Chondrichthyes
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2008-07-16T12:24:55Z
70.63.235.163
/* Metabolism */
{{Taxobox
| name = Cartilaginous fish
| fossil_range = Early [[Silurian]] - Recent
| image = White shark.jpg
| image_width = 250px
| image_caption = [[Great white shark]], ''Carcharodon carcharias''
| regnum = [[Animalia]]
| phylum = [[Chordata]]
| subphylum =
| infraphylum = [[Gnathostomata]]
| classis = '''Chondrichthyes'''
| classis_authority = [[Thomas Henry Huxley|Huxley]], 1880
| subdivision_ranks = [[Subclass]]es and [[Orders (biology)|Orders]]
| subdivision =
See text.
}}
Chondrichthyes or '''cartilaginous fishes''' are jawed [[fish]] with paired fins, paired nostrils, scales, two-chambered hearts, and skeletons made of [[cartilage]] rather than [[bone]]. They are divided into two subclasses: [[Elasmobranchii]] (sharks, rays and skates) and Holocephali ([[chimaera]], sometimes called ghost sharks).
== Characteristics ==
Animals from this group have a brain weight relative to body size that comes close to that of mammals, and is about 10 times that of [[Osteichthyes|bony fishes]]. There are exceptions: the [[mormyrid]] bony fish have a relative brain size comparable to humans, while the primitive [[megamouth shark]] has a brain of only 0.002 percent of its body weight. One of the explanations for their relatively large brains is that the density of nerve cells is much lower than in the brains of bony fishes, making the brain less energy demanding and allowing it to be bigger. Extant cartilaginous fishes range in size from the [[Dwarf lanternshark]], at 16 cm (6.3 in), to the [[Whale Shark]], growing to at least 13.6 m (45 feet).
Their digestive systems have [[spiral valve]]s, and with the exception of Holocephali, they also have a [[cloaca]].
As they do not have bone [[marrow]], [[red blood cell]]s are produced in the [[spleen]] and special tissue around the [[gonad]]s. They are also produced in an organ called [[Leydig's Organ]] which is only found in cartilaginous fishes, although some have lost it. Another unique organ is the epigonal organ which probably has a role in the immune system. The subclass Holocephali, which is a very specialized group, lacks both of these organs. 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 pubioischiadic bars evolved. In [[Batoidea|ray]]s, the pectoral fins have connected to the head and are very flexible.
A [[spiracle]] is found behind each eye on most species.
Their tough skin is covered with [[Denticle|dermal teeth]] (again with Holocephali as an exception as the teeth are lost in adults, only kept on the clasping organ seen on the front of the male's head), also called placoid scales or [[dermal denticle]]s, 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. It is assumed that their oral teeth evolved from dermal denticles which migrated into the mouth. But it could be the other way around as the [[Teleostei|teleost]] bony fish ''[[Denticeps clupeoides]]'' has most of its head covered by dermal teeth (as do probably ''[[Atherion elymus]]'', another bony fish). This is most probably 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 which of the dermal or oral teeth evolved first. Neither is it sure how many times it has happened if it turns out to be the case. It has even been suggested that the original bony plates of all the vertebrates are gone and that the present scales are just modified teeth, even if both teeth and the body armor have a common origin a long time ago. But for the moment there is no evidence of this.
One of the main characteristics present in sharks is their heterocercal tail, which aids the shark in locomotion.
==Respiratory system==
Chondrichthyes all breathe through 5-7 [[gills]], depending on species. However, they differ on how they get water to pass over the gills. Chondrichthyes mostly use their mouths, as do chimaeras and skates, but rays get water through spiracles, which are small holes on top of their head that run to the gills.
==Metabolism==
Chondrichthyes are [[ectothermic]] or cold blooded, meaning they do not have to warm themselves through eating. Therefore, metabolism is slow as well as the fact that Chondrichthyes members do not have to eat as much.
==Body covering==
Chondrichthyes have toothlike scales called [[denticles]]/placoid scales. Denticles provide two functions, protection, and in most cases streamlining. Mucous glands exist in some species as well.
==Appendages==
All Chondrichthyes have dorsal, caudal, anal, pelvic, and pectoral fins. Anal, pelvic, and pectoral fins are all paired. Pectoral fins are usually fixed.
==Skeleton==
The skeleton is cartilaginous. The [[notochord]], which is present in the young, is gradually replaced by cartilage. Chondrichthyes also lack ribs, so in the event that they left the water, their own body weight would crush their internal organs long before they would suffocate.
==Reproduction==
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 Chondrichthyes do guard their eggs.
==Members==
*[[Chimaera]]s
*[[Sharks]]
*[[Skate]]s
*[[Rays]]
== Taxonomy ==
* Class '''Chondrichthyes'''
** Subclass [[Elasmobranchii]] (sharks, rays and skates)
*** Superorder [[Batoidea]] (rays and skates), containing the orders:
***# [[Rajiformes]] (common rays and skates)
***# [[Pristiformes]] ([[Sawfish (fish)|Sawfishes]])
***# [[Torpediniformes]] ([[electric ray]]s)
*** Superorder [[Selachimorpha]] ([[shark]]s), containing the orders:
***# [[Hexanchiformes]] Two families are found within this order. Species of this order are distinguished from other sharks by having additional gill slits (either six or seven). Examples from this group include the [[cow shark]]s, [[frilled shark]] and even a shark that looks on first inspection to be a marine snake.
***# [[Squaliformes]] Three families and more than 80 species are found within this order. These sharks have two dorsal fins, often with spines, and no anal fin. They have teeth designed for cutting in both the upper and lower jaws. Examples from this group include the [[bramble shark]]s, [[dogfish]] and [[roughshark]]s.
***# [[Pristiophoriformes]] One family is found within this order. These are the ''sawsharks'', with an elongate, toothed snout that they use for slashing the fishes that they then eat.
***# [[Squatiniformes]] One family is found within this order. These are flattened sharks that can be distinguished from the similar appearing skates and rays by the fact that they have the gill slits along the side of the head like all other sharks. They have a caudal fin (tail) with the lower lobe being much longer in length than the upper, and are commonly referred to as ''angel sharks''.
***# [[Heterodontiformes]] One family is found within this order. They are commonly referred to as the ''bullhead'', or ''horn sharks''. They have a variety of teeth allowing them to grasp and then crush [[shellfish]]es.
***# [[Orectolobiformes]] Seven families are found within this order. They are commonly referred to as the ''carpet sharks'', including [[zebra shark]]s, [[nurse sharks]], [[wobbegong]]s and the largest of all fishes, the [[whale shark]]s. They are distinguished by having [[barbels]] at the edge of the nostrils. Most, but not all are nocturnal.
***# [[Carcharhiniformes]] Eight families are found within this order. It is the largest order, containing almost 200 species. They are commonly referred to as the ''groundsharks'', and some of the species include the [[blue shark|blue]], [[tiger shark|tiger]], [[bull shark|bull]], [[reef shark|reef]] and [[oceanic whitetip shark]]s (collectively called the [[requiem shark]]s) along with the [[houndshark]]s, [[catshark]]s and [[hammerhead shark]]s. They are distinguished by an elongated snout and a nictitating membrane which protects the eyes during an attack.
***# [[Lamniformes]] Seven families are found within this order. They are commonly referred to as the ''mackerel sharks''. They include the [[goblin shark]], [[basking shark]], [[megamouth]], the [[Thresher Shark|thresher]], [[mako shark]] and [[great white shark]]. They are distinguished by their large jaws and [[Ovoviviparity|ovoviviparous]] reproduction. The Lamniformes contains the extinct [[Megalodon]] (''Carcharodon megalodon''), which like most extinct sharks is only known by the teeth (the only bone found in these cartilaginous fishes, and therefore are often the only [[fossil]]s produced) and a few [[vertebrae]]. The largest of the teeth of this shark can measure (up to more than 7 inch in length) and through modern research, it has been determined that this shark could exceed 50 feet in length.
** Subclass [[Holocephali]] ([[chimaera]])
::# [[Chimaera|Chimaeriformes]] (ghost sharks)
==References==
{{Wikispecies|Chondrichthyes}}
{{Wikibookspar|Dichotomous Key|Chondrichthyes}}
* [http://www.fmnh.helsinki.fi/users/haaramo/Metazoa/Deuterostoma/Chordata/Chondrichthyes/Chondrichthyes.htm#Elasmobranchii Taxonomy of Chondrichthyes]
* [http://www.morphbank.net/Browse/ByImage/?tsn=159785 Images of many sharks, skates and rays on Morphbank]
{{Chordata}}
{{Chondrichthyes}}
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