RIVER TREASURE

There 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.