Unio crassus

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^[4] (video).^[5]

The thick shelled river mussel declined during the 20th century everywhere in Europe due to deteriorating water quality,^[6] habitat fragmentation and host fish limitation.^[7]

Its native distribution is Europe and Western Asia.^[1][8]

• It is mentioned in annexes II and IV of Habitats Directive.
• Croatia
• Czech Republic – in Bohemia, in Moravia,^[9] endangered (EN).^[10][11] Its conservation status in 2004-06 was unfavourable (U2) according to a report for the European Commission in accordance with the Habitats Directive.^[12]
• Finland – in southern Finland. Vulnerable.^[13]
• France^[14]
• Germany – critically endangered (vom Aussterben bedroht)^[15] In Germany this bivalve has disappeared from 90% of its former range.
• Netherlands – locally extinct. In the Netherlands it has not been seen alive after 1968 and is most likely extinct in that country.^[16][17]
• Poland – endangered^[18]
• Slovakia – nearly threatened^[19]
• Sweden – very rare^[20]
• Denmark – believed to have been extirpated until rediscovered in Odense River in 2003. The Odense River population is estimated to number c. 3000 individuals and subsequently it has been rediscovered from Suså River where the population size is unknown (might also survive in a couple of other rivers, but this remains unconfirmed).^[21][22]

• The Mussel Project Web Site
• Fauna Europaea