Naiad fauna of the River Schwalm (Bivalvia: Unionidae).

Results are presented from an investigation on the naiad fauna (river mussels, F. Unionidae) of the river Schwalm, a small river in the central mountainous region of western Germany (river Weser catchment). Six species of river mussels were found: Anodonta piscinalis, A. cygnea, Pseudanodonta elongata, Unio crassus, U. pictorum, U. tumidus . Only one tributary was inhabited by mussels besides the river Schwalm itself. The few results from earlier field collections are reviewed. Most of those findings, especially of Unio crassus in the upper part of river Schwalm and in Efze stream could not be confirmed. These deficiencies are due to water pollution and stream correcting measures which led to deteriorations in the quality of mussel habitats. The factors determining the actual pattern of distribution are discussed. The constancy rates were calculated for every species. Unio pictorum and Anodonta piscinalis were the most abundant species with constancy rates of 95% and 68% respectively. Unio crassus (47%) and Pseudanodonta elongata (42%) revealed surprisingly high constancy rates.