The species inhabits brooks, streams and rivers in Sweden but can also be found in the in- and outflow of lakes. It prefers a bottom with sand and gravel. The glochidia are released in “packages” taken in by the fish as a “nutrient”. Some of the larvae will then attach to the gills of the host fish. The Thick shelled river mussel is the most endangered species of the large freshwater mussels in Sweden. It is Red listed as an “Endangered” (EN). It has probably disappeared from several places of occurrence during recent years, especially from habitats in the northern part of its distribution area. The species has recently been discovered in two additional stream waters in the eastern parts of the Country. Pollution and acidification of the habitats, destroying of suitable bottom substrate and loss of host fish are the main threats to the species.
The Unio crassus Life-cycle (according to Hochwald och Bauer, 1988, 1990). Illustration: Rita Larje (Ur Von Proschwitz and Lundberg 2004).