RIVER TREASURE

The freshwater pearl mussel is now rare in Britain. But a conservation team in Cumbria is determined to restore this gem to our rivers

- ANDREW GRIFFITHS

ABOUT THE AUTHOR

Andrew Griffiths is a freelance writer who specialises in environmental issues, conservation and fly fishing. Andrew lives in the Peak District and is an amateur naturalist as well as a keen angler.

WHERE ARE FEW PARASITES that quicken the pulse of conservationists, and fewer still that are highlighted as flagship species, but that is the fate of the freshwater pearl mussel on English and Welsh rivers and streams.

This is in part because of their extraordinary life-cycle: vast numbers of tiny larvae, known as glochidia, are released by the mature female into the stream. They must then be inhaled by young salmon or trout and fix onto their gills - this is the parasitic stage - where a cyst forms and they spend about nine months in this rich, oxygenated environment before dropping off into the river gravels and beginning their journey to adulthood.

And what a journey this is, in terms of time if not distance. To begin to understand the life of the freshwater pearl mussel - and its plight - we need to tune our minds to a different temporal beat. The mussel can live to be 120 years old or more, does not become sexually mature until about 15 years old, and does not emerge from the river gravels to filter feed in their beds for about five years, after dropping off the gills of its host.

The mature bivalve mollusc, in its gnarly shell, looks as if it has sprung life from the centre of a cleaved stone. The name 'pearl mussel hints at the jewel that may lie within and, though pearl fishing is now illegal, it is this prospect of value, historically sought out by poachers, that is in part responsible for its precipitous decline.