Freshwater pearl mussel

The freshwater pearl mussel (Margaritifera margaritifera) is an endangered species of freshwater mussel, an aquatic bivalve mollusc in the family Margaritiferidae.

Although the name "freshwater pearl mussel" is often used for this species, other freshwater mussel species (e.g. Margaritifera auricularia) can also create pearls and some can also be used as a source of mother of pearl. Most cultured pearls today come from Hyriopsis species in Asia, or Amblema species in North America, both members of the related family Unionidae; pearls are also found within species in the genus Unio.

The interior of the shell of Margaritifera margaritifera has thick nacre (the inner mother of pearl layer of the shell). This species is capable of making fine-quality pearls, and was historically exploited in the search for pearls from wild sources. In recent times, the Russian malacologist Valeriy Zyuganov received worldwide reputation after he discovered that the pearl mussel exhibited negligible senescence and he determined that it had a maximum lifespan of 210–250 years. The data of V. V. Zyuganov have been corroborated by multiple observations of 250 year old individuals in Finland.

Subspecies within the species Margaritifera magaritifera include:

The freshwater pearl mussel is one of the longest-living invertebrates in existence. The oldest known specimen in Europe was caught in 1993 in Estonia when it was 134 years old.

Like all bivalve molluscs, the freshwater pearl mussel has a shell consisting of two parts that are hinged together, which can be closed to protect the animal's soft body within. The shell is large, heavy and elongated, typically yellowish-brown in colour when young and becoming darker with age. Older parts of the shell often appear corroded, an identifying feature of this mussel species. The inner surface of the shell is pearl white, sometimes tinged with attractive iridescent colours. Like all molluscs, the freshwater pearl mussel has a muscular 'foot'; this very large, white foot enables the mussel to move slowly and bury itself within the bottom substrate of its freshwater habitat.

The native distribution of this species is Holarctic. The freshwater pearl mussel can be found on both sides of the Atlantic, from the Arctic and temperate regions of western Russia, through Europe to northeastern North America.

Clean, fast-flowing streams and rivers are required for the freshwater pearl mussel, where it lives buried or partly buried in fine gravel and coarse sand, generally in water at depths between 0.5 and 2 metres, but sometimes at greater depths. Clean gravel and sand is essential, particularly for juvenile freshwater pearl mussels, for if the stream or river bottom becomes clogged with silt, they cannot obtain oxygen and will die. Also essential is the presence of a healthy population of salmonids, a group of fish including salmon and trout, on which the freshwater pearl mussel relies for part of its life cycle.