This strain has adapted to cold water, to the Mediterranean, and now California and it could spread elsewhere

Dr Jan Pawlowski

Growing at a rate of up to eight centimetres (3.5 inches) per day, the alga spreads like wildfire, depriving other plants and animals of vital sunlight and oxygen.

Marine experts are concerned that the seaweed could colonise other parts of the ocean, with devastating consequences for natural ecosystems.

The green fern-like seaweed is known as Caulerpa taxifolia.

Widespread invasion

It was accidentally released about 16 years ago in the Cote d'Azur and now covers around 6,000 hectares (15,000 acres) of seabed off France.

Outbreaks have also been recorded along the coast of Spain and Italy and as far afield as Croatia.

The weed has to be removed by divers

Now, two independent genetic assessments have proved that a similar invasive alga that is spreading off the coast of southern California is the same strain as the one that has wreaked havoc in the Mediterranean.

"It is a typically tropical alga, normally tropical alga cannot live in cold water," Dr Jan Pawlowski of the University of Geneva, Switzerland, told BBC News Online.

"This strain has adapted to cold water, to the Mediterranean, and now California and it could spread elsewhere," he added.

"In places where it becomes dominant, it changes the ecosystem."

Writing in the journal Nature, the team warns that the weed must be rapidly eradicated to prevent a new invasion.

Unwelcome stowaway

Scientists believe the seaweed first took up residence in the warm waters of the Cote d'Azur after escaping from the Oceanographic Institute of Monaco while the facility's fish tanks were being cleaned.

The California seaweed has so far covered 3,500 square metres (37,500 sq ft) at Carlsbad and is dispersed over 20,000 sq metres (215,000 sq. ft) at Huntington Harbour.

The scientists are not sure how the seaweed could have spread to the West Coast of the United States but one theory is that it travels large distances attached to the anchors of boats.

Once present, the alga is notoriously difficult to eradicate.

France, the worst hit country, is using a range of costly methods to destroy it.

These include the use of plastic sheeting or aluminium film to cover the weed and starve it of sunlight, or employing teams of scuba divers to uproot the plant by hand.