Freshwater snail assemblages of semi-isolated brackish water bays on the Åland Islands, SW Finland

The aim of the study was to relate the distribution of freshwater snails in 21 semi-isolated bays (within a salinity range of 0.7–7.4 practical salinity units, PSU) to environmental variables of the Åland Islands, SW Finland. Altogether 16 species of freshwater snails were found when the bays were investigated by hand-picking and a rod sieve. Of these, six or seven species may be regarded as almost ubiquitous. The salinity limiting most freshwater snails is approximately 4–5 PSU. The marine snail Hydrobia ventrosa was found in only two of the bays. The highest number of snail species occurred in bays with large catchment areas, especially where there were freshwater lakes in the local area. The number of species increased with decreasing salinity (Spearman's [rho] = –0.513). The maximum number of species was found in a bay with almost freshwater-like conditions, inferring that tolerance to higher salinities in some species is restricted.