Shark is the common name applied to any of the elasmobranchs constituting the superorder Selachoidea. Sharks are chiefly marine fishes and are found in all seas, but are especially abundant in tropical and subtropical waters; some species regularly inhabit fresh water. Unlike their close relatives, the rays and chimeras, sharks are fishlike in form and have lateral gill clefts. Most of them are of moderate or large size, the largest living sharks attaining a length of over 40 feet; fossils have been discovered of an extinct species of Carcharodon which attained an estimated length of 90 feet. The majority of sharks are gray in color, and have leathery skin covered with sharp, pointed scales which are known as placoid scales. Sharkskin is used in commerce as a source of a form of leather known as shagreen.
A shark's mouth is located on the underside of the head, below the anteriorly projecting snout, and is studded with numerous sharp teeth similar in structure to the placoid scales of the skin. The fins are not unusually large; contrary to popular belief, sharks' fins rarely protrude above the surface of the water when the fish are swimming close to the surface. The tail has two lobes; the cartilaginous backbone extends into the upper lobe of the tail. Shark flesh is coarse but edible. The fins abound in gelatin and are used in the Orient in the preparation of a rich, viscous soup. Shark livers yield an oil which is rich in vitamins A and D.
Most sharks are carnivorous, feeding on fishes. A few, such as the basking shark and the whale shark, feed on plankton, straining it out of the water with their sievelike gill rakers. The creatures are active and agile. Contrary to general belief, they do not turn on their backs when about to bite. Although some species are known to attack man, the great majority of sharks are not man-eaters; the basking shark and the whale shark, which are the largest living sharks, are harmless and do not attack man even when harpooned. Of the man-eating sharks, many are normally timid, attacking only when incited by the smell of blood. One of the dangerous species is a fresh-water gray shark which lives in Lake Nicaragua, Central America, and attains a length of 8 feet.
