sharks are elasmobranch
fishes with cartilaginous skeletons. Elasmobranchs include about 368 species of selachians (true sharks) belonging to 30 families, and a further 470 species of batioids (skates and rays, including
manta rays). They range in size from the pygmy lantern shark (Etmopterus perryi), which is 15 centimetres (6 in.) long, to the whale shark (Rhinchodon typus), which can grow to lengths of 12 metres (39 ft) and is the world's largest fish.
Sharks have gill slits rather than covered gills as in bony fishes. Instead of having swim-bladders they keep their buoyancy close to that of sea water by having oily livers, blood rich in urea, and fins that act as hydrofoils. The larger species, such as the whale shark, the basking shark (Cetorhinus maximus), and the megamouth (Megachasma pelagias), are
plankton feeders, but the other species feed on fish,
marine mammals, or even young
seabirds leaving a nesting colony for the first time. They have poorly developed eyesight, but have an acute sense of smell, being able to detect a drop of blood in an area the size of a full-size swimming pool. They can also detect vibrations over long distances, by means of a lateral system that runs down each side of their bodies and onto their heads. Their skin comprises toothlike structures, known as denticles, which are shed as the shark grows, and are replaced by larger ones. They have triangular-shaped teeth that are frequently shed and replaced; a shark typically replaces many thousands of teeth during its lifetime.
The most curiously shaped is the hammerhead shark (Sphyrna lewini), which has a wing-shaped head (cephalafoil) with eyes at its extremities. This shape helps the hammerhead manoeuvre, but it also acts as a large sensor which can detect the electromagnetic fields of its prey, such as
herring,
squid, and other sharks. The giant hammerhead can grow to 6 metres (20 ft) in length.
The great white shark (Carcharodon carcharias) tends to frequent waters close to seal colonies or fish spawning grounds. This species, and others that are most dangerous to man, forage in shallow coastal waters where the waters are murky. Because they hunt using their senses of smell and vibration to detect their prey they sometimes mistakenly attack humans that are swimming or wading. Worldwide, in 2002 there were 86 incidents attributed to shark attacks of which only 60 were unprovoked and only three resulted in the death of the victim. So the dangers of shark attack are often exaggerated.
Sharks have internal fertilization, and male are easily distinguished from females by their pelvic fins being modified into claspers with which they impregnate the females. Female sharks either lay a small number of eggs in mermaid's purses or the development of the eggs is internal and the young are born live. There is also evidence that the young sometimes feed cannibalistically on their siblings within the female's body. Their low reproduction rate makes them very susceptible to over-exploitation, and many sharks are killed as by-catch in
trawls or on
long-lines, and some species are now under such threat that they are protected in some parts of the world. Many taken on long-lines are finned—their fins are cut off and dried for shark-fin soup and the rest of the carcass is discarded. Such removal of top predators from any food web totally alters its structure and dynamics, and reduces its diversity.
Bibliography
Allen, T. , The Shark Almanac: A Fully Illustrated Natural History of Sharks, Skates and Rays (1999).
Carwardine, M., and and Watteron, K. , The Shark-Watchers Handbook: A Guide to Sharks and Where to See them (2002).
M. V. Angel
