Unio crassus, an endangered freshwater mussel species, inhabits streams with diversified physical structure, subject to frequent changes in channel morphology. It is usually regarded as a sedentary animal, although individual movement has been observed. The hypothesis that adults can use movement in order to actively select a microhabitat was tested in two experiments simulating dislodgement of mussels by a stream. In the first experiment, begun in 2004, individually tagged Unio crassus individuals were distributed in a regular pattern throughout a 17 m long riffle-pool reach of Cedron River (S. Poland). The mussels changed their positions after displacement. The distances travelled differed significantly between parts of the channel: they covered longer distances in the pool (ca 2 m) than in the riffle (ca 0.6 m), with minimal movements on steep banks and dead water areas and large movements in shallows and deep areas. Individuals from shallow places were found to have moved to the steep bank of the pool, built of fine sediments, and to the area of dead water in the lee of boulders. The movements were not related to the water depth gradient: individuals moved to the steep bank even from the deepest parts of the pool, a finding experimentally confirmed with radio-tracked individuals. The second experiment was begun in 2008 in a smaller reach 5 m long. The mussels were put in the deepest part of the reach. Most of them climbed to a steep bank built of fine sediment with minimal flow, and some of them hid beneath the gravel armouring the fast-flowing part of the channel. It can be concluded that adult Unio crassus individuals can move to any place in a channel after being disturbed by natural or artificially created fluvial processes. Steep banks of the pool, built of fine sediments, represented the best place of refuge.
river channel structure