Environment Agency officers are hoping to give North Yorkshire's very own pearl mussel population a new lease of life - by moving them out of the county.
Freshwater pearl mussels are extremely rare in Britain but small pockets of them can still be found including a tiny population on the River Esk above Glaisdale.
Moving the pearl mussels was really the last resort
Sue Pacey - Environment Agency
Staff have now moved some of the few remaining specimens to a Windermere hatchery where 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 originate from a time when the country was part of the greater European land mass, around 10,000 years ago.
Environment Agency 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."
A deterioration in habitat quality, and unsuitable river conditions such as too much silt has also taken its toll on pearl mussel populations.
The team, based in York, found around 200 pearl mussels during the delicate operation on Tuesday, September 4, and officers removed 29 of them for transfer under the watchful eye of a national pearl mussel expert.
The Environment Agency has been working with staff from North York Moors National Park on the project which is trying to encourage more salmon in the Esk, as well as pearl mussels.
Fraser Hugill, Farm Conservation Officer at the North York Moors National Park Authority, 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 around five years time. We are currently 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.