"Jellyfish are an excellent bellwether for the environment," explains Jacqueline Goy, of the Oceanographic Institute of Paris. "The more jellyfish, the stronger the signal that something has changed."

Brainless creatures composed almost entirely of water, the primitive animals have quietly filled a vacuum created by the voracious human appetite for fish.

Dislodging them will be difficult, marine biologists say.

"Jellyfish have come to occupy the place of many other species," notes Ricardo Aguilar, research director for Oceana, a international conservation organisation.

Nowhere is the sting of these poorly understood invertebrates felt more sharply than the Mediterranean basin, where their exploding numbers have devastated native marine species and threaten seaside tourism.

And while much about the lampshade-like creatures remains unknown, scientists are in agreement: Pelagia noctiluca -- whose tentacles can paralyse prey and cause burning rashes in humans -- will once again besiege Mediterranean coastal waters this summer.

That, in itself, is not unusual. It is the frequency and persistence of these appearances that worry scientists.

Two centuries worth of data shows that jellyfish populations naturally swell every 12 years, remain stable four or six years, and then subside.

2008, however, will be the eighth consecutive year that medusae, as they are also known, will be present in massive numbers.

The over-exploitation of ocean resources by man has helped create a near-perfect environment in which these most primitive of ocean creatures can multiply unchecked, scientists say.

"When vertebrates, such as fish, disappear, then invertebrates -- especially jellyfish -- appear," says Aguilar.

The collapse of fish populations boost this process in two important ways, he added. When predators such as tuna, sharks, and turtles vanish, not only do fewer jellyfish get eaten, they have less competition for food.

Jellyfish feed on small fish and zooplankton that get caught up in their dangling tentacles.

"Jellyfish both compete with fish for plankton food, and predate directly on fish," explains Andrew Brierley from the University of St Andrews in Scotland. "It is hard, therefore, to see a way back for fish once jellyfish have become established, even if commercial fishing is reduced."

Which is why Brierley and other experts were not surprised to find a huge surge in the number of jellyfish off the coast of Namibia in the Atlantic, one of the most intensely fished oceans in the world.

Climate change has also been a boon to these domed gelatinous creatures in so far as warmer waters prolong their reproductive cycles.

But just how many millions, or billions, of jellyfish roam the seas is nearly impossible to know, said scientists.

For one things, the boneless, translucent animals -- even big ones grouped in large swarms -- are hard to spot in satellite images or sonar soundings, unlike schools of fish.

They are also resist study in captivity, which means a relative paucity of academic studies.

"There are only 20 percent of species of jellyfish for which we know the life cycle," said Goy.

And the fact that jellyfish are not commercially exploited, with the exception of a few species eaten by gastronomes in East Asia, has also added to this benign neglect.

But the measurable impact of these stinging beasts on beach-based tourism along the Mediterranean has begun to spur greater interest in these peculiar creatures whose growing presence points to dangerous changes not just in the world's oceans, but on the ground and in the air as well.

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