Thick-shelled river mussel

The thick-shelled mussel (Unio crassus) is a freshwater mussel with a thick, elliptical shell that grows to a lenght of 4-7 centimeters. The species mainly lives in running waters with sand and gravel bottoms and is found only in a few areas in eastern Sweden, 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 in which 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, bullhead and bleak. Once developed, the juvenile mussels then detach from the host fish and live for several years buried in sediment before becoming adults. The thick-shelled river mussel can live up to 90 years.

Critically endangered species
Thick-shelled river mussel is highly threatened, both in Sweden and internationally, due to acidification, eutrophication and physical impacts on its habitat. Human impact such as river regulation, dredging and clearing 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 populations, resulting a lack of recruitment of juvenile mussels which over timeja, faktiskt både Lukas, Karin och Juilis causes mussel populations to slowly die out.

Host fish help
The species is protected by law in Sweden and listed under the EU’s Species and Habitats Directives, which requires the establishment of Special  Conservation Areas, and actions to strengthen populations. One effective method is to catch host fish and place them in tanks with gravid mussels to increase the chance of larvae attaching to the fish gills. The fish are then released back into the river where the larvae can grow into small mussels that drop of and burrow themselves into the bottom. After a few years, they develop into adult mussels and can contribute to reproduction.