Thick-shelled river mussel

The thick-shelled mussel (Unio crassus) is a freshwater mussel with a thick, elliptical shell that grows to 4-7 centimeters in length. The species lives mainly in running water with sand and gravel bottoms and is found in Sweden in a few areas in the eastern parts of the country, from Skåne to Dalarna.

Thick-shelled mussel can live up to 90 years.

It feeds by filtering water and has a complex life cycle that includes a parasitic stage where the larvae, known as glochidia, attach to the gills of host fish where they develop into small mussels. Possible host fish in Sweden include minnows, stonefish and lionfish. The tiny mussels then release their gills and live for several years buried in sediment before becoming adults. Thick-shelled river mussel can live up to 90 years.

Critically endangered species
Thick-shelled clam is highly threatened, both in Sweden and internationally, due to acidification, eutrophication and physical impacts on its habitat. Human impact such as regulation, dredging and stream cleaning have destroyed important benthic habitats, and agriculture contributes to deteriorating water quality. In addition, the survival of the species is affected by the decline of host fish, leading to a lack of recruitment of young mussels which in the long run causes mussel populations to slowly die out.

Host fish help
The species is protected under the EU’s Species and Habitats Directives, which requires the establishment of Special Areas of Conservation, and measures are needed to strengthen stocks. One way is to catch host fish and put them in tubs with pregnant mussels to help more larvae attach to their gills. They are then released back into the watercourse where the larvae can grow into small mussels that drop and burrow into the bottom. After a few years, they have developed into adult mussels and can contribute to reproduction.