Xiphosura (/zɪfoʊˈsjʊərə/;[2] from Ancient Greek ξίφος (xíphos) 'sword', and οὐρά (ourá) 'tail', in reference to its sword-like tail) is an order of arthropods related to arachnids. They are more commonly known as horseshoe crabs (a name applied more specifically to the only extant family, Limulidae). They first appeared in the Hirnantian (Late Ordovician). Currently, there are only four living species. Xiphosura contains one suborder, Xiphosurida, and several stem-genera.
The group has hardly changed in appearance in hundreds of millions of years; the modern horseshoe crabs look almost identical to prehistoric genera and are considered to be living fossils. The most notable difference between ancient and modern forms is that the abdominal segments in present species are fused into a single unit in adults.
Xiphosura were historically placed in the class Merostomata, although this term was intended to encompass also the eurypterids, whence it denoted what is now known to be an unnatural (paraphyletic) group (although this is a grouping recovered in some recent cladistic analyses).[3] Although the name Merostomata is still seen in textbooks, without reference to the Eurypterida, some have urged that this usage should be discouraged.[4] The Merostomata label originally did not include Eurypterida, although they were added in as a better understanding of the extinct group evolved. Now Eurypterida is classified within Sclerophorata together with the arachnids, and therefore, Merostomata is now a synonym of Xiphosura.[5] Several recent phylogenomic studies place Xiphosura within Arachnida, often as the sister group of Ricinulei; included among them are taxonomically comprehensive analyses of both morphology and genomes, which have recovered Merostomata as a derived clade of arachnids.[6][7][8]
Modern xiphosurans reach up to 60 cm (24 in) in adult length, but the Paleozoic species were often far smaller, some as small as 1 to 3 cm (0.39 to 1.18 in) long.
Their bodies are divided into an anterior prosoma and a posterior opisthosoma, or abdomen. The upper surface of the prosoma is covered by a semicircular carapace, while the underside bears five pairs of walking legs and a pair of pincer-like chelicerae. The mouth is located on underside of the center of the prosoma, between the bases of the walking legs, and lies behind a lip-like structure called the labrum.[9][10] The exoskeleton consist of a tough cuticle, but do not contain any crystalline biominerals.[11] Like scorpions, xiphosurans have an exocuticular layer of hyaline which exhibits UV fluorescence.[12]
Xiphosurans have up to four eyes, located in the carapace. Two compound eyes are on the side of the prosoma, with one or two median ocelli towards the front. The compound eyes are simpler in structure than those of other arthropods, with the individual ommatidia not being arranged in a compact pattern. They can probably detect movement, but are unlikely to be able to form a true image. In front of the ocelli is an additional organ that probably functions as a chemoreceptor.[10]
The first four pairs of legs end in pincers, and have a series of spines, called the gnathobase, on the inner surface. The spines are used to masticate the food, tearing it up before passing it to the mouth. The fifth and final pair of legs, however, has no pincers or spines, instead having structures for cleaning the gills and pushing mud out of the way while burrowing. Behind the walking legs is a sixth set of appendages, the chilaria, which are greatly reduced in size and covered in hairs and spines.[13] These are thought to be vestiges of the limbs of an absorbed first opisthosomal segment.[10]
The opisthosoma is divided into a forward mesosoma, with flattened appendages, and a metasoma at the rear, which has no appendages. In modern forms, the whole of the opisthosoma is fused into a single unsegmented structure.[14] The underside of the opisthosoma carries the genital openings and five pairs of flap-like gills.[10]
The opisthosoma terminates in a long caudal spine, commonly referred to as a telson (though this same term is also used for a different structure in crustaceans). The spine is highly mobile, and is used to push the animal upright if it is accidentally turned over.[10]
The mouth opens into a sclerotised oesophagus, which leads to a crop and gizzard. After grinding up its food in the gizzard, the animal regurgitates any inedible portions, and passes the remainder to the true stomach. The stomach secretes digestive enzymes, and is attached to an intestine and two large caeca that extend through much of the body, and absorb the nutrients from the food. The intestine terminates in a sclerotised rectum, which opens just in front of the base of the caudal spine.[10]
Xiphosurans have well-developed circulatory systems, with numerous arteries that send blood from the long tubular heart to the body tissues, and then to two longitudinal sinuses next to the gills. After being oxygenated, the blood flows into the body cavity, and back to the heart. The blood contains haemocyanin, a blue copper-based pigment performing the same function as haemoglobin in vertebrates, and also has blood cells that aid in clotting.[10]
The excretory system consists of two pairs of coxal glands connected to a bladder that opens near the base of the last pair of walking legs. The brain is relatively large, and, as in many arthropods, surrounds the oesophagus. In both sexes, the single gonad lies next to the intestine and opens on the underside of the opisthosoma.[10]
Xiphosurans move to shallow water to mate. The male climbs onto the back of the female, gripping her with his first pair of walking legs. The female digs out a depression in the sand, and lays from 200 to 300 eggs, which the male covers with sperm. The pair then separates, and the female buries the eggs.[10]
The egg is about 2–3 mm (0.08–0.12 in) across. Inside the egg the embryo goes through four molts before it hatches into a larva often called a 'trilobite larva', due to its superficially resemblance to a trilobite. At this stage it has no telson yet, and the larva is lecithotrophic (non-feeding) and planktonic, subsisting on the maternal yolk before settling to the bottom to molt, which is when the telson first appear.[15][16] Through a series of successive moults, the larva develops additional gills, increases the length of its caudal spine, and gradually assumes the adult form. Modern xiphosurans reach sexual maturity after about three years of growth.[10]
The oldest known stem-Xiphosuran, Lunataspis, is known from the late Ordovician of Canada, around 445 million years ago.[17] No xiphosurans are known from the following Silurian. Xiphosurida first appears during the late Devonian. A major radiation of freshwater xiphosurids, the Belinuridae is known from the Carboniferous, with the oldest representatives of the modern family Limulidae also possibly appearing during this time, though they only appear in abundance during the Triassic. Another major radiation of freshwater xiphosurans, the Austrolimulidae, is known from the Permian and Triassic.[18]
Xiphosuran classification as of 2018:[19][20]
Order Xiphosura Latreille, 1802
Two groups were originally included in the Xiphosura, but since have been assigned to separate classes:
Cladogram after Lasmdell 2020.[1]
Xiphosura Limulina Limulidae AustrolimulidaeBatracholimulus fuchsbergensis
Paleolimulidae BelinurinaAndersoniella sp.
Xiphosura (/zɪfoʊˈsjʊərə/; from Ancient Greek ξίφος (xíphos) 'sword', and οὐρά (ourá) 'tail', in reference to its sword-like tail) is an order of arthropods related to arachnids. They are more commonly known as horseshoe crabs (a name applied more specifically to the only extant family, Limulidae). They first appeared in the Hirnantian (Late Ordovician). Currently, there are only four living species. Xiphosura contains one suborder, Xiphosurida, and several stem-genera.
The group has hardly changed in appearance in hundreds of millions of years; the modern horseshoe crabs look almost identical to prehistoric genera and are considered to be living fossils. The most notable difference between ancient and modern forms is that the abdominal segments in present species are fused into a single unit in adults.
Xiphosura were historically placed in the class Merostomata, although this term was intended to encompass also the eurypterids, whence it denoted what is now known to be an unnatural (paraphyletic) group (although this is a grouping recovered in some recent cladistic analyses). Although the name Merostomata is still seen in textbooks, without reference to the Eurypterida, some have urged that this usage should be discouraged. The Merostomata label originally did not include Eurypterida, although they were added in as a better understanding of the extinct group evolved. Now Eurypterida is classified within Sclerophorata together with the arachnids, and therefore, Merostomata is now a synonym of Xiphosura. Several recent phylogenomic studies place Xiphosura within Arachnida, often as the sister group of Ricinulei; included among them are taxonomically comprehensive analyses of both morphology and genomes, which have recovered Merostomata as a derived clade of arachnids.
Los xifosuros (Xiphosura) son un orden de quelicerados marinos de la clase Merostomata. La especie más conocida es Limulus polyphemus o cangrejo herradura, aunque son todos muy semejantes entre sí. Se consideran fósiles vivientes porque han evolucionado muy poco en sus más de 475 millones de años de existencia; aparecieron por primera vez en el registro fósil en el Ordovícico inferior.[1][2]
El orden comprende un gran número de familias de las cuales solo sobrevive una, Limulidae, que incluye tres géneros y cuatro especies:[3]
Vista superior.
Vista inferior.
Macho enganchado a la hembra para el desove.
Fósil de cangrejo herradura.
Los xifosuros (Xiphosura) son un orden de quelicerados marinos de la clase Merostomata. La especie más conocida es Limulus polyphemus o cangrejo herradura, aunque son todos muy semejantes entre sí. Se consideran fósiles vivientes porque han evolucionado muy poco en sus más de 475 millones de años de existencia; aparecieron por primera vez en el registro fósil en el Ordovícico inferior.
Merostomata, Mérostomes, Xiphosures • Limules
Les Xiphosures (Xiphosura) sont une classe d'arthropodes chélicérés regroupant l'ensemble des limules, modernes et éteintes. Ce taxon était auparavant classé comme un ordre, le rang de classe étant attribué au taxon Merostomata (les Mérostomes) qui incluait l'ordre fossile des Euryptérides (bien qu'il ne contenait à sa création que les limules et les espèces fossiles apparentées). Les Euryptérides s'étant révélés plus proches des Arachnides, Merostomata est aujourd'hui considéré comme un synonyme junior de Xiphosura.
Une étude phylogénétique de 2019 vient bouleverser cette conception en concluant que les limules serait en réalité des arachnides aquatiques[1].
Xiphosura signifie « queue en épée », du grec ξίφος, épée, et οὐρά, queue.
Ce qui différencie la classe des Xiphosures de celle des Arachnides est la fonction du prosome (partie antérieure du corps) et de l'opisthosome (partie postérieure du corps). Le prosome joue le rôle d'un grand bouclier indivis, et l'opisthosome est constitué d'appendices biramés.
L'ancienne conception incluant le sous-ordre fossile Synziphosurida s'est révélée être paraphylétique aux regard des Arachnides. Certains genres de ce sous-ordre sont aujourd'hui placés au sein de Planaterga, avec Arachnida et Eurypterida, et d'autres comme simples chélicérés.
Classification des genres en dehors des limules modernes, d'après Lamsdell, 2013[2] et Paleobiology Database (septembre 2016)[3] :
Phylogénie des grands groupes de chélicérés, d'après Lamsdell, 2013[5] :
Chelicerata
Xiphosura (limules)
Arachnida (araignées, scorpions, acariens...)
†Eurypterida (scorpions de mer)
En 2019, une étude sur la phylogénie des Chélicérés vient bouleverser cette conception en plaçant les limules au sein des Arachnides, comme groupe frère des Ricinules[1].
Merostomata, Mérostomes, Xiphosures • Limules
Les Xiphosures (Xiphosura) sont une classe d'arthropodes chélicérés regroupant l'ensemble des limules, modernes et éteintes. Ce taxon était auparavant classé comme un ordre, le rang de classe étant attribué au taxon Merostomata (les Mérostomes) qui incluait l'ordre fossile des Euryptérides (bien qu'il ne contenait à sa création que les limules et les espèces fossiles apparentées). Les Euryptérides s'étant révélés plus proches des Arachnides, Merostomata est aujourd'hui considéré comme un synonyme junior de Xiphosura.
Une étude phylogénétique de 2019 vient bouleverser cette conception en concluant que les limules serait en réalité des arachnides aquatiques.
Xiphosura signifie « queue en épée », du grec ξίφος, épée, et οὐρά, queue.
Gli xifosuri (Xiphosura) sono un ordine di artropodi, nella classe dei merostomi. Gli animali che vi appartengono hanno un cefalotorace ricoperto da un ampio carapace convesso a forma di ferro di cavallo, l'addome non segmentato e dodici appendici fogliacee. Il sistema circolatorio è molto ben sviluppato, con un cuore allungato e una ricca rete vasale. I primi animali di questa sottoclasse sono comparsi nel periodo Ordoviciano e sono tuttora esistenti. Attualmente sono presenti solo cinque specie; Limulus polyphemus può raggiungere i 60 centimetri.
Nel corso dell'era Paleozoica e dell'era Mesozoica gli xifosuri raggiunsero una notevole espansione; sono note numerose forme fossili, che vivevano in una grande varietà di ambienti. Tra i più noti xifosuri estinti, da ricordare Lunataspis, Euproops e Mesolimulus.
Gli xifosuri (Xiphosura) sono un ordine di artropodi, nella classe dei merostomi. Gli animali che vi appartengono hanno un cefalotorace ricoperto da un ampio carapace convesso a forma di ferro di cavallo, l'addome non segmentato e dodici appendici fogliacee. Il sistema circolatorio è molto ben sviluppato, con un cuore allungato e una ricca rete vasale. I primi animali di questa sottoclasse sono comparsi nel periodo Ordoviciano e sono tuttora esistenti. Attualmente sono presenti solo cinque specie; Limulus polyphemus può raggiungere i 60 centimetri.
Nel corso dell'era Paleozoica e dell'era Mesozoica gli xifosuri raggiunsero una notevole espansione; sono note numerose forme fossili, che vivevano in una grande varietà di ambienti. Tra i più noti xifosuri estinti, da ricordare Lunataspis, Euproops e Mesolimulus.
Xiphosura é uma ordem de artrópodes merostomados das águas rasas asiáticas e da América do Norte. Apresentam um corpo divido em prossoma e opistossoma com carapaça arqueada em forma de ferradura e pernas com sete artículos (coxa, trocanter, fêmur, patela, tíbia, tarso e pré-tarso). [1]
Límulos, caranguejos-ferradura. Considerados fósseis vivos, as cinco espécies de Xiphosura pertencem a três gêneros geograficamente distintos, todos incluídos na família Limulidae. Nordeste da América do norte; Limulus (L. polyphemus) e no sudeste da Ásia; Tachypleus (T. tridentatus) e Carcinoscorpius coletado apenas na Malásia, no Sião e nas Filipinas. Habitam águas marinhas rasa, geralmente sobre fundos arenosos límpidos onde rastejam e se enterram se alimentando de detritos. [2]
O prossoma, maior do que o opistossoma, é composto por seis segmentos fusionados, que são constituídos por seis apêndices, sendo eles: 5 pernas e uma quelícera triarticulada. Entre as pernas do prossoma está localizado o espaço bucal, que atrás do mesmo está o endóstomio, também denominado de placa esternal, que é continuo até os quilários, par de apêndices reduzidos seguinte ao último par de pernas com função ainda não conhecida. [1]
Opistossoma composto por 9 segmentos fundidos e 7 apêndices, que como o prossoma tem uma carapaça recobrindo-o, com um par de quilários, seis pares de apêndices laminares, sendo o primeiro par fundido medianamente e formando um opérculo genital sobre os gonóporos, e os últimos cinco pares natatórias, adaptadas com patas braquíferas (ou lamelas branquiais). [1]
Seguinte ao opistossoma, situa-se o télson, uma estrutura móvel e extensa que pode chegar a uma tamanho maior que o restante do corpo de um xifosuro. Apesar do formato semelhante a espinho, o télson não é utilizado contra predados ou para capturar presas, sendo ausente de veneno e funcional para locomoção. [1]
Pedipalpos e pernas com quelas, IV par de pernas chanfrado na posição distal para apoio em substratos moles. Possuem gnatobases nos ênditos da coxas dos pedipalpos e dos três primeiros pares de pernas. E flabelos para limpeza das brânquias partindo das coxas do IV par de pernas. [2]
Os límulos costumam migrar em grande quantidade para as praias próximas durante a época de acasalamento. As fêmeas depositam seus ovos, que variam entre 60 mil a 120 mil ovos, em buracos cavados por elas, quais são fertilizados pelos machos posteriormente. Em média de duas semanas após a fertilização, esses ovos eclodem e as larvas seguem em direção ao mar, quando sobrevivem a predação, principalmente as realizadas por pássaros. [3]
Uma característica marcante do grupo é a coloração azul do sangue. Essa ocorrência é devido a presença da proteína hemocianina que possui cobre na composição, que na combinação com oxigênio carregado pela célula gera a colação azul no sangue dos límulos. [3]
Os Limulus possui um sangue de grande valor para a industria farmacêutica, pois é usado na detecção de endotoxinas bacterianas. O sangue do Limulus coagula quando entra em contato com as endotoxinas liberadas pelas bactérias, essas endotoxinas são substâncias tóxicas ao serem humanos, devido isso o grande valor econômico. [3]
A concepção antiga, incluindo a subordem fóssil Synziphosurida, foi considerada parafilética em relação aos aracnídeos. Alguns géneros desta subordem são agora colocados dentro de Planaterga, com Arachnida e Eurypterida, e outros como simples Chelicerata. A classificação mais moderna do grupo, separando os Limulidae modernos, é a seguinte:[4]
Xiphosura é uma ordem de artrópodes merostomados das águas rasas asiáticas e da América do Norte. Apresentam um corpo divido em prossoma e opistossoma com carapaça arqueada em forma de ferradura e pernas com sete artículos (coxa, trocanter, fêmur, patela, tíbia, tarso e pré-tarso).
검미목(Xiphosura)은 투구게 등을 포함하고 있는 협각아문에 속하는 절지동물의 분류이다. 현존하는 1개 과(투구게과)에 3개 속을 포함하고 있다.[1]
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