Freshwater Pearl Mussel
As pearl mussels often live in fast-flowing rivers and burns there is always a danger that, over time, floods will sweep the population downstream to the sea. To counter this, they have developed a complex and intricate means of reproducing that includes hitching a ride on young salmon or trout in order to recolonise upstream areas. In doing so the overall population is able to remain in the river.
Reproduction begins in the early summer when male pearl mussels release sperm. These are drawn in by females and the fertilised eggs develop into larvae within the females until mid to late summer. The larvae (glochidia) are effectively microscopic mussels (0.06mm long) and are released by the females and swept downstream. Most of the glochidia are swept away and die, for to survive the larvae must be inhaled by a young salmon or trout and attach themselves to the gills. The chances of this happening are extremely low (less than 0.0004%) and therefore each mussel produces vast numbers of larvae, between one and four million.
Those larvae that do manage to become attached to the gills of a salmon or trout remain there and grow for almost a year where they have no harmful effect on the wild fish. Then, the following spring or summer, the larvae drop from the gills and settle onto the river bed.
The young pearl mussels rely on landing on areas of river bed that are suitable habitat. This is fine gravel or coarse sand. If they do land on such areas, they bury themselves in the sediment where they grow for up to 100 years, reaching sexual maturity after 12-15 years.