LONDON - The largest population in English waters of an endangered species of mollusc has almost been destroyed.
Insect charity Buglife has called for an inquiry after the death of up to 90% of the freshwater pearl mussels at the Ennerdale Water Reservoir in Cumbria.
It is thought that water levels in the outflow of the reservoir fell, causing the temperature to go up and oxygen levels to go down.
"This is devastating news," said Buglife chief executive Matt Shardlow.
Meet the mussel:
The freshwater pearl mussel is a type of mollusc, of which there are around 85,000 recognised extant species
The freshwater pearl mussel can grow up to 15cm long
Their larvae resemble tiny mussels with hinged shells that snap shut when they find a suitable juvenile fish to attach themselves to
It is thought that the freshwater pearl mussel may once have been the most numerous bivalve mollusc in the world
In bivalve molluscs, the gills both "breathe" and produce a water flow through the animal, which is used for both reproduction and excretion
About 80,000 freshwater mussels were lost in this single instance, out of an estimated total population in England and Scotland of about 12m, according to Buglife.
Mr Shardlow compared the loss to wiping out a medium-sized city in the UK, in human terms. The species is protected under UK and international legislation.
"The UK supports a large proportion of the world population for this species and we have an international responsibility to protect these animals," said Matt Shardlow.
The freshwater pearl mussel recently joined giant pandas and Javan rhinos in a list of the world's 365 "most endangered species", assembled by the International Union for Conservation of Nature (IUCN) last year.
The species is thought to be one of the longest-living invertebrates in existence ,with individuals surviving for up to 150 years.
"The bloated corpses of animals born when Charles Darwin was alive have been floating out of their beds and (are) being swept into the Irish Sea," said Matt Shardlow.
The mussels take 10-12 years to reach sexual maturity, meaning that it takes time to build up numbers.
"If you get to the point where the river is just uninhabitable, you cause 150 years' worth of damage, and that's what's happened in this instance," said Matt Shardlow.
Matt Shardlow said the pearl mussel had been in "relatively steep decline", in recent years, partly because of poaching and illegal pearl fishing.
But he said it was exacerbated by the mussel's complicated life cycle.
The molluscs take 10-12 years to reach sexual maturity "The larvae come out and they attach to the gills of salmon and trout and live there for a while," said Matt Shardlow.
"They then drop off and they live in gravel in the riverbed and then gradually grow into the big mussels that one recognises as pearl mussels."
It means that changes to fish populations or silt and soil being washed into the river system have negative effects on the species.
Pollutants from agriculture have also affected the species.
"On top of that they've got issues to do with water flow and oxygen availability, which is obviously the problem that they've had here," said Matt Shardlow.
"We must understand the cause of this disaster so that it never happens again," he said.