The seemingly simple jellyfish

By Nutan Shukla

JELLYFISH is a fascinating creature of the sea.It looks like a beautifully coloured, semi-transparent, blob which can be seen floating aimlessly on the surface of water. However, all the jellyfish have 95 per cent water in their body and they seem harmless because they never attack anybody. Some species are, however, quite poisonous and their stinging cells can even kill human beings.

These pulsating, inverted saucer like creatures have bushes of tentacles around the rim. These tentacles are capable of stretching downward for 50 feet or more. If they come in contact with the skin, they produce a fiery pain and raise great weals wherever they have touched, for in their tissues they carry great numbers of tiny structures known as nematocysts or stinging cells.

There are many different kinds of nematocysts to be found among jellyfish. These perform different functions, therefore it is not easy to describe these cells. Among these one kind of cells are the ones that sting, but all of them appear to work on the same principle. As far as a description of these cells is concerned, it is often found in the books. If one takes a rubber glove, pushes in the fingers so that they are inside-out, and then blows sharply into the glove, the fingers will pop out again. This is the sort of explanation we often get about nematocysts. But a nematocyst is not like a glove: it has only one very long, thread-like finger in a flask-like capsule with no opening through which air can be blown.

Under an appropriate stimulus (touching some living body), a fluid containing a poisonous substance is forced through the wall of the capsule. It rapidly builds up pressure and fills the coiled up, inverted, hollow thread so that it is thrust out violently. It has a dart at its tip so that it can pierce the skin and inject the venom. The thread usually has little spines arranged around it in a spiral, enabling it to attach itself or to wind itself around the prey and draw it up to the mouth. Although frightening, it is an intriguing mechanism. What is surprising is the complexity of this lowly, simple creature.

Jellyfish are, of course, not fish. They are properly called medusae and they belong to a rather lowly group of animals usually called coelenterates, meaning literally "hollow inside", but used to indicate that the body cavity is also the digestive organ. Roughly speaking, a coelenterate is just a stomach surmounted by a mouth surrounded by tentacles.

A nematocyst or stinging cell is tiny and scarcely visible without a microscope. It does not operate alone. The lightest touch against a tentacle will trigger off a battery of hundreds or may be thousands of these venomous darts, as though an area is being seared with a hot iron.

More widely recognised as a menace is the so-called Portuguese Man-of-War, a relative of jellyfish, of the tropical Atlantic and the Mediterranean. Despite the float in the body (which appears more like a flask) this fish is not always seen by the swimmer. By adjusting the amount of gas, it can sink below the surface. Even if it is seen, it may already be too close. To brush against the tentacles is to be stung over a considerable area of skin by millions of venomous threads. According to one expert, 55,000,000 nematocysts weigh only one gm and over 62 gm of them are to be found in one gallon of tentacle tissue; how many gallons there are of the jelly-like mass removed from the water depends on the size of the creature.

Nematocysts are not under any nervous or muscular control: each one fires off its dart under a direct stimulus from outside. This stimulus is not tactile but chemical, for if a relatively inert substance like glass is brushed against a tentacle the nematocysts do not respond. A food substance — generally protein or fatty matter — is what turns them on. It seems there may be a sort of control through the cnidoblast, that is, the mother cell. When the animal is fully fed, the stinging apparatus does not work so well, it seems that if food substance is plentiful inside the tissues, the flow of fluid is reduced and this prevents the nematocysts from functioning. Without this inhibition, the presence of food on the outside causes a rapid intake of fluid that builds up considerable pressure.

The thread, coiled inside-out in the capsule, is often pleated to reduce its volume and the barbs are crowded inside with their tips towards the centre. The capsular fluid now fills the thread, thrusting it rapidly outward; the pleats unfold so that the length of the thread is greatly increased as it exerts and twists, spacing the barbs out in the characteristic spiral whorl round the outside; the long, hollow tube springs out straight, strengthened by the twist, and drives the dart at its tip into the source of food.

Because nematocysts often behave as though they are independent creatures lodged in the tissues of the jellyfish, it is imprudent to handle a speciman that appears to be dead (unless it is known to be a relatively harmless kind) for the nematocysts may still be capable of stinging. Some kinds of molluscs feed on these animals and can actually pass the nematocysts into special spaces in their skin, where they function on behalf of their predator.

Sometimes after a storm when many jellyfish have been broken against the stones, fragments of tentacles may litter the beaches and it can be dangerous to walk with bare feet. This is particularly true in tropical regions.

This fortnightly feature was published on December 6, 1998