Chondrichthyes (cartilaginous fish, including sharks, rays & chimaeras)

Life > Eukaryotes > Opisthokonta > Metazoa (animals) > Bilateria > Deuterostomia > Chordata > Craniata > Vertebrata (vertebrates)  > Gnathostomata (jawed vertebrates)

What is a cartilaginous fish?

Cartilaginous fishes (Class Chondrichthyes, from Greek chondros, cartilage, and ichthos, fish) are a class of aquatic, gill-breathing, finned vertebrates (animals with a vertebral column or `backbone'), equivalent to the bony fishes (Class Osteichthyes: osteos, bone, and ichthos, fish). Cartilaginous fishes include the living sharks, rays, and chimaeras, and have true upper and lower jaws, a sensory snout that overhangs the mouth and nostrils on the underside of the head, teeth in conspicuous transverse rows or in fused tooth plates that are replaced from inside the mouth, no bony plates on the head, scales in the form of small, toothlike dermal denticles or placoid scales, fins without bony fin rays, and a simplified internal cartilaginous skeleton without bone.

All cartilaginous fishes have a pair each of symmetrical pectoral and pelvic fins on the sides of the body, which correspond to the forelimbs and hands or wings, and hindlimbs and feet of amphibians, reptiles, birds and mammals (tetrapods or `four-footed' land vertebrates). All sharks and chimaeras have a caudal fin on the end of the tail, which may be reduced or absent in rays. An anal fin, on the underside of the tail between the paired pelvic fins and the caudal fin, is present in many sharks and chimaeras but absent in others, while all rays and skates lack this fin. Most sharks and all chimaeras have two dorsal fins on the back between the head and caudal fin while a few sharks have only one dorsal fin and rays vary with two, one, or no dorsal fins. Some sharks have fin spines on the front edges of their dorsal fins.

All cartilaginous fishes have 4 to 7 pairs of gill slits on the sides or underside of the head, which independently open to the exterior in sharks and rays but in chimaeras are covered by a soft gill cover similar to the hard, bony gill cover of bony fishes. All living cartilaginous fishes have internal fertilization of eggs through paired claspers (copulatory organs) on the pelvic fins of males, and are egg-layers or live-bearers. Cartilaginous fishes collect urea and other metabolic waste products in the blood and body fluids for osmotic balance, and have a specialized rectal gland for removal of excess salt in their body fluids.

There are over 900 named species of cartilaginous fishes, and at least 1100 known species including species new to science that are being described by researchers in museums and other research organizations. The living cartilaginous fishes are subdivided into two living groups, the large Subclass Elasmobranchii (elasmos, plates, and branchos, gills), which includes several groups of fossil sharks and the dominant Subcohort Neoselachii (neos, new, and selachos, a shark) or `modern' SHARKS and RAYS, and the small Subclass Holocephali (chimaeras). Modern sharks and rays (neoselachians) have 5 to 7 pairs of external gill slits, upper jaws not fused to the skull and usually capable of protrusion from the mouth, rows of teeth not fused into tooth plates, an erect 1st dorsal fin which cannot fold backwards, and no accessory claspers on the head or pelvic fins.

What is a shark?

Sharks are cylindrical or flattened neoselachians with 5 to 7 gill slits on the sides of their heads, moderate-sized pectoral fins that are not attached to the head above the gill slits, a large, stout tail with a large caudal fin, one or two dorsal fins with or without spines, and an anal fin variably present or absent. The term shark is also used loosely for members of many fossil elasmobranch groups that are not members of the Neoselachii but have a sharklike form.

There are over 370 species of living sharks worldwide, almost entirely in marine waters. A few requiem sharks (Carcharhinidae), including the Zambezi shark (Carcharhinus leucas) regularly ascend tropical rivers and occur in lakes with access to the sea.

What is a ray?

Rays are flattened neoselachians derived from sharks, `winged sharks' that have their pectoral fins expanded forwards and fused to the sides of their heads over the gill openings, so that their gill openings are on the undersides of their heads. They have short, flat bodies, 5 or 6 gill openings, tails that vary from large, thick, and sharklike to slender and whiplike, 2, 1 or no dorsal fins which lack spines when present, no anal fin, and the caudal fin varying from large and sharklike to absent. The pectoral fins of rays supplement or replace the caudal fin as propulsive organs, and are greatly enlarged in the more specialized rays, particularly the stingrays and skates, which have reduced slender tails and caudal fins.

The skeletons of rays are more complex than those of sharks, with attachments of the pectoral girdle to the vertebral column and the anterior basals of the pectoral fins to the skull or neurocranium, and fusion of the vertebral column between cranium and pectoral girdle into a tube. This arrangement serves to support the broadened pectoral fins and to resist their propulsive thrust.

At least 500 species of rays occur worldwide, mostly in marine waters. The sawfishes (Family Pristidae) and whiptailed stingrays (Family Dasyatidae) readily enter fresh water and are found in tropical rivers and lakes, while some whiptailed stingrays are confined to fresh water in Africa and Asia. The river stingrays (Family Potamotrygonidae) are confined to freshwater river systems in tropical South America.

What is a chimaera?

Chimaeras are cartilaginous fishes belonging to the Subclass Holocephali (holos, entire, and kephalos, head), in reference to the fusion of the upper jaws to the skull in these fishes. These compressed, often silvery cartilaginous fishes differ from sharks and rays in having 4 pairs of gill slits, which are covered by a soft gill cover and communicate to the exterior via a single pair of external gill slits. Chimaeras have largely naked skin, with denticles missing over most of the body, their teeth fused into three pairs of ever-growing tooth plates like rodent incisors (hence the names ratfish or rabbitfish for some of the species), the first dorsal fin with a spine that can be erected or depressed, and a simplified gut with the stomach merged with the intestine. In addition to the pelvic claspers, male chimaeras have an unpaired clasper (frontal tentaculum) on the forehead and paired claspers (prepelvic tentaculum) in front of the pelvic fins, all of which have hooklike dermal denticles to help the male hold the female during copulation. Chimaeras propel themselves with their large, fan-shaped pectoral fins.

Over 31 species of chimaeras exist, and most live in deep water on the continental slopes. Some chimaeras, including the local St. Joseph or elephant fish (Callorhinchus capensis), occur on the continental shelves and may range close inshore, but none are found in fresh water. All lay eggs in distinctive spindle-shaped, finned egg-cases.

What to do if you find a rare or unusual species

If you obtain an unusual or rare cartilaginous fish, particularly one that does not fit any of the species covered in Biodiversity Explorer, please contact any of the organizations below. If at all possible save the specimen and bring it fresh or preferably frozen to the nearest research organization. Small cartilaginous fishes can be preserved in a solution of one part concentrated (40%) formaldehyde to 9 parts water; if formaldehyde is not available use 50% isopropyl or n-propyl alcohol or 75% ethyl alcohol as a preservative. If the animal is too big to keep intact, take photographs of it, take its weight, total length, precaudal length (for sharks and chimaeras), and disk width for rays, and cut off its head intact or remove its jaws and save them. Photographs should be taken of side views of the entire animal and the undersides of the heads for normally shaped sharks and chimaeras; photograph the upper and lower surfaces of angel sharks and rays. If you record your fish on videotape or motion picture film take shots of it from several angles.

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Text by Leonard J.V. Compagno, David A. Ebert and Malcolm J. Smale


  Iziko Museums of Cape Town, 2008

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