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Many sharks look more lethal than they are

Posted in Fish, Nature, Sea, Wildlife on Tuesday, 27 November 2012

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This edited article about selachians originally appeared in Look and Learn issue number 788 published on 19th February 1977.

Blue shark, picture, image, illustration

Blue shark and dolphin

No creature of the sea has a more terrifying reputation than the shark. Many a story, and more recently the highly successful film Jaws, have helped to make the word shark a frightening one.

Yet there are several species of shark, including some of the biggest, which offer no threat to human beings.

Sharks are normally classed as fish, but they belong to a group having features which clearly distinguish its members from the majority of fish. Most types of fish, like mammals, have skeletons of bone. But the selachians, the group to which sharks, rays and skates belong, are “cartilaginous”. This means that their skeletons are composed of a kind of gristle.

In addition, their gills, by means of which they extract oxygen from the water, lack the gill-covers present on other fish. Another feature is that the selachians have to keep in constant motion to keep afloat, as they lack the “swim-bladder” which gives buoyancy to the other types.

A fourth difference is to be seen in the shark’s scales. These are constructed like minute teeth, and instead of overlapping they are joined by leathery skin.

Finally, all the sharks breed their young in a manner rarely found among bony-skeletoned fish. Normally, the female fish lays eggs, which are afterwards fertilised by the male, and in due course hatch out to produce the young.

The eggs of selachians – and of a few other species, such as the tropical guppy – are fertilised inside the female. In some cases the fertilised eggs are laid – as with birds or reptiles. The Greenland shark and the skates are among the egg-laying selachians.

Most sharks, however, are “viviparous” – that is, their young leave the egg inside the female, and are born alive. Some big sharks produce as many as 20 young at one birth.

We are inclined to think of sharks as denizens of tropical waters. In fact they are found in every ocean, often far from equatorial regions. The great white shark, the 40-foot monster with the worst record of all for attacks on human beings, prefers the warm waters, but it has been sighted as far north as Newfoundland in the Atlantic, and in eastern waters it has been seen off Japan.

The blue shark ranges even farther, from Norway to Australia, and sometimes appears in the Mediterranean. However, it is unlikely that any individual specimens wander as far as this – it is the species as a whole that has this extensive habitat.

Yet it is known that sharks will travel long distances. Blue sharks follow ships for many miles, their constant presence betrayed by the dorsal, or back, fins which cleave the surface. They are not, as some believe, patiently waiting to devour some person unlucky enough to fall overboard. What they are after is the food refuse which, as they know by instinct, will sooner or later be discharged from the ship.

Some sharks migrate from one area to another because the fish or other creatures on which they feed are present in different areas at different seasons. An example of this is the basking shark, which moves into the northern Atlantic, following its food supply, in spring and summer. This selachian, as large as the great white shark, has sometimes caused panic among bathers along Britain’s shores. In fact it is harmless.

The basking shark swims slowly round, its huge mouth gaping wide, gulping down the tiny plankton which is its only diet. At other times it “basks” on the surface, generally with its back uppermost, but sometimes turning over to expose its white under-belly. It is believed to return to deep waters for breeding.

Other sharks seen in British waters are the porbeagle, the mako and the thresher. The latter uses its long, crescent-shaped tail to drive shoals of fish into shallow water, where it is more difficult for them to escape its jaws.

Most sharks are true carnivores, and are equipped with many formidable teeth. They are only loosely rooted in the gums, so that as they are worn down they drop out, to be replaced by new teeth. The teeth of the great white shark are saw-edged, and two or three inches in length. Though it is not without reason that it is called the “man-eater” shark, it feeds mostly on fish.

In almost every shark species, the mouth is located on the underside of the head. This means that, in order to attack a prey above them, they have to turn over on their backs.

The strength of sharks’ teeth is shown by the fact that they have been found embedded in a submarine cable. This was at a depth of 4,500 feet (1,370 m) – an indication that sharks can withstand the water pressure at great depths.

A curiosity among sharks is the hammerhead. The front part of its head is extended on each side to resemble the head of a hammer, with the eyes set in the extremities.

The Greenland shark, some 20 feet long, is also called the “sleeper”, because of its slow and lazy movements. Though it preys on fish and young seals, it also seeks out carrion, from dead whales to the fish refuse discharged from the Alaskan salmon canneries.

The other main branch of the selachians, the rays, also includes impressive monsters – some rather dangerous to Man, others only frightening to look at. They are wide and flat in shape, the fins merging with the body. In movement, the action of the side extensions of the body resembles that of wings.

The electric ray, or torpedo, has a roughly disc-shaped body. It exists on the sea-bed, moving only sluggishly. In its front, or pectoral, fins it carries cells which generate an electric current. This is probably used both to deter attackers and to paralyse its prey. Varieties of torpedo are found in the Mediterranean, and on both coasts of the USA. They can give a painful shock to anyone coming in contact with them.

More dangerous are various sting-rays. Near the base of their long, thong-like tails, they carry sharp stings, which can inject poison into anyone who disturbs them, causing unpleasant wounds.

Most terrifying of all is the devil-fish, or manta ray. Twenty feet (6 m) wide, it is a sinister sight as it lazily flaps its way through the water, close to the surface. Its’ appearance is not improved by two projections, or “horns”, on its head. In colour it is blue-black above, and a ghastly white on the underside.

Yet the manta is in no way aggressive. It is content to graze peacefully on plankton. When it has caused casualties, they have been accidental – as when it has fouled the air-pipe of a diver.

None of the bony fish rivals in size the largest of the selachians. The world’s largest fish is the so-called “whale shark”, one specimen of which was found to measure nearly 60 feet (18 m). But the biggest of all sea creatures – and indeed of all animals on earth – are to be found among the whales.

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