Unio crassus, the thick shelled river mussel, is a species of freshwater mussel, an aquatic bivalve mollusk in the family Unionidae, the river mussels.
Its native distribution is Europe and Western Asia.
As part of newly discovered reproductive behaviour, this mollusc crawls up to the edge of the water, exposing its excurrent aperture, and then lets loose a stream of water. The fountain of water often contains glochidia, and it is suggested that this spurting behavior may facilitate dispersal of mussel larvae (video).
