Alien mussels invade Thames

The creatures, which are no more than the size of a thumbnail, are capable of devastating the capital's water supply and native water life.

An audit of "alien species" in the river carried out by the society found thousands of zebra mussels, named for their stripy shells, present in just a short stretch of the river near Richmond.

Although the creatures are tiny, they cling together - forming large "reefs" which can block outlet pipes from power stations and sink mooring buoys.

Thousands of tonnes of them have already been found in the capital's water pipes, constricting the flow and forcing Thames Water to hire specialists to chip them off.

It is believed the inedible mussels reached the Thames via cargo ships, which take in ballast water full of mussel eggs and transport it across the world before discharging here.

One zebra mussel can produce 30,000 offspring in a single season and they have no natural predators in Britain, allowing them to colonise vast areas. Jean-Luc Solandt, biodiversity policy officer at the conservation society, said: "The last time we counted them at Richmond there were only 25 zebra mussels. This year, we counted thousands."

In one square metre of drained river bed scientists from the society, London Zoo and Thames Landscape Strategy, a conservation umbrella group, found 832 mussels.

Other alien species found at Richmond Lock include Asiatic clams and Chinese mitten crabs.