sharks

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Shark, any of some 340 species of fishes that, together with the skates, rays, and chimaeras, are sharply distinguished from the vast number of teleost (bony) fish species by their cartilaginous skeletons. Sharks are versatile and keen-sensed fishes, many species of which are able to hunt and eat nearly all the larger marine animals in both shallow and deep seas. These two features account for their long evolutionary history; many of the shark species living today are quite similar to abundant species that swam in seas of the Cretaceous period more than 100 million years ago. Sharks also reveal great diversity in behavior and size. The whale shark is the largest shark and also the largest fish in the sea, measuring up to 15 m (49 ft) in length; the cookie-cutter shark measures less than 50 cm (19 in) in length. Sharks are chiefly marine fishes found in all seas and are especially abundant in tropical and subtropical waters. Many species migrate up rivers, however, and one, the bull shark, reaches Lake Nicaragua in Central America. Sharks are best known as aggressive carnivores that even attack members of their own species, but two of the largest sharks-the basking shark and the whale shark-are docile feeders on plankton, which they strain from the water with sievelike gill rakers.

II PHYSICAL CHARACTERISTICS

Most sharks are gray in color and have leathery skin covered with small, sharp, pointed placoid scales, which, unlike those of bony fish, do not enlarge during the animal's growth. Usually five gill slits lie behind the head. The tail is asymmetrical, with the vertebral column extended into the upper lobe. Many species have general rows of sharp teeth embedded in fibrous membranes instead of in the jawbones; the teeth, which are frequently lost in the flesh of prey, are quickly replaced by other teeth that shift into position. The fins and tails of sharks are rigid instead of erectile, like those of the bony fish. Contrary to a popular image, the dorsal fins rarely protrude above water when the fish are close to the surface.

Sharks do not have swim bladders and, when motionless, sink to the bottom. They have strong digestive enzymes and a specialized epithelial fold that spirals the length of the small intestine, enabling the fishes to absorb a great diversity of foods. Sharks, to a large extent, are scavengers, eating injured fishes, carrion, garbage, and other waste from ships as well as animals such as seals, turtles, birds, whales, crabs, and a wide range of fishes. Males can be identified by claspers, which are extensions of the pelvic fins that serve as copulatory organs.

III REPRODUCTION

Unlike bony fishes, which usually spawn great masses of tiny, immature young, most sharks produce large, well-developed offspring numbering, at the most, 100 to a litter. The sand tiger shark, for example, bears only two young at a time. Fertilization takes place internally, the male inserting one of its claspers into the female. Most sharks are ovoviviparous, hatching the eggs within the female and bearing live young. Some sharks are oviparous, however, laying their eggs externally; the eggs are often encased in leathery shells with tendrils that anchor them to rocks or weeds. Others are viviparous, the young developing in utero, analogous to development in mammals; the yolk sac becomes a yolk placenta in the folds of the uterine wall and conducts nutrients to the embryo. Embryonic development takes more than six months, and in the Atlantic spiny dogfish it takes close to two years. At birth, young sharks of some larger species are more than 1 m (more than 3 ft) long and are swift, capable swimmers that feed on the same prey as adults. The young are frequently born in protected inshore areas away from the males. Sharks commonly fast for long periods during the breeding season and live on the vast reserves of lipids stored in their livers; cannibalizing of the young and of one another is thereby avoided.

IV SENSES AND FEEDING

Sharks have an acute sense of smell; they are able to detect minute substances, such as blood in water, and trace them to their source. Seeing, although less acute, allows the shark to catch dim movements of shadow and light in dark waters as it approaches its prey. Sharks are particularly sensitive to sounds of low frequency and have fine directional hearing. Organs along their lateral lines and on the snout enable sharks to pick up weak electrical stimuli from the muscle contractions of bony fish. This combination of keen senses accounts for the evolutionary success of sharks.

When hunting in schools, sharks can incite one another into a feeding frenzy. They circle their prey and make sudden crisscrosses, frequently striking victims from below (but not turning on their backs, as is popularly believed). Considering the number of scuba divers, swimmers, and water-skiers who now venture into shark-infested waters, however, relatively few shark attacks on humans occur; of those that do, about one-third result in fatalities. Among the sharks most dangerous to humans are the great white shark, the hammerhead shark, the tiger shark, and the blue shark.

V ECOLOGY

Sharks play an important ecological role in oceans, similar to that of large predators on land. Besides ridding waters of wastes, sharks prey on weaker or maimed members of populations, thereby helping a species to maintain its genetic strength. Because modern fishing methods have helped to deplete many food-fish species, however, the industry now views sharks as competitors, to the extent that shrimp trawlers frequently employ electrical shields by their trawls to keep sharks from destroying the catch. On the other hand, with the increasing scarcity and rising price of many food fishes, especially for United States consumers, shark meat is itself now selling at prices comparable to those for traditional table fishes. The meat (as well as shark fins for soup) has long been eaten, especially in East Asian countries, but it is now increasingly being featured in United States markets and is often compared to swordfish.

Scientific classification: Sharks belong to the class Chondrichthyes. The whale shark is classified as Rhincodon typus, the cookie-cutter shark as Squaliolus laticaudus, and the bull shark as Carcharhinus leucas. The basking shark is classified as Cetorhinus maximus, the tiger shark as Galeocerdo cuvier, the spiny dogfish as Squalus ancanthias, and the great white shark as Carcharodon carcharias. Hammerhead sharks make up the family Sphyrnidae. The blue shark is classified as Prionace glauca.