Potential modes of range shifts in euryhaline snails from the Baltic Sea and fresh water lakes in northern Germany.

The widely distributed snail species <italic>Theodoxus</italic> <italic>fluviatilis</italic> (Linneaus, 1758) is rather diverse in shell size and patterning. It has formed regional subgroups in northern Germany that appear separate in either fresh- or brackish water, yet, according to literature, are indistinguishable by morphology, anatomy or mtRNA markers. Tolerance towards challenging salinities, however, is distinct. The question as to whether the observed difference in salinity tolerance is a result of local adaptation or of phenotypic plasticity remains. In this study, physiological limits to salinity stress and tolerance shift abilities were compared between individuals from both subgroups in a salinity transfer experiment and acclimation regime followed by a survival analysis. Brackish water animals struggled in freshwater but performed much better than freshwater individuals when directly transferred into higher salinities. With acclimation to their challenging salinities, individuals of both subgroups achieved shifts in the slopes of their reaction norms and were able to survive salinity levels otherwise lethal to them. Even with this shift freshwater animals were not able reach the tolerance levels of brackish water individuals. The results indicate that phenotypic plasticity as well as genetic adaptation may determine osmotolerance in regional subgroups of <italic>T. fluviatilis</italic>. [ABSTRACT FROM AUTHOR]