Among sci-fi's most tired conventions is the one in which some latter-day cataclysm releases from an aeons-long sleep a monstrous prehistoric creature who rampages around for eight or nine reels until the combined brains of the military-scientific-industrial complex figure out a novel ploy to dispatch the thing.
Such strained fictions have always seemed a lot of fuss to ichthyologists. Why bother to wake the creatures of unimaginably distant geologic ages when you can find, in a condition essentially unchanged for 63 million years, a creature cruising handily off every beach in the world who once shared the planet with dinosaurs, ichthyosaurs and pterosaurs and is as strange, unpredictable and dangerous as those bad old boys?
That creature is, of course, the shark. Doubtless one of the reasons it has long exercised such a powerful hold on the imagination of everyone (except, until recently, novelists and movie makers) is that it attacks not merely out of the depths of the ocean but out of the depth of prehistory as well. The other great source of its near mythic fascination is that despite ever-growing attention by marine scientists, there is precious little reliable information about sharks. It is not even known how many varieties of sharks there are (best estimate: around 300) or how many of them must be regarded as definitely lethal to man (best guess: about a dozen, with the great white and the tiger leading most lists). It is almost impossible to make wide-ranging behavioral generalizations from the way the creatures act in captivity and even more difficult to study them in their natural habitat. Science therefore knows more about the natural history and physiology of sharks, which can be gathered from autopsies, than about how they actually live in the wild.
Ranging in size from six inches to 60 feet, all shark species lack skeletons. Essentially they are masses of cartilage covered by a remarkably tough hide (in itself a nasty rasplike weapon in the larger species). Theirs is an indiscriminating appetite. Everything from a keg of nails to a 100-lb. sea lion has been found in shark entrails. The biblical Jonah was there too, today's marine scientists theorize. Sharks are condemned by nature to a life without sleep or even rest. The reason is that they lack the swim bladders of the bony fishes, which permit the latter to float when they need to. A shark must literally swim or sink. If you wanted to anthropomorphize the beast, you could account for its wretched disposition by that fact alone.