Shark, any of various species of cartilaginous fishes of predatory habit that constitute the order Selachii (class Chondrichthyes).
Sharks, together with rays and skates, make up the subclass Elasmobranchii of the Chondrichthyes. Sharks differ from other elasmobranchs, however, and resemble ordinary fishes, in the fusiform shape of their body, and in the location of their gill clefts on each side of the head. Though there are exceptions, sharks typically have a hard skin that is dull gray in color and is roughened by toothlike scales. They also usually have a muscular, asymmetrical, upturned tail; pointed fins; and a pointed snout reaching forward and over a crescentic mouth set with sharp angular teeth. Sharks have no swim bladder and must swim enduringly to keep from sinking to the depths.
There are more than 400 living species of sharks, taxonomically grouped into 14–30 families, according to many experts. Numerous larger species can be fatal to humans. Many sharks are fished commercially. However, overfishing in the late 20th and early 21st centuries substantially reduced the populations of some shark species.
Shark species are nondescript in color, varying from gray to cream, brown, yellow, slate, or blue and often patterned with spots, bands, marbling, or protuberances. The oddest-looking sharks are the hammerheads (Sphyrna), whose heads resemble double-headed hammers and have an eye on each stem, and the wobbegongs (family Orectolobidae), whose skin flaps and shielding coloration closely match the seafloor. The vernacular of shark names show colors in living species, such as the blue (Prionace glauca), the white (Carcharodon carcharias; also known as the great white shark), and the lemon (Negaprion brevirostris) shark.
The whale shark (Rhincodon typus) and the basking shark (Cetorhinus maximus), both of which may weigh several tons, are harmless giants that subsist on plankton strained from the sea through modified gill rakers. Whale sharks may grow up to 18 meters (59 feet) in length, whereas basking sharks may reach 14 meters (46 feet) fully grown. All other sharks prey on smaller sharks, fish, squid, octopuses, shellfish, other invertebrates, and, in some species, trash. The largest among the more carnivorous species is the voracious 6-meter (20-foot) white shark, which attacks seals, dolphins, sea turtles, large fish, and occasionally people. The more slow Greenland shark (Somniosus microcephalus) of cold deep waters feeds on seals, large fish, and even swimming reindeer; they may also scavenge whale carcasses.
Normally, sharks feed on fish, often attacking in schools. Open-ocean species such as the mackerel (Lamna), mako (Isurus), and thresher (Alopias) sharks frequently feed near the surface and are much sought after with rod and reel for sport. Beautifully streamlined and powerful swimmers, those open-ocean sharks are adept at feeding on fast tuna, marlin, and the like. Bottom-feeding species of sharks are stout, blunt-headed forms that tend to have more-sluggish habits. The shellfish eaters among them have coarse, pavement like crushing teeth.