Notorious seaweed

The waters along France's Cote d'Azur conceal a secret. As another hot summer passes, the Big Blue is becoming tinged with a livid green.

It is caulerpa taxifolia - the so-called killer algae. This notorious seaweed is a tropical invader, an alien species which is fast carpeting the Mediterranean in a dense marine shag pile, smothering the native flora.

The BBC's Joanne Gilhooly: "Killer algae is covering the Mediterranean"

And that means less food for fish and a drastic change to the natural ecosystem.

At least that is the theory of marine biologist Professor Meinesz.

"In 1984, one square metre; in 1990, three hectares; in 1991, 30 hectares; 1992, 400 hectares; and now we are more than 5,000-6,000 hectares more or less covered," he says.

"Every year more and more, and every year new countries, new regions are colonised, so this is only the beginning of the story."

The Cote d'Azur is reportedly being invaded

And as Professor Meinesz explains in a forthcoming book, this is no ordinary algae. Caulerpa taxifolia can grow two centimetres a day, and up to three metres in length, yet it's made up of just one continuous cell.

Like other algae, it can photosynthesise but also has roots that can draw nutrients from the substrate.

If the water is warm enough, it grows just about anywhere - on rocks, sand and mud, and even 30m down. A broken off stem can rapidly grow back into dense vegetation.

It likes to cluster around sewage outlets, having a penchant for anything putrefying. And its expansion has created a scientific stink of oceanic proportions, pitching marine biologist against marine biologist and leaving the French Environment Ministry facing charges of inaction.

'Exaggeration'

It has all been exaggerated according to Professor Jean Jaubert, who is using remote sensing of the seabed to check out the algae's purported invasion along a stretch of the Cote d'Azur.

He says there has been no expansion in the last two years and claims Professor Meinesz is wrong to suggest that there is an ecological disaster.

This slug could provide the solution

But if Professor Jaubert is wrong, what can be done? Pulling the weed up by hand just spreads it further. Covering it with plastic sheeting kills it off, but now it is far too late for that.

Professor Meinesz is not alone in being convinced that the situation is urgent. But he thinks he's found a solution - a slug. Asco Glossum, he says, may be able to eat its way through the problem.