‘White death’ at sea
By Nutan Shukla

IN the sea, the teeth of sharks have contributed much in sustaining their reputation as nature’s ultimate killers, but not all sharks have the same sort of teeth. There are large and small teeth, sharp and blunt, saw-like and knife-like, grasping and holding teeth and grinding and cutting teeth; but above all, shark’s teeth are endlessly replaceable. They are on a kind of conveyorbelt, with the newly developing teeth at the back of the completed teeth at the front. As the teeth are lost — at a rate of a row in 8 days for the lemon shark — the developing teeth move forward to replace them. In this way a shark is always ready for action. Like many other predators, though, it can not chew, and in order to take a chunk out of a large victim it must shake its head vigorously until the teeth have sliced through flesh and bones.

Mako and sand tiger sharks have long, pointed, awl-like teeth for grasping fish and squid. Great whites have serrated, triangular-shaped, saw-like teeth for hacking through mammalian flesh and blubber. Tiger sharks have characteristic L-shaped, serrated teeth with which they cut through turtle shells. Dogfish have blunt teeth for crushing mollusc shells, and horned sharks have two types of teeth —small, sharp ones at the front of the mouth for catching small fish and flat, tile-like crushing teeth at the back of the mouth do deal with shellfish. The tiny 17-inch-long, large-toothed, cookie-cutter shark, which takes circular chunks of flesh from the bodies of squid, has the largest teeth for its body size of any known shark. The relatively enormous, triangular teeth are 25 per cent of the head length.

A characteristic feature of most sharks is that their body is covered with tooth-like structures, or denticles, which give the shark skin its rasping qualities. In the mouth these denticles enlarge to form the teeth. It is this factor which contributes to the fearsome appearance of many sharks.

White pointer sharks are bluish grey above and white below. Their teeth are good identifying feature. These are large and triangular with coarse serrations along the edges. There may be as many as 200 in one set of jaws.

Some zoologists compare these animals to aeroplanes. They use energy to keep from sinking just as an aeroplane must use energy to keep in the air. Bony fish use swim bladders for the same purpose.

Another feature of the shark shape is that the upper tail fin lobe is longer than the lower. The backbone does not stop at the beginning of the tail fin as in bony fish but continues into the upper lobe. This lobe not only serves to drive the shark forward but acts as an "elevator" to lift the rear end of the body. In bony fish there is a single external opening at the place of gills whereas sharks have five to seven visible gill slits on the head.

Sharks are cartilaginous fish. Their skeleton is made of cartilage (gristle) instead of bone. There are 250 species of them and they range in size from about 70 cm to 18 metres. Most of them are harmless to humans and many are edible. Bony fish are those whose skeleton is made of bone and this group includes about 20,000 species found in all oceans, seas and fresh water habitat.

The white pointer shark is one of the giants of the seas, roaming the open oceans. Preferring the temperate and tropical seas this creature follows ships and swallows anything thrown overboard or falling in by accident. A South Australian Government handbook lists the following items as having been found in the stomachs of captured white pointers or the great whites; a mako shark one metre long, a large Newfoundland dog, an 18 kg fish and an entire human corpse.

White pointers are so fearsome that they have earned names such as ‘white death’. Largest of them can reach a length of up to 7.5 metres, with a weight of up to 3,500 kg. Even these giants are dwarfed by fossil specimens of the same genus, which possibly measured 12 metres in length and may have weighed about 125 tonnes.

If we look at all the species of sharks then even the great white will be dwarfed by another species, the whale shark which is also the world’s largest fish. Adults of this massive fish can grow up to a length of 18 metres.

The teeth in whale sharks are small and re-curved, carried in two bands right across the jaws, which are up to 1.5 metres across. The gape is big enough for a man to crouch inside. Like the baleen whales, whale sharks feed on small forms of planktonic life, including krill.