An unwelcome invader is threatening native species on the Norfolk Broads.

Scientists have raised concerns about the rapid spread of the Asian clam which is thought to have come in from Europe.

Experts say that once established, Corbicula fluminea is almost impossible to eradicate.

The Asian clam was first discovered on the Broads in 1998 along the River Chet.

'Minimise' risk

It has now spread up the River Yare to Norwich, and into the rivers Bure, Ant and Thurne.

The main concern is that the clam, which has a thick yellow shell, will supersede slower-growing native freshwater species.

Andrea Kelly, conservation officer for the Broads Authority, said: "This area is the clam's only stronghold in the UK.

"People need to be aware of it so they can help prevent it spreading."

Boat owners and anglers are being asked to pay particular attention to the clams if they clean their boats, trailers and other equipment.

"Clams can survive out of water for long periods so we are asking people to help us minimise the risk of it spreading to other waterways."

The clam was introduced into the USA where it has been blamed for blocking irrigation channels and the water intake pipes on power stations.

In the last 50 years it has spread all over the world, either in ballast water from ships or via the exotic fish or plant trades.

The Broads Authority is so concerned about the threat posed by the Asian clam that anyone seeing the organism outside its known distribution area is asked to contact the Authority or the Environment Agency.