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Chondrichthyes

(Class)

Overview

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Chondrichthyes (; from Greek ???d?- chondr- 'cartilage', ????? ichthys 'fish') or cartilaginous fishes are jawed fish with paired fins, paired nares, scales, two-chambered hearts, and skeletons made of cartilage rather than bone. The class is divided into two subclasses: Elasmobranchii (sharks, rays and skates) 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 extant members of which all fall into Teleostomi.

Anatomy

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Skeleton

The skeleton is cartilaginous. The notochord, which is present in the young, is gradually replaced by cartilage. Chondrichthyes also lack ribs, so if they leave water, the larger species' own body weight would crush their internal organs long before they suffocate.

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 cartilaginous fishes, although some do not possess it. The subclass Holocephali, which is a very specialized group, lacks both the Leydig's and epigonal organ.

Appendages

Their tough skin is covered with 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 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. Another exception are the electric rays, which have a thick and flabby body, with soft, loose skin devoid of dermal denticles and thorns.

Originally the pector al 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 rays, the pectoral fins have 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.[2]

Body covering

Chondrichthyes have toothlike scales called denticles or placoid scales. Denticles provide two functions, 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 which 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 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.

Respiratory system

All Chondrichthyes breathe through 5-7 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.

Biology

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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.

Phylogeny

[ Back to top ] 
Subphylum Vertebrata+-Infraphylum Gnathostomata      +-Class Placodermi ? extinct (armored gnathostomes)      +Microphylum Eugnathostomata (true jawed vertebrates)         +-Class Chondrichthyes (cartilaginous fish)
      +-(unranked) Teleostomi (Acanthodii & Osteichthyes)             +-Class Acanthodii ? extinct ("spiny sharks")             +Superclass Osteichthyes (bony fish)                +-Class Actinopterygii (ray-finned fish)                +-Class Sarcopterygii (lobe-finned fish)                      Note: lines show evolutionary relationships.

Taxonomy

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The extant members of the Chondrichthyes are the sharks and rays, belonging to the subclass Elasmobranchii, and the chimaeras, belonging to the subclass Holocephali. Nelson's 2006 Fishes of the World arranges the class as follows:

* position uncertain

The notochord, which is present in the young, is gradually replaced by cartilage. Chondrichthyes also lack ribs, so if they leave water, the larger species' own body weight would crush their internal organs long before they suffocate.

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 prod uced in the Leydig's organ which is only found in cartilaginous fishes, although some do not possess it. The subclass Holocephali, which is a very specialized group, lacks both the Leydig's and epigonal organ.

Appendages

Their tough skin is covered with 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 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. Another exception are the electric rays, which have a thick and flabby body, with soft, loose skin devoid of dermal denticles and thorns.

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 pubio ischiadic bars evolved. In rays, the pectoral fins have 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.[2]

Body covering

Chondrichthyes have toothlike scales called denticles or placoid scales. Denticles provide two functions, 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 which 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 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.

Respiratory system

All Chondrichthyes breathe through 5-7 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.

Biology

[ Back to top ]

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.

Phylogeny

[ Back to top ] 
Subphylum Vertebrata+-Infraphylum Gnathostomata      +-Class Placodermi ? extinct (armored gnathostomes)      +Microphylum Eugnathostomata (true jawed vertebrates)         +-Class Chondrichthyes (cartilaginous fish)         +-(unranked) Teleostomi (Acanthodii & Osteichthyes)             +-Class Acanthodii ? extinct ("spiny sharks")             +Superclass Osteichthyes (bony fish)
      +-Class Actinopterygii (ray-finned fish)                +-Class Sarcopterygii (lobe-finned fish)                      Note: lines show evolutionary relationships.

Taxonomy

[ Back to top ]

The extant members of the Chondrichthyes are the sharks and rays, belonging to the subclass Elasmobranchii, and the chimaeras, belonging to the subclass Holocephali. Nelson's 2006 Fishes of the World arranges the class as follows:

* position uncertain

References

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  1. ^ Botella, H., P.C.J. Donoghue and C. Mart?nez-P?rez (May 2009). "Enameloid microstructure in the oldest known chondrichthyan teeth". Acta Zoologica 90 (Supplement 1): 103?108.
  2. ^ Function of the heterocercal tail in sharks: quantitative wake dynamics durin g steady horizontal swimming and vertical maneuvering - The Journal of Experimental Biology 205, 2365?2374 (2002)

Taxonomy

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The Class Chondrichthyes is further organized into finer groupings including:

Orders

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Carcharhiniformes

The ground sharks, order Carcharhiniformes, are the largest order of sharks. With over 270 species, carcharhiniforms include a number of common types, such as the blue shark, catsharks, swellsharks, and sandbar shark. [more]

Chimaeriformes

Chimaeras are cartilaginous fish in the order Chimaeriformes, known informally as ghost sharks, ratfish (not to be confused with the rattails), spookfish (not to be confused with the true spookfish of the family Opisthoproctidae), or rabbitfishes (not to be confused with the true rabbitfishes of the family Siganidae). They may be the "oldest and most enigmatic groups of fishes alive today." At one time a "diverse and abundant" group (based on the fossil record), their closest living relatives are sharks, though in evolutionary terms they branched off from sharks nearly 400 million years ago and have remained isolated ever since. Today they are largely confined to deep water. [more]

Cladoselachida

[more]

Cochliodontiformes

[more]

Ctenacanthiformes

[more]

Echinorhiniformes

Echinorhinus is a genus of squaliform sharks, the only extant genus in the family Echinorhinidae. The name is from Greek echinos meaning "spiny" and rhinos meaning "nose". Both species are uncommon, little known sharks. They are found worldwide in cold temperate to tropical seas down to 900 metres (3,000 ft) depth. [more]

Eugeneodontida

[more]

Galeomorphii

[more]

Heterodontiformes

The bullhead sharks are a small order (Heterodontiformes) of basal modern sharks (Neoselachii). There are nine living species in a single genus, Heterodontus, in the family Heterodontidae. All are relatively small, with the largest species being just 150 centimetres (59 in) in adult length. They are bottom feeders in tropical and subtropical waters. [more]

Hexanchiformes

Hexanchiformes is the order consisting of the most primitive types of sharks, and numbering just six extant species. Fossil sharks that were apparently very similar to modern sevengill species are known from Jurassic specimens. [more]

Hybodontiformes

The hybodonts are an extinct group of sharks and are the sister taxa to the Neoselachii (all modern sharks, skates and rays). They were very successful in their own right and existed as a group for more than 200 million years. Their fossil record extends from the Carboniferous to the Late Cretaceous. The hybodonts dominated the shark faunas of the Early Mesozoic and unlike modern sharks were abundant in both freshwater and marine habitats. The group went into decline in the second half of the era and finally became extinct near to the end of the Cretaceous, close to, or as part of the same extinction that wiped out the non-avian Dinosaurs. [more]

Lamniformes

Lamniformes is an order of sharks commonly known as mackerel sharks (which may also refer specifically to the family Lamnidae). It includes some of the most familiar species of sharks, such as the great white shark, as well as more unusual representatives, such as the goblin shark and the megamouth shark. [more]

Myliobatiformes

Myliobatiformes is one of the four orders of batoids, cartilaginous fishes related to sharks. They were formerly included in the order Rajiformes, but more recent phylogenetic studies have shown that the myliobatiforms are a monophyletic group, and that its more derived members evolved their highly flattened shapes independently of the skates. [more]

Orectolobiformes

The carpet sharks are an order, Orectolobiformes, of sharks, so called because many members have ornate patterns reminiscent of carpets. Sometimes the term "carpet shark" is used interchangeably with wobbegong, which are a subgroup of the order. [more]

Petalodontida

[more]

Pristiformes

Sawfish, also known as the Carpenter Shark, are a family of rays, characterized by a long, toothy nose extension snout. Several species can grow to approximately 7 metres or 23 feet. The family as a whole is largely unknown and little studied. They are members of the sole living family Pristidae within the order Pristiformes, from the Ancient Greek pristes (p??st??) meaning "a sawyer" or "a saw". [more]

Pristiophoriformes

The sawsharks or saw sharks are an order (Pristiophoriformes) of sharks bearing long blade-like snouts edged with teeth, which they use to slash and disable their prey. Most occur in waters from South Africa to Australia and Japan, at depths of 40 metres (130 ft) and below; in 1960 the Bahamas sawshark was discovered in the deeper waters (640 m to 915 m) of the northwestern Caribbean. [more]

Rajiformes

Rajiformes is one of the four orders of batoids, flattened cartilaginous fishes related to sharks. [more]

Rhiniformes

[more]

Rhinobatiformes

The guitarfish are a family, Rhinobatidae, of rays. The guitarfish are known for an elongated body with a flattened head and trunk and small ray like wings. The combined range of the various species is tropical, subtropical and temperate waters worldwide. They often travel in large schools. [more]

Squaliformes

Squaliformes is an order of sharks that includes about 97 species in seven families. [more]

Squatinactida

[more]

Squatiniformes

The angel sharks are an unusual genus of sharks with flattened bodies and broad pectoral fins that give them a strong resemblance to rays. The more than 16 known species are in the genus Squatina, the only genus in its family, Squatinidae, and order Squatiniformes. They occur worldwide in temperate and tropical seas. Most species inhabit shallow temperate or tropical seas, but one species inhabits deeper water, down to 1,300 metres (4,300 ft). [more]

Symmoriida

[more]

Torpediniformes

The electric rays are a group of rays, flattened cartilaginous fish with enlarged pectoral fins, comprising the order Torpediniformes. They are known for being capable of producing an electric discharge, ranging from as little as 8 volts up to 220 volts depending on species, used to stun prey and for defense. There are 69 species in four families. [more]

Xenacanthiformes

[more]

At least 4 species and subspecies belong to the Order Xenacanthiformes.

More info about the Order Xenacanthiformes may be found here.

References

[ Back to top ]
  1. ^ Botella, H., P.C.J. Donoghue and C. Mart?nez-P?rez (May 2009). "Enameloid microstructure in the oldest known chondrichthyan teeth". Acta Zoologica 90 (Supplement 1): 103?108.
  2. ^ Function of the heterocercal tail in sharks: quantitative wake dynamics during steady horizontal swimming and vertical maneuvering - The Journal of Experimental Biology 205, 2365?2374 (2002)

Sources

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Last Revised: August 24, 2012
2012/08/24 13:05:37