Health: Beware the underwater Terminator: Even in seas close to home a jellyfish sting may be nasty, but some exotic species can kill. A bottle of vinegar could save you, says Ed Needham

WHILE SWIMMING in clear Mediterranean seas on a beautiful morning, a bather suddenly thought she was being attacked by something that felt like a cheese wire. A swimmer in India said it was like having burning chip fat poured over him. A third holiday-maker, swimming off Hawaii, could not understand why, in the middle of the Pacific, he appeared to be getting powerful electric shocks.

This is not the plot of a submarine slasher movie, but three experiences of a particularly anti-social form of marine life: the jellyfish. With the holiday season about to begin, many more people will be finding out for themselves how unfriendly the jellyfish can be. Others will get a nastier surprise when they reach their tropical paradise to find the sea completely out of bounds.

Although the 250 species of jellyfish are scattered throughout the world, brochures for holiday playgrounds such as Miami, Acapulco, Hawaii and Goa fail to mention that the more venomous varieties gather to sting for their supper in tropical waters beloved of sunseekers. Closer to home, Mediterranean jellyfish can also provide unpleasant and unwanted souvenirs.

The sting of the jellyfish is made up of millions of poisoned darts, called nematocysts. As their tentacles trail through the water, contact with a foreign body triggers the tiny barbs, injecting the prey with a paralysing toxin. Many are strong enough to penetrate human flesh, and leave painful and distinctive wounds. Even contact with floating, broken-off tentacles can cause an unpleasant rash.

Some holiday-makers return looking as though they have had cigarettes stubbed out on them, while others appear to have been whipped. Fortunately, these lesions eventually fade.

Some jellyfish, though, can kill, presenting a problem for some of the most exotic holiday locations. The Australian Tourist Commission, for example, advises tourists to avoid swimming from coastal beaches in its danger zones (shallow waters north of the Tropic of Capricorn) between November and March.

The reason is simple: Chironex fleckeri, known as the sea wasp or box jelly, is an underwater Terminator. Found in waters from Australia to Japan, the box jelly can kill within minutes. Research in the Sixties showed that box jellies had claimed four times as many victims as sharks in Australia over the century. And you don't have to swim to become a target. One man died at Townsville, Queensland, in 1953 while setting a fish trap in knee-deep water, just 10 yards from the shore. The city now broadcasts television warnings in the style of Jaws.

Although the box jelly rarely measures more than 25cm across, it trails tentacles up to six metres behind it, and carries as many as 40 million nematocysts. A massive sting is said to cause the most excruciating pain known to man. Such a sting would produce intense muscular pain and violent spasms, followed by a rapid weak pulse, respiratory failure and, within a few minutes, death.

The Chironex venom was found to be so strong in tests that even diluted 10,000 times it was strong enough to kill small mammals before the syringe injecting the solution was removed. For anyone unfortunate enough to meet a box jelly, the sting area should be flooded with vinegar to release tentacle fragments from the skin. Attempts to move slime or fragments by hand will result in more extensive stinging. Australian beaches in the danger zone are usually equipped with bottles of vinegar at regular intervals for this purpose. Mouth-to-mouth and cardiac resuscitation may also be required.

According to the Medical Advisory Service for Travellers Abroad, the effects of box jelly poison are transient, so rapid action gives a victim a good chance of survival. The burns from even a minor sting, though, can take weeks to heal, and the victim is often scarred for life.

Australian lifeguards can be seen emerging from the water looking like futuristic dispatch riders, wearing colourful body suits and helmets to protect themselves from this jellyfish. A previous form of protection was to cover as much of the body as possible with nylon tights, which prevent the tentacles from gripping.

Most British jellies are too feeble to cause trouble, although a close relation, the Portuguese man-of-war, can still pack a powerful punch. The man-of-war looks like a blue or violet-coloured float, up to a foot long, which dangles tentacles beneath it to trap food.

Contact with humans can produce a burning pain and a large weal that eventually fades, although small children and adults with weak hearts are particularly sensitive to the venom. Other side-effects are high fever and vomiting, and severe shock may in extreme cases produce drowning.

Remarkably, there appear to be few remedies for jellyfish stings and, as the vast majority are extremely painful yet far from life- threatening, advice comes mainly in the grin-and-bear-it mould. The Queensland Health Department recommends vinegar to inactivate stinging cells, followed by iced water to relieve pain. A general spray for stings from Australia called Stingose is also recommended for pain relief. Folk remedies include flooding the sting with petrol or urine instead of vinegar, although doctors are unable to confirm their effectiveness.

The best form of protection is to avoid the creatures completely, a tricky task when tides, winds and currents drive them in towards shore in vast shoals. Swimmers should be especially attentive after storms or strong winds blowing into the coast, as previously empty waters can fill with swarms of drifting jellies overnight.

As conditions vary widely according to country and season, the best guide is to ask a local before jumping in. All the same, bathers can take comfort in the fact that they are unlikely ever to meet the world's biggest jellyfish, the lion's mane and the Arctic lion's mane. These monsters can reach a diameter of up to two metres, and their tentacles cover areas of up to 500 square metres. To catch its prey the lion's mane slowly sinks with its tentacles spread like a great poisonous net. The fate of a trapped swimmer hardly bears thinking about.

Fortunately, the lion's mane prefers the deeper waters of the world's oceans. Anyone reckless enough to dive in with this Arctic horror should be more concerned about hypothermia than a vicious sting.