The freshwater mussel (FWPM) (Margaritifera margaritifera) is a fascinating animal with a very interesting lifecycle.

The family of bivalve molluscs, Margaritiferidae, to which the FWPM belongs is very ancient, and has changed little in 65 million years. The FWPM can be found on both sides of the Atlantic, from the Arctic and temperate regions of western Russia, through Europe to north-eastern North America.

Mussels are declining rapidly internationally and are listed as critically endangered, equivalent to the risk of extinction of the Orang-utan and Gorilla. In the last 90 years, they have declined globally by 61.5% and by 87% in Europe; the importance of the River Ehen mussel population can therefore not be overstated.

The FWPM as a bivalve mollusc, has its body almost completely enclosed between a pair of shells hinged together. The FWPM 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 outer shell is generally yellowish-brown in colour, darkening with age, and the inner surface, the mantle, is pearl white. The mantle protects the soft parts of the mussel from any parasites that get into the shell, by growing over them and covering them. As the layers build up a pearl may be formed.

FWPM has a long lifespan; they can live for over 100 years and can grow up to 155 mm in length. The adult mussel burrows to two-thirds of its shell depth into sandy/gravelly substrates, often between boulders and cobbles in fast-flowing oligotrophic (low nutrient) streams and rivers and is mostly sessile in nature. They are filter feeders, large quantities of water are pumped in through the animal’s inhalant siphon, food-material is sifted out and passed to the mussel’s mouth and then the water is expelled through the exhalant siphon. They require cool, clean and well-oxygenated water.

FWPM are typically dioecious (separate sexes), with males releasing sperm into the water column in June-July; the female inhales the sperm and fertilises the eggs. Between July and September, the female releases several million tiny (approximately 0.6-0.7 mm) larvae, called glochidia, into the water column. Glochidia require a juvenile (fry) salmonid (Atlantic salmon (Salmo salar) or trout (Salmo trutta)) fish host for the next stage of their development. The preferred salmonid host in the River Ehen is Atlantic salmon. The tiny shells of the glochidia are held open until they are inhaled by the host, then snap shut on the host’s gill filaments. Glochidia become encysted within the gill tissue and grow in this oxygen-rich environment until the following spring, when they drop off. These juvenile mussels must land and burrow into clean, sandy or gravelly substrates in order to settle and start to grow. This association does not appear to harm the fish, and facilitates mussel dispersal. Due to the essential role salmonid fish play in the life of the FWPM, the conservation of salmonids is also central to the survival of this endangered species.

Mussels reach sexual maturity between 10 and 15 years of age, when the shell length exceeds 6.5 cm. It is thought adult mussels do not have an upper age limit for reproductive activity and continue to reproduce until they die.

Please see attached documents for more detailed information about the ecology and conservation of FWPM.

The River Ehen in West Cumbria supports the largest population of mussels in England and as a result it is designated as a Special Area of Conservation (SAC) and Site of Special Scientific Interest (SSSI). The SSSI has been assessed by Natural England as being in “unfavourable declining” condition due to insufficient freshwater mussel recruitment, making the current population unsustainable.

The links below lead to the short films about freshwater mussels in the River Ehen produced as part of the Pearls in Peril Project which completed in 2016 -…

Photo Gallery


Name Type Size
Ecology of Freshwater Mussel pdf 1.16 MB
Conservation of Freshwater Mussel and Relationship with Salmonids pdf 937.17 KB
Maggie the Mussel - children's story pdf 1.35 MB