THEY can live for more than 100 years - but despite their longevity pearl mussels have still become one of the rarest creatures in the country.

The shellfish were once widespread in Britain's rivers, but many former populations are now virtually extinct.

However, efforts are now being made to give them a new lease of life by encouraging them to breed many miles from their home.

Among the small pockets of them which still exist is a tiny population in the River Esk, above Glaisdale on the North York Moors.

And the Environment Agency has painstakingly removed some of the few remaining specimens to a Windermere hatchery.

There they will be encouraged to breed in captivity and then returned to the Esk when their situation improves.

The Esk pearl mussel is thought to be part of a genetically unique population on England's east coast, which experts believe originates from a time when the country was part of the greater European land mass, about 10,000 years ago.

Wildlife officer Sue Pacey said: "Moving the pearl mussels was really the last resort, but if we hadn't done this they could have become extinct in the next 25 years.

"The pearl mussels have become so dispersed in the wild that breeding is virtually impossible. Even when they do breed, the process is so complicated that there is little chance of success, so we've had to step in to help them survive."

The agency team, based in York, found about 200 pearl mussels during the delicate operation and removed 29 of them for transfer under the watchful eye of a national pearl mussel expert.

Fraser Hugill, the National Park's farm conservation officer, said: "The challenge for us is to get the River Esk into a more suitable condition so that we can re-introduce the pearl mussels in about five years' time. We are bidding for funding to put in place the improvements needed."

Pearl mussels were recorded in British rivers at the time of the Roman invasion and their unusual breeding process involves males releasing sperm into the water, which is then filtered by the females.

After fertilisation, the eggs ripen in the female's breeding pouch before being released as larvae.

The larvae then attach themselves to the gills of young salmon, then drop off into the gravels to feed on algae and to grow into juvenile pearl mussels.