CHONDRICHTHIAN

any fish that is a member of the class Chondrichthyes (or Selachii). The chondrichthians, or cartilaginous fishes, include the sharks, rays, skates, and chimaeras. Before dying out, the armour-plated placoderms of the Devonian Period (408 to 360 million years ago) gave rise to the osteichthians (bony fishes) and the chondrichthians (cartilaginous fishes). Though successful as a group, the chondrichthians number far fewer species (about 550) than do the teleosts, or modern bony fishes (about 20,000 species). Chondrichthians differ from bony fishes in several respects. Most importantly, their skeleton consists of cartilage and, though partly calcified in the vertebrae, lacks true bone. Their other distinctive characteristics include five to seven pairs of gill clefts, a skin covered with placoid scales that have a toothlike structure, and numerous teeth that are arranged in rows in the mouth. Most chondrichthians live in marine waters, though there are some freshwater species of shark. They are unimportant as a food source for humans. There are about 225 species of sharks. All are carnivorous, and about 27 species are identified as being dangerous to humans. The most ferocious of the man-eaters is the white shark (Carcharodon carcharias). Most sharks and rays tend to stay in water no cooler than 21 C (70 F). The largest species, the whale shark (Rhincodon typus) and basking shark (Cetorhinus maximum), feed on plankton. About 320 species of rays and skates are known. Most have a flattened, disklike body and greatly enlarged, winglike pectoral fins. They are chiefly bottom dwellers that feed on other fish and invertebrates such as mollusks and crustaceans. Skates partially bury themselves on the bottom, emerging suddenly to pursue small prey. Chimaeras are also bottom dwellers; they feed at night almost entirely on small fishes and invertebrates. Fertilization is internal among most species of chondrichthians. The male has a pair of copulatory organs called claspers that are attached to the pelvic fin, and which are used to insert sperm into the female. Many species of sharks and rays other than skates hatch their eggs within the female. Only a few species of chondrichthians are egg layers. See also ray; shark. class name Chondrichthyes, or Selachii, any member of the group of cartilaginous fishes that includes the sharks, skates, chimaeras, and rays. The class is one of the two great groups of living fishes, the other being the osteichthians, or bony fishes. The term Selachii is also sometimes used as the name for the order of sharks. Many structural, physiological, biochemical, and behavioral peculiarities make these fishes of particular interest to scientists. The dissection of a small shark is the biology student's introduction to vertebrate anatomy. These fishes are, in a sense, living fossils, for many of the living sharks and rays are assigned to the same genera as species that swam the Cretaceous seas over 100,000,000 years ago. Although by any reckoning a successful group, the modern chondrichthians number far fewer species than the more advanced bony fishes, or teleosts; 200 to 250 species of sharks and 300 to 340 species of rays are known. The danger some sharks and stingrays present to humans makes these animals fascinating and, at the same time, abhorrent. Perhaps for this reason, they figure prominently in the folklore and art of many tropical peoples whose living depends on the sea. The danger from shark attack, while very real, is easily sensationalized, and quite frequently little attempt is made to distinguish between dangerous and harmless species. Additional reading J.S. Babel, "Reproduction, Life History, and Ecology of the Round Stingray, Urolophus halleri Cooper," Fish Bull. Calif. 127 (1967), an excellent natural history study of a ray; H.B. Bigelow and W.C. Schroeder, "Sharks," Fishes of the Western North Atlantic, Mem. Sears Fdn. Mar. Res., no. 1, pt. 1, pp. 59-546 (1948); "Sawfishes, Guitarfishes, Skates and Rays," and "Chimaeroids," ibid., no. 1, pt. 2 (1953), two cosmopolitan syntheses in spite of emphasis on the western North Atlantic; "A Study of the Sharks of the Suborder Squaloidea," Bull. Mus. Comp. Zool. Harv., vol. 117, no. 1 (1957), a revision of the group; P. Budker, La Vie des requins (1947; rev. Eng. trans., The Life of Sharks, 1971), a nontechnical, authoritative work; J.F. Daniel, The Elasmobranch Fishes, 3rd rev. ed. (1934), a classic treatise on the anatomy of sharks and rays; D.H. Davies, About Sharks and Shark Attack (1964), an authoritative, nontechnical work about sharks, principally South African species; P.W. Gilbert (ed.), Sharks and Survival (1963), a collection of 22 papers about sharks, with emphasis on dangerous species, including a list of documented attacks throughout the world; P.W. Gilbert, R.F. Mathewson, and D.P. Rall (eds.), Sharks, Skates and Rays (1967), a collection of technical papers covering a wide range of subjects; E.W. Gudger (ed.), Archaic Fishes, Bashford Dean Memorial Volume (1937), contains articles on the anatomy, reproduction, and development of the frilled shark (Chlamydoselachus anguineus), and on natural history and development of Heterodontid sharks; H.W. McCormick and T. Allen, with W.E. Young, Shadows in the Sea: The Sharks, Skates, and Rays (1963, reissued 1978), an authoritative, well-illustrated, nontechnical work; L.H. Matthews and H.W. Parker, "Notes on the Anatomy and Biology of the Basking Shark (Cetorhinus maximus)," Proc. Zool. Soc. Lond., 120:535-576 (1950); P.M. Roedel and W.E. Ripley, "California Sharks and Rays," Fish Bull. Calif. 75 (1950), a handbook for identification of California species (of general use on the Pacific coast), illustrated with excellent photographs; A.S. Romer, Vertebrate Paleontology, 3rd ed. (1966), a comprehensive review, with a classification of archaic and recent fishes; H.W. Smith, From Fish to Philosopher (1961), a chapter on the elasmobranchs discusses their evolution and physiological adaptations to marine environment; D.W. Strasburg, "Distribution, Abundance, and Habits of Pelagic Sharks in the Central Pacific Ocean," Fish Bull. U.S. Dep. Interior, 58:335-416 (1958), contains field observations and quantitative data on 12 species; G.P. Whitley, The Fishes of Australia, Part I, The Sharks, Rays, Devil-Fish, and Other Primitive Fishes of Australia and New Zealand (1940), a nontechnical book about Australian species; Carl Gaus and T.S. Parsons, A Photographic Atlas of Shark Anatomy: The Gross Morphology of 'Squalus Acanthias' (1964, reprinted 1981).