Sunday, September 2, 2001		Nature

JELLYFISHES are unique marine creatures. With about 95 per cent of the body being water these innocent looking, free-floating animals are deadly. They are often seen floating in thousands in the open sea and swimming through them is like advancing through a minefield. To find food they do not pursue the prey. Instead, they wait for it to blunder into the tentacles that hang below the bell or their disc-shaped body.

With about 200 species, having transparent bodies, they can grow to immense sizes. The Arctic jellyfish is found in shallow bays in the North Atlantic, and one specimen was found in Massachusetts Bay in 1865 that was about 8 feet in diametre and had tentacles hanging 120 feet below the umbrella-shaped body, which could cover an area of 500 sq yards.

Jellyfishes are invertebrates with soft body and no bones. Found in all the world’s oceans, their bodies are made up of two layers of cells, with a jelly-like substance between the layers. They feed on plankton and small animals which they catch in their tentacles. They reproduce asexually by budding (a form of reproduction in which a new individual develops within the body wall or cell membrane of the parent, causing a bud-like swelling, then detaches itself to commence an independent life).

Another species of jellyfish, the lion’s mane jellyfish, reaches one metre across and has yellow tentacles up to 50 ft long. Occasionally, half-a-mile swarms are seen drifting towards the Norwegian coast, and the species prompted Sir Arthur Conan Doyle to employ its sting for criminal ends in the Sherlock Holmes’ mystery Adventures with the Lion’s Mane.

In reality, lion’s mane jellyfish are not that dangerous to man, but others are. Huge plague of a red jellyfish sometimes make life a misery for holiday makers in the Mediterranean. Shoals of 100,000 at a time have been seen drifting off holiday beaches along the Greek Coast, and in 1987 Egyptian beaches were jammed with slicks of jellyfish 1 km long, a phenomenon which has been blamed both on pollution and the reduction in numbers of sea turtles, but in reality has been going on for centuries. Jellyfish blooms were recorded over 100 years ago.

The business end of the jellyfish is its tentacles. The bell itself is more concerned with getting the jellyfish to where it wants to go and digesting any food its tentacles might catch. In the main a collision with prey is an accident, but some species arrange themselves so that an accident is more likely. One species swimming off the US Pacific coast drives itself up to the surface with contractions of its saucer-shaped bell until it reaches the surface. Then it turns upside down and drifts slowly down, trailing its 60 or so tentacles behind it. It is thought that by manoeuvring in this way, the jellyfish creates vortices in the water that sweep small organisms into the tentacles. Another species in the same waters swims a zig-zag course so that its tentacles sweep an area greater than the width of the bell and increases the chances of snarling prey.

Some jellyfish follow the daily vertical migration of plankton and small fish. At night they rise to the surface and by day they sink to the depths. But jellyfish cannot adjust their buoyancy and so they must actively swim all the way. One Mediterranean species, just 1½ inches across, swims a daily vertical round trip of 3,600 ft, the equivalent of a man swimming 33 miles a day every day.

Related to the jellyfish, comb jellies and sea gooseberries are technically known as ctenophores. They are a half-way stage between the round-shaped jellyfish and sea anemones and the bilaterally symmetrical flatworms. They have transparent bodies and float about with the zooplankton, their tentacles trailing like fishing lines.

Comb jellies are tiny, 4-cm-long, transparent barrels which propel themselves through the water with the help of rows of moving hairs (cilia). They trail 50-cm-long adhesive tentacles with lateral filaments containing adhesive cells that can stick to prey that might blunder into them. Pipefish, which are a similar size to the comb jelly, have been seen to be ‘played’ until exhausted.

Sea gooseberries live near the surface and tend to bump into prey. A sense organ near the mouth detects when a contact is made and the body immediately contorts. The body shortens, the gut widens, and the prey is swept into the mouth by the sudden inrush of water. The mouth is then closed and the unfortunate victim is trapped.

Some comb jellies have ‘lasso’ cells. These burst on contact, releasing a ‘lasso’ to capture small prey organisms such as rotifers.