Chondrichthyes (/kɒnˈdrɪkθi.iːz/; from Ancient Greek χόνδρος (khóndros) 'cartilage', and ἰχθύς (ikhthús) 'fish') is a class that contains the cartilaginous fishes that have skeletons primarily composed of cartilage. They can be contrasted with the Osteichthyes or bony fishes, which have skeletons primarily composed of bone tissue. Chondrichthyes are jawed vertebrates with paired fins, paired nares, scales, and a heart with its chambers in series. Extant chondrichthyes range in size from the 10 cm (3.9 in) finless sleeper ray to the 10 m (32 ft) whale shark.
The class is divided into two subclasses: Elasmobranchii (sharks, rays, skates, and sawfish) and Holocephali (chimaeras, sometimes called ghost sharks, which are sometimes separated into their own class).
Within the infraphylum Gnathostomata, cartilaginous fishes are distinct from all other jawed vertebrates.
The skeleton is cartilaginous. The notochord is gradually replaced by a vertebral column during development, except in Holocephali, where the notochord stays intact. In some deepwater sharks, the column is reduced.[3]
As they do not have bone marrow, red blood cells are produced in the spleen and the epigonal organ (special tissue around the gonads, which is also thought to play a role in the immune system). They are also produced in the Leydig's organ, which is only found in certain cartilaginous fishes. The subclass Holocephali, which is a very specialized group, lacks both the Leydig's and epigonal organs.
Apart from electric rays, which have a thick and flabby body, with soft, loose skin, chondrichthyans have tough skin covered with dermal teeth (again, Holocephali is an exception, as the teeth are lost in adults, only kept on the clasping organ seen on the caudal ventral surface of the male), also called placoid scales (or dermal denticles), making it feel like sandpaper. In most species, all dermal denticles are oriented in one direction, making the skin feel very smooth if rubbed in one direction and very rough if rubbed in the other.
Originally, the pectoral and pelvic girdles, which do not contain any dermal elements, did not connect. In later forms, each pair of fins became ventrally connected in the middle when scapulocoracoid and puboischiadic bars evolved. In rays, the pectoral fins are connected to the head and are very flexible.
One of the primary characteristics present in most sharks is the heterocercal tail, which aids in locomotion.[4]
Chondrichthyans have tooth-like scales called dermal denticles or placoid scales. Denticles usually provide protection, and in most cases, streamlining. Mucous glands exist in some species, as well.
It is assumed that their oral teeth evolved from dermal denticles that migrated into the mouth, but it could be the other way around, as the teleost bony fish Denticeps clupeoides has most of its head covered by dermal teeth (as does, probably, Atherion elymus, another bony fish). This is most likely a secondary evolved characteristic, which means there is not necessarily a connection between the teeth and the original dermal scales.
The old placoderms did not have teeth at all, but had sharp bony plates in their mouth. Thus, it is unknown whether the dermal or oral teeth evolved first. It has even been suggested that the original bony plates of all vertebrates are now gone and that the present scales are just modified teeth, even if both the teeth and body armor had a common origin a long time ago. However, there is currently no evidence of this.
All chondrichthyans breathe through five to seven pairs of gills, depending on the species. In general, pelagic species must keep swimming to keep oxygenated water moving through their gills, whilst demersal species can actively pump water in through their spiracles and out through their gills. However, this is only a general rule and many species differ.
A spiracle is a small hole found behind each eye. These can be tiny and circular, such as found on the nurse shark (Ginglymostoma cirratum), to extended and slit-like, such as found on the wobbegongs (Orectolobidae). Many larger, pelagic species, such as the mackerel sharks (Lamnidae) and the thresher sharks (Alopiidae), no longer possess them.
In chondrichthyans, the nervous system is composed of a small brain, 8-10 pairs of cranial nerves, and a spinal chord with spinal nerves.[5] They have several sensory organs which provide information to be processed. Ampullae of Lorenzini are a network of small jelly filled pores called electroreceptors which help the fish sense electric fields in water. This aids in finding prey, navigation, and sensing temperature. The Lateral line system has modified epithelial cells located externally which sense motion, vibration, and pressure in the water around them. Most species have large well-developed eyes. Also, they have very powerful nostrils and olfactory organs. Their inner ears consist of 3 large semicircular canals which aid in balance and orientation. Their sound detecting apparatus has limited range and is typically more powerful at lower frequencies. Some species have electric organs which can be used for defense and predation. They have relatively simple brains with the forebrain not greatly enlarged. The structure and formation of myelin in their nervous systems are nearly identical to that of tetrapods, which has led evolutionary biologists to believe that Chondrichthyes were a cornerstone group in the evolutionary timeline of myelin development.[6]
Like all other jawed vertebrates, members of Chondrichthyes have an adaptive immune system.[7]
Fertilization is internal. Development is usually live birth (ovoviviparous species) but can be through eggs (oviparous). Some rare species are viviparous. There is no parental care after birth; however, some chondrichthyans do guard their eggs.
Capture-induced premature birth and abortion (collectively called capture-induced parturition) occurs frequently in sharks/rays when fished.[8] Capture-induced parturition is often mistaken for natural birth by recreational fishers and is rarely considered in commercial fisheries management despite being shown to occur in at least 12% of live bearing sharks and rays (88 species to date).[8]
The class Chondrichthyes has two subclasses: the subclass Elasmobranchii (sharks, rays, skates, and sawfish) and the subclass Holocephali (chimaeras). To see the full list of the species, click here.
Cartilaginous fish are considered to have evolved from acanthodians. Originally assumed to be closely related to bony fish or a polyphyletic assemblage leading to both groups, the discovery of Entelognathus and several examinations of acanthodian characteristics indicate that bony fish evolved directly from placoderm like ancestors, while acanthodians represent a paraphyletic assemblage leading to Chondrichthyes. Some characteristics previously thought to be exclusive to acanthodians are also present in basal cartilaginous fish.[13] In particular, new phylogenetic studies find cartilaginous fish to be well nested among acanthodians, with Doliodus and Tamiobatis being the closest relatives to Chondrichthyes.[14] Recent studies vindicate this, as Doliodus had a mosaic of chondrichthyian and acanthodiian traits.[15]
Dating back to the Middle and Late Ordovician Period, many isolated scales, made of dentine and bone, have a structure and growth form that is chondrichthyan-like. They may be the remains of stem-chondrichthyans, but their classification remains uncertain.[16][17][18]
The earliest unequivocal fossils of cartilaginous fishes first appeared in the fossil record by about 430 million years ago, during the middle Wenlock Epoch of the Silurian period.[19] The radiation of elasmobranches in the chart on the right is divided into the taxa: Cladoselache, Eugeneodontiformes, Symmoriida, Xenacanthiformes, Ctenacanthiformes, Hybodontiformes, Galeomorphi, Squaliformes and Batoidea.
By the start of the Early Devonian, 419 million years ago, jawed fishes had divided into three distinct groups: the now extinct placoderms (a paraphyletic assemblage of ancient armoured fishes), the bony fishes, and the clade that includes spiny sharks and early cartilaginous fish. The modern bony fishes, class Osteichthyes, appeared in the late Silurian or early Devonian, about 416 million years ago. The first abundant genus of shark, Cladoselache, appeared in the oceans during the Devonian Period. The first Cartilaginous fishes evolved from Doliodus-like spiny shark ancestors.
A Bayesian analysis of molecular data suggests that the Holocephali and Elasmoblanchii diverged in the Silurian (421 million years ago) and that the sharks and rays/skates split in the Carboniferous (306 million years ago).
Subphylum Vertebrata └─Infraphylum Gnathostomata ├─Placodermi — extinct (armored gnathostomes) └Eugnathostomata (true jawed vertebrates) ├─Acanthodii (stem cartilaginous fish) └─Chondrichthyes (true cartilaginous fish) ├─Holocephali (chimaeras + several extinct clades) └Elasmobranchii (shark and rays) ├─Selachii (true sharks) └─Batoidea (rays and relatives)
{{cite web}}: CS1 maint: archived copy as title (link) Fossil Fish of Bear Gulch 2005 by Richard Lund and Eileen Grogan Accessed 2009-01-14 Chondrichthyes (/kɒnˈdrɪkθi.iːz/; from Ancient Greek χόνδρος (khóndros) 'cartilage', and ἰχθύς (ikhthús) 'fish') is a class that contains the cartilaginous fishes that have skeletons primarily composed of cartilage. They can be contrasted with the Osteichthyes or bony fishes, which have skeletons primarily composed of bone tissue. Chondrichthyes are jawed vertebrates with paired fins, paired nares, scales, and a heart with its chambers in series. Extant chondrichthyes range in size from the 10 cm (3.9 in) finless sleeper ray to the 10 m (32 ft) whale shark.
The class is divided into two subclasses: Elasmobranchii (sharks, rays, skates, and sawfish) and Holocephali (chimaeras, sometimes called ghost sharks, which are sometimes separated into their own class).
Within the infraphylum Gnathostomata, cartilaginous fishes are distinct from all other jawed vertebrates.
Les chondrichtyens (Chondrichthyes) ou poissons cartilagineux forment une classe de poissons caractérisés par des nageoires appariées, des narines appariées, des écailles, un cœur à chambres en série et un squelette constitué de cartilage plutôt que d'os. La classe est divisée en deux sous-classes: Elasmobranchii (requins, raies et poissons-scie) et Holocephali (chimères). Au sein de l'infra-embranchement Gnathostomata, les poissons cartilagineux sont distincts de tous les autres vertébrés à mâchoires.
Le terme Chondrichthyes dérive du grec ancien : χόνδρος, khóndros, « cartilage » et de ἰχθύς, ikhthús, « poisson ». Ce terme vient du fait que le squelette est formé de divers cartilages, sans os calcifié.
Contrairement à son groupe frère, les ostéichthyens (Osteichthyes), qui ont un squelette osseux.
Ces poissons possèdent un squelette cartilagineux (le mot grec ancien Hóndros signifie cartilage) et une peau recouverte d'écailles placoïdes (en forme de dents). Le cartilage, environ deux fois plus léger que l'os, confère aux Chondrichthyes un squelette léger, ce qui, avec leur foie énorme[1] gorgé de lipides (matière grasse moins dense que l'eau), compense leur flottabilité négative due à l'absence de vessie natatoire[2]. Ils ne sont toutefois pas les seuls poissons pourvus d'un squelette cartilagineux, puisque c'est également le cas des Chondrostéens parmi les Osteichthyes. Le degré de minéralisation des tissus cartilagineux et osseux reflète des adaptations pour des caractéristiques telles que l'équilibre, la flottabilité, la conservation de la température et l'énergie[3].
Chacune de leurs branchies possède une ouverture propre. Ils ont aussi une paire d'évents. Leur tube digestif possède une valve spiralée. Le système dentaire des chimères est différent de celui des Elasmobranches : les dents des chimères ne tombent pas, alors que pour les autres espèces de la classe, les dents tombent et repoussent en continu. Les dents de ces espèces sont souvent les seuls organes calcifiés[4]. Ils sont le plus souvent ovipares mais certaines espèces de requins pratiquent la gestation interne, soit sans lien mère-embryon (ovovivipare) soit avec un lien (vivipare). Double, l'organe de copulation est issu de la transformation des nageoires pelviennes.
Contrairement aux Osteichthyes, les Chondrichtyens ne possèdent pas ou très peu d'os périchondraux[5] (os qui remplace le cartilage et se forme au cours du développement chez les ostéichthyens). En revanche, ils présentent une couche de cartilage calcifié prismatique à la périphérie de leurs pièces squelettiques cartilagineuses. Ils n'ont pas non plus de vessie natatoire, et utilisent l'huile de leur foie pour assurer leur flottabilité.
Les poissons cartilagineux sont très variés en taille comme le sagre elfe qui mesure 17 centimètres et le requin baleine qui lui atteint 18 mètres de long.
Les Chondrichthyens sont divisés en deux sous-classes : la sous-classe des Elasmobranchii (anciennement nommés : Sélaciens), à laquelle appartiennent les raies, les torpilles et les requins ; et la sous-classe des Holocéphales, à laquelle appartiennent les chimères. Les Holocéphales ont leur mâchoire supérieure complètement rattachée au neurôcrane alors qu'elle ne l'est que partiellement chez les Elasmobranchii.
Les Elasmobranchii sont divisés en deux groupes :
Mais cette distinction n'est pas phylogénétique : le groupe des raies et torpilles est apparu au sein d'un des clades de requins.
À noter : le genre éteint de poissons cartilagineux Gladbachus est rattaché aux Chondrichthyes, mais n'a reçu depuis sa découverte aucun classement en termes d'ordre et de famille. La seule espèce connue est Gladbachus adentatus.
Selon ITIS :
Les chondrichtyens (Chondrichthyes) ou poissons cartilagineux forment une classe de poissons caractérisés par des nageoires appariées, des narines appariées, des écailles, un cœur à chambres en série et un squelette constitué de cartilage plutôt que d'os. La classe est divisée en deux sous-classes: Elasmobranchii (requins, raies et poissons-scie) et Holocephali (chimères). Au sein de l'infra-embranchement Gnathostomata, les poissons cartilagineux sont distincts de tous les autres vertébrés à mâchoires.
연골어류(軟骨魚類)는 연골어강(軟骨魚綱, Chondrichthyes)에 속하는 물고기로 딱딱한 뼈 대신에 질긴 피부와 가벼운 물렁뼈를 가지고 있다. 뼈가 단단한 경골어류는 기체로 채워진 부레(풍선모양의 주머니)로 물 속에서 떠오르거나 가라앉거나 하지만, 연골어류는 부레가 없다. 때문에 연골어류는 가벼운 지방을 축적하여 약간의 부력을 얻고, 계속 물에 떠 있기 위해 쉬지 않고 몸을 움직인다.
다음은 유악류 계통 분류이다.[1]
척추동물 유악류 진유악류 진구류 경골어류 육기어류 네발동물 양막류 용궁류 단궁류