What can we infer from the shell dimensions of the thick-shelled river mussel Unio crassus?

We studied shell variation in the thick-shelled river mussel (Unio crassus Philipson, 1788) sampled from two sites of very different character: (1) a rocky channel (San river) and (2) a fine-sediment channel (Zborowianka river). The analyzed mussels differed significantly between the sampled channels in almost all analyzed phenotypic traits and shape indices. Intersexual variation was so low that it cannot be used effectively for sexing in this species. The growth rate was much higher in the fine-sediment and nutrient-rich channel than in the rocky one, but the asymptotic shell length for the rocky channel was larger than for the fine-sediment channel, suggesting higher survival in the mountainous, unmodified river. Shell size differed significantly within the rocky channel, depending on microhabitat: shell size and dorsal arching were much greater in mussels living in the strong current of the rocky midchannel than in those inhabiting still water at the nearest bank. The results demonstrate that microhabitat conditions significantly determine shell shape.