Böyrək bir cütlüdür. Əksəriyyəti müxtəlifcinslidir. Dənizlərdə və şirin sularda yaşayan əhəmiyyətli bir çox nümayəndələri vardır.

Təsnifatı

• Anomalodesmata yarımsinfi
• Bərabərdişlilər (Heterodonta) yarımsinfi
• Paleoheterodonta yarımsinfi
• Protobranchia yarımsinfi
• Pteriomorphia yarımsinfi

An daougleureged eo ar blotviled a ya d'ober ar gevrennad Bivalvia. Perzh boutin an holl loened-se eo o c'hrogenn rannet e div lodenn stag an eil ouzh eben hag a c'hell serriñ ha digeriñ.

Bevañ a reont holl en dour (dous pe sall).

Rummatadur

N'emañ ket an holl loenoniourien a-du diwar-benn rummatadur an daougleureged. E-touez an urzhiadoù, spesadoù bev enno hiziv an deiz, emañ:

• Nuculoida
• Solemyoida
• Arcoida
• Mytiloida (an darn vrasañ eus ar meskl)
• Ostreoida (skouer : an istr)
• Pterioida
• Unionoida
• Myoida
• Veneroida (skouer : kontelleged)
• Pholadomyoida

Gastròpode fòssil i bivalves fragmentades en pedra calcària juràssic al sud d'Israel.

Els bivalves (Bivalvia) són un grup de mol·luscs. Habitualment presenten una conquilla separada en dues parts, amb les dues valves simètriques respecte a la línia de la frontissa. Aquesta classe conté unes 30.000 espècies, incloent ostres, vieires, cloïsses, petxines i musclos.

Els bivalves són exclusivament aquàtics; n'hi ha formes marines i d'aigua dolça. De totes maneres algunes, com els musclos, poden sobreviure fora de l'aigua per a períodes curts de temps tancant les valves.

Sistemàtica

La sistemàtica exposada aquí està d'acord amb la classificació de Newell (1965) basada en la morfologia de les dents de la frontissa que formen ambdues valves. No hi ha una filogènia robusta.^[1]

Subclasse Anomalosdesmata

• Pholadomyoida

Subclasse Cryptodonta

• Cardiola †
• Praecardioida †
• Solemyoida

Subclasse Heterodonta †

• Cycloconchidae †
• Hippuritoida †
• Lyrodesmatidae †
• Myoida
• Redoniidae †
• Veneroida

Subclasse Paleoheterodonta

• Modiomorpha †
• Trigonioida †
• Unionoida (típics músclos d'aigua dolça)

Subclasse Palaeotaxodonta

• Ordre Nuculoida

Subclasse Pteriomorphia (ostres, músclos, etc.)

• Arcoida
• Cyrtodontoida †
• Mytiloida
• Ostreoida
• Praecardioida †
• Pterioida

També existeix un altre esquema sistemàtic alternatiu d'acord amb la morfologia de les brànquies (Franc 1960). Aquest distingeix entre Protobranchia, Filibranchia i Eulamellibranchia. El primer grup correspon a Palaeotaxodonta + Cryptodonta de Newell, el segon a Pteriomorphia, i l'últim conté la resta de grups.

L'any 2010, Bieler, Carter i Coanes van proposar un nou sistema de classificació dels bivalves.^[2]

Bivalves

• Subclasse Autobranchia
  • Superordre Heteroconchia
    • Clade Heterodonta
      • Ordre Actinodontoida †
      • Ordre Carditoida
      • Ordre Hippuritoida †
      • Ordre Lucinoida
      • Ordre Myoida
      • Ordre Orthonotoida †
      • Ordre Pholadomyoida (=Anomalodesmata)
      • Ordre Veneroida
    • Clade Palaeoheterodonta
      • Ordre Trigonioida
      • Ordre Unionoida
  • Superordre Nepiomorphia †
    • Ordre Antipleuroida †
    • Ordre Praecardioida †
  • Superordre Pteriomorphia
    • Ordre Arcoida
    • Ordre Cyrtodontoida †
    • Ordre Limoida
    • Ordre Mytiloida
    • Ordre Ostreoida
    • Ordre Pectinoida
    • Ordre Pterioida
• Subclasse Protobranchia (=Palaeotaxodonta)
  • Ordre Nuculanoida
  • Ordre Nuculoida
  • Ordre Solemyoida

Referències

Haeckel Acephala

Grŵp o organebau anfudol di-ben yw cregyn deuglawr (unigol: cragen ddeuglawr; Saesneg: bivalves) sy'n perthyn i'r dosbarth Bivalvia o fewn y ffylwm Mollusca. Mae'r dosbarth yma'n cynnwys:

• cragen fylchog (cregyn bylchog) a elwir hefyd yn 'gragen Berffro' (Saesneg: clams)
• wystrysen (wystrys), weithiau 'llymarch' (llymeirch)
• cocosen (cocos)
• cragen las neu 'misglen' (ll. cregyn gleision; Saesneg: mussels)
• cragen fylchog (ll. cregyn bylchog; Saesneg: 'scallops)

Yn y gorffennol, cyfeirir atynt fel Lamellibranchiata a Pelecypoda. Maent i'w canfod mewn dŵr hallt a dŵr croyw. Mae'r gair 'clawr' yn yr enw 'cragen ddeuglawr' yn cyfeirio at ddau blât (neu 'haenau') cymesur o galsiwm carbonad a gynhyrchir gan secretiad o'i chwarennau, gyda cholfach yn eu cyplysu. Mae gan y gragen ddeuglawr hefyd ddwy falf er mwyn ffiltro'r bwyd o'r dŵr ac sy'n ei galluogi i anadlu a bwyta; esblygodd y tegyll yn organau arbenigol a elwir yn ctenidia i hwyluso hyn.

Mae rhai rhywogaethau'n claddu eu hunain mewn tywod neu bridd, gyda'r seiffon yn unig yn ymestyn i'r wyneb er mwyn iddi anadlu. Mae mathau eraill yn angori eu cyrff i graig neu'n gorwedd ar wely'r môr, a gall rhai mathau nofio e.e. cregyn bylchog.

Wystrysen

Wystrysen mwya’r byd?

Wystrysen fawr sy’n cystadlu i fod o faint sy’n torri record

Gall hwn fod y wystrysen fwyaf i'w gweld ym Mhrydain?^[1] Dyma adroddiad yn y Guardian yn 2009 am un “mwyaf” arall llai: It measures 18cm (7ins) across and weighs 1.36kg (3lbs), apparently the largest recorded in British waters. The previous largest, found in Scotland, weighed 0.83kg (1.8lbs).

Misglen neu’r gragen las

Pysgodfa cregyn gleision aber yr afon Conwy

Labordy Conwy

Mae cregyn gleision Conwy yn fyd enwog. Yng nghanol yr 1980au bu Stephen Lockwood yn gyfarwyddwr Labordy Pysgodfeydd Conwy hyd nes iddo gau ym 1999. Rol Labordy Conwy oedd gwneud gwaith ymchwil ar feithrin pysgod a physgod cregyn ag effeithiau pysgota ar yr amgylchedd morol. Bu pysgodfa a physgotwyr cregyn gleision Conwy â chysylltiad agos â'r labordy a bu labordy Conwy yn chwarae rhan flaenllaw wrth ddatblygu technegau ar gyfer cynhyrchiad masnachol y rhywogaethau hyn, technegau sy'n cael eu defnyddio ledled y byd.^[2]

Rheoleiddio’r gwelyau cregyn

Mae cregyn gleision yn amrywio yn nhermau eu nodweddion a'r gwahanol fathau ohonynt. Mae angen rheoleiddio gwelyau'r cregyn gleision mewn mannau penodol ar aber yr afon Conwy. Mae dull lleol a lled unigryw o gasglu’r cregyn gleision sydd yn cael ei rannu a dull tebyg yn Long Island yn UDA, sef cribin arbennig.^[2].

Haint

Yn oes aur y diwydiant ar y Gonwy amlygwyd problemau y gellid cael eu hachosi trwy fwyta cregyn gleision. Bu cynnydd ym mhoblogaeth yr ardal yn y 19G ac ar ddechrau'r 20G. Nid oedd carthion yn cael eu trin cyn iddynt gael eu harlIwys i Afon Conwy ac fe achoswyd yn sgil hynny broblemau coluddol i'r rhai oedd yn bwyta'r cregyn. Ym 1912, caewyd y bysgodfa gan yr awdurdodau iechyd. Blwyddyn yn ddiweddarach, penodwyd bacteriolegydd enwog, Robert Dodgson (dim yn berthynas I Charles "Lewis Carroll" Dodgson) gan Gyngor Conwy. Ei waith oedd datrys y broblem. Cynlluniodd ddull syml a dibynadwy i buro'r cregyn gleision. Mabwysiadwyd y model ar draws y byd.^[2].

Cyfeiriadau

Mlži (Bivalvia) jsou vodní živočichové, řazení do kmene měkkýšů (Mollusca). Je známo kolem 30 000 druhů mlžů. Patří k nim tak obecně známí obyvatelé mořských a sladkovodních prostředí, jako ústřice, slávky, srdcovky, škeble a perlorodky.

Stavba těla

Stavba lastury mlžů

Mlži jsou měkkýši s výraznou bilaterální symetrií. Jsou přitom obvykle silně laterálně zploštělí. Plášť tvoří obvykle dva laloky, každý vylučuje jednu lasturu. Lastury jsou spojeny zámkem, vazy a silnými svaly. S okolním prostředím je vnitřek lastury spojen pouze třemi otvory, přijímacím, vyvrhovacím a otvorem pro vysouvání svalnaté nohy. Hlava je redukována – nenese ani oči či radulu. Přesto se mohou oči na jiných místech těla vyvinout; to samé platí pro statocystu. Uvnitř schránky se rovněž rozprostírá rozsáhlá plášťová dutina.^[1]

Dýchají pomocí žaber. Mají srdce, jejich tělní tekutina (krev) se nazývá „hemolymfa“ a obsahuje barvivo hemocyanin, jehož molekuly obsahují měď. Nervová soustava je gangliová. Mlži mají čichové ústrojí, podle kterého se orientují, a hmatové buňky. Vylučovací soustavu představuje jeden pár^[1] nefridií.

Rozličné tvary lastur mlžů

Pomocí nohy se mlži dokážou zahrabat do písku, nebo se přichytí k podkladu pomocí byssových vláken. Někteří mlži dokážou plavat. Mlži jsou filtrátoři, živí se částečkami potravy, které se zachytí na jejich žábrách.

Rozmnožování

Mlži jsou obvykle gonochoristé. Mají většinou nepřímý vývin, oplození je vnější. Mlži mohou mít dva druhy larev. Veliger je mořská a glochidie je sladkovodní a cizopasí na rybách.

Klasifikace a zástupci

Klasifikace mlžů je v současnosti v pohybu a málokteří dva autoři se shodnou na jedné verzi.^[1] Molekulární studie z r. 2015 analyzující transkriptom mlžů nabízí následující fylogenetický strom mlžů:^[2]

Protobranchia

Pteriomorpha

Palaeoheterodonta

Archiheterodonta

Anomalodesmata

Imparidentia

Níže jsou uvedeny jen příklady zástupců:

Sladkovodní:

• Perlorodka říční (Palaeoheterodonta)
• Velevrub malířský (Palaeoheterodonta)
• Škeble rybničná (Palaeoheterodonta)
• Slávička mnohotvárná

Mořští:

• Srdcovka jedlá (Imparidentia)
• Střenka jedlá (Imparidentia)
• Šášeň lodní (Imparidentia)
• Ústřice jedlá (Pteriomorpha)
• Slávka jedlá (Pteriomorpha)
• Perlotvorka mořská (Pteriomorpha)
• Hřebenatka jakubská (Pteriomorpha)
• Zéva obrovská
• Hřebenatka islandská

Odkazy

Reference

1. ↑ ^{a b c} BRUSCA, Richard C.; BRUSCA, Gary J. Invertebrates. 2. vyd. [s.l.]: Sinauer, 2003. ISBN 0878930973.
2. ↑ GONZÁLEZ, Vanessa L.; ANDRADE, Sónia C. S.; BIELER, Rüdiger, COLLINS, Timothy M.; DUNN, Casey W.; MIKKELSEN, Paula M.; TAYLOR, John D.; GIRIBET, Gonzalo. A phylogenetic backbone for Bivalvia: an RNA-seq approach. Proceedings of the Royal Society B [online]. 14. leden 2015. Svazek 282, čís. 1801: 20142332, s. 1-9. Dostupné online. ISSN 1471-2954. DOI:10.1098/rspb.2014.2332. (anglicky)

Externí odkazy

• Obrázky, zvuky či videa k tématu mlži ve Wikimedia Commons
• Slovníkové heslo mlž ve Wikislovníku
• Taxon Bivalvia ve Wikidruzích
• Galerie - středoevropští sladkovodní mlži
• Taxonomie mlžů na BioLibu
• Mlži ve Virtuálním muzeum ČGS

Muslinger (Bivalvia) er en klasse af dyr, der hører til bløddyrene. De er kendetegnet ved at have en hård skal, mens de på indersiden er bløde. De fleste muslinge-arter lever i saltvand, men der findes også arter i ferskvand.

I Danmark er der registeret i alt 152 arter af muslinger, men blåmuslingen er den mest udbredte art. ^[1]

Klassifikation

Klasse: Bivalvia

• Familie: Xenoturbellidae
• Underklasse: Protobranchia
  • Orden: Nuculoida
  • Orden: Solemyoida
• Underklasse: Metabranchia
  • Overorden: Filibranchia
    • Orden: Trigonioida
    • Orden: Pteriomorpha (fx blåmusling, østers)
  • Overorden: Eulamellibranchia
    • Orden: Unionida (fx dammusling)
    • Orden: Veneroida (fx almindelig hjertemusling, knivmuslinger, østersømusling)
    • Orden: Lucinoida
    • Orden: Myoida (fx sandmusling, pæleorm)
    • Orden: Anomalodesmata
    • Orden: Septibranchia

Muslinger som Fødevare

Arkæologiske udgravninger af f.eks. køkkenmøddinger har vist, at mennesket har spist muslinger i tusindvis af år og muslinger er stadig en vigtig fødevare på verdensplan den dag i dag.

Muslinger til konsum, både dyrkes og fanges. Når muslinger dyrkes, sker det ofte i havbrug med varierende teknikker, hvor dyrkning på liner har vundet meget frem de senere år, bl.a. på grund af miljøfordelene. Muslingefangst i naturen sker normalt ved at skrabe velkendte muslingebanker med bundtrawl, hvorefter kolonierne får lov at genskabes i en længere periode. Muslingeskrab, som metoden kaldes, har dog været kritiseret fra mange sider for at skade havmiljøet på mange niveauer. ^{[2] [3]}

Muslinger indeholder mange vigtige næringsstoffer og anses normalt for en sund fødevare. De indeholder især megen selen (46 µg), zink (3,4 mg), B12 (10,2 µg), folat (41,8 µg) og ikke mindst omega-3 fedtsyrer (890 mg), som er essentielle for os. Indholdsmængden er målt per 100 g kogt blåmusling (Mytilus edulis) og er middelværdier. ^[4]

Sikkerhed

Der er en vis sundhedsrisiko forbundet med selv at indsamle muslinger fra naturen og en minimal risiko ved de erhvervsfiskede muslinger. Dels kan der være problemer med fordærvede muslinger, dels kan de i sjældne tilfælde indeholde algegifte eller mikrobiologisk forurening fra f.eks. sygdomsfremkaldende bakterier eller virus. Fordærvede eller forurenede muslinger kan fremkalde skaldyrsforgiftning.

Fordærvede muslinger kan man normalt selv tjekke for ved at se om dyret er levende og reagerer på fysiske stimuli. Muslinger indeholdende biologiske gifte fra alger el.lign. kan man ikke selv umiddelbart tjekke.

Huskeråd

Muslinger som ikke åbner, når de er kogte, kan stadig være fuldt ud kogte og sikre at spise.^[5] Der er ellers en forståelse inden for kokkefaget om det modsatte, hvilket muligvis kan tildeles frygt for at gøre kunder syge, og fordelen ved at være på den sikre side. Til gengæld kan muslingerne være fordærvede efter at have ligget i beskidt vand.

Referencer

Eksterne henvisninger

• Fakta om fisk: blåmusling
• Mollusca på Curlie (som bygger videre på Open Directory Project)
• Systema naturae 2000 (classification) – Taxon: Class Bivalvia

Dieser Artikel erläutert die zoologisch als Muscheln bezeichnete Klasse der Weichtiere. Für weitere Bedeutungen siehe Muschel.

Die Muscheln (Bivalvia von dem lateinischen Wort bi-valvius „zwei-klappig“ abgeleitet) sind eine Klasse der Weichtiere (Mollusca). Merkmale der Klasse sind eine aus zwei kalkigen Klappen bestehende Schale und ein weitgehend reduzierter Kopf. Sie leben weltweit in Salzwasser (zu 80 Prozent), Brackwasser und Süßwasser und sind meist zwischen 0 und 100 m, selten bis 11.000 m Wassertiefe zu finden. Muscheln leben im Meeresgrund, sind an ihm festgewachsen oder liegen frei auf ihm. Die meisten Muschelarten ernähren sich von Plankton, das sie mit ihren Kiemen aus dem Wasser filtern. Die Lebenserwartung reicht von ca. 1 Jahr bis zu mehr als 500 Jahren.

Der Mensch nutzt Muscheln als Nahrungsmittel und Muschelschalen als Ausgangsmaterial für Schmuck z. B. als Perlenlieferant, Souvenir und früher auch als Muschelgeld. Darüber hinaus werden Muscheln auch in Teichen zur Reinigung verwendet.

Die Klasse enthält etwa 7500 bis 10.000 rezente und 20.000 fossile Arten, die in 106 Familien eingeteilt werden. Nach Huber (2010) ist heute von weltweit etwa 8000 marinen Arten in vier Unterklassen und 99 Familien mit 1100 Gattungen auszugehen. Die Familie der Venusmuscheln ist mit mehr als 680 Arten die größte marine Familie. Aus dem Süßwasser sind weitere etwa 1200 Arten in sieben zusätzlichen Familien bekannt. Die Fluss- und Teichmuscheln stellen hier mit etwa 700 Arten die artenreichste Familie.

Wissenschaftliche Bezeichnung

Der wissenschaftliche Name „Bivalvia“ (Carl von Linné, 1758) ist von der zweigeteilten Kalkschale abgeleitet, die ihren Körper je nach Art mehr oder weniger schützend umhüllt.

Die Bezeichnung „Lamellibranchia“ Henri Marie Ducrotay de Blainville 1824 wurde nach der Gestaltung der Kiemen (Blattkiemen) gewählt und ist in der wissenschaftlichen Literatur am häufigsten zu finden. Weitere vor allem historische Namen sind „Pelecypoda“ August Goldfuß 1820 nach der Gestaltung des Fußes oder „Acephale“ Georges Cuvier 1798 aufgrund des fehlenden Kopfes oder „Conchifera“ Jean-Baptiste de Lamarck 1818, wegen der Muschelschalen (Schalenträger).

Die Begriffe sind nicht ganz synonym, Probleme bilden die Abgrenzungen der Gruppen, so umfasste z. B. „Bivalvia“ für Linné auch die Armfüßer (Brachiopoden). Die deutsche Bezeichnung Muscheln wird nicht nur bei Weichtieren verwendet, sondern auch etwa für die zu den Krebsen gehörenden Entenmuscheln.

Körperbau

Weichkörper

Bei Muscheln ist der Kopf reduziert. Der Weichkörper der Muschel wird beidseitig von den Mantellappen bedeckt und umschließt die Kiemen und die meist getrenntgeschlechtlichen Gonaden. Das Kreislaufsystem mit einem Herzen ist offen. Typische Muskeln sind die zwei getrennten Schließmuskeln, die die beiden Schalenhälften zusammenziehen, die Fußrückziehmuskeln und die Mantelrückziehmuskeln. Der Fuß der Muscheln ist beweglich und mit Schleimdrüsen ausgestattet. Das Nervensystem besitzt zwei Nervenzellhäufungen: das Pedal- und das Viszeralganglion.

Rudimente des Kopfes

Aufgrund der ursprünglich im Sediment grabenden Lebensweise ist der Kopf mit Ausnahme der Mundregion zurückgebildet (deswegen nannte Frédéric Cuvier 1798 die Muscheln „Acephala“, die Kopflosen). Im Vergleich mit den anderen Weichtieren nur noch rudimentär sind die Tentaktel, die Kiefer, die Radula und die Zunge. Die Schlunddrüsen sind weitgehend reduziert. Neben der Mundöffnung sitzen noch paarige, flache Mundlappen. Auf diesen Mundlappen sitzen bei ursprünglichen Formen (wie den Nussmuscheln) noch bewimperte Taster, die Nahrungspartikel zur Mundöffnung transportieren.

Mantel

Sandklaffmuschel mit Sipho. Aus Herklots, 1859

Der Weichkörper der Muschel wird beidseitig von den Mantellappen bedeckt und geschützt. Der von den beiden Mantellappen gebildete Raum wird als Mantelraum bezeichnet.

Der Mantelrand besteht aus drei Falten, die unterschiedliche Aufgaben erfüllen: Die äußerste Randfalte bildet Schale und Schalenhaut (Periostracum), die mittlere dient der Wahrnehmung sensorischer Aufgaben (z. B. Punktaugen) und die innere Falte reguliert den Wasserstrom in den Mantelraum zu den Kiemen.

Bei dieser Kammmuschel sind die Punktaugen zu erkennen

Der Mantelrand ist bei den meisten Arten hell-dunkel-empfindlich. Bei schwimmenden Muscheln, wie Kamm- (Pecten) und Feilenmuscheln (Lima), die genauere Informationen über ihre Umgebung brauchen, ist der Mantelrand mit einfachen Ocellen (Punktaugen) besetzt. Bei der Gattung Arca können es über 200 Augen sein.

Ursprünglich verlaufen die Mantelränder getrennt und parallel zum Schalenrand (integripalliat). Abgeleitet sind bei den meisten Arten die aneinander liegenden Mantelränder mehr oder weniger miteinander verwachsen. Zum Teil bleiben nur drei Öffnungen bestehen: zwei Öffnungen am hinteren Ende der Muschel, durch die Atemwasser und Nahrung in den Mantelraum ein- und – durch die Kiemen gefiltert – wieder ausströmen, sowie einer Öffnung für den Fuß.

Der Mantelrand um die Atemöffnungen des Mantels sind bei grabenden oder bohrenden Muschelarten oft schlauchförmig verlängert, so dass die Muschel auch im Substrat mit Atemwasser und Nahrung versorgt ist. Die schlauchförmigen Mantelfortsätze bezeichnet man als Siphonen. Man unterscheidet einen zuführenden Sipho (Ingestionssipho) und einen ausführenden Sipho (Egestionssipho). Beide können zu einer einziehbaren Doppelröhre verwachsen sein, die im ausgestreckten Zustand länger sein kann als die Muschel selbst.

Damit dieser Sipho bei Gefahr in die Schale zurückgezogen werden kann, musste die Ansatzlinie der Mantelrandmuskeln nach innen ausweichen. Diese zurückgewichene Ansatzlinie ist in der Schale anhand einer mehr oder weniger ausgeprägten Mantelbucht der Mantellinie zu erkennen (sinupalliat).

Sandklaffmuscheln (Mya arenaria) beispielsweise leben eingegraben im Substrat des Wattenmeeres und versorgen sich durch den Sipho. Werden sie aus dem Substrat ausgespült, müssen sie sterben. Im Gegensatz dazu sitzt die Gemeine Miesmuschel (Mytilus edulis) angeheftet auf festen Untergrund und benötigt deshalb keinen Sipho. Sie stirbt daher, wenn sie von Substrat begraben wird.

Im Mantelrand der Riesenmuscheln (Tridacna) leben symbiotische Algen (Zooxanthellen), die von der Muschel geschützt werden. Im Gegenzug dafür kommt die Muschel in den Genuss der Photosyntheseprodukte der Algen.

Kiemen

Muscheln mit Sipho

Evolutionär haben sich aus einfachen Fiederkiemen zur Atmung die komplexeren Faden- und Blattkiemen zur Filtration entwickelt. Die im Mantelraum liegenden Kiemen der Muschel dienen dann nicht nur zur Atmung, sondern bei den meisten Arten außerdem zur Nahrungsaufnahme. Die Kiemen sind – wie die ganze Mantelhöhle – mit Wimpern besetzt, die einen Atemwasserstrom produzieren, der auch Nahrungspartikel einstrudeln kann. Die Kiemenbögen produzieren einen Schleim, in dem die Partikel hängenbleiben und auf Wimpernbändern zum Mund transportiert werden. Ungeeignete Partikel werden jedoch nicht verschluckt, sondern in Schleim zu größeren Scheinkotballen – Pseudofaeces – zusammengerollt und mit dem ausströmenden Wasser nach außen abgegeben.

Kiementypen:

• Die paarigen Fiederkiemen (Ctenidien) bestehen jeweils aus einem Schaft mit mehreren Kiemenblättchen.^[1]
• Bei den Fadenkiemen (Filibranchien) hängen vor und hinter dem Fuß W-förmige Kiemenfäden in zwei Reihen in den Mantelraum. Die Kiemenfäden sind dorsal an der mittleren Spitze des W befestigt und die Schenkel des W untereinander durch kleine Wimpernstrukturen verbunden.
• Die netzartigen Schein-Blattkiemen (Pseudolamellibranchien) sind durch seitliche Verwachsung der Kiemenfäden, also Verbindungen zwischen aufeinanderfolgenden „Ws“ entstanden.
• Die echten Blattkiemen (Eulamellibranchien) sind durch echte, von Blutgefäßen durchzogene, Gewebebrücken zwischen den Kiemenfäden gekennzeichnet.
• Die Netzkiemen (Septibranchien) sind schmal, netzförmig und seitlich mit dem Mantel verwachsen. Diese Verwachsung ist muskulös. Bei einigen Formen sitzen die Kiemen in Lochreihen dieser Verwachsung die dann eine Scheidewand (Septum) ausbildet, selten sind die Kiemen ganz reduziert. Typischerweise sind die Netzkiemen-Arten Tiefseebewohner und saugen z. T. aktiv Nahrung ein.

Kreislaufsystem

Das Blutgefäßsystem der Muscheln ist, wie bei den meisten Weichtieren, offen. Das Herz hat zwei Vorkammern und eine Hauptkammer. Die Hauptkammer wird primär und sekundär vom Enddarm durchzogen.

Fuß

Der ursprünglich flach beilförmige Fuß der Muscheln kann, angepasst an Lebensweise und Fortbewegung, unterschiedliche Formen wie balkenförmig, zungenförmig oder wurmförmig annehmen. Schwimmende und festsitzende Muschelarten haben oft einen weitgehend zurückgebildeten Fuß. Der Fuß trägt die Byssusdrüsen, die vor allem bei Jungmuscheln Haftfäden (70 % Kollagen) produzieren. Bei einigen Arten produzieren auch die ausgewachsenen Tiere Byssusfäden, mit denen sich die Muscheln am Untergrund verankern (z. B. bei Miesmuscheln (Mytilidae), Archenmuscheln (Arcidae), Kammmuscheln (Pectinidae) und Steckmuscheln (Pinnidae)). Die Byssusverbindung kann später gelöst werden, indem die Muschel die Fäden sekretorisch abtrennt (Mytilus) oder ganz abstößt (Perlmuscheln (Pinctada)). Miesmuscheln nutzen ihre Byssusfäden auch zur Verteidigung, indem sie kleinere Schnecken, z. B. Reusenschnecken (Hinia) damit einspinnen.

Gehäuse

Innenseite einer linken Muschelschale

Muschel von dorsal (von oben), rechte Schalenklappe unten

Schale einer Herzmuschel, Innenansicht der linken Schalenklappe

Das Gehäuse der Muscheln besteht aus einer rechten und einer linken Klappe und umgibt den Weichkörper der Muschel. Beide Klappen werden auf dem Rücken (also oben) mit einem Schloss und einem Schlossband (Ligament) zusammengehalten. Der Wirbel (Umbo) ist der älteste Teil des Muschelgehäuses. Farben und Formen sind sehr variabel. Das Gehäuse wird vom Mantel, einer Hautfalte der Muschel, gebildet und zwar in drei Schichten: der farbigen Schalenhaut (Periostracum), der Prismenschicht (Ostracum) und der inneren Kalkschicht (Hypostracum). Die beiden Hälften können durch zwei innere Schließmuskeln zusammengezogen werden.

Material

Die inneren Schalenschichten bestehen hauptsächlich aus Kalk, dessen Kristalle mittels einer organischen Substanz, dem Conchin, verkittet werden. Die Art der gebildeten Kalkstrukturen hat sich im Laufe der Evolution verändert: Ursprüngliches Merkmal ist die Bildung von perlmuttrigem Aragonit. Bei den heute dominierenden Heterodonta und bei den Taxodonta wird als abgeleitetes Merkmal ein kreuzlamellärer Aragonit gebildet. Parallel dazu entwickelten die Pteriida die Einlagerung von Calcit.

Calcit tritt auch abgeleitet in der mittleren Prismenschicht auf, und zwar bei den Anisomyaria und den ausgestorbenen Hippuritoida. Die Calcit- und Aragonitkristalle sind in der Prismenschicht in kleinen, eckigen Säulen (Prismen) angeordnet, die mehr oder weniger senkrecht nach außen zeigen.

Das äußere Periostracum besteht aus organischem Conchiolin und kann verschieden ausgeprägt sein. Am häufigsten wird eine dünne Haut ausgebildet, es kommen aber auch „Haare“ (z. B. Bärtige Archenmuschel) und lappige Vorhänge (z. B. Solemya togata) vor.

Ligament

Eigentlich ist die Schale nur im Bereich der beiden Kalkschichten (Ostracum und Hypostracum) in zwei Hälften geteilt. Die äußerste Schalenschicht, das Periostracum, überzieht die Schale auch hier, verstärkt sich zum Ligament und verbindet so rückenseitig die beiden Schalenhälften. Dieses Ligament zieht oft in den Scharnierbereich hinein und verdickt sich zum Resilinum (siehe Resilin und Abductin). Dieses Schlossband besteht aus nicht verkalkendem Conchin. Dieser Ligament-Resilinum-Komplex ist sehr elastisch und arbeitet antagonistisch zu den Schließmuskeln. Nach dem Tod des Tieres klaffen daher die beiden Schalen auseinander und werden durch mechanische Beanspruchung, wie etwa die Brandung, leicht getrennt, sodass man meist nur mehr einzelne Schalenhälften findet – im Gegensatz zur selteneren Dublette, die noch aus beiden Klappen besteht.

Schloss

Damit die beiden Hälften seitlich nicht verrutschen, tragen sie meist sogenannte Schlosszähne, deren Ausbildung auch Grundlage systematischer Zuordnungen ist. Diese Schlosszähne bestehen aus zahn- oder leistenförmigen Erhebungen am inneren Rückenrand der Klappen beziehungsweise entsprechenden Gruben auf der Gegenklappe, die ineinandergreifen. Auch ineinandergreifende Einkerbungen am unteren Schalenrand können ein seitliches Verrutschen verhindern. Diese Einkerbungen sind ebenfalls bestimmende Merkmale einer Muschelart.

Es werden bis zu neun verschiedene Schlosstypen unterschieden:

• taxodont (reihenzähnig): zahlreiche Zähne sind in einer (meist abnehmenden) Reihe am Rand der Muschel platziert (z. B. bei Archenmuscheln)
• heterodont (verschiedenzähnig): wenige große Zähne und max. vier Nebenzähne (z. B. bei Cerastoderma, Venusmuschel)
• desmodont (bandzähnig): Zwei Zähne zu einem verwachsen mit löffelförmigen Chondrophor, einem vorstehenden Träger des inneren Ligaments (z. B. bei Mya)
• pachyodont (dickzähnig): ein bis drei dicke Zapfen, die in tiefe Gruben der Gegenseite passen
• dysodont (schlecht bezahnt): ohne Zähne – nur kleine Erhebungen (z. B. bei Ostrea)
• schizodont (spaltzähnig): mittlerer Zahn der linken Klappe meist gespalten und rechts von zwei keilförmigen Zähnen umrahmt
• isodont (gleichzähnig): zwei bis vier Zähne symmetrisch auf beiden Klappen
• hemidapedont: schwach ausgebildete Hauptzähne, nur selten Nebenzähne
• anomalodesmatisch: ohne oder nur mit schwach ausgebildeten Zähnen, mit Chondrophor, einem vorstehenden Träger des inneren Ligaments (Schließknorpel oder Resilium)

Analog spricht man bei den Schalen der Articulata, die keine Muscheln sind, sondern zu den Armfüßern gehören, vom Schloss.

Muskelabdrücke

Die Ansatzstellen der Schließmuskeln (Schließmuskelabdrücke) und der Mantelrückziehmuskeln (die Mantellinie) spielen für Systematik und Bestimmung eine wichtige Rolle. Von den Schließmuskelabdrücken sind ursprünglich zwei gleich große vorhanden (isomyar). Im Lauf der Evolution entstanden Arten mit unterschiedlich großen Schließmuskelabdrücken (anisomyar oder heteromyar) und solche mit nur einem Abdruck (monomyar).

Die Mantellinie veränderte sich von einer einfachen gebogenen Linie, die die beiden Schließmuskelabdrücke verbindet, (integripalliat, vgl. das Foto einer Herzmuschel) hin zu einer eingebuchteten Form (sinupalliat, vgl. Zeichnung).

Formen

Konvergente Schalenformen bei Muscheln: Panopea und Mya leben vergraben im Sediment

In Farbe, Form und Beschaffenheit sind die Schalen der einzelnen Arten sehr unterschiedlich. Weiße und stachlige existieren ebenso wie längliche, schwarze und glatte Schalen.

Die Schalenform der Muscheln ist an ihre jeweilige Lebensweise angepasst. Da Muscheln aus unterschiedlichen Gruppen oft ähnliche Lebensweisen besitzen, etwa im Sediment vergraben durch einen Sipho Wasser filtrieren, bilden sie trotz geringer Verwandtschaft oft ähnliche Schalenformen aus (Konvergenz). Der Zusammenhang zwischen Schalenform und Lebensweise ermöglicht auch die Rekonstruktion der Lebensweise fossiler Formen und eventuell auch deren Habitate.

Löcher

Am Strand angespülte Muschelschalen weisen manchmal kreisrunde, 1–3 mm große Löcher auf. Diese werden von Raubschnecken (meist Naticidae) erzeugt, welche die Muschel mit ihrer Raspelzunge aufbohren und dann die Weichteile verzehren.^[2]

Lebensweise

Lebensräume

Riesenmuscheln sind mit bis zu 140 cm Länge und 400 kg Gewicht die größten noch lebenden Muscheln

Muscheln sind an das Leben im Wasser gebunden. Sie kommen sowohl im Salzwasser, Brackwasser als auch Süßwasser vor, von der Arktis und Antarktis bis in die Tropen. Besonders im Wattenmeer findet man sehr große Mengen an Muscheln (beispielsweise Herzmuscheln und Miesmuscheln) im beziehungsweise auf dem Watt. Sie stellen eine wichtige Nahrung für Seevögel dar.

Die Art Enigmonia aenigmatica (HOLTEN, 1803) (eine Anomiidae) lebt in der Gezeitgischt auf Mangrovenblättern im Indopazifik und entspricht am ehesten einer Lebensweise an Land.

Muscheln leben als erwachsene Tiere überwiegend sessil (festsitzend), teils an festen Oberflächen, beispielsweise Felsen oder Steinen, teils im Sand oder Schlick. Sie gehören also zum Benthos. Sie sind von der Gezeitenzone bis in die Tiefsee verbreitet.

Fortbewegung und Verankerung

Folgende Fortbewegungsarten sind bei Muscheln beobachtet worden:

• Fortbewegung mit Hilfe des Fußes

Die häufigste Fortbewegungsweise bei Muscheln ist die Fortbewegung mit Hilfe ihres Fußes. Dieser Körperteil kann zum Beispiel bei Herzmuscheln (Cardiidae) auf die dreifache Länge des Schalendurchmessers gestreckt werden. Dank ihres Fußes können sich Muscheln schnell eingraben, ruckweise kriechen oder sogar springen (Knotige Herzmuschel (Acanthocardia tuberculata) aus einem 20 cm hohen Aquarium). Für die Fortbewegung brauchen die Muscheln ein geeignetes Substrat, da sie im Gegensatz zu Schnecken nie einen echten Kriechfuß besitzen. Arten der Gattung Sphaerium sind in der Lage, sich auf Pflanzen spannerartig fortzubewegen, indem der Fuß ausgestreckt, die Spitze festgeklebt und der Körper nachgezogen wird.

• Durch Schalenklappern freies Schwimmen auf kurzen Strecken

Arten der Gattung Lima schwimmen gut, einige der Familie Kammmuscheln (Pectinidae) ebenfalls. Sie können durch ruckartiges Zusammenklappen der Schalenhälften einen gerichteten Wasserstrom erzeugen und sich so nach dem Raketenprinzip ein Stückchen durch das Wasser bewegen. Kammmuscheln können angeblich Wasser auch gezielt durch die Öhrchen an den Seiten ausstoßen und so ihre Bewegungen, unterstützt durch ihre Linsenaugen, feiner koordinieren.

• Verankern mit Byssusfäden

Einige Muscheln (unter anderem Miesmuscheln (Mytilus), Steckmuscheln (Pinnidae), Archenmuscheln (Arcidae) und Sattelmuschel (Anomia ephippium)) spinnen mit einem Sekret der Byssusdrüse an ihrem Fuß so genannte Byssusfäden, mit denen sie sich aneinander und an der Unterlage festkleben. Für Positionswechsel können zumindest Miesmuscheln die Byssusfäden mit einem Sekret wieder auflösen.

• Feste Verankerung mit der Schale am Untergrund

Bekanntestes Beispiel sind die Austern (Ostreidae). Weitere festgewachsene Gattungen: Chama, Pseudochama, einige Anomia-Arten, Stachelaustern (Spondylus).

• Keine feste Verankerung, da von Substrat geschützt

Bei Arten, die in Substrat wie Holz oder Stein bohren, kann die Bewegungsfähigkeit eingeschränkt und die Schale als Schutz reduziert sein. Beispiele: Schiffsbohrwürmer und Krause Bohrmuschel.

Ernährung

Die Muscheln waren ursprünglich Sedimentfresser, abgeleitet entstanden filtrierende Planktonfresser und Sonderformen wie Holzfresser, Jäger und Bakterienzüchter.

Die urtümlichen Muscheln aus der Unterklasse Protobranchia sammeln mit verlängerten Mundlappen vom umgebenden Substrat essbare Partikel, wie Protozoen, Eier, Larven und verdaulichen Detritus ein. Die Nahrung gelangt anschließend über eine Wimperrinne zur Mundöffnung.

Die meisten höher entwickelten Muscheln ernähren sich ausschließlich durch Filtration ihres Atemwassers. Im Mantelraum und auf den Kiemen gelegene Wimpern erzeugen einen gerichteten Wasserstrom, der durch eine Atemöffnung eintritt und durch die andere wieder austritt. Feste Teilchen im Atemwasser werden an den Kiemen aufgefangen und gelangen in einem Schleimpaket zur Mundöffnung. Die verwertbaren Partikel, meist Plankton, werden verdaut, der Rest in Kotschnüren ausgeschieden. Aufgrund ihrer Ernährung durch Filtration kommen Muscheln mit sehr großen Wassermengen in Kontakt, was sie besonders empfänglich für im Wasser enthaltene Schadstoffe macht und als Bioindikatoren prädestiniert.

Die Bohrmuscheln (Teredinidae) verwerten mit speziellen Enzymen das Holz, in dem sie bohren.

Einige, möglicherweise auch die meisten Arten der zu den Anomalodesmata gehörenden Septibranchia (darunter die Keulenmuscheln) sind Jäger, die mithilfe ihres muskulösen Septums in der Mantelhöhle aktiv kleine Krebstiere einsaugen.

Die Riesenmuscheln (Tridacna) und Arten der Gattung Solemya leben mit symbiotischen, Photosynthese betreibenden Algen im Mantelrand, und Tiefseearten an Black Smokern halten sich Sulfidbakterien in speziellen Strukturen ihrer Kiemen.

Fortpflanzung und Entwicklung

Die meisten Muschelarten sind getrennt geschlechtlich; es gibt sowohl männliche, als auch weibliche Tiere. Befruchtung und anschließende Larvenentwicklung finden äußerlich im Wasser statt. Der Ausstoß von Eiern und Samenzellen kann etwa bei den Riesenmuscheln (Tridacna) hormonell koordiniert sein. Nach einer Entwicklung über ein Larvenstadium vom Trochophora- oder Veliger-Typus entsteht aus der Larve nach einer Metamorphose die Jungmuschel, die sich während ihres Heranwachsens einen passenden Ort sucht, an dem sie das Erwachsenenleben verbringen kann. Bei Miesmuscheln (Mytilus), Austern (Ostrea) und anderen koloniebildenden Arten bleiben die Jungmuscheln meist in der Nähe der Kolonie und befestigen sich anschließend nicht nur am Untergrund, sondern auch an anderen Muscheln. So entstehen beispielsweise die Muschelbänke der Miesmuschel wie man sie aus dem Wattenmeer kennt.

Die im Süßwasser lebenden Muschelarten zeigen sehr unterschiedliche Fortpflanzungs- und Entwicklungsmethoden. Diese Muschelgruppen, ebenso wie die im Süßwasser und an Land lebenden Schneckengruppen, haben sich im Verlauf ihrer Entwicklung stark an die wechselhaften Lebensbedingungen, die das Süßwasser von den relativ konstanten Bedingungen im Meer unterscheiden, angepasst. Die Gruppe der Unionida, zu denen die in Mitteleuropa heimischen Großmuscheln (Maler-, Bach-, Teich- und Flussperlmuscheln) gehören, entwickelt sich über ein parasitisches Larvenstadium, die so genannten Glochidien, die sich zur weiteren Entwicklung erfolgreich an einem vorbeischwimmenden Fisch festheften müssen.

Im Gegensatz dazu sind die meisten im Süßwasser lebenden Kleinmuscheln (Erbsenmuscheln (Pisidium) und Kugelmuscheln (Sphaerium)) zwittrige Tiere, die lebende Larven gebären (Ovoviviparie). Die zu den Dreikantmuscheln gehörende Wandermuschel (Dreissena polymorpha) hingegen entwickelt sich, wie ihre meereslebenden Verwandten, über ein veligerähnliches planktontisches Larvenstadium.

Gefährdung

Durch Gewässerverschmutzung und Flussbegradigungen sind die großen Süßwassermuscheln (Unionida), zu denen auch die Flussperlmuschel (Margaritifera margaritifera) zählt, vielerorts stark in ihrem Bestand bedroht. Hinzu kommt die weitere Dezimierung durch die 1905 nach Eurasien eingeschleppte Bisamratte, die als Haupt-Fraßfeind dieser Muschelarten zählt.

Stammesgeschichte und Systematik

Evolution

Verteilung von 2032 Muschelgattungen vom Kambrium bis zur geologischen Gegenwart

Muscheln erscheinen fossil erstmals im Kambrium vor etwa 500 Millionen Jahren. Die frühen Formen waren noch einfach gebaute Weichtiere mit einer einklappigen Schale. Im mittleren Ordovizium erschienen erstmals Vertreter aller modernen Unterklassen.

Die rezenten Muscheln sind aus sedimentgrabenden Vorfahren entstanden. Diese hatten als Anpassung an das Leben im Meeresboden den Kopf bis auf die Mundöffnung und die Mundlappen reduziert. Jüngere Formen stellten auf eine Ernährung durch Filtration um. Evolutionär haben sich aus einfachen Fiederkiemen zur Atmung die komplexeren Faden- und Blattkiemen zur Filtration entwickelt.

Während der Kreidezeit bildeten schnell wachsende Muscheln riffähnliche Strukturen, die mit heutigen Korallenriffen vergleichbar sind. Diese Muscheln, deren Schalen Licht leitende Elemente enthielten, lebten teilweise in Symbiose mit Photosynthese betreibenden Einzellern.

In vielen Gesteinen zählen Muscheln zu den besonders häufigen Fossilien, da die harte Schale sich gut erhält. Sie dienen deswegen oft als Leitfossilien.

Die Evolutionsgeschwindigkeit der Muscheln ist sehr unterschiedlich. Während als Leitfossil dienende Arten im Durchschnitt 0,3 bis 1 Million Jahre existieren, sind einzelne Gattungen erheblich langlebiger. Die Gattung Gryphaea ist seit dem Unterjura (195 Mio. Jahre), die Gattung Spondylus seit dem Perm (285 Mio. Jahre) und Lima seit dem Jura nachgewiesen.

Das ursprüngliche, taxodonte Schloss der Muscheln besitzt eine große Zahl kleiner, gleichförmiger Zähnchen, während hoch abgeleitete, heterodonte Formen aus wenigen Hauptzähnen und bis zu vier leistenförmige Seitenzähnen bestehen oder gar keine Zähne besitzen.

Systematik der Großgruppen

Die Systematik der Muscheln war in der Vergangenheit größeren Veränderungen unterworfen. Auch heute kommen vor allem durch molekulargenetische Untersuchungen neue Daten hinzu, die selbst die Großsystematik noch geringfügig verändern könnten. Allerdings hat sich die Systematik und auch die Phylogenie der Großgruppen in den letzten 15 Jahren jedoch – mit wenigen Ausnahmen – weitgehend stabilisiert. Die Unterschiede in den verschiedenen Klassifikationen beruhen vor allem in der sehr subjektiven hierarchischen Stellung einiger weniger Gruppen (z. B. Ordnung Ostreida gegen Überfamilie Ostreoidea, Anomalodesmata Ordnung oder Überordnung).

Die Klassifikation der Muscheln nach Bieler & Mikkelsen (2006) (nur rezente Gruppen) und Amler et al. (2000) (auch fossile Gruppen) sowie nach dem Zoological Record stellt sich zurzeit so dar:

Klasse Bivalvia Linnaeus, 1758

• Infraklasse Protobranchi(at)a Pelseneer, 1889
  • • • Ordnung Nuculida Dall, 1889
      • Ordnung Solemyida Dall, 1889
      • Ordnung Nuculanida Carter, Campbell & Campbell, 2000 (diese neue Ordnung hat noch keine allgemeine Akzeptanz gefunden (nach Zoological Record); alternativ können die in dieser Ordnung vereinigten Familien in die Ordnung Nuculida gestellt werden)
• Infraklasse Autolamellibranchi(at)a/Autobranchi(at)a Grobben, 1894
  • Unterklasse Pteriomorphia Beurlen, 1944
    • • Ordnung † Praecardioida Newell, 1965
      • Ordnung † Cyrtodontoida Scarlato & Starobogatov, 1971
      • (Ordnung Ostreida Férussac, 1822) (diese Ordnung wird von Bieler & Mikkelsen (2006) nicht anerkannt, sondern zu einer Überfamilie innerhalb der Ordnung Pteriida „degradiert“. Demgegenüber verwendet die überwiegende Mehrzahl der Malakologen diese Großgruppe im Ordnungsrang; laut Zoological Record)
      • Ordnung Arcida Stoliczka, 1870
      • Ordnung Mytilida Férussac, 1822
      • Ordnung Pteriida Newell, 1965
      • Ordnung Limida Waller, 1978
      • Ordnung Pectinida H. Adams & A. Adams, 1857
  • Unterklasse Heteroconchia Hertwig, 1895
    • Überordnung Palaeoheterodonta Newell, 1965
      • Ordnung † Actinodontoida Douvillé, 1912
      • Ordnung † Modiomorphoida Newell, 1969
      • Ordnung Trigonoida Dall, 1889
      • Ordnung Unionida Stoliczka, 1870
    • Überordnung Heterodonta Neumayr, 1883
      • Ordnung Carditida Dall, 1889
      • Ordnung † Hippuritida Newell, 1965
      • Ordnung Venerida H. Adams & A. Adams, 1856
      • Ordnung Myida Stoliczka, 1870
      • Ordnung Anomalodesmata Dall, 1889 (Diese Gruppe wird von den meisten Malakologen noch als Überordnung betrachtet; Harper et al. (2006) begründen die Rückstufung auf Ordnungsrang jedoch mit vielen neuen molekulargenetischen Daten).

Für noch ältere Klassifikationen, die gelegentlich noch in populärwissenschaftlichen Muschelhandbüchern auftauchen, sei auf den Artikel Systematik der Muscheln verwiesen.

Stammbaum

Über die Phylogenie bzw. die Kladistik der heutigen Gruppen besteht insofern weitgehende Einigkeit unter den Malakologen, als die Protobranchi(at)a als die Schwestergruppe der restlichen Muscheln, den Autolamellibranchi(at)a (oder auch Autobranchi(at)a) angesehen werden. Einige halten die Protobranchia jedoch für eine paraphyletische Gruppierung. Innerhalb der Autolamellibranchia bilden Pteriomorphia und Heteroconchia wiederum Schwestergruppen. Die weitere Schwesterngruppengliederung innerhalb der Pteriomorphia ist noch unsicher. Innerhalb der Heteroconchia bilden Palaeoheterodonta und Heterodonta Schwestergruppen.

Zahl der Arten nach Großgruppen

Huber (2010) geht von weltweit 106 Familien mit insgesamt 1260 Gattungen und etwa 9200 rezenten Muschelarten aus. Diese verteilen sich wie folgt auf die jeweiligen Gruppen:

Muscheln und der Mensch

Muscheln als Nahrung

Austern gehören zu den wenigen Tierarten, die auch in der westlichen Welt lebend gegessen werden

Austernkultur im Fluss Bélon, Frankreich

Menschen aus vielen historischen und prähistorischen Kulturen haben in ihren Küchenabfällen Hinweise darauf hinterlassen, dass Muscheln konsumiert wurden. Die ältesten Belege hierfür sind rund 165.000 Jahre alt und stammen von den so genannten Pinnacle-Point-Menschen aus Südafrika. In Dänemark werden solche urgeschichtlichen Muschelhaufen Køkkenmøddinger genannt.

Auch heute noch dienen viele Arten von Muscheln – in der westlichen Welt besonders Miesmuscheln, Austern, Venusmuscheln, Messermuscheln (wie Schwertförmige Scheidenmuschel) und Kammmuscheln – als Nahrung des Menschen. Allein in Europa werden jährlich rund 100.000 t Miesmuscheln verzehrt. Die Zucht von Muscheln auf Muschelbänken ist heute (besonders Austern und Miesmuscheln, die auch natürlich solche Bänke bilden) ein wichtiges Gewerbe an nahezu allen Küsten der Welt.

Die Angaben zum Nährwert unterscheiden sich zwischen Muschelarten nur geringfügig. Der Energiegehalt in kJ wurden nach den für Menschen gültigen Formeln berechnet. Wenn Muscheln als Nahrung für Tiere dienen, muss eventuell der Aufwand für die Verdauung der Muschelschale bedacht werden. Je nach Schalendicke kann daher die Energiebilanz negativ ausfallen.

(verzehrbarer Anteil für Frischware)

Perlen und Perlmutt

Neben der Nutzung als Nahrungsmittel ist die Verwendung von Muschelteilen, besonders Perlen und Perlmutt, als Schmuck von Bedeutung. Auch diese hat eine lange Tradition. Beispielsweise wurden im Neolithikum in Europa Muschelschalen über weite Strecken als Handelsware transportiert, wie Funde der Schalen der Stachelauster (Spondylus gaederopus) in der Bandkeramischen und Theiß-Kultur zeigen. Spondylus princeps, die an der Küste von Ecuador vorkommt, besaß eine große Bedeutung für die Menschen der präkolumbischen Zeit. Zu den Funden der asiatischen Mehrgarh-Kultur des frühesten Neolithikums gehört ebenfalls Schmuck aus Meeres-Muscheln.

Vertreter der Unionida und Pteriidae sind von jeher natürliche Quellen für Perlen und Perlmutt. Der hohe Wert von Perlen der Flussperlmuschel führte dazu, dass sie im Mittelalter in einigen Regionen der heutigen Bundesrepublik Deutschland unter Schutz gestellt und Wilderei massiv – etwa durch das Abhacken der Hand – bestraft wurde. Ab dem 5. Jahrhundert n. Chr. wurden in China so genannte „Buddha-Perlen“ in Süßwassermuscheln (wie z. B. Hyriopsis cumingii) gezüchtet, aber erst dem Japaner Kokichi Mikimoto gelang es in den frühen 1920er Jahren, die ersten vollrunden Zuchtperlen auf den Markt zu bringen und damit die Grundlage der heutigen Perlenzucht und des Handels mit Perlen zu legen.

Muschelwährungen

Viele Kulturen, etwa die Ureinwohner Nordamerikas, besaßen eine eigene Muschelwährung, da Muscheln wegen ihrer Seltenheit und ihres Schmuckwertes geschätzt wurden. So entstand der englische Begriff „shell out“ für „bezahlen“.

Schäden durch den Schiffsbohrwurm

Auch negative Auswirkungen von Muscheln auf den Menschen und seine Technik sind bekannt. Ein Beispiel für Bioerosion bietet der Schiffsbohrwurm (Teredo navalis), der sich vom Holz von Schiffen, Stegen, Buhnen oder hölzernen Deichtoren ernährt und durch seine langen Gänge das Holz brüchig macht. Seine Einschleppung aus Amerika führte zu einer Revolution des Schiffbaus, sein derzeitiges Vordringen in die Ostsee ist ein aktuelles Problem der Unterwasserarchäologie, da er dort historisch bedeutsame Holzschiffreste zerstört.

Symbole und Mythologie

Die Geburt der Venus (1485), Sandro Botticelli

Jakobspilger mit Muscheln an Umhängen und Hüten (Darstellung von 1568)

Muscheln finden sich als Symbol in der Kunst und Mythologie, die wiederum die Benennung von Muscheln inspirierte. Zum Beispiel sind die Venusmuscheln (zu denen auch die Muschelgattung Venus gehört) benannt nach Venus, der antiken römischen Göttin der Liebe, des erotischen Verlangens und der Schönheit. Darstellungen zeigen oft die Geburt der Venus aus einer Muschel; allerdings wird dabei oft nicht eine Venusmuschel dargestellt, sondern eine Kammmuschel der Gattung Pecten (so auch im Bild rechts).

Seit dem Mittelalter dienen die stärker gewölbten rechten Schalenhälften der Großen Pilgermuschel Pecten maximus (bzw. Zinnabgüsse solcher Muscheln) den Jakobspilgern, die das Grab des heiligen Jakobus in Santiago de Compostela besuchen, als Erkennungszeichen. Die Mittelmeer-Pilgermuschel Pecten jacobaeus, die der Bezeichnung „Jakobsmuschel“ näher kommt, kann aufgrund ihres Verbreitungsgebietes nicht die von den Pilgern genutzte Art sein. Im Artikel Jakobsmuschel werden daher sowohl Pecten maximus als auch Pecten jacobaeus als Jakobsmuscheln bezeichnet.

Das Wappen von Guinea-Bissaus enthält eine stilisierte Kammmuschel als Symbol für die Lage des Landes an der Küste Afrikas. Der Ölkonzern Shell (englisch shell = Muschel) verwendet sie als Firmenlogo.

Das „Meeresrauschen“ in der Muschel

Es heißt, dass in Muscheln das Rauschen des Meeres zu hören sein soll. Man kann nämlich das Rauschen der Muschel mit einem Mikrofon aufnehmen, ohne dass sich ein Mensch in der Nähe befindet. Auf der Aufnahme wäre das Rauschen dann zu hören.

Tatsächlich handelt es sich hierbei um ein Resonanzphänomen: Wie in einem Blasinstrument befindet sich in der Schale eine Luftsäule, die eine bestimmte Eigenfrequenz besitzt. Das ist jener Ton, welcher entsteht, wenn man die Muschel zum Schwingen bringt. Die Luftsäule wird durch kaum merkliche Geräusche in der Umgebung, deren Frequenzen ungefähr der Eigenfrequenz der Muschel entsprechen, in Schwingung versetzt. Dadurch werden diese sonst nicht hörbaren Umgebungsgeräusche erheblich verstärkt. Das entstehende Tongemisch wird vom Menschen als Rauschen wahrgenommen.^[3][4][5]

Siehe auch

• Muschelschale
• Muschelmuseum
• Køkkenmøddinger (prähistorische Muschelhaufen)

Literatur

• R. Tucker Abbott, S. Peter Dance: Compendium of Seashells. 1990, ISBN 0-915826-17-8
• Markus Huber: Compendium of Bivalves. A Full-Color Guide to 3'300 of the World's Marine Bivalves. A Status on Bivalvia after 250 Years of Research. 901 S. + CD. ConchBooks, Hackenheim 2010, ISBN 978-3-939767-28-2
• Rudolf Kilias: Marine Muscheln und Schnecken, Lexikon. 2. Auflage, Ulmer, Stuttgart 2000 (Erstausgabe 1997), ISBN 978-3-8001-3105-1
• Gert Lindner: Muscheln und Schnecken der Weltmeere, Aussehen, Vorkommen, Systematik [Bestimmungsbuch], 5., überarbeitete und erweiterte Auflage. BLV, München / Wien / Zürich 1999, ISBN 3-405-15438-3
• Gert Lindner: Muscheln und Schnecken sammeln und bestimmen: Die häufigsten und schönsten Arten. Für die Strände Europas [Bestimmungsbuch], In: Der zuverlässige Naturführer. 3. Auflage, BLV, München 2008, ISBN 978-3-8354-0374-1
• Arno Hermann Müller: Lehrbuch der Paläozoologie. Band. II, Teil 1, Protozoa – Mollusca 1, Fischer, Jena 1993, ISBN 3-334-60409-8
• Alfred Kaestner (Begründer): Lehrbuch der Speziellen Zoologie. Band I, Teil 3 Mollusca etc., Fischer, Stuttgart 1982, ISBN 3-437-20260-X
• Rüdiger Bieler, Paula M. Mikkelsen: Bivalvia – a look at the Branches. Zoological Journal of the Linnean Society, 148: 223–235, London 2006.
• Michael Amler, Rudolf Fischer, Nicole Rogalla: Muscheln. Haeckel-Bücherei, Band 5. Enke Verlag, Stuttgart 2000, ISBN 3-13-118391-8
• Elisabeth M. Harper, Hermann Dreyer, Gerhard Steiner: Reconstructing the Anomalodesmata (Mollusca: Bivalvia): morphology and molecules. Zoological Journal of the Linnean Society, 148(3): 395–420, London 2006
• D. C. Campbell: Paleontological and molecular evidence on Bivalve phylogeny. In: Evolutionary Paleobiology and Paleoecology of the Bivalvia 2002
• J. A. Schneider: Bivalve systematics in the 20th century. Journal of Paleontology, 75: 1119–1127 2001.
• Gonzalo Giribet, Daniel L. Distel: Bivalve phylogeny and molecular data. In: Lydeard, Charles; Lindberg, David R. [Eds]. Molecular systematics and phylogeography of mollusks. pp.45–90. [Smithsonian Series in Comparative Evolutionary Biology.]. Smithsonian Books, Washington and London 2003.
• Robert Prezant: Classification. In: P.L. Beesley, G. J. B. Ross & A. Wells (Hrsg.): Mollusca. The Southern Synthesis, Part A Fauna of Australia, vol.5, pp. 224–226, Collingwood, CSIRO Publishing, 1998
• J. G. Carter, D. C. Campbell, M. R. Campbell: Cladistic perspectives of early bivalve evolution. In: Geological Society Special Publications, pp. 47–79, Geological Society, London 2000.
• Norman D. Newell: Classification of Bivalvia. In: R.C. Moore (Hrsg.): Treatise on Invertebrate Paleontology, Part N, Mollusca 6, pp. N205-N218, University of Kansas Press, Lawrence, Kansas 1969.

Literarische Darstellung

• Birgit Vanderbeke: Das Muschelessen [Erzählung]. Fischer-Taschenbuch 13783, Frankfurt am Main 1997 (Erstausgabe: Rotbuch, Berlin 1990, ISBN 3-88022-757-8), ISBN 978-3-596-13783-1 (Ingeborg Bachmann-Preis 1990).

Einzelnachweise

1. ↑ Pia Andrea Egger: Anatomisch-histologische Untersuchung der Kiemenretraktoren bei ausgewählten Arten der Caudofoveata (Mollusca). (PDF) Diss. Universität Wien, 2008.
2. ↑ Wolfgang von Buddenbrook: Vergleichende Physiologie. Band III: Ernährung, Wasserhaushalt und Mineralhaushalt der Tiere. Springer Basel 1956, S. 262
3. ↑ 121 – Akustische Hohlraumschwingungen. (Nicht mehr online verfügbar.) Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät, 13. August 2010, archiviert vom Original am 13. Mai 2012; abgerufen am 19. Oktober 2011 (Versuch mit einer leeren Flasche).
4. ↑ Vladimir Rydl: Wissensfrage. Meeresrauschen oder Ohrensausen – was hört man, wenn man sich das Gehäuse einer Meeresschnecke ans Ohr hält? In: Planet Wissen. 1. Juni 2009, abgerufen am 19. Oktober 2011.
5. ↑ Was rauscht in der Muschel? (Nicht mehr online verfügbar.) Schweizer Fernsehen, 19. April 2007, ehemals im Original; abgerufen am 19. Oktober 2011.@1@2Vorlage:Toter Link/www.srf.ch (Seite nicht mehr abrufbar, Suche in Webarchiven)

Bivalvas

La conca vacua de la bivalva jigante (Tridacna gigas)

Concas vacua de la rasador spadin (Ensis ensis)

La bivalvas es un clase de moluscas de mares, rios, e lagos cual ave corpos compresada enclluida par un conca composada de du partes xarnierida. Los no ave un testo e manca alga de la organos comun de otra moluscas, como la radula e la odontofor. Los inclui bivalvas comun, ostras, cardios, mules, petenes, e multe otra familias. La majoria come tra un filtre. La brancias ia evolui a ctenidia, organos spesialida per come e respira. La plu de bivalvas entera se en deponeda, do los es relativa secur de predores. Otras reposa sur la leto de la mar o afisa se a rocas o otra surfases dur. Alga bivalvas, como petenes, pote nada. La vermes de barcones fora se en lenio, arjila, o petra e vive en esta materiales.

La conca de un bivalva es composada de carbonato de calsio, e es du partes, tipal simil, nomida valvas. Estas es juntada longo un borda par un ligamento flexable cual, tipal con "dentes" intertexeda sur cada valva, composa la xarnier. Esta ordinada permete ce la conca pote es abrida e cluida sin ce la du duis desfisa. La conca es tipal simetre biladal, con la xarnier en la plana sajital. La grandia de concas matur varia de fratas de un milimetre a plu ca un metre en longa, ma la majoria de spesies no esede 10 cm.

Arbor filojenetica

 └─o Bivalvia o Pelesipoda ├─o Nuculoida (bivalvas nozin) ├─o Solemioida └─o Autolamelibranciata ├─o Pteriomorfia │ ├─o Mitilide (mules) │ ├─o Prionodonta (petenes) │ └─o Eupteriomorfa │ ├─o Pterioida (ostras plumin/penin/martelin) │ └─o Ostreoida (ostras) └─o ├─o Paleoeterodonta │ └─o Unionoida (mules de acua dulse) └─o Eterodonta ├─o Anomalodesmata (bivalvas de mar) └─o Veneroida (cardios e otra bivalvas de mar) 

Ang Bivalvia ay isang ng maliliit na mollusca na nakakain at may magkasalikop na pares ng kabibe na naibubuka at naipipinid.

Ang lathalaing ito na tungkol sa Hayop ay isang usbong. Makatutulong ka sa Wikipedia sa nito.

Prazna gigantska školjka Tridacna gigas

Prazna školjka Ensis ensis

Četiri filamenta škrga plave dagnje ( Mytilus edulis )
a) dio filamenata koji prikazuju trepljaste interfilamentne spojeve (cj) b) dijagram jednog filamenta sa dvije lamele u intervalima vezanim interlamelarnim spojevima (ilj) i položaj cilijskih interfilamentarnih spojeva (cp)

Margaritifera margaritifera

1 — Zadnja šarka,
2 — Prednja šarka,
3 — Gornja polovina,
4 — Donja polovina,
5 — Izlaz sifona,
6 — Otvor sifona,
7 — Stopalo,
8 — Brava,
9 — Zaokruženje,
10 — Plašt,
11 — Vrh školjke

Školjka venera sa sifonima

Bivalvia – u prošlim stoljećima označavane i kao Lamellibranchiata i Pelecypoda, a na bosanskom jeziku školjke ili školjkaši – razred morskih I slatkovodnih mehkušaca koji imaju bočno spljošteno tijelo u oklopu koji se sastoji od dva zglobna dijela. Bivalvia kao grupa nemaju glavu i nedostaju im uobičajeni organi mehkušaca, poput radule i odontofora. Uključuju školjke, ostrige, krape i brojne druge porodice koje žive u slanoj vodi, kao i određeni broj porodica slatkovodnih mehkušaca. Većina je se hrami filtracijom hranjivih čestica iz vodenog okruženja. Škrge su evoluirale u ktenidije, specijalizirane organe za ishranu i disanje. Većina školjkaša se zadržava u sedimentima, gdje su relativno sigurni od predatora. Ostali leže na morskom dnu ili se pričvršćuju na stijene ili druge tvrde površine. Neke školjke, kao što su skute i ljuskaste, mogu plivati. Brodski crvi prodiru u drvo, glinu ili kamen i žive unutar ovih supstrata.^[1]

Životinjska školjka bivalvija sastoji se od kalcij-karbonata, u dva, obično slična dijela koja se nazivaju valve. Oni se spajaju duž jednog ruba (zglobna linija) fleksibilnim ligamentom koji je obično u kombinaciji sa isprepletanim "zubima" na svakoj od valve, tvoreći šarku (“baglamu”). Ovakav raspored omogućava da se školjka otvori i zatvori bez odvajanja dviju polovina. Školjka je tipično dvostrano (bilateralno) simetrična, sa šarkama u sagitalnoj ravni. Veličine ljuske odraslih ljuštura variraju, zavisno od grufe, od milimetra do preko metra, ali većina vrsta ne prelazi 10 cm.^[2]

Školjke su odavno dio prehrane obalne i priobalne ljudske populacije. Ostrige su uzgajane i u ribnjacima starih Rimljana, a marikultura je u novije vreme postala važan izvor školjki za hranu. Suvremeno poznavanje reprodukcijskih ciklusa mehkušaca dovelo je do razvoja mrijestilišta i novih tehnika marikulture. Bolje razumijevanje potencijalnog trovanja školjkama uzimanjem sirovih ili nekuhanih školjki dovelo je do boljeg skladištenja i prerade. Biserne ostrige (zajedničko ime dvije vrlo različite porodice u slanoj i slatkoj vodi) su najčešći izvor prirodnih bisera. Valve školjki koriste se u zanatskim radovima, te za proizvodnji nakita i dugmadi. Također su koristne i kao biosenzori, indikatori u biokontroli zagađenja.

U fosilnim zapisima, bivalve se pojavljuje prvi put u kambriju, prije više od 500 miliona godina. Ukupni broj poznatih živih vrsta je oko 9.200. Svrstavaju se u okviru 1.260 rodova i 106 porodica. Morski školjkaši (uključujući bočatnu vodu i estuarijske vrste) predstavljaju oko 8.000 vrsta, kombiniranih u četiri potklase i 99 porodica s 1.100 rodova. Najveće današnje morske porodice su Veneridae, s više od 680 vrsta i Tellinidae i Lucinidae, od kojih svaka ima preko 500 vrsta. Slatkovodne školjke uključuju sedam porodica, od kojih je najveća Unionidae, sa oko 700 vrsta.^[3][4][5]

Većina školjkaša je razdvojenih spolova a oplodnja je vanjska. Prijenos očinske mtDNK kod školjkaša redovna je pojava, za razliku od ostalih organizama.

Biodiverzitet

Maksimalna veličina odraslih postojećih vrsta školjki kreće se od 0,52 mm u Condylonucula maya,^[6] do dužine od 1.532 mm kod Kuphus polythalamia, izduženog "brodskog crva".^[7] Međutim, vrsta koja se obično smatra najvećom živom školjkom je divovska školjka Tridacna gigas, koja može narasti do dužine od oko 1.200 mm i težine veća od 200 kg.^[8] Najveća poznata izumrla školjka je iz roda Platyceramus čiji su fosili bili dugi i do 3.000 mm.^[9]

U svom traktatu iz 2010. godine Compendium of Bivalves, Markus Huber daje ukupan broj živih vrsta školjkaša oko 9.200 u 106 porodica.^[10] Huber navodi broj 20.000 živih vrsta, a da neke, koje se često susreće u literaturi, nije moguće provjeriti i prikazuje sljedeću tabelu radi ilustracije poznate raznolikosti:

Taksonomija

Razred Bivalvia

^[11]

Također pogledajte

• Mehkušci

Reference

Bivalvia ya iku kelas jinis moluska kang kagolong kerang-kerangan: duwé sapasang thothok (jeneng "bivalvia" artiné thothok loro). jeneng liyané ya iku Lamellibranchia, Pelecypoda, utawa bivalva. Ning kelompok iki kaya ta kerang, kupang, remis, kijing, lokan, simping, tiram, sarta kima; meski macemé akèh banget.

Kerang-kerangan akèh manfaaté ing kauripané manungsa awit jaman purba. Dagingé dipangan, dadi sumber protéin. Thothoké dimanfaataké dadi pacetan, kerajinan tangan, bekal kubur, kanggo gantiné dhuwit nalika jaman biyèn. Mutiara dikasilaké déning tiram. Pemanfaatan modhèren uga ndadikaké kerang-kerangan dadi biofilter déning polutan.

Sistematika

Klasifikasi Newel (1965) kang didasaraké ngèlmu morfologi. Ana kelompok parafiletik, kaya ta Anomalodesmata. Ana uga sistematika alternatif didasaraké morfologi angsang saka Franc (1960) lan disebutaké ing rengrengan ngisor iki. Franc nggolongaké Septibranchia saklompok dhéwé, meski kanthi molekular malah nggawe Eulamellibranchia dadi parafiletik.

Subkelas Palaeotaxodonta (Protobranchia miturut Franc)

• Ordo Nuculoida

Subkelas Cryptodonta (Protobranchia miturut Franc)

• †Praecardioida
• Solemyoida

Subkelas Pteriomorphia (tiram, kupang, dll., Filibranchia miturut Franc)

• Arcoida
• †Cyrtodontoida
• Mytiloida
• Ostreoida – kagolong Pterioida
• †Praecardioida
• Pterioida

Subkelas Paleoheterodonta (Eulamellibranchia miturut Franc)

• †Trigonioida
• Unionoida (jinis-jinis kupang air tawar)
• †Modiomorpha

Subkelas Heterodonta (tuladhané remis, lokan, lan kerang-kerang kang biyasa dikenal, Eulamellibranchia miturut Franc)

• †Cycloconchidae
• †Hippuritoida
• †Lyrodesmatidae
• Myoida
• †Redoniidae
• Veneroida

Subkelas Anomalodesmata (Eulamellibranchia miturut Franc)

• Pholadomyoida

Strukturé Awak

thothoké loro nyambung amarga ana ligamen kang kuwat lan mulur, ligamen iku lumrahé mbukak terus nanging, manawa ana gangguan, ligamen mau bakal nutup. Ligamen mau bisa nutup amarga ana otot kang bantu owah, ya iku ning anterior lan posterior. Bakal ana garis konsentris kang sajajar, garis iki karan uga garis pertumbuhan kang nunjukaké kepriye pertumbuhané suwi utawa ora ana pertumbuhan. Garis iki berselangseling karo pita pertumbuhan kang nunjukaké pertumbuhan kang cepat. Tambah akèh pita pertumbuhan, umuré kéwan tambah tuwa. Cangkang kang tuwa dhéwé lumrahé paling kandhel, ana ing persendiaan jenengé umbo. ning pérangan posterior thothok ana rong macem celah kang karan sifon. Celah kang ana ing cedhaké anus dijenengi sifon, fungsiné kanggo ngetokaké lan nglebokaké banyu lan zat-zat sisa. Sawaliké sifon kang ana ing ngisoré sifon fungsiné kanggo nglebokaké oksigèn, banyu, lan panganan.^[1]

Cangkang Pelecypoda

Cangkang Pelecypoda bagiané iku.^[1]

1. Periostrakum, Periostrakum lapisan paling njaba, diwangun saka zat kitin kang karan konkiolin fungsiné dadi pelindung. Manawa teles warnané biru tuwa, nanging manawa gaing warnané soklat.
2. Prisma, Prisma lapisan tengah kang kasusun saka kristal kalsit.
3. Nakre, Nakre karan uga lapisan induk mutiara kang kasusun saka lapisanlapisan tipis paralel lan kalsit (karbonat) kang katon mengkilat.
4. Mantel, Mantel ana ing ngisoré nakreas kang kaya ta sèl-sèl nakreas (kang

sekreté mbentuk lapisan nakreas lan mbentuk mutiara) jaringan ikat, lan sèl-sèl epitelium kang bersilia.

Sistem Organ

Dimulai saka cangkem, kerongkongan, lambung, usus, lan anus. Cangkem lan anusé ana ing gronggongan mantel. Sistem ekskresiné ngunakaké sapasang nefridium kang fungsiné kaya ginjel. Sistem sarafé ya iku utek, simpul saraf kaki, lan simpul saraf otot. Sistem peredaran getihé kabuka, jantungé ana bilik lan ana rong serambi. Respirasiné migunakaké angsang.^[1]

Daur Hidup

Kéwan iki ana kang sipaté hermaprodit lan lumrahé kéwan iki duwé piranti kelamin kang kapisah. Nalika kawin ,piranti kelamin lanang bakal ngetokaké sperma ning banyu lan bakalé mlebu ing awaké kéwan wadon. Ovum kang nempel ning ruwang mantel, nuli netes lan metu larva kang karan glokidium. Larva iki bakalé metu saka awaké kéwan wadon liwat sifon metuné banyu, nuli larva mau bakal nempel ning angsang itawa sirip iwak lan larva mau bakal dibungkus karo lendir saka kulit iwak. Larva ini sifaté parasit nganti 3 minggu suwené. Sawisé gedhé, larva bakal ucul saka angsang utawa sirip iwak lan bakal urip bébas.^[1]

Rujukan

• Franc, A. (1960): Classe de Bivalves. In: Grassé, Pierre-Paul: Traite de Zoologie 5/II.
• Newell, N.D. (1969): [Bivalvia systematics]. In: Moore, R.C.: Treatise on Invertebrate Paleontology Part N.
• Jay A. Schneider (2001). "Bivalve Systematics During the 20th Century". Journal of Paleontology 75 (6): 1119–1127. doi:10.1666/0022-3360(2001)075<1119:BSDTC>2.0.CO;2. Paramèter |month= sing ora kawruhan dikiwakaké (pitulung)

Bivalvia, in previous centuries referred tae as the Lamellibranchiata an Pelecypoda, is a cless o marine an freshwatter molluscs that hae laterally compressed bouks enclosed bi a shell conseestin o twa hinged pairts. Bivalves as a group hae na heid an they lack some uisual molluscan organs lik the radula an the odontophore.

References

Bivalvo

Bivalvo esas molusko-klaso tipala kun du simetresoza parti.

Le bivalvos (bivalvia) es un classe ex le phylo del molluscos. Illos ha un concha que consiste de duo partes generalmente symmetric.

Ikki pallalilar, plastinkajabralilar (Bivalvia, Lamel-libranchiat) — mollyuskalar tipining sinfi. Tanasining uz. bir necha mm dan 1,5 m gacha, vazni 30 kg , baʼzan 200 kg gacha (tridakna), bilaterial simmet-riyali, yon tomondan yassilangan gavda va oyoqdan iborat. Boshi boʻlmaydi. Koʻpchiligining oyogʻi bitta, ponasimon; tuban tuzilgan turlarida yassi; harakatsiz yopishib yashaydigan turlarining oyogʻi reduksiyaga uchragan (mi-diyalar, dreyssenalar) yoki yoʻqolib ketgan (ustritsalar). Koʻpchilik I.p. oyogʻidagi bissus bezlari ishlab chiqaradigan bissus iplari yordamida sub-stratga mahkam yopishib olib yashaydi. Tanasi mantiya bilan qoplangan. Chigʻanogʻi tanasini yon tomondan yopib turuvchi ikkita palladan iborat. Chigʻanogʻining ichki tomoni koʻpincha sadaf bilan qoplangan. Pallalar chetida chigʻanoqni yopuvchi qulf (tish) hosil qiladigan oʻsimtasi boʻladi. Chigʻanoq pallalarining oldingi va keyingi ichki yuzasida yopuvchi muskullar bor. Ayrim I.p. (marvariddorlar, midiyalar) mantiya-siga tushgan yot zarralar (qum)ni sadaf bilan oʻrab olib, marvarid hosil qiladi. Jigari 3 boʻlakdan iborat. Jabralari koʻpchiligida plastinkaga oʻxshash (sinfning ikkinchi nomi ana shundan kelib chiqqan). Nerv sistemasi 3 juft gangliydan iborat. Sezgi organlari kuchsiz rivojlangan. Qon aylanish sistemasi ochiq; ayirish sistemasi uzun tasmasimon buyrakdan iborat. Koʻpchilik I.p. ayrim jinsli; ayrim turlarida jinsiy dimorfizm yaxshi rivojlangan. Oʻrugʻlanishi tashqi, lichinkasi erkin suzib yuradigan veligir yoki gloxidiy. Ayrim turlari tuxumlari va lichinkalarini jabralarida olib yuradi. 20000 ga yaqin turi bor, asosan, dengizlarda tarqalgan. Chuchuk suvlarda sadafdorlar, marvariddorlar va baqachanoqlar tarqalgan. Suv tubida yashaydi. Suvni filtrlovchi detritofaglar, plantofaglar, baʼzi turlari yirtqich. Baliqlar va b. dengiz hayvonlariga oziq boʻladi. Ayrim turlari tosh yoki yogʻochni teshish xususiyatiga ega. Bir qancha turlari kemalar korpusi va gidrotexnik qurilmalarga katta ziyon keltiradi. Goʻshti uchun ovlanadi (midiyalar, ustritsalar). Ayrim turlari (marvariddorlar) marvarid olish va goʻsht uchun boqiladi. Baʼzi I.p. suv qushlari parazitlarining oraliq xoʻjayini hisoblanadi. Baqachanoqning lichinkasi chuchuk suv baliklari terisida parazitlik qiladi. Oʻzbekiston suv hav-zalarida I.p. mollyuskalarning 52 turi tarqalgan, jumladan, 5 turi endemik boʻlib, Oʻzbekiston Qizil kitobiga kiritilgan.

Adabiyot

• Mavlonov O., Hurramov Sh., Umurtqasizlar zoologiyasi, T., 1998.^[1]

Manbalar

Ñawisapa ñaqch'a lakachu

Lakachu icha Iskaysillp'i (Bivalvia) nisqakunaqa alli-allillamanta suchuq, ch'iñicha kawsaqkunata mikhunapaq yaku hurquspa suysuq, iskay isku rumi sillp'iyuq llamp'ukakunam, yakupi kawsaqkuna.

Hawa t'inkikuna

• Commons nisqaqa multimidya kapuyninkunayuqmi kay hawa: Lakachu.

• Wikispecies nisqaqa qillqasqa p'anqayuqmi kay hawa: Lakachu

D'Muschele sinn eng Klass vum Stamm vun de Weechdéieren. Et gëtt ongeféier 30.000 Aarten déi an 105 Familljen, an 10 Uerdnungen an a 5 Ënnerklassen agedeelt sinn. Bekannt Aarte sinn d'Mullen an d'Austeren.

Um Spaweck

Musseln (Bivalvia) sünd kopplose Weekdeerten in harte Schalen, de dör Sluutmuskeln slaten weern köönt. Ene scharneeroordige, hurnige Huut, dat Slottband, hollt de beiden Schalen tosamen.

De wieke Körper is an de Buuksiet as en Foot utbillt, up den de Mussel sik bewegen kann. In den anspöölten Teek an’n Noordseestrand sünd vele verschedene Musseloorden to finnen. Faken kaamt vör de Mies-, Hart-, Sand-, Trog- un Pepermusseln. De Miesmusseln sünd 6 bet 8 cm groot, seht blau ut un leevt dicht ünner den Sietwaterstand. De Foot scheed en Sekreet ut, mit dat sik dat Deert an ene Unnerlaag fastkitten deit. Miesmusseln weert as Spiesmusseln hannelt. Ene smackhafte Mussel is de Ööster. Se leevt blots up schlickfre’en, fasten Grund un is an de Unnerlaag fastwussen. Bit 1925 geev dat ok an de noordfreeschen Inseln noch Öösterbänk. De eenzige, de dat dar noch gifft, is up Sylt. In een Jahr weert dar so um de ene Million Ööstern verkofft. Vundaag weert de Ööstern tomehrst in Öösterparks an de Küst von de Middellannsche See tücht un as düre Delikatess in’n Hannel brocht.

• Ünnerklass Protobranchia
  • Solemyoida
  • Nuculoida
• Ünnerklass Pteriomorphia - Ööstern
  • Arcoida
  • Mytiloida
  • Pterioida
• Ünnerklass Paleoheterodonta - Musseln
  • Trigoinoida
  • Unionoida
• Ünnerklass Heterodonta - Venus-, Zebramusseln
  • Veneroida
  • Myoida
• Ünnerklass Anomalosdesmata
  • Pholadomyoida

Umi mymba ijyta mokõi (bivalvia jueheguasã) ha'e opa syrymbe oiko yguasúpe oreko ijapekue hatãre mokõi yta ijapýre oñembojoapýva. Ko atýpe oiko amo 13 000 mymba juehegua.

Ko'ã mymbakuéra oiko yvyku'íre térã itakuéra rupi, yguasúpe ha yguasu rembe'ýre.

Mymba ijyta mokõi niko ijyva'ỹva, ijytakuéra hatãite ha sa'y hetáva. Umi mymba ijyta mokõi ojeikuaavéva hína umi ytã, tamba ha améha, yvypóra oipuru rembi'úicha.

Mandu'apy

• Jonathan Elphick, Jen Green, Berbara Taylor, Richard Walker (2001). Gran Enciclopedia de los Animales. Santiago de Chile: La Tercera.

Siang-khak-kong (hàn-jī: 雙殼綱, Latin-gí: Bivalvia) sī 1 lūi ū 2 sìⁿ khak ê nńg-thé tōng-bu̍t.

Ô-á, hún-giô lóng sio̍k-î chit lūi.

Hun-lūi

(Ē-kha ê hun-lūi m̄ sī î-it 1 chióng.)

• Kong: Bivalvia
  • A-kong: Protobranchia
    • Bo̍k: Solemyoida
    • Bo̍k: Nuculoida
  • A-kong: Pteriomorphia
    • Bo̍k: Arcoida
    • Bo̍k: Mytiloida
      • Chhiau-kho: Mytilacea
        • Kho: Mytilidae
    • Bo̍k: Pterioida
      • Chhiau-kho: Ostreacea
        • Kho: Ostreidae (ô-á)
  • A-kong: Paleoheterodonta
    • Bo̍k: Trigoinoida
    • Bo̍k: Unionoida (pāng)
  • A-kong: Heterodonta
    • Bo̍k: Veneroida
      • Chhiau-kho: Veneracea
        • Kho: Veneridae (hún-giô)
      • Chhiau-kho: Corbiculoidea
        • Kho: Corbiculidae (lâ-á)
    • Bo̍k: Myoida
  • A-kong: Anomalosdesmata
    • Bo̍k: Pholadomyoida

Gōa-pō͘ liân-kiat

• Tâi-oân Pōe-lūi Chu-liāu-khò͘

Twiiskaalagen (Bivalvia) san en klas uun a biologii an hiar tu a wokdiarten. Hör nööm haa's diarfaan, dat jo tau skaalen haa. Jo lewe miast ei jiper üs 100 m oner weeder. At jaft amanbi 10.000 slacher.

Beskriiwang

Slacher

• (Myoida)

(Myidae)

Paser (Mya arenaria)

• (Mytiloida)

(Mytilidae)

onerfamile(Mytilinae)

slacherHeesen (Mytilus)

Hees (Mytilus edulis)

• (Ostreoida)

Uastrangen (Ostreidae)

Pazifik-Uastrang (Crassostrea gigas)

• (Veneroida)

Kaamken (Cardiidae)

Kaamk (Cerastoderma edule)

Ferwis efter bütjen

– Saamlang faan bilen of filmer

Wikispecies hää en artiikel tu:

Twêekleppign (Bivalvia) zyn e klasse van de molluuskn (Mollusca) die in zoet woater en zêewoater leevn, ook wel Pelecypoda of Lamellibranchia genoemd. De klasse omvat mussels, oesters, kokkels, scheirmesschelpn, sint-jakobsschelpn en vele andere families. Under lyf is an de zykant ofgeplat en bedekt met e schelpe die bestoat uut twêe verschillige dêeln die min of mêer symmetrisch zyn. Ze zittn an mekoar vaste en kunn oopn en toe goan. De mêeste sôortn èn gin kop of radula (rasptoenge) en krygn under eetn binn deur woater te filtern. De kieuwn, ctenidia genoemd, speeln e rolle by 't oasmn moar ook by 't vangn van de voediengstoffn in 't woater. De mêeste twêekleppign groavn under in ip de zêebodem in zand of slyk, woa dan ze veilig zyn vo roofbêestn. Andere leevn ip de zêebodem of zettn under vaste ip stêens of andere arte ippervlakkn. Sommigte kunn ook zwemmn.

Taxonomie

Volgens World Register of Marine Species (1 april 2016):

• Subklasse Heterodonta
  • Infraklasse Archiheterodonta
    • Orde Actinodontida †
    • Orde Carditida
  • Infraklasse Euheterodonta
    • Superorde Anomalodesmata
      • 8 superfamilies zoender orde
    • Superorde Imparidentia
      • Orde Adapedonta
      • Orde Cardiida
      • Orde Lucinida
      • Orde Myida
      • Orde Venerida
      • Orde Hippuritida †
      • Orde Megalodontida †
      • Orde Modiomorphida †
      • + 10 superfamilies zoender orde
• Subklasse Palaeoheterodonta
  • Orde Trigoniida
  • Orde Unionida
• Subklasse Protobranchia
  • Orde Afghanodesmatida †
  • Orde Nuculanida
  • Orde Nuculida
  • Orde Solemyida
• Subklasse Pteriomorphia
  • Orde Arcida
  • Orde Colpomyida †
  • Orde Cyrtodontida †
  • Orde Limida
  • Orde Myalinida †
  • Orde Mytilida (Mytiloida), mussels
  • Orde Ostreida, oesters
  • Orde Pectinida

Ике́ кабырчыклы́ моллю́склар (Bivalvia) - кабырчыклы моллюсклар классы. 80 меңгә якын төре билгеле, Татарстан территориясендә 50 гә якын (авыр ачыкланучы бөрчеклеләр - Pisididae семьялыгын исәпләмичә) төре яши. Иң күп таралганнары - тешсез әкәм-төкәмнәр, дрейсеналар, перловицалар, шаровиклар, бөрчеклеләр. Бу төркем моллюсклары өчен арка ягында кушылган ян кабырчыклы ике капкач, шулай ук баш һәм кыргычлы йоткылык редукциясе хас. Су мохитендә яши. Пластинкалы саңаклар пары (пластинкасаңаклылар - Lamellebranchia исеме шуннан) ярдәмендә сулыйлар. Суны йөзмә кисәкчәләрдән чистартуда актив катнашып, фильтрация ысулы белән тукланалар.

Эре ике кабырчыклы моллюсклар - азык буларак, перловицалар һәм тешсез әкәм-төкәм кабырчыклары төймәләр җитештерүдә кулланыла.

Чыганаклар

• ТР Фәннәр академиясенең Татар энциклопедиясе институты
• Татар энциклопедиясе. Казан, 2008.

Ике ҡабырсаҡлы моллюскылар (лат. Bivalvia, (рус. Двустворчатые) — ер йөҙөнөң бөтә урындарында тоҙло һәм сөсө һыуҙа йәшәй, ҡайһы берҙәренең ҡабырсаҡтары перламутр һәм ынйы бирә торған ике ҡабырсаҡлы моллюскылар йәки ҡусҡарҙар ^[1]. Cөсө һыуҙа йәшәүсе ике ҡабырсаҡлы моллюсклар синыфы. Ике ҡабырсаҡлылар синыфының иң кескенә төрҙәре, кәүҙә оҙонлоғо 20 мм-ға (йыш ҡына 5 мм) ҡәҙәр етә. Ҡабырсағы оҙонса түңәрәк йәки овалыраҡ. Ағым һәм ағымһыҙ һыуҙа таралған.

Башҡа моллюскаларҙан айырмалы булараҡ, Bivalvia синыфының баша һәм радулаһы (аҙыҡты ваҡлау торған ағза) юҡ. Пластинкалы айғолаҡтары ныҡ үҫешкән, айғолаҡтар һулыш алыу фукнцияһын һәм һыуҙағы аҙыҡ киҫәктәрен фильтрлау функцияһын үтәй.

Иҫкәрмәләр

1. ↑ Терминологический словарь по зоологии. Русско-башкирский и башкирско-русский (Т.Г.Баишев, 1952)

Сығанаҡ

• Татар энциклопедияһы (тат.)

Әҙәбиәт

• Евсеев Г. А., Яковлев Ю. М. Двустворчатые моллюски дальневосточных морей России. — Владивосток: Институт биологии моря, 2006 . — 120 с.

Кош капкалуу моллюскалар (лат. Bivalvia) — омурткасыз жаныбарлардын моллюскалар тибинин классы. Узундугу бир нече ммден 1,5 мге, салмагы 30 кгга чейин жетет. Башы жок, оозунда эки жуп калакчасы бар. Карынында туюк өсүндү болуп, ичиндеги кристалл бутакчасы азык сиңирүүгө жардамдашат жана анаэроб шартында жашаган моллюскаларды кычкылтек менен камсыздайт. Жүрөгү үч камералуу, кан айлануусу туюк эмес, бөйрөгү экөө, нерв системасы 3 жуп ганглийден турат. Сезүү органдары начар өнүккөн. Денесинин сырткы эки капталы капка (раковина) менен капталган. Капкалардын курсак жак четтери ачылып, жабылат, жон жагы бири бирине тиш-тиш болуп ашташкан. Капканын ички бети седеп заты менен жылмаланып, жалтырап турат. Моллюсканын денесинин сыртынан жаап турган мантиясы (кементайы) бар. Кээ бир түрлөрү бермет пайда кылат. Деңизде жана тузсуз сууларда топуракка көмүлүп, кээси бош же субстратка бекип жашайт. Суунун түбүндөгү детрит, планктондор менен азыктанат. Булар дүйнөлүк океандарда кеңири тараган. 14 түркүмү, 130 тукуму, 1000дей уруусу, 20 миңдей түрү бар. Көз Карандысыз мамлекеттер шериктештигинин деңиздеринде жана сууларында 400дөй түрү кезигет. Кош капкактуу молюскалардын көпчүлүгү айрым жыныстуу, кээ бири — гермафродиттер. Промыселдик мааниси бар: устрица, мидиялар тамак катары пайдаланылат, айрымдарынан бермет алынат. Көпчүлүгү деңиз түбүндөгү жаныбарларга жем. Кош капкактуу молюскалардын айрымдары жыгачты, ташты тешет, кемелерге, гидротехникалык курулмаларга зыян келтирет.

Колдонулган адабияттар

• А. А. Алдашев. Биология терминдеринин жана айбанат аттарынын орусча-кыргызча сөздүгү. - Бишкек, 1998. ISBN 9967-11-027-9
• “Кыргызстан” улуттук энциклопедиясы: 4-том. Башкы редактору Асанов Ү. А. К 97. Б.: Мамлекеттик тил жана энциклопедия борбору, 2012. 832 бет, илл. ISBN 978 9967-14-104-9

Школките (односно Bivalvia, синоними Lamellibranchia, Pelecypoda) се класа на мекотели. Тие се карактеризираат со странично сплескано тело и черупка која се состои од два капака (подвижни делови), зглобени со еластични филаменти (односно мускули). Школките се важен извор на храна за луѓето, како и за полжавите, рибите и приморските птици.

Черупки на школките

Двата капака на черупката од школката ја препокриваат десната и левата страна на животното; тие се дорзално споени (над телото) и се отвораат вентрално (под телото). Обично двата капака се слични и еднакви по големина, но кај некои форми, како остригите кои се прикрепуваат за супстратот со едниот капак, левиот (горниот) капак е поголем отколку десниот (долниот). Два мускула, наречени адуктори, поминуваат меѓу внатрешните површини на двата капака. Тие делуваат антагонистички на зглобните лигаменти со што овозможуваат школката да се затвори.

Поради големото разнообразие на големини, форми, површински скулптури и бои, карактеристиките на школките се од големо значење при процесот на нивна идентификација. Школките се најразлични по големина, и тоа од ситните (2 mm) школки карактеристични за членовите од слатководната фамилија Sphaeriidae, па до гигантските видови од Tridacna застапени во Јужниот Пацифик, кои достигнуваат должина од преку 120 cm и можат да тежат преку 225 kg.

Анатомија на школките

Група на живи примероци од слатководната бисерна школка Margaritifera margaritifera на дното од река во Шведска.

Шематски приказ на анатомијата на слатководната бисерна школка Margaritifera margaritifera.

Под черупката се наоѓа месест слој на ткиво наречено мантија; постои празнина (мантијна празнина) помеѓу мантијата и телесниот ѕид. Мантијата ги секретира слоевите на черупката, вклучувајќи го и седефестиот (бисерен) слој. Понекогаш се формира бисер како реакција на иритација. Притоа се наслојува седефест слој околу туѓата честичка. Главениот регион е многу редуциран, така што скоро и да отсуствува. На него нема очи или пипала. Од предниот крај на животното излегува мускулно, секировидно стапало кое се протега меѓу капаците од черупката. Тоа се користи за закопување и, кај некои школки, за пливање. Повеќето школки имаат две цевки, или сифони, кои се издолжуваат од задниот крај: едниот (внесувачки) сифон служи за преземање на оксигенираната вода и храна, и другиот (изнесувачки) сифон служи за екскреција. Двете цевки можат да бидат споени во единствен сифон, наречен врат.

Жабрите, суспендирани низ мантијната празнина, обично се многу големи и функционираат при собирањето на храната (филтерно хранење) како и при дишењето. Како што водата минува низ жабрите, ситни органски честички се заробуваат и се носат кон устата. Членовите на редот Septibranchia, меѓутоа, немаат жабри и се хранат со мали ракови и црви.

Школките имаат целосен дигестивен систем; редуциран нервен систем; целосен, отворен тип на циркулаторен систем со срце кое поседува комора, артерии, вени и крвни синуси; како и екскреторни и репродуктивни органи. Повеќето видови се гонохористи, а јајце клетките и сперматозоидите се испуштаат во водата каде се случува оплодувањето. Ларвалниот стадиум е слободнопливачки и кај него отсуствува черупка.

Специјализација на школките

Школките се разликуваат во нивните навики: некои, како остригите и морските мусели, имаат редуцирано стапало и постојано се прикачени за супстрат; некои, пак, како слатководните мусели се закопуваат во песокот или калта со помош на стапалото; трети живеат на или близу поврпината на океанското дно; останатите, како бродскиот црв, ги копаат карпите или дрвата барајќи заштита, при што можат да оштетат и некои морски структури. Скалопите пливаат со голема брзина, со безо наизменично отворање и затворање на черупката, со што се исфрлува водата од мантијната празнина. Школките кои се изложени на мала плима ги одржуваат своите жабри влажни со помош на складираната вода во мантијната празнина.

Школките како тема во уметноста и во популарната култура

• „Школки“ - песна на францускиот поет Пол Верлен.^[1]
• „Школките и времето“ - расказ на италијанскиот писател Итало Калвино.^[2]
• „Спирала“ - расказ на италијанскиот писател Итало Калвино.^[3]
• „Школка“ (Caracola) - песна на шпанскиот поет Федерико Гарсија Лорка.^[4]
• „Школка“ - кус расказ на македонскиот писател Иван Шопов од 2010 година.^[5]

Наводи

द्विकपाटी प्राणी

द्विकपाटी, या पटलक्लोमी (Lamellibranchia/लैमेलिब्रैंकिया या Bivalvia) अकशेरुकी तथा जलीय प्राणी हैं। यह मोलस्का (Molausca) संघ का एक वर्ग (class) है। इसे लैमेलिब्रैंकियाटा, द्विकपाटी (Bivalve), या पेलेसिपोडा (Pelecypoda) भी कहते हैं। चूँकि इनके पाद चपटे होने के बजाय नवतलित अधरीय होते हैं, इसलिए ये 'पैलेसिपोडा' कहलाते हैं। इस वर्ग के प्राणियों में सिर नहीं होता, अत: यह वर्ग मोलस्का के अन्य वर्गों से भिन्न है। इनमें लेबियल स्पर्शकों (labial palp) के द्वारा सिर का प्रतिनिधित्व होता है। ये द्विपार्श्व सममित प्राणी है। इनके सभी अंश जोड़े में अथवा मध्यस्थ होते हैं। लैमेलिब्रैंकिया स्थानबद्ध प्राणी हैं। कुछ द्विकपाटी चट्टानों से बद्ध रहते हैं, जब कि अन्य धागे सदृश पुलिंदे से जमीन से संलग्न रहते हैं। इस पुलिंदे को सूत्रगुच्छ (Byssus) कहते हैं। यह सूत्रगुच्छ पाद की एक गुहिका से स्रवित होता है। अधिकांश द्विकपाटियों के पाद बिल बनाने, या गमन के लिए व्यवहृत होते हैं। कुछ द्विकपाटी अपने कवचों को एकाएक बंद कर, पानी को बाहर निकालने के द्वारा तैरते हैं।

लैमेलिब्रैंकिया के १०० से अधिक कुल एवं ७,००० स्पीशीज ज्ञात हैं।

परिचय

इनके कवक में दो प्रारूपिक, समान कपाट होते हैं। दोनों कपाट एक प्रत्यास्थ स्नायु (elastic ligament) के द्वारा जुड़े रहते हैं। यदि स्नायु आंतरिक होते हैं, तो ये रेसिलियम (resilium) कहलाते हैं। ये स्नायु कपाटों को अलग रखते हैं, जबकि दो अभिवर्तनी (adductor) पेशियाँ कवचों का बंद रखने का प्रयास करती हैं। कवच के आंतर पृष्ठीय भाग, या हिंजपट्ट (hinge plate) में हिंज दाँत होते हैं, जो अंतर्कीलित होते हैं। दाँतों का साधारण रूप अनेक समान दाँतों का बहुदंती (taxodont) हिंज है। कुछ विभेदित दाँतों का उच्चतम विकास हुआ हैं। अनेक द्विकपाटियों में अधर और पार्श्विक उपांत के सूक्ष्म दंत द्वारा कपाटों का ठीक-ठीक बंद होना सहापित होता है।

प्रावार (mantle) के स्राव के कवच का निर्माण होता है। प्रावार संपूर्ण शरीर को ढँक लेता है। इसकी दाईं एवं बाईं दो पालियाँ होती हैं। ये पालियाँ प्रावार पेशियों (pallial muscles), या वर्तुल (orbicular) पेशियों के द्वारा कपाटों से जुड़ी रहती हैं। कवच का प्रावार (pallial) क्षतचह्न संलगनी रेखा (line of attachment) को प्रकट करता है। प्रावार रेखा के अंत में अनुप्रस्थ अभिवर्तनी पेशियाँ होती हैं। प्रावार पालि के स्वतंत्र अधर, सीमांत दो, तीन या चार छिद्रक छोड़ते हुए, अंशत: जुड़े रहते हैं।

अपवाही (exhalant) तथा अंतर्वाही (inhalant) धाराओं के लिए पश्चछिद्रक होते है। इन दो छिद्रकों पर प्रावार प्राय: दो पेशीय ट्यूब के रूप में बढ़ा रहता है। ऊपरवाला ट्यूब अपवाही या गुदानाल तथा नीचेवाला ट्यूब अंतर्वाही या क्लोमनाल (देखें चित्र २.) होता है। तीसरे छिद्र से पाद का बहिर्बेधन होता है। प्रावार गुहिका में दो मुख्य धाराएँ होती है। अंतर्वाही छिद्रक से मुँह को ढँकनेवाले लेवियल स्पर्शकों तथा गिलों की ओर एक धारा पश्चत: दिष्ट होती है। दूसरी धारा उलटी दिशा में अपवाही नाल की ओर दिष्ट होती है। बालू, या बजरी में गड़े रहनेवाले पिन्ना (pinna) और सोलेन (solen) में अपवाही धाराएँ पक्ष्माभिकामय नाल द्वारा जाती हैं। प्रावार की कोर पर प्राय: ग्रंथियाँ, स्पर्शक, वर्णक चकत्ता (pigment spot) तथा आँखें होती हैं।

प्राय: लैमेलिब्रैंकिया के गिल, या क्लोम, कंकत क्लोम (Ctenidium) कहलाते हैं, क्योंकि अब इनका मुख्य कार्य श्वसन नहीं है। श्वसन मुख्यत: प्रावार से होता है। ये पक्ष्माभिकी गति के द्वारा अंतर्वाही छिद्रक से एक धारा उत्पन्न करते हैं, जो, सूक्ष्म जीवों को भोजन के लिए छाँटकर लेबियल स्पर्शक पर पहुँचा देती है। लेबियल स्पर्शक मुहँ के ओष्ठ, या युग्मित पालियुक्त प्रक्षेपण है। दो गिलों में से प्रत्येक में एक केंद्रीय अक्ष होता है, जिसमें तंतुओं की दो श्रेणियाँ होती हैं, जिन्हें अर्धक्लोम, (demibranchs) कहते हैं। प्रोटोब्रैंक, (protobranch) द्विकपाटियों में तंतु साधारण पट्टिकाएँ दंड होते हैं, फिलिब्रैंक (filibranch) गिलों में तंतु समांतर दंड होते हैं, जो अंतर्कीलित पक्ष्माभिकी टफ (ciliary tuff) द्वारा जुड़े रहते हैं तथा यूलैमेलिब्रैक गिलों में दंड संवहनी (vascular) संधियों द्वारा जुड़े रहते हैं।

प्राय: नर और मादा पृथक् पृथक् होते हैं। समुद्री लैमेलिब्रैंकिया में ट्रोकोस्फीयर (trochosphere) एवं वेलीजर (veliger) लार्वां होते हैं। अलवण जल के लैमेलिब्रैंकिया की विशेषता ऊष्मायन (incubation) है।

वर्गीकरण

हिंज दाँतों के रूपों, गिलों की संरचनाओं तथा विशेषत: पक्ष्माभिकी गुणों के आधार पर लैंमेलिब्रैंकिया को चार गणों (orders) में विभक्त किया गया है, जो निम्नलिखित हैं :

• (१) प्रोटोब्रैंकिएटा (Protobranchiata) - इस गण के लैमिलिब्रैकियाओं के गिल में चपटे अपरावर्तित तंतु होते हैं, जो क्लोम अक्ष की उल्टी ओर, दो पक्तियों में विन्यस्त रहते हैं। इस गण के उदाहरण हैं : सोलेनोमिया (Solenomya), न्यूकुला (Nucula) तथा योल्डिया (Yoldia)।

• (२) फिलिब्रैंकिएटा (Filibranchiata) - इस गण के लैमेलिब्रैंकियाओं में गिल समांतर, अधरीय दिष्ट तथा परावर्तित तुतु बनाता है। आंतर पक्ष्माभिकी संधियों द्वारा क्रमिक तंतु आपस में जुड़े रहते हैं। इस गण के उदाहरण हैं : अनोमिया (Anomia), आर्का (Arca), मिटिलस (Mytilus) तथा पेक्टेन (Pecten)। जनक के कवच से बाहर निकलने पर दो से छह सप्ताह तक बच्चे मछलियों के परजीवी रहते हैं। इस मसलों (Mussel) में बहुधा मोती पाए जाते हैं।

अलवणीय मसल (Margaritifera margaritifera) की शरीररचना

• (३) यूलैमेलिब्रैंकिएटा (Eulamellibranchiata) - इस गण के यूलैमेलिब्रैंकियाओ के गिल के तंतु समान अंतरों पर संवाहनी संधियों द्वारा जुड़े रहते हैं। ये संधियाँ रेखीय तंतु जैसे स्थान की गवाक्षों (fenestrae) में रूपांतरित कर देती है। इस गण के उदाहरण हैं : ऐनोडोंटा (Anodonta), ऑस्ट्रिया (Ostrea), टेलिना (Tellina), कार्डियम (Cardium) तथा फोलैस (Pholas)।

• (४) सेप्टिब्रैकिएटा (Septibranchiata) - इस गण के प्राणियों के गिल श्वसन अंग के रूप में नहीं रहते हैं। अब ये पेशीय पट (septum) बनाते हैं, जो अभिवर्तनी पेशी से लेकर साइफनों के परस्पर पृथक् होने के स्थान तक जाते हैं। इस गण के उदाहरण हैं : पोरोमाइया (Poromya) तथा कस्पीडेरिया (Cuspidaria)।

Empty shell of the giant clam
(Tridacna gigas)

Empty shells of the sword razor
(Ensis ensis)

ਦੋਕਪਾਟੀ, ਜਾਂ ਪਟਲਕਲੋਮੀ (Lamellibranchia/ਲੈਮੇਲਿਬਰੈਂਕਿਆ ਜਾਂ Bivalvia) ਅਕਸ਼ੇਰੁਕੀ ਅਤੇ ਜਲੀ ਪ੍ਰਾਣੀ ਹਨ। ਇਹ ਮੋਲਸਕਾ (Molausca) ਸੰਘ ਦੀ ਇੱਕ ਸ਼੍ਰੇਣੀ ਹੈ। ਇਸਨੂੰ ਲੈਮੇਲਿਬਰੈਂਕਿਆਟਾ, ਦੋਕਪਾਟੀ (Bivalve), ਜਾਂ ਪੇਲੇਸਿਪੋਡਾ (Pelecypoda) ਵੀ ਕਹਿੰਦੇ ਹਨ। ਹਾਲਾਂਕਿ ਇਨ੍ਹਾਂ ਦੇ ਪਾਦ ਚਪਟੇ ਹੋਣ ਦੇ ਬਜਾਏ ਨਵਤਲਿਤ ਅਧਰੀਏ ਹੁੰਦੇ ਹਨ, ਇਸਲਈ ਇਹ ਪੈਲੇਸਿਪੋਡਾ ਕਹਾਂਦੇ ਹਨ। ਇਸ ਵਰਗ ਦੇ ਪ੍ਰਾਣੀਆਂ ਦੇ ਸਿਰ ਨਹੀਂ ਹੁੰਦੇ, ਇਸ ਲਈ ਇਹ ਵਰਗ ਮੋਲਸਕਾ ਦੇ ਹੋਰ ਵਰਗਾਂ ਨਾਲੋਂ ਭਿੰਨ ਹੈ। ਇਹਨਾਂ ਵਿੱਚ ਲੇਬੀਅਲ ਸਪਰਸ਼ਕ (labial palp) ਸਿਰ ਦੀ ਤਰਜਮਾਨੀ ਕਰਦੇ ਹਨ। ਇਹ ਦੋਪਾਸੜ ਸਮਮਿਤ ਪ੍ਰਾਣੀ ਹੈ। ਇਨ੍ਹਾਂ ਦੇ ਸਾਰੇ ਅੰਸ਼ ਜੋੜਿਆਂ ਵਿੱਚ ਅਤੇ ਮਧਿਅਸਥ ਹੁੰਦੇ ਹਨ। ਲੈਮੇਲਿਬਰੈਂਕਿਆ ਸਥਾਨਬੱਧ ਪ੍ਰਾਣੀ ਹਨ। ਕੁੱਝ ਦੋ ਕਪਾਟੀ ਚਟਾਨਾਂ ਨਾਲ ਫਸੇ ਰਹਿੰਦੇ ਹਨ, ਜਦੋਂ ਕਿ ਹੋਰ ਧਾਗਿਆਂ ਵਰਗੇ ਪਲੰਦਿਆਂਨਾਲ ਜ਼ਮੀਨ ਨਾਲ ਨੱਥੀ ਰਹਿੰਦੇ ਹਨ। ਇਨ੍ਹਾਂ ਪਲੰਦਿਆਂ ਨੂੰ ਸੂਤਰਗੁੱਛ (Byssus) ਕਹਿੰਦੇ ਹਨ। ਇਹ ਸੂਤਰਗੁੱਛ ਪਾਦ ਦੀ ਇੱਕ ਗੁਹਿਕਾ ਨਾਲ ਸਰਵਿਤ ਹੁੰਦਾ ਹੈ। ਬਹੁਤੇ ਦੋਕਪਾਟੀਆਂ ਦੇ ਪਾਦ ਬਿਲ ਬਣਾਉਣ, ਜਾਂ ਗਮਨ ਲਈ ਇਸ਼ਤੇਮਾਲ ਕੀਤੇ ਜਾਂਦੇ ਹਨ। ਕੁੱਝ ਦੋਕਪਾਟੀ ਆਪਣੇ ਕਵਚਾਂ ਨੂੰ ਅਚਾਨਕ ਬੰਦ ਕਰਕੇ, ਪਾਣੀ ਨੂੰ ਬਾਹਰ ਕੱਢਣ ਰਾਹੀਂ ਤੈਰਦੇ ਹਨ।

ਲੈਮੇਲਿਬਰੈਂਕਿਆ ਦੇ ਕੁਲ 7,000 ਸਪੀਸ਼ੀਆਂ ਵਿੱਚੋਂ 100 ਤੋਂ ਜਿਆਦਾ ਗਿਆਤ ਹਨ।

ਜਾਣ ਪਛਾਣ

ਇਨ੍ਹਾਂ ਦੇ ਕਵਚ ਵਿੱਚ ਦੋ ਪ੍ਰਾਰੂਪਿਕ, ਸਮਾਨ ਕਪਾਟ ਹੁੰਦੇ ਹਨ। ਦੋਨੋਂ ਕਪਾਟ ਇੱਕ ਲਚਕਦਾਰ ਤੰਤੂ (elastic ligament) ਦੇ ਰਾਹੀਂ ਜੁੜੇ ਰਹਿੰਦੇ ਹਨ। ਜੇਕਰ ਤੰਤੂ ਆਂਤਰਿਕ ਹੁੰਦੇ ਹਨ, ਤਾਂ ਇਹ ਰੇਸਿਲੀਅਮ (resilium) ਕਹਾਂਦੇ ਹਨ। ਇਹ ਤੰਤੂ ਕਪਾਟਾਂ ਨੂੰ ਵੱਖ ਰੱਖਦੇ ਹਨ, ਜਦੋਂ ਕਿ ਦੋ ਐਡਕਟਰ ਪੇਸ਼ੀਆਂ ਕਵਚਾਂ ਨੂੰ ਬੰਦ ਰੱਖਣ ਦੀ ਕੋਸ਼ਿਸ਼ ਕਰਦੀਆਂ ਹਨ। ਕਵਚ ਦੇ ਇੰਟਰਡੋਰਸਲ ਭਾਗ, ਜਾਂ ਹਿੰਜਪੱਟ (hinge plate) ਵਿੱਚ ਹਿੰਜ ਦੰਦ ਹੁੰਦੇ ਹਨ, ਜੋ ਇੰਟਰਲਾਕ ਹੁੰਦੇ ਹਨ। ਦੰਦਾਂ ਦਾ ਸਧਾਰਣ ਰੂਪ ਅਨੇਕ ਸਮਾਨ ਦੰਦਾਂ ਦਾ ਬਹੁਦੰਦੀ (taxodont) ਹਿੰਜ ਹੈ। ਕੁੱਝ ਵਿਭੇਦਿਤ ਦੰਦਾਂ ਦਾ ਉੱਚਤਮ ਵਿਕਾਸ ਹੋਇਆ ਹੈ। ਅਨੇਕ ਦੋਕਪਾਟੀਆਂ ਵਿੱਚ ਅਧਰ ਅਤੇ ਪਾਰਸ਼ਵਿਕ ਉਪਾਂਤ ਦੇ ਸੂਖਮ ਦੰਦ ਦੁਆਰਾ ਕਪਾਟਾਂ ਦਾ ਠੀਕ ਠੀਕ ਬੰਦ ਹੋਣਾ ਅਸਾਨ ਹੁੰਦਾ ਹੈ।

ਪਰਨਾ (mantle) ਦੇ ਰਿਸਣ ਨਾਲ ਕਵਚ ਦਾ ਨਿਰਮਾਣ ਹੁੰਦਾ ਹੈ। ਪਰਨਾ ਸੰਪੂਰਣ ਸਰੀਰ ਨੂੰ ਢਕ ਲੈਂਦਾ ਹੈ। ਇਸਦੀ ਸੱਜੀਆਂ ਅਤੇ ਖੱਬੀਆਂ ਦੋ ਪਾਲੀਆਂ ਹੁੰਦੀਆਂ ਹਨ। ਇਹ ਪਾਲੀਆਂ ਪਰਨਾ ਪੇਸ਼ੀਆਂ (pallial muscles), ਜਾਂ ਵਰਤੁਲ (orbicular) ਪੇਸ਼ੀਆਂ ਦੇ ਰਾਹੀਂ ਕਪਾਟਾਂ ਨਾਲ ਜੁੜੀਆਂ ਰਹਿੰਦੀਆਂ ਹਨ। ਕਵਚ ਦਾ ਪਰਨਾ (pallial) ਜੁੜਨ ਦੀ ਰੇਖਾ (line of attachment) ਨੂੰ ਜ਼ਾਹਰ ਕਰਦਾ ਹੈ। ਪਰਨਾ ਰੇਖਾ ਦੇ ਅੰਤ ਵਿੱਚ ਪਿਛੇ ਹਟਣ ਵਾਲਿਆਂ ਪੇਸ਼ੀਆਂ ਹੁੰਦੀਆਂ ਹਨ। ਪਰਨਾ ਪਾਲ ਦੇ ਆਜ਼ਾਦ ਅਧਰ, ਸੀਮਾਂਤ ਦੋ, ਤਿੰਨ ਜਾਂ ਚਾਰ ਮੁਸਾਮਾਂ ਨੂੰ ਛਡ ਕੇ ਅੰਸ਼ਕ ਤੌਰ ਤੇ ਜੁੜੇ ਰਹਿੰਦੇ ਹਨ।

ਅਪਵਾਹੀ (exhalant) ਅਤੇ ਅੰਤਰਵਾਹੀ (inhalant) ਧਾਰਾਵਾਂ ਲਈ ਮੁਸਾਮਹੁੰਦੇ ਹਨ। ਇਨ੍ਹਾਂ ਦੋ ਮੁਸਾਮਾਂ ਉੱਤੇ ਪਰਨਾ ਆਮ ਤੌਰ ਤੇ ਦੋ ਪੇਸ਼ੀ ਟਿਊਬ ਦੇ ਰੂਪ ਵਿੱਚ ਵਧਿਆ ਰਹਿੰਦਾ ਹੈ। ਉਪਰ ਵਾਲੀ ਟਿਊਬ ਅਪਵਾਹੀ ਜਾਂ ਗੁਦਾ ਨਾਲ ਅਤੇ ਹੇਠਾਂ ਵਾਲੀ ਟਿਊਬ ਅੰਤਰਵਾਹੀ ਜਾਂ ਕਲੋਮ ਨਾਲ (ਵੇਖੋ ਚਿੱਤਰ 2) ਹੁੰਦਾ ਹੈ। ਤੀਸਰੇ ਮੁਸਮਤੋਂ ਪਾਦ ਦਾ ਬਹਿਰਬੇਧਨ ਹੁੰਦਾ ਹੈ। ਪਰਨਾ ਗੁਹਿਕਾ ਵਿੱਚ ਦੋ ਮੁੱਖ ਧਾਰਾਵਾਂ ਹੁੰਦੀਆਂ ਹਨ। ਅੰਤਰਵਾਹੀ ਛਿਦਰਕ ਤੋਂ ਮੂੰਹ ਨੂੰ ਢਕਣ ਵਾਲੇ ਲੇਬੀਅਲ ਸਪਰਸ਼ਕਾਂ ਅਤੇ ਗਿਲੀਆਂ ਦੇ ਵੱਲ ਇੱਕ ਧਾਰਾ ਪਸ਼ਚ ਤੌਰ ਤੇ ਦਿਸ਼ਟ ਹੁੰਦੀ ਹੈ। ਦੂਜੀ ਧਾਰਾ ਉਲਟੀ ਦਿਸ਼ਾ ਵਿੱਚ ਅਪਵਾਹੀ ਨਾਲ ਦੇ ਵੱਲ ਦਿਸ਼ਟ ਹੁੰਦੀ ਹੈ। ਰੇਤਾ, ਜਾਂ ਬਜਰੀ ਵਿੱਚ ਧਸੇ ਰਹਿਣ ਵਾਲੇ ਪਿੰਨਾ (pinna) ਅਤੇ ਸੋਲੇਨ (solen) ਵਿੱਚ ਅਪਵਾਹੀ ਧਾਰਾਵਾਂ ਪਕਸ਼ਮਾਭਿਕਾਮਏ ਨਾਲ ਦੁਆਰਾ ਜਾਂਦੀਆਂ ਹਨ। ਪਰਨਾ ਦੀ ਕੋਰ ਉੱਤੇ ਆਮ ਤੌਰ ਤੇ ਗਰੰਥੀਆਂ, ਸਪਰਸ਼ਕ, ਰੰਗ ਚੱਕ (pigment spot) ਅਤੇ ਅੱਖਾਂ ਹੁੰਦੀਆਂ ਹਨ।

ਆਮ ਤੌਰ ਤੇ ਲੈਮੇਲਿਬਰੈਂਕਿਆ ਦੇ ਗਿਲ, ਜਾਂ ਕਲੋਮ, ਕੰਕਤ ਕਲੋਮ (Ctenidium) ਕਹਾਂਦੇ ਹਨ, ਕਿਉਂਕਿ ਹੁਣ ਇਨ੍ਹਾਂ ਦਾ ਮੁੱਖ ਕਾਰਜ ਸਾਹਕਿਰਿਆ ਨਹੀਂ ਹੈ। ਸਾਹਕਿਰਿਆ ਮੁੱਖ ਤੌਰ ਤੇ ਪਰਨਾ ਨਾਲ ਹੁੰਦੀ ਹੈ। ਮੂੰਹ ਦੇ ਬੁੱਲ੍ਹ, ਜਾਂ ਯੁਗਮਿਤ ਪਾਲੀਯੁਕਤ ਪਰਖੇਪਣ ਹੈ। ਦੋ ਗਿਲੀਆਂ ਵਿੱਚੋਂ ਹਰ ਇੱਕ ਵਿੱਚ ਇੱਕ ਕੇਂਦਰੀ ਅਕਸ਼ ਹੁੰਦਾ ਹੈ, ਜਿਸ ਵਿੱਚ ਤੰਤੁਵਾਂ ਦੀਆਂ ਦੋ ਸ਼ਰੇਣੀਆਂ ਹੁੰਦੀਆਂ ਹਨ, ਜਿਨ੍ਹਾਂ ਨੂੰ ਅਰਧਕਲੋਮ, (demibranchs) ਕਹਿੰਦੇ ਹਨ। ਪ੍ਰੋਟੋਬਰੈਂਕ, (protobranch) ਦੋਕਪਾਟੀਆਂ ਵਿੱਚ ਤੰਤੁ ਸਧਾਰਣ ਪੱਟਿਕਾਵਾਂ ਸਜਾ ਹੁੰਦੇ ਹਨ, ਫਿਲਿਬਰੈਂਕ (filibranch) ਗਿਲੀਆਂ ਵਿੱਚ ਤੰਤੁ ਸਮਾਂਤਰ ਸਜਾ ਹੁੰਦੇ ਹਨ, ਜੋ ਇੰਟਰਲਾਕ ਸਿਲੀਅਰੀ ਟਫ (ciliary tuff) ਦੁਆਰਾ ਜੁੜੇ ਰਹਿੰਦੇ ਹਨ ਅਤੇ ਯੂਲੈਮੇਲਿਬਰੈਕ ਗਿਲੀਆਂ ਵਿੱਚ ਸਜਾ ਸੰਵਹਨੀ (vascular) ਸੰਧੀਆਂ ਦੁਆਰਾ ਜੁੜੇ ਰਹਿੰਦੇ ਹਨ।

ਆਮ ਤੌਰ ਤੇ ਨਰ ਅਤੇ ਮਾਦਾ ਭਿੰਨ ਭਿੰਨ ਹੁੰਦੇ ਹਨ। ਸਮੁੰਦਰੀ ਲੈਮੇਲਿਬਰੈਂਕਿਆ ਵਿੱਚ ਟਰੋਕੋਸਫੀਇਰ (trochosphere) ਅਤੇ ਵੇਲੀਜਰ (veliger) ਲਾਰਵਾ ਹੁੰਦੇ ਹਨ। ਲੂਣੇ ਪਾਣੀ ਦੇ ਲੈਮੇਲਿਬਰੈਂਕਿਆ ਦੀ ਵਿਸ਼ੇਸ਼ਤਾ ਊਸ਼ਮਾਇਨ (incubation) ਹੈ।

ఆల్చిప్పలు మొలస్కా జాతికి చెందిన జంతువులు. ఇవి బైవాల్వియా తరగతికి చెందినవి. ఈ తరగతిని పెలిసిపోడా అని కూడా పిలుస్తారు. వీటికి ద్విపార్శ్వ సౌష్టవం కలిగిన రెండు భాగాలు కలిగిన కర్పరాలు ఉంటాయి. ఈ తరగతిలో సుమారు 30,000 జాతులు ఉన్నాయి.

ఆల్చిప్పలు నీటిలో నివసించే జీవులు. ఇవి సముద్రంలోను, మంచి నీటి ఆవాసాలలోను నివసిస్తాయి.

Bivalves are unique among the molluscs for lacking a radula; they feed by siphoning and filtering large particles from water. Some bivalves are epifaunal: that is, they attach themselves to surfaces in the water, by means of a byssus or organic cementation. Others are infaunal: they bury themselves in sand or other sediments; these forms typically have a strong digging foot. Some bivalves, such as scallops, can swim.

Empty shell of the giant clam
(Tridacna gigas)

Empty shells of the sword razor
(Ensis ensis)

Bivalvia (/baɪˈvælviə/), in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of marine and freshwater molluscs that have laterally compressed bodies enclosed by a shell consisting of two hinged parts. As a group, bivalves have no head and they lack some usual molluscan organs, like the radula and the odontophore. The class includes the clams, oysters, cockles, mussels, scallops, and numerous other families that live in saltwater, as well as a number of families that live in freshwater. The majority are filter feeders. The gills have evolved into ctenidia, specialised organs for feeding and breathing. Most bivalves bury themselves in sediment, where they are relatively safe from predation. Others lie on the sea floor or attach themselves to rocks or other hard surfaces. Some bivalves, such as the scallops and file shells, can swim. The shipworms bore into wood, clay, or stone and live inside these substances.

The shell of a bivalve is composed of calcium carbonate, and consists of two, usually similar, parts called valves. These are joined together along one edge (the hinge line) by a flexible ligament that, usually in conjunction with interlocking "teeth" on each of the valves, forms the hinge. This arrangement allows the shell to be opened and closed without the two halves detaching. The shell is typically bilaterally symmetrical, with the hinge lying in the sagittal plane. Adult shell sizes of bivalves vary from fractions of a millimetre to over a metre in length, but the majority of species do not exceed 10 cm (4 in).

Bivalves have long been a part of the diet of coastal and riparian human populations. Oysters were cultured in ponds by the Romans, and mariculture has more recently become an important source of bivalves for food. Modern knowledge of molluscan reproductive cycles has led to the development of hatcheries and new culture techniques. A better understanding of the potential hazards of eating raw or undercooked shellfish has led to improved storage and processing. Pearl oysters (the common name of two very different families in salt water and fresh water) are the most common source of natural pearls. The shells of bivalves are used in craftwork, and the manufacture of jewellery and buttons. Bivalves have also been used in the biocontrol of pollution.

Bivalves appear in the fossil record first in the early Cambrian more than 500 million years ago. The total number of known living species is about 9,200. These species are placed within 1,260 genera and 106 families. Marine bivalves (including brackish water and estuarine species) represent about 8,000 species, combined in four subclasses and 99 families with 1,100 genera. The largest recent marine families are the Veneridae, with more than 680 species and the Tellinidae and Lucinidae, each with over 500 species. The freshwater bivalves include seven families, the largest of which are the Unionidae, with about 700 species.

Etymology

The taxonomic term Bivalvia was first used by Linnaeus in the 10th edition of his Systema Naturae in 1758 to refer to animals having shells composed of two valves.^[3] More recently, the class was known as Pelecypoda, meaning "axe-foot" (based on the shape of the foot of the animal when extended).

The name "bivalve" is derived from the Latin bis, meaning "two", and valvae, meaning "leaves of a door".^[4] ("Leaf" is an older word for the main, movable part of a door. We normally consider this the door itself.) Paired shells have evolved independently several times among animals that are not bivalves; other animals with paired valves include certain gastropods (small sea snails in the family Juliidae),^[5] members of the phylum Brachiopoda^[6] and the minute crustaceans known as ostracods^[7] and conchostracans.^[8]

Anatomy

Drawing of freshwater pearl mussel (Margaritifera margaritifera) anatomy:

1. posterior adductor
2. anterior adductor
3. outer left gill demibranch
4. inner left gill demibranch
5. excurrent siphon
6. incurrent siphon
7. foot
8. teeth
9. hinge
10. mantle
11. umbo

Interior of the left valve of a venerid

Main parts of a bivalve shell:

1. sagittal plane
2. growth lines
3. ligament
4. umbo

Bivalves have bilaterally symmetrical and laterally flattened bodies, with a blade-shaped foot, vestigial head and no radula.^[9][10] At the dorsal or back region of the shell is the hinge point or line, which contain the umbo and beak and the lower, curved margin is the ventral or underside region. The anterior or front of the shell is where the byssus (when present) and foot are located, and the posterior of the shell is where the siphons are located. With the hinge uppermost and with the anterior edge of the animal towards the viewer's left, the valve facing the viewer is the left valve and the opposing valve the right.^[11][12] Many bivalves such as clams, which appear upright, are evolutionarily lying on their side.

Mantle and shell

The shell is composed of two calcareous valves held together by a ligament. The valves are made of either calcite, as is the case in oysters, or both calcite and aragonite. Sometimes, the aragonite forms an inner, nacreous layer, as is the case in the order Pteriida. In other taxa, alternate layers of calcite and aragonite are laid down.^[13] The ligament and byssus, if calcified, are composed of aragonite.^[13] The outermost layer of the shell is the periostracum, a thin layer composed of horny conchiolin. The periostracum is secreted by the outer mantle and is easily abraded.^[14] The outer surface of the valves is often sculpted, with clams often having concentric striations, scallops having radial ribs and oysters a latticework of irregular markings.^[15]

In all molluscs, the mantle forms a thin membrane that covers the animal's body and extends out from it in flaps or lobes. In bivalves, the mantle lobes secrete the valves, and the mantle crest secretes the whole hinge mechanism consisting of ligament, byssus threads (where present), and teeth.^[16] The posterior mantle edge may have two elongated extensions known as siphons, through one of which water is inhaled, and the other expelled.^[17] The siphons retract into a cavity, known as the pallial sinus.^[18]

The shell grows larger when more material is secreted by the mantle edge, and the valves themselves thicken as more material is secreted from the general mantle surface. Calcareous matter comes from both its diet and the surrounding seawater. Concentric rings on the exterior of a valve are commonly used to age bivalves. For some groups, a more precise method for determining the age of a shell is by cutting a cross section through it and examining the incremental growth bands.^[19]

The shipworms, in the family Teredinidae have greatly elongated bodies, but their shell valves are much reduced and restricted to the anterior end of the body, where they function as scraping organs that permit the animal to dig tunnels through wood.^[20]

Muscles and ligaments

The main muscular system in bivalves is the posterior and anterior adductor muscles. These muscles connect the two valves and contract to close the shell. The valves are also joined dorsally by the hinge ligament, which is an extension of the periostracum. The ligament is responsible for opening the shell, and works against the adductor muscles when the animal opens and closes.^[21] Retractor muscles connect the mantle to the edge of the shell, along a line known as the pallial line.^[22][16] These muscles pull the mantle though the valves.^[16]

In sedentary or recumbent bivalves that lie on one valve, such as the oysters and scallops, the anterior adductor muscle has been lost and the posterior muscle is positioned centrally.^[23] In species that can swim by flapping their valves, a single, central adductor muscle occurs. These muscles are composed of two types of muscle fibres, striated muscle bundles for fast actions and smooth muscle bundles for maintaining a steady pull.^[24] Paired pedal protractor and retractor muscles operate the animal's foot.^[11][25][26]

Nervous system

The sedentary habits of the bivalves have meant that in general the nervous system is less complex than in most other molluscs. The animals have no brain; the nervous system consists of a nerve network and a series of paired ganglia. In all but the most primitive bivalves, two cerebropleural ganglia are on either side of the oesophagus. The cerebral ganglia control the sensory organs, while the pleural ganglia supply nerves to the mantle cavity. The pedal ganglia, which control the foot, are at its base, and the visceral ganglia, which can be quite large in swimming bivalves, are under the posterior adductor muscle. These ganglia are both connected to the cerebropleural ganglia by nerve fibres. Bivalves with long siphons may also have siphonal ganglia to control them.^[27][28]

Senses

The sensory organs of bivalves are largely located on the posterior mantle margins. The organs are usually mechanoreceptors or chemoreceptors, in some cases located on short tentacles. The osphradium is a patch of sensory cells located below the posterior adductor muscle that may serve to taste the water or measure its turbidity. Statocysts within the organism help the bivalve to sense and correct its orientation.^[28] In the order Anomalodesmata, the inhalant siphon is surrounded by vibration-sensitive tentacles for detecting prey.^[29] Many bivalves have no eyes, but a few members of the Arcoidea, Limopsoidea, Mytiloidea, Anomioidea, Ostreoidea, and Limoidea have simple eyes on the margin of the mantle. These consist of a pit of photosensory cells and a lens.^[30] Scallops have more complex eyes with a lens, a two-layered retina, and a concave mirror.^[31] All bivalves have light-sensitive cells that can detect a shadow falling over the animal.^[27]

Circulation and respiration

Four filaments of the gills of the blue mussel (Mytilus edulis) a) part of four filaments showing ciliated interfilamentar junctions (cj) b) diagram of a single filament showing the two lamellae connected at intervals by interlamellar junctions (ilj) and the position of the ciliated interfilamentar junctions (cp)

Bivalves have an open circulatory system that bathes the organs in blood (hemolymph). The heart has three chambers: two auricles receiving blood from the gills, and a single ventricle. The ventricle is muscular and pumps hemolymph into the aorta, and then to the rest of the body. Some bivalves have a single aorta, but most also have a second, usually smaller, aorta serving the hind parts of the animal.^[32] The hemolymph usually lacks any respiratory pigment.^[33] In the carnivorous genus Poromya, the hemolymph has red amoebocytes containing a haemoglobin pigment.^[34]

The paired gills are located posteriorly and consist of hollow tube-like filaments with thin walls for gas exchange. The respiratory demands of bivalves are low, due to their relative inactivity. Some freshwater species, when exposed to the air, can gape the shell slightly and gas exchange can take place.^[35][36] Oysters, including the Pacific oyster (Magallana gigas), are recognized as having varying metabolic responses to environmental stress, with changes in respiration rate being frequently observed.^[37]

Digestive system

Modes of feeding

Most bivalves are filter feeders, using their gills to capture particulate food such as phytoplankton from the water. Protobranchs feed in a different way, scraping detritus from the seabed, and this may be the original mode of feeding used by all bivalves before the gills became adapted for filter feeding. These primitive bivalves hold on to the bottom with a pair of tentacles at the edge of the mouth, each of which has a single palp, or flap. The tentacles are covered in mucus, which traps the food, and cilia, which transport the particles back to the palps. These then sort the particles, rejecting those that are unsuitable or too large to digest, and conveying others to the mouth.^[38]

In more advanced bivalves, water is drawn into the shell from the posterior ventral surface of the animal, passes upwards through the gills, and doubles back to be expelled just above the intake. There may be two elongated, retractable siphons reaching up to the seabed, one each for the inhalant and exhalant streams of water. The gills of filter-feeding bivalves are known as ctenidia and have become highly modified to increase their ability to capture food. For example, the cilia on the gills, which originally served to remove unwanted sediment, have become adapted to capture food particles, and transport them in a steady stream of mucus to the mouth. The filaments of the gills are also much longer than those in more primitive bivalves, and are folded over to create a groove through which food can be transported. The structure of the gills varies considerably, and can serve as a useful means for classifying bivalves into groups.^[38][39]

A few bivalves, such as the granular poromya (Poromya granulata), are carnivorous, eating much larger prey than the tiny microalgae consumed by other bivalves. Muscles draw water in through the inhalant siphon which is modified into a cowl-shaped organ, sucking in prey. The siphon can be retracted quickly and inverted, bringing the prey within reach of the mouth. The gut is modified so that large food particles can be digested.^[34]

The unusual genus, Entovalva, is endosymbiotic, being found only in the oesophagus of sea cucumbers. It has mantle folds that completely surround its small valves. When the sea cucumber sucks in sediment, the bivalve allows the water to pass over its gills and extracts fine organic particles. To prevent itself from being swept away, it attaches itself with byssal threads to the host's throat. The sea cucumber is unharmed.^[40]

Digestive tract

The digestive tract of typical bivalves consists of an oesophagus, stomach, and intestine. Protobranch stomachs have a mere sac attached to them while filter-feeding bivalves have elongated rod of solidified mucus referred to as the "crystalline style" projected into the stomach from an associated sac. Cilia in the sac cause the style to rotate, winding in a stream of food-containing mucus from the mouth, and churning the stomach contents. This constant motion propels food particles into a sorting region at the rear of the stomach, which distributes smaller particles into the digestive glands, and heavier particles into the intestine.^[41] Waste material is consolidated in the rectum and voided as pellets into the exhalent water stream through an anal pore. Feeding and digestion are synchronized with diurnal and tidal cycles.^[42]

Carnivorous bivalves generally have reduced crystalline styles and the stomach has thick, muscular walls, extensive cuticular linings and diminished sorting areas and gastric chamber sections.^[43]

Excretory system

The excretory organs of bivalves are a pair of nephridia. Each of these consists of a long, looped, glandular tube, which opens into the pericardium, and a bladder to store urine. They also have pericardial glands either line the auricles of the heart or attach to the pericardium, and serve as extra filtration organs. Metabolic waste is voided from the bladders through a nephridiopore near the front of the upper part of the mantle cavity and excreted.^[44][45]

Reproduction and development

The sexes are usually separate in bivalves but some hermaphroditism is known. The gonads and either open into the nephridia, or through a separate pore into a chamber over the gills.^[46][47] The ripe gonads of males and females release sperm and eggs into the water column. Spawning may take place continually or be triggered by environmental factors such as day length, water temperature, or the presence of sperm in the water. Some species are "dribble spawners", releasing gametes during protracted period that can extend for weeks. Others are mass spawners and release their gametes in batches or all at once.^[48]

Fertilization is usually external. Typically, a short stage lasts a few hours or days before the eggs hatch into trochophore larvae. These later develop into veliger larvae which settle on the seabed and undergo metamorphosis into adults.^[46][49] In some species, such as those in the genus Lasaea, females draw water containing sperm in through their inhalant siphons and fertilization takes place inside the female. These species then brood the young inside their mantle cavity, eventually releasing them into the water column as veliger larvae or as crawl-away juveniles.^[50]

Most of the bivalve larvae that hatch from eggs in the water column feed on diatoms or other phytoplankton. In temperate regions, about 25% of species are lecithotrophic, depending on nutrients stored in the yolk of the egg where the main energy source is lipids. The longer the period is before the larva first feeds, the larger the egg and yolk need to be. The reproductive cost of producing these energy-rich eggs is high and they are usually smaller in number. For example, the Baltic tellin (Macoma balthica) produces few, high-energy eggs. The larvae hatching out of these rely on the energy reserves and do not feed. After about four days, they become D-stage larvae, when they first develop hinged, D-shaped valves. These larvae have a relatively small dispersal potential before settling out. The common mussel (Mytilus edulis) produces 10 times as many eggs that hatch into larvae and soon need to feed to survive and grow. They can disperse more widely as they remain planktonic for a much longer time.^[51]

Freshwater bivalves have different lifecycle. Sperm is drawn into a female's gills with the inhalant water and internal fertilization takes place. The eggs hatch into glochidia larvae that develop within the female's shell. Later they are released and attach themselves parasitically to the gills or fins of a fish host. After several weeks they drop off their host, undergo metamorphosis and develop into adults on the substrate.^[46]

Some of the species in the freshwater mussel family, Unionidae, commonly known as pocketbook mussels, have evolved an unusual reproductive strategy. The female's mantle protrudes from the shell and develops into an imitation small fish, complete with fish-like markings and false eyes. This decoy moves in the current and attracts the attention of real fish. Some fish see the decoy as prey, while others see a conspecific. They approach for a closer look and the mussel releases huge numbers of larvae from its gills, dousing the inquisitive fish with its tiny, parasitic young. These glochidia larvae are drawn into the fish's gills, where they attach and trigger a tissue response that forms a small cyst around each larva. The larvae then feed by breaking down and digesting the tissue of the fish within the cysts. After a few weeks they release themselves from the cysts and fall to the stream bed as juvenile molluscs.^[52]

Comparison with brachiopods

Anadara, a bivalve with taxodont dentition from the Pliocene of Cyprus

A fossil Jurassic brachiopod with the lophophore support intact

Brachiopods are shelled marine organisms that superficially resembled bivalves in that they are of similar size and have a hinged shell in two parts. However, brachiopods evolved from a very different ancestral line, and the resemblance to bivalves only arose because they occupy similar ecological niches. The differences between the two groups are due to their separate ancestral origins. Different initial structures have been adapted to solve the same problems, a case of convergent evolution. In modern times, brachiopods are not as common as bivalves.^[53]

Both groups have a shell consisting of two valves, but the organization of the shell is quite different in the two groups. In brachiopods, the two valves are positioned on the dorsal and ventral surfaces of the body, while in bivalves, the valves are on the left and right sides of the body, and are, in most cases, mirror images of one other. Brachiopods have a lophophore, a coiled, rigid cartilaginous internal apparatus adapted for filter feeding, a feature shared with two other major groups of marine invertebrates, the bryozoans and the phoronids. Some brachiopod shells are made of calcium phosphate but most are calcium carbonate in the form of the biomineral calcite, whereas bivalve shells are always composed entirely of calcium carbonate, often in the form of the biomineral aragonite.^[54]

Evolutionary history

The Cambrian explosion took place around 540 to 520 million years ago (Mya). In this geologically brief period, all the major animal phyla diverged and these included the first creatures with mineralized skeletons. Brachiopods and bivalves made their appearance at this time, and left their fossilized remains behind in the rocks.^[55]

Possible early bivalves include Pojetaia and Fordilla; these probably lie in the stem rather than crown group. Watsonella and Anabarella are perceived to be (earlier) close relatives of these taxa.^[56] Only five genera of supposed Cambrian "bivalves" exist, the others being Tuarangia, Camya and Arhouriella and potentially Buluniella.^[57]

Bivalve fossils can be formed when the sediment in which the shells are buried hardens into rock. Often, the impression made by the valves remains as the fossil rather than the valves. During the Early Ordovician, a great increase in the diversity of bivalve species occurred, and the dysodont, heterodont, and taxodont dentitions evolved. By the Early Silurian, the gills were becoming adapted for filter feeding, and during the Devonian and Carboniferous periods, siphons first appeared, which, with the newly developed muscular foot, allowed the animals to bury themselves deep in the sediment.^[58]

By the middle of the Paleozoic, around 400 Mya, the brachiopods were among the most abundant filter feeders in the ocean, and over 12,000 fossil species are recognized.^[59] By the Permian–Triassic extinction event 250 Mya, bivalves were undergoing a huge radiation of diversity. The bivalves were hard hit by this event, but re-established themselves and thrived during the Triassic period that followed. In contrast, the brachiopods lost 95% of their species diversity.^[54] The ability of some bivalves to burrow and thus avoid predators may have been a major factor in their success. Other new adaptations within various families allowed species to occupy previously unused evolutionary niches. These included increasing relative buoyancy in soft sediments by developing spines on the shell, gaining the ability to swim, and in a few cases, adopting predatory habits.^[58]

For a long time, bivalves were thought to be better adapted to aquatic life than brachiopods were, outcompeting and relegating them to minor niches in later ages. These two taxa appeared in textbooks as an example of replacement by competition. Evidence given for this included the fact that bivalves needed less food to subsist because of their energetically efficient ligament-muscle system for opening and closing valves. All this has been broadly disproven, though; rather, the prominence of modern bivalves over brachiopods seems due to chance disparities in their response to extinction events.^[60]

Diversity of extant bivalves

The adult maximum size of living species of bivalve ranges from 0.52 mm (0.02 in) in Condylonucula maya,^[61] a nut clam, to a length of 1,532 millimetres (60.3 in) in Kuphus polythalamia, an elongated, burrowing shipworm.^[62] However, the species generally regarded as the largest living bivalve is the giant clam Tridacna gigas, which can grow to a length of 1,200 mm (47 in) and a weight of more than 200 kg (441 lb).^[63] The largest known extinct bivalve is a species of Platyceramus whose fossils measure up to 3,000 mm (118 in) in length.^[64]

In his 2010 treatise, Compendium of Bivalves, Markus Huber gives the total number of living bivalve species as about 9,200 combined in 106 families.^[65] Huber states that the number of 20,000 living species, often encountered in literature, could not be verified and presents the following table to illustrate the known diversity:

Distribution

Zebra mussels encrusting a water velocity meter in Lake Michigan

The bivalves are a highly successful class of invertebrates found in aquatic habitats throughout the world. Most are infaunal and live buried in sediment on the seabed, or in the sediment in freshwater habitats. A large number of bivalve species are found in the intertidal and sublittoral zones of the oceans. A sandy sea beach may superficially appear to be devoid of life, but often a very large number of bivalves and other invertebrates are living beneath the surface of the sand. On a large beach in South Wales, careful sampling produced an estimate of 1.44 million cockles (Cerastoderma edule) per acre of beach.^[66]

Bivalves inhabit the tropics, as well as temperate and boreal waters. A number of species can survive and even flourish in extreme conditions. They are abundant in the Arctic, about 140 species being known from that zone.^[67] The Antarctic scallop, Adamussium colbecki, lives under the sea ice at the other end of the globe, where the subzero temperatures mean that growth rates are very slow.^[68] The giant mussel, Bathymodiolus thermophilus, and the giant white clam, Calyptogena magnifica, both live clustered around hydrothermal vents at abyssal depths in the Pacific Ocean. They have chemosymbiotic bacteria in their gills that oxidise hydrogen sulphide, and the molluscs absorb nutrients synthesized by these bacteria.^[69] The saddle oyster, Enigmonia aenigmatica, is a marine species that could be considered amphibious. It lives above the high tide mark in the tropical Indo-Pacific on the underside of mangrove leaves, on mangrove branches, and on sea walls in the splash zone.^[70]

Some freshwater bivalves have very restricted ranges. For example, the Ouachita creekshell mussel, Villosa arkansasensis, is known only from the streams of the Ouachita Mountains in Arkansas and Oklahoma, and like several other freshwater mussel species from the southeastern US, it is in danger of extinction.^[71] In contrast, a few species of freshwater bivalves, including the golden mussel (Limnoperna fortunei), are dramatically increasing their ranges. The golden mussel has spread from Southeast Asia to Argentina, where it has become an invasive species.^[72] Another well-travelled freshwater bivalve, the zebra mussel (Dreissena polymorpha) originated in southeastern Russia, and has been accidentally introduced to inland waterways in North America and Europe, where the species damages water installations and disrupts local ecosystems.^[73]

Behaviour

A large number of live venerid bivalves underwater with their siphons visible

Pacific oyster equipped with activity electrodes to follow its daily behaviour

Most bivalves adopt a sedentary or even sessile lifestyle, often spending their whole lives in the area in which they first settled as juveniles. The majority of bivalves are infaunal, living under the seabed, buried in soft substrates such as sand, silt, mud, gravel, or coral fragments. Many of these live in the intertidal zone where the sediment remains damp even when the tide is out. When buried in the sediment, burrowing bivalves are protected from the pounding of waves, desiccation, and overheating during low tide, and variations in salinity caused by rainwater. They are also out of the reach of many predators.^[74] Their general strategy is to extend their siphons to the surface for feeding and respiration during high tide, but to descend to greater depths or keep their shell tightly shut when the tide goes out.^[74] They use their muscular foot to dig into the substrate. To do this, the animal relaxes its adductor muscles and opens its shell wide to anchor itself in position while it extends its foot downwards into the substrate. Then it dilates the tip of its foot, retracts the adductor muscles to close the shell, shortens its foot and draws itself downwards. This series of actions is repeated to dig deeper.^[75]

Other bivalves, such as mussels, attach themselves to hard surfaces using tough byssus threads made of collagen and elastin proteins.^[76] Some species, including the true oysters, the jewel boxes, the jingle shells, the thorny oysters and the kitten's paws, cement themselves to stones, rock or larger dead shells.^[77] In oysters the lower valve may be almost flat while the upper valve develops layer upon layer of thin horny material reinforced with calcium carbonate. Oysters sometimes occur in dense beds in the neritic zone and, like most bivalves, are filter feeders.^[78]

Bivalves filter large amounts of water to feed and breathe but they are not permanently open. They regularly shut their valves to enter a resting state, even when they are permanently submerged. In oysters, for example, their behaviour follows very strict circatidal and circadian rhythms according to the relative positions of the moon and sun. During neap tides, they exhibit much longer closing periods than during spring tides.^[79]

Although many non-sessile bivalves use their muscular foot to move around, or to dig, members of the freshwater family Sphaeriidae are exceptional in that these small clams climb about quite nimbly on weeds using their long and flexible foot. The European fingernail clam (Sphaerium corneum), for example, climbs around on water weeds at the edges of lakes and ponds; this enables the clam to find the best position for filter feeding.^[80]

Predators and defence

The thick shell and rounded shape of bivalves make them awkward for potential predators to tackle. Nevertheless, a number of different creatures include them in their diet. Many species of demersal fish feed on them including the common carp (Cyprinus carpio), which is being used in the upper Mississippi River to try to control the invasive zebra mussel (Dreissena polymorpha).^[81] Birds such as the Eurasian oystercatcher (Haematopus ostralegus) have specially adapted beaks which can pry open their shells.^[82] The herring gull (Larus argentatus) sometimes drops heavy shells onto rocks in order to crack them open.^[83] Sea otters feed on a variety of bivalve species and have been observed to use stones balanced on their chests as anvils on which to crack open the shells.^[84] The Pacific walrus (Odobenus rosmarus divergens) is one of the main predators feeding on bivalves in Arctic waters.^[85] Shellfish have formed part of the human diet since prehistoric times, a fact evidenced by the remains of mollusc shells found in ancient middens. Examinations of these deposits in Peru has provided a means of dating long past El Niño events because of the disruption these caused to bivalve shell growth.^[86] Further changes in shell development due to environmental stress has also been suggested to cause increased mortality in oysters due to reduced shell strength. ^[37]

Invertebrate predators include crustaceans, starfish and octopuses. Crustaceans crack the shells with their pincers and starfish use their water vascular system to force the valves apart and then insert part of their stomach between the valves to digest the bivalve's body. It has been found experimentally that both crabs and starfish preferred molluscs that are attached by byssus threads to ones that are cemented to the substrate. This was probably because they could manipulate the shells and open them more easily when they could tackle them from different angles.^[77] Octopuses either pull bivalves apart by force, or they bore a hole into the shell and insert a digestive fluid before sucking out the liquified contents.^[87] Certain carnivorous gastropod snails such as whelks (Buccinidae) and murex snails (Muricidae) feed on bivalves by boring into their shells. A dog whelk (Nucella) drills a hole with its radula assisted by a shell-dissolving secretion. The dog whelk then inserts its extendible proboscis and sucks out the body contents of the victim, which is typically a blue mussel.^[88]

Razor shells can dig themselves into the sand with great speed to escape predation. When a Pacific razor clam (Siliqua patula) is laid on the surface of the beach it can bury itself completely in seven seconds ^[89] and the Atlantic jackknife clam, Ensis directus, can do the same within fifteen seconds.^[90] Scallops and file clams can swim by opening and closing their valves rapidly; water is ejected on either side of the hinge area and they move with the flapping valves in front.^[91] Scallops have simple eyes around the margin of the mantle and can clap their valves shut to move sharply, hinge first, to escape from danger.^[91] Cockles can use their foot to move across the seabed or leap away from threats. The foot is first extended before being contracted suddenly when it acts like a spring, projecting the animal forwards.^[92]

In many bivalves that have siphons, they can be retracted back into the safety of the shell. If the siphons inadvertently get attacked by a predator, in some cases they snap off. The animal can regenerate them later, a process that starts when the cells close to the damaged site become activated and remodel the tissue back to its pre-existing form and size.^[93] On the other hand in some cases it does not snap off. If the siphon is exposed it is the key for a predatory fish to obtain the entire body. This tactic has been observed against bivalves with an infaunal lifestyle.^[94][95]

File shells such as Limaria fragilis can produce a noxious secretion when stressed. It has numerous tentacles which fringe its mantle and protrude some distance from the shell when it is feeding. If attacked, it sheds tentacles in a process known as autotomy. The toxin released by this is distasteful and the detached tentacles continue to writhe which may also serve to distract potential predators.^[96]

Mariculture

Oyster culture in Brittany, France

Oysters, mussels, clams, scallops and other bivalve species are grown with food materials that occur naturally in their culture environment in the sea and lagoons.^[97] One-third of the world's farmed food fish harvested in 2010 was achieved without the use of feed, through the production of bivalves and filter-feeding carps.^[97] European flat oysters (Ostrea edulis) were first farmed by the Romans in shallow ponds and similar techniques are still in use.^[98] Seed oysters are either raised in a hatchery or harvested from the wild. Hatchery production provides some control of the broodstock but remains problematic because disease-resistant strains of this oyster have not yet been developed. Wild spats are harvested either by broadcasting empty mussel shells on the seabed or by the use of long, small-mesh nets filled with mussel shells supported on steel frames. The oyster larvae preferentially settle out on the mussel shells. Juvenile oysters are then grown on in nursery trays and are transferred to open waters when they reach 5 to 6 millimetres (0.20 to 0.24 in) in length.^[98]

Many juveniles are further reared off the seabed in suspended rafts, on floating trays or cemented to ropes. Here they are largely free from bottom-dwelling predators such as starfish and crabs but more labour is required to tend them. They can be harvested by hand when they reach a suitable size. Other juveniles are laid directly on the seabed at the rate of 50 to 100 kilograms (110 to 220 lb) per hectare. They grow on for about two years before being harvested by dredging. Survival rates are low at about 5%.^[98]

The Pacific oyster (Crassostrea gigas) is cultivated by similar methods but in larger volumes and in many more regions of the world. This oyster originated in Japan where it has been cultivated for many centuries.^[99] It is an estuarine species and prefers salinities of 20 to 25 parts per thousand. Breeding programmes have produced improved stock that is available from hatcheries. A single female oyster can produce 50–80 million eggs in a batch so the selection of broodstock is of great importance. The larvae are grown on in tanks of static or moving water. They are fed high quality microalgae and diatoms and grow fast. At metamorphosis the juveniles may be allowed to settle on PVC sheets or pipes, or crushed shell. In some cases, they continue their development in "upwelling culture" in large tanks of moving water rather than being allowed to settle on the bottom. They then may be transferred to transitional, nursery beds before being moved to their final rearing quarters. Culture there takes place on the bottom, in plastic trays, in mesh bags, on rafts or on long lines, either in shallow water or in the intertidal zone. The oysters are ready for harvesting in 18 to 30 months depending on the size required.^[99]

Similar techniques are used in different parts of the world to cultivate other species including the Sydney rock oyster (Saccostrea commercialis), the northern quahog (Mercenaria mercenaria), the blue mussel (Mytilus edulis), the Mediterranean mussel (Mytilus galloprovincialis), the New Zealand green-lipped mussel (Perna canaliculus), the grooved carpet shell (Ruditapes decussatus), the Japanese carpet shell (Venerupis philippinarum), the pullet carpet shell (Venerupis pullastra) and the Yesso scallop (Patinopecten yessoensis).^[100]

Production of bivalve molluscs by mariculture in 2010 was 12,913,199 tons, up from 8,320,724 tons in 2000. Culture of clams, cockles and ark shells more than doubled over this time period from 2,354,730 to 4,885,179 tons. Culture of mussels over the same period grew from 1,307,243 to 1,812,371 tons, of oysters from 3,610,867 to 4,488,544 tons and of scallops from 1,047,884 to 1,727,105 tons.^[101]

Use as food

Flat oysters (Ostrea edulis) from France

Bivalves have been an important source of food for humans at least since Roman times^[102] and empty shells found in middens at archaeological sites are evidence of earlier consumption.^[86] Oysters, scallops, clams, ark clams, mussels and cockles are the most commonly consumed kinds of bivalve, and are eaten cooked or raw. In 1950, the year in which the Food and Agriculture Organization (FAO) started making such information available, world trade in bivalve molluscs was 1,007,419 tons.^[103] By 2010, world trade in bivalves had risen to 14,616,172 tons, up from 10,293,607 tons a decade earlier. The figures included 5,554,348 (3,152,826) tons of clams, cockles and ark shells, 1,901,314 (1,568,417) tons of mussels, 4,592,529 (3,858,911) tons of oysters and 2,567,981 (1,713,453) tons of scallops.^[103] China increased its consumption 400-fold during the period 1970 to 1997.^[104]

It has been known for more than a century that consumption of raw or insufficiently cooked shellfish can be associated with infectious diseases. These are caused either by bacteria naturally present in the sea such as Vibrio spp. or by viruses and bacteria from sewage effluent that sometimes contaminates coastal waters. As filter feeders, bivalves pass large quantities of water through their gills, filtering out the organic particles, including the microbial pathogens. These are retained in the animals' tissues and become concentrated in their liver-like digestive glands.^[104][105] Another possible source of contamination occurs when bivalves contain marine biotoxins as a result of ingesting numerous dinoflagellates. These microalgae are not associated with sewage but occur unpredictably as algal blooms. Large areas of a sea or lake may change colour as a result of the proliferation of millions of single-cell algae, and this condition is known as a red tide.^[104]

Viral and bacterial infections

In 1816 in France, a physician, J. P. A. Pasquier, described an outbreak of typhoid linked to the consumption of raw oysters. The first report of this kind in the United States was in Connecticut in 1894. As sewage treatment programmes became more prevalent in the late 19th century, more outbreaks took place. This may have been because sewage was released through outlets into the sea providing more food for bivalves in estuaries and coastal habitats. A causal link between the bivalves and the illness was not easy to demonstrate because the illness might come on days or even weeks after the ingestion of the contaminated shellfish. One viral pathogen is the Norwalk virus. This is resistant to treatment with chlorine-containing chemicals and may be present in the marine environment even when coliform bacteria have been killed by the treatment of sewage.^[104]

Since the 1970s, outbreaks of oyster-vectored diseases have occurs throughout the world. The mortality rate of one disease causing bacteria Vibrio vulnificus, was high at 50%.^[104] In 1978, an oyster-associated gastrointestinal infection affecting more than 2,000 people occurred in Australia. The causative agent was found to be the Norwalk virus and the epidemic caused major economic difficulties to the oyster farming industry in the country.^[106] In 1988, an outbreak of hepatitis A associated with the consumption of inadequately cooked clams (Anadara subcrenata) took place in the Shanghai area of China. An estimated 290,000 people were infected and there were 47 deaths.^[107] In the United States and the European Union, since the early 1990s regulations have been in place that are designed to prevent shellfish from contaminated waters entering restaurants.^[104]

Paralytic shellfish poisoning

Paralytic shellfish poisoning (PSP) is primarily caused by the consumption of bivalves that have accumulated toxins by feeding on toxic dinoflagellates, single-celled protists found naturally in the sea and inland waters. Saxitoxin is the most virulent of these. In mild cases, PSP causes tingling, numbness, sickness and diarrhoea. In more severe cases, the muscles of the chest wall may be affected leading to paralysis and even death. In 1937, researchers in California established the connection between blooms of these phytoplankton and PSP.^[108] The biotoxin remains potent even when the shellfish are well-cooked.^[108] In the United States, there is a regulatory limit of 80 µg/g of saxitoxin equivalent in shellfish meat.^[108]

Amnesic shellfish poisoning

Amnesic shellfish poisoning (ASP) was first reported in eastern Canada in 1987. It is caused by the substance domoic acid found in certain diatoms of the genus Pseudo-nitzschia. Bivalves can become toxic when they filter these microalgae out of the water. Domoic acid is a low-molecular weight amino acid that is able to destroy brain cells causing memory loss, gastroenteritis, long-term neurological problems or death. In an outbreak in the western United States in 1993, finfish were also implicated as vectors, and seabirds and mammals suffered neurological symptoms.^[108] In the United States and Canada, a regulatory limit of 20 µg/g of domoic acid in shellfish meat is set.^[109]

Ecosystem services

Nutrient extraction services provided by bivalves. Blue mussels are used as examples but other bivalves like oysters can also provide these nutrient extraction services.^[110]

Ecosystem services provided by marine bivalves in relation to nutrient extraction from the coastal environment have gained increased attention to mitigate adverse effects of excess nutrient loading from human activities, such as agriculture and sewage discharge. These activities damage coastal ecosystems and require action from local, regional, and national environmental management. Marine bivalves filter particles like phytoplankton, thereby transforming particulate organic matter into bivalve tissue or larger faecal pellets that are transferred to the benthos. Nutrient extraction from the coastal environment takes place through two different pathways: (i) harvest/removal of the bivalves – thereby returning nutrients back to land; or (ii) through increased denitrification in proximity to dense bivalve aggregations, leading to loss of nitrogen to the atmosphere. Active use of marine bivalves for nutrient extraction may include a number of secondary effects on the ecosystem, such as filtration of particulate material. This leads to partial transformation of particulate-bound nutrients into dissolved nutrients via bivalve excretion or enhanced mineralization of faecal material.^[110]

When they live in polluted waters, bivalve molluscs have a tendency to accumulate substances such as heavy metals and persistent organic pollutants in their tissues. This is because they ingest the chemicals as they feed but their enzyme systems are not capable of metabolising them and as a result, the levels build up. This may be a health hazard for the molluscs themselves, and is one for humans who eat them. It also has certain advantages in that bivalves can be used in monitoring the presence and quantity of pollutants in their environment.^[111]

Economic value of bivalve nutrient extraction, linking processes to services to economic values.^[110]

There are limitations to the use of bivalves as bioindicators. The level of pollutants found in the tissues varies with species, age, size, time of year and other factors. The quantities of pollutants in the water may vary and the molluscs may reflect past rather than present values. In a study near Vladivostok it was found that the level of pollutants in the bivalve tissues did not always reflect the high levels in the surrounding sediment in such places as harbours. The reason for this was thought to be that the bivalves in these locations did not need to filter so much water as elsewhere because of the water's high nutritional content.^[112]

A study of nine different bivalves with widespread distributions in tropical marine waters concluded that the mussel, Trichomya hirsuta, most nearly reflected in its tissues the level of heavy metals (Pb, Cd, Cu, Zn, Co, Ni, and Ag) in its environment. In this species there was a linear relationship between the sedimentary levels and the tissue concentration of all the metals except zinc.^[113] In the Persian Gulf, the Atlantic pearl-oyster (Pinctada radiata) is considered to be a useful bioindicator of heavy metals.^[114]

Crushed shells, available as a by-product of the seafood canning industry, can be used to remove pollutants from water. It has been found that, as long as the water is maintained at an alkaline pH, crushed shells will remove cadmium, lead and other heavy metals from contaminated waters by swapping the calcium in their constituent aragonite for the heavy metal, and retaining these pollutants in a solid form.^[115] The rock oyster (Saccostrea cucullata) has been shown to reduce the levels of copper and cadmium in contaminated waters in the Persian Gulf. The live animals acted as biofilters, selectively removing these metals, and the dead shells also had the ability to reduce their concentration.^[116]

Other uses

Carved shell miniatures

Conchology is the scientific study of mollusc shells, but the term conchologist is also sometimes used to describe a collector of shells. Many people pick up shells on the beach or purchase them and display them in their homes. There are many private and public collections of mollusc shells, but the largest one in the world is at the Smithsonian Institution, which houses in excess of 20 million specimens.^[117]

1885 wampum belt

Freshwater mussel shell used for making buttons

Carved nacre in a 16th-century altarpiece

Shells are used decoratively in many ways. They can be pressed into concrete or plaster to make decorative paths, steps or walls and can be used to embellish picture frames, mirrors or other craft items. They can be stacked up and glued together to make ornaments. They can be pierced and threaded onto necklaces or made into other forms of jewellery. Shells have had various uses in the past as body decorations, utensils, scrapers and cutting implements. Carefully cut and shaped shell tools dating back 32,000 years have been found in a cave in Indonesia. In this region, shell technology may have been developed in preference to the use of stone or bone implements, perhaps because of the scarcity of suitable rock materials.^[118]

The indigenous peoples of the Americas living near the east coast used pieces of shell as wampum. The channeled whelk (Busycotypus canaliculatus) and the quahog (Mercenaria mercenaria) were used to make white and purple traditional patterns. The shells were cut, rolled, polished and drilled before being strung together and woven into belts. These were used for personal, social and ceremonial purposes and also, at a later date, for currency.^[119] The Winnebago Tribe from Wisconsin had numerous uses for freshwater mussels including using them as spoons, cups, ladles and utensils. They notched them to provide knives, graters and saws. They carved them into fish hooks and lures. They incorporated powdered shell into clay to temper their pottery vessels. They used them as scrapers for removing flesh from hides and for separating the scalps of their victims. They used shells as scoops for gouging out fired logs when building canoes and they drilled holes in them and fitted wooden handles for tilling the ground.^[120]

Buttons have traditionally been made from a variety of freshwater and marine shells.^[121] At first they were used decoratively rather than as fasteners and the earliest known example dates back five thousand years and was found at Mohenjo-daro in the Indus Valley.^[122]

Sea silk is a fine fabric woven from the byssus threads of bivalves, particularly the pen shell (Pinna nobilis). It used to be produced in the Mediterranean region where these shells are endemic. It was an expensive fabric and overfishing has much reduced populations of the pen shell.^[123]

Crushed shells are added as a calcareous supplement to the diet of laying poultry. Oyster shell and cockle shell are often used for this purpose and are obtained as a by-product from other industries.^[124]

Pearls and mother-of-pearl

Mother-of-pearl or nacre is the naturally occurring lustrous layer that lines some mollusc shells. It is used to make pearl buttons and in artisan craftwork to make organic jewellery. It has traditionally been inlaid into furniture and boxes, particularly in China. It has been used to decorate musical instruments, watches, pistols, fans and other products. The import and export of goods made with nacre are controlled in many countries under the International Convention of Trade in Endangered Species of Wild Fauna and Flora.^[125]

A pearl is created in the mantle of a mollusk when an irritant particle is surrounded by layers of nacre. Although most bivalves can create pearls, oysters in the family Pteriidae and freshwater mussels in the families Unionidae and Margaritiferidae are the main source of commercially available pearls because the calcareous concretions produced by most other species have no lustre. Finding pearls inside oysters is a very chancy business as hundreds of shells may need to be pried open before a single pearl can be found. Most pearls are now obtained from cultured shells where an irritant substance has been purposefully introduced to induce the formation of a pearl. A "mabe" (irregular) pearl can be grown by the insertion of an implant, usually made of plastic, under a flap of the mantle and next to the mother-of-pearl interior of the shell. A more difficult procedure is the grafting of a piece of oyster mantle into the gonad of an adult specimen together with the insertion of a shell bead nucleus. This produces a superior, spherical pearl. The animal can be opened to extract the pearl after about two years and reseeded so that it produces another pearl. Pearl oyster farming and pearl culture is an important industry in Japan and many other countries bordering the Indian and Pacific Oceans.^[126]

Symbolism

The scallop is the symbol of St James and is called Coquille Saint-Jacques in French. It is an emblem carried by pilgrims on their way to the shrine of Santiago de Compostela in Galicia. The shell became associated with the pilgrimage and came to be used as a symbol showing hostelries along the route and later as a sign of hospitality, food and lodging elsewhere.^[127]

Roman myth has it that Venus, the goddess of love, was born in the sea and emerged accompanied by fish and dolphins, with Botticelli depicting her as arriving in a scallop shell. The Romans revered her and erected shrines in her honour in their gardens, praying to her to provide water and verdant growth.^[128] From this, the scallop and other bivalve shells came to be used as a symbol for fertility.^[129] Its depiction is used in architecture, furniture and fabric design and it is the logo of Royal Dutch Shell, the global oil and gas company.^[130]

Bivalvian taxonomies

Mussels in the intertidal zone in Cornwall, England

Mytilarca is a distant relative of the mussels; from the Middle Devonian of Wisconsin.

Fossil gastropod and attached mytilid bivalves in a Jurassic limestone (Matmor Formation) in southern Israel

Aviculopecten subcardiformis; a fossil of an extinct scallop from the Logan Formation of Wooster, Ohio (external mold)

For the past two centuries no consensus has existed on bivalve phylogeny from the many classifications developed. In earlier taxonomic systems, experts used a single characteristic feature for their classifications, choosing among shell morphology, hinge type or gill type. Conflicting naming schemes proliferated due to these taxonomies based on single organ systems. One of the most widely accepted systems was that put forward by Norman D. Newell in Part N of the Treatise on Invertebrate Paleontology,^[131] which employed a classification system based on general shell shape, microstructures and hinge configuration.^[132] Because features such as hinge morphology, dentition, mineralogy, shell morphology and shell composition change slowly over time, these characteristics can be used to define major taxonomic groups.

Since the year 2000, taxonomic studies using cladistical analyses of multiple organ systems, shell morphology (including fossil species) and modern molecular phylogenetics have resulted in the drawing up of what experts believe is a more accurate phylogeny of the Bivalvia.^{[133][134][135][136][137]} Based upon these studies, a new proposed classification system for the Bivalvia was published in 2010 by Bieler, Carter & Coan.^[138] In 2012, this new system was adopted by the World Register of Marine Species (WoRMS) for the classification of the Bivalvia. Some experts still maintain that Anomalodesmacea should be considered a separate subclass, whereas the new system treats it as the order Anomalodesmata, within the subclass Heterodonta. Molecular phylogenetic work continues, further clarifying which Bivalvia are most closely related and thus refining the classification.^[139][140]

Practical taxonomy of R.C. Moore

R.C. Moore, in Moore, Lalicker, and Fischer, 1952, Invertebrate Fossils, gives a practical and useful classification of pelecypods (Bivalvia) even if somewhat antiquated, based on shell structure, gill type, and hinge teeth configuration. Subclasses and orders given are:

Subclass:Prionodesmacea

Order

Paleoconcha

Taxodonta: Many teeth (e.g. order Nuculida)

Schizodonta: Big bifurcating teeth (e.g. Trigonia spp.)

Isodonta: Equal teeth (e.g. Spondylus spp.)

Dysodonta: Absent teeth and ligaments joins the valves.

Subclass:Teleodesmacea

Order

Heterodonta: Different teeth (e.g. family Cardiidae). [ Lower Ordovician – Recent]

Pachydonta: Large, different, deformed teeth (e.g. rudist spp.). [ Late Jurassic – Upper Cretaceous]

Desmodonta: Hinge-teeth absent or irregular with ligaments (e.g. family Anatinidae).

Prionodesmacea have a prismatic and nacreous shell structure, separated mantle lobes, poorly developed siphons, and hinge teeth that are lacking or unspecialized. Gills range from protobranch to eulamellibranch. Teleodesmacea on the other hand have a porcelanous and partly nacreous shell structure; Mantle lobes that are generally connected, well developed siphons, and specialized hinge teeth. In most, gills are eulamellibranch.

1935 taxonomy

In his 1935 work Handbuch der systematischen Weichtierkunde (Handbook of Systematic Malacology), Johannes Thiele introduced a mollusc taxonomy based upon the 1909 work by Cossmann and Peyrot. Thiele's system divided the bivalves into three orders. Taxodonta consisted of forms that had taxodont dentition, with a series of small parallel teeth perpendicular to the hinge line. Anisomyaria consisted of forms that had either a single adductor muscle or one adductor muscle much smaller than the other. Eulamellibranchiata consisted of forms with ctenidial gills. The Eulamellibranchiata was further divided into four suborders: Schizodonta, Heterodonta, Adapedonta and Anomalodesmata.^[141][142]

Taxonomy based upon hinge tooth morphology

The systematic layout presented here follows Newell's 1965 classification based on hinge tooth morphology (all taxa marked † are extinct) :^[132]

The monophyly of the subclass Anomalodesmata is disputed. The standard view now is that it resides within the subclass Heterodonta.^{[133][136][143]}

Taxonomy based upon gill morphology

An alternative systematic scheme exists using gill morphology.^[144] This distinguishes between Protobranchia, Filibranchia and Eulamellibranchia. The first corresponds to Newell's Palaeotaxodonta and Cryptodonta, the second to his Pteriomorphia, with the last corresponding to all other groups. In addition, Franc separated the Septibranchia from his eulamellibranchs because of the morphological differences between them. The septibranchs belong to the superfamily Poromyoidea and are carnivorous, having a muscular septum instead of filamentous gills.^[145]

2010 taxonomy

In May 2010, a new taxonomy of the Bivalvia was published in the journal Malacologia. In compiling this, the authors used a variety of phylogenetic information including molecular analysis, anatomical analysis, shell morphology and shell microstructure as well as bio-geographic, paleobiogeographic and stratigraphic information. In this classification 324 families are recognized as valid, 214 of which are known exclusively from fossils and 110 of which occur in the recent past, with or without a fossil record.^[138] This classification has since been adopted by WoRMS.^[146]

Proposed classification of Class Bivalvia (under the redaction of Rüdiger Bieler, Joseph G. Carter and Eugene V. Coan) (all taxa marked † are extinct) :^[147]

References