The mussels in the River Esk are more than 80 years old and are the last remaining individuals in Yorkshire, according to those behind a new project in the North York Moors national park.
One adult freshwater pearl mussel can filter 50 litres of water per day and the species provides a range of critical services to ecosystems, Yorkshire Water said.
Conservationists have collected the adult mussels from the Esk and temporarily relocated them to a specialist facility to encourage them to release their young, according to the firm, which is funding the new project run by the Freshwater Biological Association (FBA).
The juvenile mussels will spend their first few years at the facility before being released back to the river when they are around seven years old.
The project is a collaboration between Yorkshire Water, the FBA and the North York Moors National Park Authority.
The company said numbers of freshwater pearl mussels – Margaritifera margaritifera – have seen a steep decline in the UK over the last 50 to 70 years and are considered endangered. Population declines have been linked to factors such as habitat loss, river engineering and reduced level of certain types of fish.
Ben Aston, principal ecologist at Yorkshire Water, said: “Freshwater pearl mussels are incredible creatures, and their continued presence in British waters is really important. We are delighted to play our part in trying to safeguard at-threat species like the pearl mussel and help the wildlife of Yorkshire to thrive.”
Louise Lavictoire, mussel reintroductions research officer at the FBA said: “We’re really excited we can help the Esk population to avoid extinction. We’ve had some success already rearing juvenile mussels from the Esk and this much-needed funding support from Yorkshire Water will help us to build upon this and really help to kick-start the population again.”
Elizabeth Clements, head of natural environment with the North York Moors National Park Authority, said: “Hopefully the adult mussels will breed successfully at the FBA and their young will be returned to the River Esk in about seven years.
“They may not be cute, but freshwater pearl mussels are a very important keystone species in our river.
“We continue to work together with land managers and farmers to get the conditions right for them, and this will have positive knock-on benefits for a wide range of other wildlife such as otters, Atlantic salmon and trout, and birds like dippers and kingfishers.”
The River Esk project is part of a wider body of work by the FBA at the Freshwater Pearl Mussel Ark.
The scheme, which has has additional funding from Natural England, the North York Moors National Park Authority and the Environment Agency, aims to propagate juveniles from some of the most imperilled populations in England