Hydrological and substrate requirements of the thick-shelled river mussel Unio crassus (Philipsson 1788)

Conservation and restoration strategies require knowledge of the autecology of target species. The thick-shelled river mussel Unio crassus is an endangered freshwater bivalve with largely unknown habitat preferences. In this study, hydrological and substrate parameters including shear stress, flow velocity and penetration resistance of substrate were examined in relation to mussel presence/absence data in six streams with recruiting and self-sustaining populations to derive information on the hydrological and substrate habitat requirements of the species. Mean flow velocities between 0.11 and 0.31 m s⁻¹ and absolute maximum values in the study streams ranging from 0.22 to 0.95 m s⁻¹ indicate that the species is tolerant of a wide range of hydrological conditions. Areas with high mussel densities were characterized by low flow velocities (<0.3 m s⁻¹), low mean penetration resistances (0.36 ± 0.52 kg cm⁻²) as well as by low near-bed shear stress (1.06 ± 0.33 x 10⁻⁶ N cm⁻²) compared with non-colonized sites. Maximum near-bed flow velocity at mussel beds was 0.33 m s⁻¹ and 0.35 m s⁻¹ at 60% depth. Higher flows with maximum velocities of 0.52 m s⁻¹ and 0.95 m s⁻¹ at 60% depth were measured at non-colonized sites. Mussel density was significantly negatively correlated with near-bed shear forces. The results indicate that the hydrological and substrate characteristics of U. crassus habitat are broader than expected. In contrast to the assumption that it depends on moderate to high flows (>0.3 m s⁻¹), streams with low water flow and soft substrate were also identified as suitable habitats. Functional characteristics of the substrate, especially stability and areas with low shear stress (<1.5 N cm⁻²), seem to be of great importance for the persistence of the species, and monitoring protocols used at present for habitat assessment of U. crassus should be updated.