Freshwater pearl mussels are extraordinary animals. Found in clean, fast-flowing rivers, they are among the world’s longest-lived invertebrates, with individuals capable of surviving for well over a century. As filter feeders, they process large quantities of water, removing suspended particles and organic matter and helping to maintain water quality and healthy river ecosystems. Their presence is also a valuable indicator of river health: where these sensitive molluscs are thriving and reproducing, conditions are generally good for a wide range of other freshwater wildlife.

Yet the freshwater pearl mussel is now critically endangered in Europe, where populations have declined by more than 90% over the last century. River modification, pollution, excess nutrients and fine sediment, habitat degradation, historic exploitation for pearls, and declines in host fish have all contributed. Young mussels are particularly vulnerable, meaning many surviving populations are dominated by ageing adults, with too few juveniles surviving to replenish them naturally.