The extraordinary life cycle of the freshwater pearl mussel Margaritifera margaritifera enabled it to thrive in rivers across most of northern Europe and north America including the UK.
The adult mussels live in gravel on the bottom of rivers with about one third of their shell sticking out into the stream. Once a year they release millions of larvae into the water. Survival depends on the unlikely chance that a passing juvenile salmon or brown trout will swallow them so that the larvae can clamp themselves on the fish’s gills and grow in the oxygen rich environment.
The few that get lucky are ready to drop off their host after about a year and will bury themselves in the gravel bed of the river. It then takes 10 to 15 years for them to mature and produce larvae of their own. In healthy oxygen-rich rivers these adult mussels can live up to 130 years so have lots of opportunities to produce young.
This strategy worked until man started hunting them for the valuable pearls they contain, damaging pristine rivers with dams and pollution, and reducing the fish populations they need as hosts. They are legally protected but are disappearing almost everywhere except Scotland, where 50 rivers contain half the world’s healthy populations.
