shark
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shark
shark, member of a group of almost exclusively marine and predaceous fishes. There are about 250 species of sharks, ranging from the 2-ft (60-cm) pygmy shark to 50-ft (15-m) giants. They are found in all seas, but are most abundant in warm waters. Some may enter large rivers, and one ferocious freshwater species lives in Lake Nicaragua. Most are predatory, but the largest species, the whale shark and the basking shark, are harmless plankton eaters, and the small bonnethead shark eats seagrass as well as crustaceans and other prey. Dogfish is the name for members of several families of small sharks; these should not be confused with the bony dogfishes of the mud minnow and bowfin families. See also hammerhead shark and thresher shark.
Shark meat is nutritious and is used for human food. In Asian cuisines a prized gelatinous soup is made from the fins of certain species; many of the millions of sharks landed annually are taken just for the fins, and finning is now believed to threaten such species. The flesh is also sold for poultry feed, and shark oils are used in industry; shark-liver oil was formerly used as a source of vitamin A. The rough skin is used as a sandpaper called shagreen, and tanned sharkskin is a durable leather. A decrease in shark populations due to overharvesting has led a number of nations to establish shark sanctuaries in their waters.
Characteristics
Predation
Classification
Bibliography
See P. E. Pope, A Dictionary of Sharks (1973); T. H. Lineaweaker and R. H. Backus, The Natural History of Sharks (1970, repr. 1986); J. A. Musick and B. McMillan, The Shark Chronicles (2002).
Shark
a fish of the Selachii order of the Elasmobranchii subclass. The shark is represented by three suborders: living, primitive, and horn sharks. The living sharks (Selachoidei) vary in length from 0.5 m (Etmopterus spinax) to 20 m (basking shark). They have a fusiform body and five gill clefts on each side; only the saw shark has six. The scale is placoid, the mouth is located in the lower part of the head, and the skeleton is cartilaginous; these fish have no swim bladders. Sharks are widespread in coastal and open waters; some inhabit rivers—for instance, the Amazon and Ganges. In the USSR they live in the Barents, Baltic, Black, and Azov seas and in the seas of the Far East. Although most sharks lay eggs (large, in a horn-shaped membrane), some are viviparous. The majority are predators, feeding on fish, deepwater invertebrates, echinoderms, mollusks, and worms; sometimes they attack man. Sharks have commercial uses. Most sharks are caught in tropical waters. Those caught in the USSR include the spiny dogfish, the Greenland shark, and the porbeagle. Fish oil is extracted from the shark’s liver, the meat is used for food, and the skeleton is used to make fish glue. Primitive sharks (Hexanchoidei) have six or seven gill clefts on each side. They consist of two families: the frill sharks (Chlamydoselachidae), represented by the single species Chlamydoselachus anguineus, which is widespread, but rarely encountered (bodies measuring about 1.5 m), and the cow sharks (Hexanchidae). Horn sharks (Heterodontoidei) can reach a length of 1.5 m. One genus, Heterodontus, includes four species, which are found in the subtropical and tropical parts of the Pacific and Indian oceans.
REFERENCE
Nikol’skii,G. V. Chastnaia ikhtiologiia, 2nd ed. Moscow, 1954.G. V. NIKOL’SKII