Gathering of the clams poised to overwhelm our waterways

It is smaller than a matchbox and as big as a potato crisp, but the tiny Asiatic clam can cripple power stations, close factories and poison the water supply - and now it's here.

It is smaller than a matchbox and as big as a potato crisp, but the tiny Asiatic clam can cripple power stations, close factories and poison the water supply - and now it's here.

Scientists have discovered that millions of the 11/2in molluscs are silently massing in East Anglia, poised to wreak havoc across Britain's inland waterways.

The clam's small size and prolific breeding enable it to infiltrate and clog the pipes and condensing systems of industries that use rivers for cooling purposes. Power stations - including nuclear ones - are particularly vulnerable.

For the time being, the Asiatic clam, Corbicula fluminea, is restricted to the Norfolk Broads. But there are fears that unless it is contained it will emulate the damage it has caused in the United States, where the clear-up operation is a multi-million-dollar industry.

They also pose a serious health risk as tons of rotting clam flesh could poison the domestic water system.

"It is only a matter of time before it spreads," said Dr Roy Baker, a retired zoologist who first identified the Asiatic clam's presence in Britain. "We can't do anything about it. The potential is there for it to be a real problem. The only thing you can do is control it by chlorinating the water."

The clam, native to the South China Sea, is a bivalve first identified in the Norfolk Broads two years ago. Then they amounted to no more than a handful but today the population is thought to be several million - and growing.

The Asiatic clam is moving forward on two flanks. Like all cockles and mussels it uses its fleshy "foot" to pull itself through the silt. At the same time the clam releases free-standing larvae that float in the water and are carried along by the tide. It has already spread from the southern Broads around the Rivers Waveney and Yare to the River Thurne 25 miles away. It could soon enter the main water pipe supplying water from the region to London and Essex and young clams and larvae will soon find their way into the condensers of power stations.

It also threatens to displace native species such as the depressed river mussel. The Environment Agency is concerned because the clam is aggressive and takes up space on the sea bed needed by the depressed river mussel, which is highly endangered, while the Norfolk Wildlife Trust has found that the population of Asiatic clams on the River Chet has created a layer on the riverbed which stops other flora from flourishing.

Julia Stansfield, a biologist with the Environment Agency, likened the clam's impact to that of the grey squirrel on red squirrels. She said: "One solution is to poison the river bed to kill off the clams but that would wipe out the depressed river mussel as well. There is no action we can take to get rid of it that isn't drastic."

The Asiatic clam is thought to have been introduced to Britain by the trade in exotic fish, in particular koi carp. The water in which the carp are imported often contains the clams and their larvae which are simply dropped into the water system along with the fish.

Its advance through Norfolk's waterways has been helped by holidaymakers on barges. The clam attaches itself to the weights the boats use and is found in great concentrations near riverside pubs.

It was first identified in the US in 1957 and spread at a phenomenal rate: 10 years later it knocked out a nuclear power plant in Russelville, Arkansas.