Chondrichthyes

Cartilaginous fishes Fossil range: Early Silurian - Recent
Great white shark, Carcharodon carcharias Great white shark, Carcharodon carcharias
Scientific classification
Kingdom: Animalia Phylum: Chordata Subphylum: Vertebrata Infraphylum: Gnathostomata Class: Chondrichthyes Huxley, 1880
Subclasses and Orders
See text.

The Chondrichthyes or cartilaginous fishes are a major class of jawed fish that includes the sharks, rays, and skates, and whose skeleton is characterized by rubbery cartilage, which is very light and flexible, rather than bone, as in the bony fishes (class Osteichthyes]] (such as cod or salmon. The chondrichthyans have jaws, paired fins, paired nostrils, scales, and two-chambered hearts.

Chondrichthyes is divided into two subclasses: Elasmobranchii (sharks, rays, and skates) and Holocephali (chimaera, sometimes called ghost sharks). The extant (living) elasmobranchs are sometimes divided into two superorders, Selachimorpha (sharks) and Batoidea (rays, skates, sawfish). Nelson (1994) notes that there is growing acceptance of the view that sharks and rays form a monophyletic group (superorder Euselachii), and that sharks without rays are a paraphyletic group.

Characteristics

Chondrichthyans are characterized by a cartilaginous skeleton. The cartilage is often partly calcified (mineralized with calcium, making it harder and more bone-like, but it is seldom if ever ossified (Nelson 1994). There is bone reported in the vertebrae of Scyliorhinus canicula (Nelson 1994).

Both swim bladder and lungs are absent in members of Chondrichthyes. Condrichthyans have digestive systems with intestinal spiral valves, and with the exception of Holocephali, they also have a cloaca. A spiracle is found behind each eye on most species.

As they do not have bone marrow, red blood cells are produced in the spleen and special tissue around the gonads. They are also produced in an organ called Leydig's Organ, which is only found in cartilaginous fishes, although some lack 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.

In the history of the class, the pectoral and pelvic girdles, which do not contain any dermal elements, were considered to orginally not be connected. In later forms, each pair of fins became ventrally connected in the middle when scapulocoracoid and pubioischiadic bars developed. In rays, the pectoral fins have connected to the head and are very flexible.

The tough skin of chondrichthyans is covered with dermal teeth . Holocephali is an exception, as the teeth are lost in adults, and only kept on the clasping organ seen on the front of the male's head. These teeth, also called placoid scales or dermal denticles, making the skin feel like sandpaper.

The oral teeth are usually not fused to the jaws, but are replaced serially (Nelson 1994). It typically 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 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.

Modern forms practice internal fertilization., with males with pelvic claspers that are inserted into the female cloaca and oviducts (Nelson 1994). The embryo is encapsulated in a leatherlike case and gestation periods of up to two years are known, which is longer than any vertebrate (Nelson 1994).

Animals from this group have a brain weight relative to body size that comes close to that of mammals, and is about ten times that of 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.

Taxonomy

A fish is a poikilothermic (cold-blooded), water dwelling vertebrate with gills throughout life, and limbs—if present—in the form of fins. Poikilothermic refers to the fact that the internal temperatures of fish vary, often matching the ambient temperature of the environment.

There are over 27,000 known extant (living) fish species, making them the most diverse group of vertebrates, with more than one-half of the total vertebrate species. A heterogeneous assemblage, modern fish are conventionally divided into the jawless fish (class or superclass Agnatha, about 75 species including lampreys and hagfish), the cartilaginous fish (class Chondrichthyes, about 800 species including sharks and rays), and the bony fish (class Osteichthyes, with over 26,000 species). Some individuals only use the term fish when referring to the jawed bony fish, and do not include Agnatha or Chondrichthyes.

Nelson recognizes among extant taxa 10 orders, 45 families, 170 genera, and about 846 species

The extant (living) orders of Elasmobranchii that are typically considered sharks are Hexanchiformes, Squaliformes, Squatiniformes, Pristiophoriformes, Heterodontiformes, Orectolobiformes, Lamniformes, and Carchariniformes (Nelson 1994; Murch 2007). The squatiniformes (angel sharks) have a ray-like body (Nelson 1994).

• Class Chondrichthyes
  • Subclass Elasmobranchii (sharks, rays and skates)
    • Superorder Batoidea (rays and skates), containing the orders:
      1. Rajiformes (common rays and skates)
      2. Pristiformes (Sawfishes)
      3. Torpediniformes (electric rays)
    • Superorder Selachimorpha (sharks), containing the orders:
      1. 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 sharks, frilled shark and even a shark that looks on first inspection to be a marine snake.
      2. 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 sharks, dogfish and roughsharks.
      3. 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.
      4. 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.
      5. 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 shellfishes.
      6. Orectolobiformes Seven families are found within this order. They are commonly referred to as the carpet sharks, including zebra sharks, nurse sharks, wobbegongs and the largest of all fishes, the whale sharks. They are distinguished by having barbels at the edge of the nostrils. Most, but not all are nocturnal.
      7. 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, tiger, bull, reef and oceanic whitetip sharks (collectively called the requiem sharks) along with the houndsharks, catsharks and hammerhead sharks. They are distinguished by an elongated snout and a nictitating membrane which protects the eyes during an attack.
      8. 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, mako shark and great white shark. They are distinguished by their large jaws and 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 fossils produced). A reproduction of the jaw was based on some of the largest teeth (up to almost 7 inches in length) and suggested a fish that could grow 120 feet in length. The jaw was realized to be inaccurate, and estimates revised downwards to around 50 feet.
  • Subclass Holocephali (chimaera)

References

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Wikispecies has information related to:

Chondrichthyes

Wikibooks Dichotomous Key has more about this subject:

Chondrichthyes

• Taxonomy of Chondrichthyes

• Murch, A. 2007. Shark taxonomy. Elasmodiver.com. Retrieved May 26, 2007.

Nelson

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