white shark, large, ferocious shark, Carcharodon carcharias. Also known as the maneater, this aggressive shark can attack swimmers and boats without provocation. Although not abundant anywhere, it is widely distributed in tropical and temperate oceans and is found in both inshore and deep waters; it is most common on the Atlantic coast of the United States. Like the other members of its family, the mako and the porbeagle, it is a fast swimmer, with large pectoral fins and a nearly symmetrical tail fin. Despite its name, the white shark is usually whitish only on the underside, the back being some shade of gray. It has dark-tipped fins and a conspicuous black spot behind the pectorals. It reaches a length of over 20 ft (6 m) and a weight of over 7,000 lb (3,180 kg). It feeds on large fish and other animals; a 100-lb (45-kg) sea lion was recovered from the stomach of one specimen. The white shark's serrated, triangular teeth were used as arrowheads by Native Americans of the Florida coast. The white shark is classified in the phylum Chordata, subphylum Vertebrata, class Chondrichthyes, order Selachii, family Isuridae.

whale shark, large, plankton-eating shark, Rhincodon typus, found in all tropical seas of the world. The largest known specimens are 50 ft (15 m) long, making them the largest fish in the world. The whale shark feeds largely on plankton, as well as on small fish and crustaceans. It is the only large shark with its mouth at the front of its head rather than on the underside. The mouth is equipped with many rows of tiny teeth, and the throat has numerous long slender structures called gill rakers, which form a fine mesh for straining food from the water. The whale shark's body is stout but streamlined, like that of a whale. It is dark brown above, with many white or yellow spots, and white or yellow below. The whale shark is a docile, torpid fish; it does not attack, even on provocation, but has been known to collide with boats. It is classified in the phylum Chordata, subphylum Vertebrata, class Chondrichthyes, order Selachii, family Rhincodontidae.

thresher shark, long-tailed, warm-water shark, genus Alopias. The upper fork of its tail is slender and sickle-shaped and is about equal in length to the rest of the body. This shark uses its tail to herd the small schooling fish on which it feeds; the tail is flailed from side to side, sweeping the prey in front of the shark. It also slaps the water with its tail to frighten the fish. Threshers are found chiefly in offshore, tropical waters, but are also known in temperate regions. The common thresher, A. vulpinus, is widely distributed throughout the Atlantic and the E Pacific; it is common off the New England coast in summer and is fished commercially on the S California coast. It may reach a length of 20 ft (6.2 m) and weigh 1,000 lb (450 kg). A second species, A. pelagicus, is found in the W Pacific. The big-eyed thresher, A. superciliosus, is a deep-sea fish of the tropical and subtropical Atlantic. It is smaller than the common thresher, but its eyes may measure 4 in. (10 cm) in diameter. Thresher sharks are classified in the phylum Chordata, subphylum Vertebrata, class Chondrichthyes, order Selachii, family Alopiidae.

shovelhead shark: see hammerhead 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. 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 estimated 100 million sharks landed annually are taken just for the fins. 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.

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.

mackerel shark: see mako.

leopard shark: see dogfish.

hammerhead shark, active, surface-living shark, genus Sphyrina. Its curious head has lateral projections resembling the crossbar of a T, and its eyes and ears are located in the outer tips of the projections. It has been suggested that the extension of the head in the plane of the pectoral fins may give the fish increased lift, but there is no evidence that hammerheads are better swimmers than other sharks. Found in inshore, brackish water, hammerheads are both predators and scavengers. They feed on a variety of fishes including skates, rays, and other hammerheads, and large hammerheads have been known to attack and eat humans without provocation. The largest species is the widely distributed great hammerhead, Sphyrina mokarran, which typically reaches 15 ft (4.6 m) in length; the distance between its eyes may be as much as 3 ft (90 cm). The common hammerhead, S. zygaena, is found in all tropical waters, summering as far N as Newfoundland. It may reach a length of 12 ft (3.7 m). The bonnet, or shovelhead, shark is a small, harmless hammerhead found in the W Atlantic from Brazil to Massachusetts. It reaches a length of 5 ft (150 cm) and its lateral lobes are much shorter than in other species, giving the head a rounded appearance. There are several other species distributed throughout tropical and temperate oceans. Hammerhead sharks are classified in the phylum Chordata, subphylum Vertebrata, class Chondrichthyes, order Selachii, family Sphyrnidae.

bonnet shark: see hammerhead shark.

basking shark, large, plankton-feeding shark, Cetorhinus maximus, inhabiting many oceans of the world, especially in temperate regions. Found singly or in schools of up to 100, it spends much of its time on or just below the surface, cruising slowly with its dorsal fin breaking water. It reaches a length of 40 ft (12 m) and weighs up to 8,500 lb (3,900 kg)—among fishes it is second in size only to the whale shark. It feeds by filtering out plankton as water passes into its mouth and out of the gills. Its gill openings are greatly enlarged to accommodate a large volume of water, and its throat is lined with numerous slender structures called gill rakers. These rakers, which are attached to the inside of the gill arches, form a fine mesh that serves as a strainer. The basking shark has a torpedo-shaped body, a nearly symmetrical tail fin, and long, conspicuous gill slits. Its color ranges from gray to black or brown. It is fished commercially, mostly by harpooning; its flesh is used for fish meal and its liver oil for certain tanning processes. It is classified in the phylum Chordata, subphylum Vertebrata, class Chondrichthyes, order Selachii, family Cetorhinidae.

or white shark

Large, aggressive shark (Carcharodon carcharias, family Lamnidae), considered the species most dangerous to humans. It is found in tropical and temperate regions of all oceans and is noted for its voracious appetite. Its diet includes fishes, sea turtles, birds, sea lions, small whales, carcasses, and ships' garbage. The great white is heavy-bodied and has a crescent-shaped tail and large, saw-edged, triangular teeth. It can reach a length of more than 20 ft (6 m) and is generally gray, bluish, or brownish, with the colour shading suddenly into a whitish belly. Though it is widely feared, only a few hundred humans are known to have ever been killed by the great white shark.

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Species (Rhincodon typus) of gigantic but harmless shark found worldwide but mainly in the tropics. The largest of living fishes, it often grows to about 30 ft (9 m) long and may reach twice that size. It is gray or brown with a pale undersurface and is distinctively marked with small spots and narrow vertical lines of yellow or white. It has tiny teeth, and eats plankton and small fishes. A sluggish animal, it generally swims slowly near the surface and has been hit by ships.

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Potentially dangerous shark (Galeocerdo cuvieri, family Carcharhinidae), found worldwide in warm oceans, from the shoreline to the open sea. Up to 18 ft (5.5 m) long, the grayish tiger shark has a long, pointed upper tail lobe, and its large teeth are deeply notched along one side. This voracious shark eats fishes, other sharks, turtles, mollusks, birds, carrion, and garbage, including coal, tin cans, and clothing. It is a source of leather and liver oil. Seealso sand shark.

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Any of five species (family Alopiidae) of sharks with a long, scythelike tail that may constitute almost half their total length. They are found in tropical and temperate seas worldwide. They eat squid and schooling fishes, attacking after circling and herding their prey into small groups. They sometimes use their tail to stun their prey or, by thrashing the water, to frighten them. They are not considered dangerous to humans. The long-tailed thresher, or fox shark (Alopias vulpinus), is a big, dark fish that grows about 20 ft (6 m) long.

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Any of more than 300 species of predatory cartilaginous fish (order Selachii). An ancient animal, it has changed little in 100 million years. The skin typically is dull gray and tough and has toothlike scales. Most sharks have a muscular, asymmetrical, upturned tail; pointed fins; a pointed snout; and sharp triangular teeth. Sharks have no swim bladder and must swim perpetually to keep from sinking. Most species bear living young. Several species can be dangerous to humans (e.g., great white shark, hammerhead shark, sand shark, tiger shark); smaller ones, called topes, hounds, and dogfishes, are fished commercially. Seealso basking shark, mackerel shark, mako shark, thresher shark, whale shark.

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Any of about six species of shallow-water, bottom-dwelling sharks in the genus Odontaspis (family Odontaspididae), found along tropical and temperate coastlines of all oceans. They are 10–20 ft (3–6 m) long and are brown or gray above, paler below. Voracious but generally sluggish, they have long, slim, pointed teeth and prey on fishes and invertebrates. Two species, the sand tiger (O. taurus) of the Atlantic and the gray nurse (O. arenarius) of Australia, are potentially dangerous.

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Only Atlantic species (Ginglymostoma cirratum) of 25 carpet shark species (family Orectolobidae). Yellow- or gray-brown, sometimes with dark spots, it may grow to over 13 ft (4 m) long. It may attack swimmers, especially when provoked, but is not related to the dangerous gray nurse (Odontaspis arenarius), a sand shark.

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Any of certain potentially dangerous sharks (genus Isurus) in the mackerel shark family (Isuridae). Two species are generally recognized: the Atlantic I. oxyrinchus and the Indo-Pacific I. glaucus. Makos range throughout tropical and temperate seas. Blue-gray, with a white belly, they are about 13 ft (4 m) long and weigh about 1,000 lbs (450 kg). They prey on fishes such as herring, mackerel, and swordfish. Outstanding game fish, they are prized for their fighting qualities and their spectacular repeated leaps out of the water.

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Any of several temperate-water sharks (genus Lamna) in a family (Isuridae) that also includes the great white shark and the mako shark group. The swift, active mackerel sharks have a crescent-shaped tail and slender teeth. They are gray or blue-gray above and paler below and about 10 ft (3 m) long. They eat fishes such as herring, mackerel, and salmon, sometimes taking fishermen's catches and damaging nets. They are fished commercially for food. Common species include the Atlantic mackerel shark, or porbeagle (L. nasus), and the Pacific mackerel shark, or salmon shark (L ditropis).

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Any of the swift, powerful sharks in the family Sphyrnidae, having a broad, flattened, hammer- or spade-shaped head, with the eyes and nostrils at the ends of the sidewise projections. Widely distributed in all oceans, in warm and temperate waters, they feed on fish, stingrays, skates, and other sharks. Some species are fished for leather and oil. Three species seem to be particularly dangerous to humans: the great hammerhead (the largest hammerhead, growing to 15 ft, or 4.5 m, or more), the scalloped hammerhead, and the smooth hammerhead. All three are grayish and found throughout the tropics.

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or chimaera

Any of the 28 species of ancient fishes constituting the subclass Holocephali (class Chondrichthyes), found in temperate to cold waters of all oceans. Like sharks and rays, chimeras have a skeleton of cartilage rather than bone, and the males possess external reproductive organs (claspers). They have a single external gill opening, covered by a flap as in the bony fishes, on each side of the body. Males have a supplemental clasping organ that is unique among fishes. Chimeras have large pectoral and pelvic fins and two dorsal fins, the first preceded by a sharp spine. They range in length from 24 to 80 in. (60 to 200 cm) and in colour from silvery to blackish. They inhabit rivers, estuaries, coastal waters, and open ocean to depths of 8,000 ft (2,500 m) or more. They eat small fishes and invertebrates.

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In Greek mythology, a fire-breathing female monster. Its foreparts resembled a lion, its middle a goat, and its hindquarters a dragon. It devastated the land around Caria and Lycia until it was killed by Bellerophon. The word is now often used to denote a fantasy or a figment of the imagination.

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Huge, sluggish shark (family Cetorhinidae) named for its habit of floating or slowly swimming at the surface. Possibly comprising more than one distinct species, it inhabits northern and temperate regions of the Atlantic, Pacific, and Indian oceans. It may grow as large as 46 ft (14 m); among fishes, only the whale shark grows larger. Despite its size, the basking shark feeds on plankton. It is gray-brown or blackish, with tiny teeth and very long gill slits. It is generally harmless and is hunted sporadically for fish meal and liver oil.

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