It was the mysterious creature from the deep. It had no head, brain, heart, eyes, nor ears. It had no bones, but possessed poisonous tentacles. And it was carnivorous.
It appeared one day in huge numbers. No one quite knew why. A globulous mass that glowed in the dark, trailing filament feelers each covered in thousands of tiny stinging cells which exploded, launching barbs laden with venom into its victims.
Tens of thousands of people were attacked as they were paddling on a seaside holiday. Many needed emergency treatment after they came into contact with the creatures' tentacles. Some died. They had been injected with a poison which was among the most deadly in the animal kingdom.
The authorities went into overdrive. Nets were strung off the coast. Danger flags were hoisted, closing beaches. Boats went out to shift the primeval protoplasm but, even as it was scooped up, the tentacles broke off and floated away in the water - their venom undiminished for at least 24 hours after the death of the creatures.
There was something apocalyptic about it. For it seemed that the creatures had been summoned, unwittingly, by human beings who knew not what they were doing.
It sounds like the plot of a disaster movie. But there is currently an explosion of jellyfish worldwide. Holiday destinations in Spain, Italy, France, Croatia and northern Africa are plagued with a creature called Pelagia noctiluca, which glows a dull yellow and packs a painful sting, hence its common name, the mauve stinger. It is burgeoning throughout the Mediterranean and the Adriatic.
A fleet of boats is working off the Costa del Sol to remove the jellyfish. Beaches where the water has up to 10 jellyfish per square metre have been closed. Alicante has a new system of jellyfish warning flags. At least 30,000 people have been stung, with someone treated for a bite every 10 minutes in Mijas.
In south-west Norway extremely large stinging jellyfish have been reported. In the United States jellyfish of all kinds are swarming everywhere from Hawaii to Cape Cod. In Korea, 3ft-wide specimens of the poisonous jellyfish Nemopilema nomurai are being beached. And last month a mass of jellyfish forced a Japanese nuclear power plant partially to shut down after the slimy animals blocked its seawater cooling system. In China, a 44-year-old woman and a nine-year-old girl died after being stung, probably by the deadly box jellyfish, off the Golden Sand Beach in Shenyang.
Even in Britain there are problems. For three months now there have been blooms of moon and barrel jellyfishes off the Welsh coast. Swarms of the compass and lion's mane jellyfish have been reported in the Irish Sea.
An increase in jellyfish is natural at this time of year as maritime waters warm and the adults breed. But there are fears that we may be seeing something irreversible, thanks to global warming. Normally the bulk of jellyfish medusas remained 40 miles off the coast where sea temperatures are higher than by the beach. But drought has depleted rivers and greatly reduced the volume of cooler river-water flowing into the sea. That means coastal waters are now considerably warmer and less salty than in the past.
Marine biologists say the temperature of the northern Mediterranean has increased by 4C this summer. It means that plankton, the main food of the jellyfish, is blooming. And at the same time the creatures' natural predators such as swordfish, tuna and turtles are in decline. A delicate eco-system is out of kilter.
Jellyfish are one of earth's more primitive life forms. Biologically they are not "fish", but are related to corals. But where the latter are polyps that grow attached to rocks, jellyfish are free-floating animals. As well as eating plankton they catch fish with their tentacles. Big jellyfish are capable of devouring large crustaceans.
They possess an elementary nervous system of receptors sensitive to light, odour, gravity and chemicals in the water. They have no respiratory system but diffuse oxygen through their skins. Some appear to have "eyes" which, since they have no brains, may give scientists clues about how vision works. Its mouth is also its anus.
They come in a wide variety of sizes - from less than an inch to 7ft across - and a range of colours though most are semi-transparent. Their bodies are at least 94 per cent water and are found in every pelagic area of the world.
Most are passive drifters dependent on currents to feed on whatever becomes caught in their tentacles, though they can propel themselves to some extent by rhythmic pulsations of the body. Their form is enormously delicate which is why their poison is so deadly - the need to kill their prey pretty instantly to prevent a thrashing victim from damaging its killer.
It delivers its venom through capsules called nematocysts of which a single tentacle can have hundreds or thousands. At even the slightest touch, these shoot out small, barbed harpoons, injecting paralyzing toxins into their prey. Judged by how many people an ounce of the venom can kill, and how long it takes the victim to succumb, some jellyfish - such as the box jellyfish - emit as deadly a poison as nature produces.
Most jellyfish live in shallow coastal waters, but they can survive to depths of 12,000ft. Australian researchers have used ultrasonic tagging to learn that the creatures sleep on the ocean floor between 3pm and dawn to conserve energy and to avoid predators. Not many things eat them anyway. Swordfish and tuna do and the ocean sunfish eats only jellyfish. Leatherback turtles eat them, migrating thousands of miles from their tropical nesting beaches to UK waters each summer to feed on them - which is why the Marine Conservation Society is appealing to the public to take part in a jellyfish survey to assist with the survival of the endangered leatherback (the turtles often die after mistaking plastic bags for their main food source).
The only other predator is man. The Chinese eat them with sesame oil and spring onions. In Vietnam they are eaten with red chilli peppers; in Korea with mustard; and in Thailand as a noodle. Western foodies mix them with cucumber and garlic, though gastro websites include squeamish Americans who insist they taste like "snot". There are also debates about whether vegetarians can eat them since they have no brain - which apparently makes them OK under Buddhist vegetarianism but not other kinds.
One thing is clear, fishermen around the world now haul in 450,000 tons of jellyfish a year, more than twice as much as a decade ago. That fact could have significance beyond demonstrating the foodie need for constant novelty. Breeding jellyfish may be taking over the ocean, and not just because they can thrive in less oxygen-rich water than fish. Research published last month at the University of St Andrews looked at the area off the coast of Namibia which was, thanks to the Benguela current, once one of the most prolific fishing areas in the world but which was plundered by foreign fishing fleets until the 1990s. What they found was that jellyfish have overtaken fish in terms of the biomass they contribute to the ocean.
Over-fishing has transformed what was once one of the world's most prolific ecosystems, apparently triggering a jellyfish explosion in what researchers fear may be a permanent "regime shift" in which they outnumber real fish by a ratio of four to one. Once jellyfish become established, says the lead researcher at St Andrews, Andrew Brierley, "it may be very difficult to revert to fish domination because jellyfish are predatory on fish eggs and juveniles".
Jeremy Jackson, a marine ecologist at the Scripps Institution of Oceanography, in the United States, said: "We're pushing the oceans back to the dawn of evolution, a half-billion years ago when the oceans were ruled by jellyfish and bacteria."
Perhaps that doom-laden disaster movie is not so far-fetched after all.
What to do if you get stung
* BRITISH JELLYFISH
An ice-pack applied for five to 15 minutes will stop the pain in 98 per cent of cases. Antihistamine by mouth may help. Do NOT use vinegar (see below) which could make the sting worse. Cortisone-based creams may suppress anti-inflammatory response. Small children should go to hospital if badly stung.
* BOX JELLYFISH
Vinegar is essential. Not a painkiller but it stops the poison spreading.Reuse content