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

Sharks are heavy fishes, possessing neither lungs nor swim bladders (see fish). Their skeletons are made of cartilage rather than bone, and this, along with large deposits of fat, partially solves their weight problem; nevertheless, most sharks must keep moving in order to breathe and to stay afloat. They are good swimmers; the wide spread of the pectoral fins and the upward curve of the tail fin provide lift, and the sweeping movements of the tail provide drive. Their tough hides are studded with minute, toothlike structures called denticles. Sharks have pointed snouts; their crescent-shaped mouths are set on the underside of the body and contain several rows of sharp, triangular teeth. They have respiratory organs called gills, usually five on each side, with individual gill slits opening on the body surface; these slits form a conspicuous row and lack the covering found over the gills of bony fishes. Like most fishes, sharks breathe by taking water in through the mouth and passing it out over the gills. Usually there are two additional respiratory openings on the head, called spiracles. A shark's intestine has a unique spiral valve, which increases the area of absorption. Fertilization is internal in sharks; the male has paired organs called claspers for introducing sperm into the cloaca of the female. Members of most species bear live young, but a few of the smaller sharks lay eggs containing much yolk and enclosed in horny shells. Compared to bony fishes, sharks tend to mature later and reproduce slowly.

Predation

Only a small number of the predatory species are definitely known to engage in unprovoked attacks on humans. The largest and most feared of these is the great white shark, which may reach 20 ft (6 m) in length and is probably responsible for more such attacks than any other species. Other sharks reputed to be especially dangerous are the tiger and blue sharks and the mako. Sharks are extremely sensitive to motion and to the scent of blood. Swimmers in areas where dangerous varieties occur should leave the water quietly if they are cut; spearfishing divers should remove bleeding fish from the water immediately. In some places bathing areas are guarded by nets. A number of substances have been used as shark repellents, including maleic acid, copper sulfate, and decaying shark flesh, but their effectiveness is variable. An electrical repellent device, exploiting the shark's sensitivity to electrical fields, has been developed in South Africa. Sharks usually circle their prey before attacking. Since they seldom swim near the surface, an exposed dorsal fin is more likely to be that of a swordfish or ray than that of a shark.

Classification

Sharks, rays (including skates), and chimaeras together form the vertebrate class Chondrichthyes, the cartilagenous fishes (see Chordata). The sharks and rays form the subclass Elasmobranchii, and the sharks form the order Selachii.

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).

The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

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

shark

[shärk]

(vertebrate zoology)

Any of about 225 species of carnivorous elasmobranchs which occur principally in tropical and subtropical oceans; the body is fusiform with a heterocercal tail and a tough, usually gray, skin roughened by tubercles, and the snout extends beyond the mouth.

shark

large and ferocious fish, sometimes man-eating. [Zoology: NCE, 2493]

See: Deadliness

shark

any of various usually ferocious selachian fishes, typically marine with a long body, two dorsal fins, rows of sharp teeth, and between five and seven gill slits on each side of the head

Sharks

(dreams)

Please remember that the water in your dreams may be a statement about your emotions and the unconscious. Sharks, water-dwelling animals, could represent unpleasant emotions or difficult and painful materials coming up from the unconscious. You may feel some emotional upset, and the shark could be the symbol of the perceived emotional danger. Old dream interpretation books say that sharks may represent dishonest friends or reflect financial troubles.