Geographical Variability in Freshwater Mollusks

摘要

In general, geographical variability is known for almost of taxa of living organisms. Practically, the phenomenon of geographical variability was doubted for freshwater mollusks, both gastropods and bivalves. It was assumed that these taxa were characterized by chaotic variations, i.e., when a local race is formed in each particular biocenosis. A statistical assessment of geographical clines in freshwater mollusks was lacking until early 1970s. However, even nowadays, only a short list of species hasbeen analyzed for this phenomenon; this does not allow one to draw conclusions on the overall regularities of the geographical variability within this group. For most species, a significant shririking of the shell size is observed when moving from the south to north. This can probably be explained, not by the straight effect of low temperatures, but rather by the shortening of the warm period of the year in northern latitudes, and, therefore, the shortening of the period favorable for mol-lusk growth.Some species of freshwater pulmonate mollusks demonstrate clinal variability of their shell indices, which leads the authors to disparage their definition as a sub-species, since the species margins disappear when thoroughly studying the geographical variability of the species-specific parameters. Even less information is available about the geographical variability of the anatomic parameters. However, the clinal variability is found for at least one species of a pond snail (Lymnaea terebra) when studying the proportions of its copulatory apparatus moving from south to north. Further studies of the geographical variation of the freshwater mollusks will allow one to conclude whether the geographical clines are truly adaptive, i.e., whether they are characterized by a genetic basis or appear under the modifying pressure of the environment.