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 - very rare[21]

• The Mussel Project Web Site
• Fauna Europaea

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