A shark is a marine fish with cartilaginous skeleton belonging to subclass Elasmobranchii, class Chondrichthyes, order Pleurotremata. Modern shark ancestry dates back at least 150 million years. About 350 species in 90 genera are found worldwide from Arctic to Antarctic waters, including ocean depths. The greatest abundance and diversity occurs in tropical and warm temperate regions. A few species invade tropical fresh waters. About 37 species occur in Canada's coastal and offshore waters including thresher, great white, porbeagle, mako, basking, blue, hammerhead and dogfish. A few species occur off both coasts throughout the year, especially in deep water; the large, predaceous forms appear mainly during summer and fall.
Sharks range from 15 cm and a few grams (Squaliolus laticaudas) to over 18 m and many tonnes (whale shark, Rhincodon typus, the world's largest fish). They are typically elongate, cylindrical fishes with 1 or 2 dorsal fins, large pectoral and moderate to small pelvic fins and, usually, an anal fin and the characteristic tail fin with its enlarged upper lobe. The crescent-shaped mouth is normally ventral. The jaws have multiple rows of teeth, which are continually replaced from inside the mouth. The skin is covered with pointed, toothlike denticles.
Male sharks have copulatory organs (claspers) on the pelvic fins for internal fertilization. Most species are live bearers (ovoviviparous) but a few deposit eggs in horny capsules (oviparous). The gestation period for the dogfish shark (Squalus acanthias), 20-22 months, is one of the longest for a vertebrate.
Sharks of family Lamnidae (eg, mako, great white) have heat-regulating mechanisms allowing maintenance of a warm body temperature and thus increasing swimming and predation efficiency. The large liver stores food reserves, allowing survival during prolonged fasting. Small species of sharks eat planktonic crustaceans and other invertebrates; large predaceous species prey on marine fishes, cephalopods, marine mammals and other vertebrates. The largest sharks (eg, basking and whale) are plankton feeders.
Significance of Fishery
Shark fisheries are conducted in many countries by longlines, gill nets and purse seines for food, fishmeal and leather. Shark meat is marketed fresh, frozen, salted and dried. Dogfish sharks are used extensively in biomedical research. Most sharks are harmless, but about 10% are a hazard to humans and another 10% a potential hazard. The danger of shark attack is greatly exaggerated.