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Pearl Mussel Project

Close up of Mussels in gravel
Large pearl mussels
Large pearl mussels

The FBA is leading two exciting projects focussing on conservation of the freshwater pearl mussel (Margaritifera margaritifera) to form an integrated conservation strategy for the species in England. Both projects are supported by Natural England and the Environment Agency and aim to halt the decline of the species in the wild through targeted restoration and increase numbers of surviving individuals in the wild through captive rearing and reintroductions.

Interested in working with Pearl Mussels? We have an opening please read our job application pack

About the species
The freshwater pearl mussel Margaritifera margaritifera is classified as critically endangered on the IUCN Red List of Endangered Species due to its unprecedented, worldwide decline during the latter part of the 20th Century. Many factors have contributed to the decline including pearl fishing, water pollution, siltation, and declines in host fish populations. The freshwater pearl mussel has a very long life-span, commonly reaching ages of over 130 years (Bauer, 1992) and individuals inhabit oligotrophic (nutrient-poor) rivers with clean, well oxygenated gravels.

M. margaritifera has a very interesting and complex life cycle which requires a host fish for their larvae (glochidia). Mussels are normally dioecious (have separate sexes). Male mussels release sperm into the water column in June – July (depending upon water temperature). Sperm is inhaled by the female mussels to fertilise their eggs. Glochidia are released into the water column between July and September (temperature dependent). A single female can release 4-16 million glochidia pear year, each measuring 60-70µm in length (Young & Williams, 1984). Glochidia require a salmonid fish host (Atlantic salmon, Salmo salar or brown/sea trout, S. trutta in the UK) for the next stage in their development. Glochidia are inhaled by the host and, as water passes over the fish’s gills, the glochidia snap shut onto the gill filaments. Glochidia become encysted within the gill tissue and grow there until the following spring when they drop off the fish in May or early June. At this point they measure approximately 400µm in length. Juveniles must land in clean, well oxygenated gravel substrates where they will burrow into the interstices to continue their development.

The number of mussels in the wild in England totals approximately 500,000 with the majority of individuals coming from only one river. Low numbers of mussels are found in around 10 other rivers. Pollution and siltation cause particular problems for pearl mussels. Despite adults reproducing successfully, there continues to be a near total loss of juveniles annually due to poor substrate conditions in rivers. The result is aging populations with the youngest individuals in some rivers being over 40 years old.

For a comprehensive list of literature relating to the habitat requirements of the freshwater pearl mussel please visit our webpage here.

To view videos of freshwater pearl mussels at the FBA click here and click below to view our video explaining some of the research work being undertaken by the FBA.

For more information on the Freshwater Pearl Mussel Ark Project please contact the Freshwater Pearl Mussel Ark Project at


  • Bauer, G. (1992). Variation in the life span and size of the freshwater pearl mussel. Journal of Animal Ecology 61: 425-436.
  • Young, M. and J. Williams (1984). The reproductive biology of the freshwater pearl mussel Margaritifera margaritifera (Linn.) in Scotland. 1. Field studies. Archiv für Hydrobiologie 99(4): 405-422.


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