The distribution of the gastropods Theodoxus fluviatilis (L.) and Potamopyrgus antipodarum (Gray) in lakes on the Åland Islands, southwestern Finland

On the Åland Islands, surrounded by the brackish Baltic Sea, the two species Theodoxus fluviatilis (L.) and Potamopyrgus antipodarum (Gray) are common and widespread. T. fluviatilis was found in lakes where the hardness values were below 3 °dH. P. antipodarum was found in four lakes at sea level and in two additional lakes of older age (higher elevation). On the Åland Islands the short distances from the lakes to the sea are advantageous for the transportation of snails, provided the lakes are connected to the sea through open ditches where fish wander or active colonization is possible. T. fluviatilis appears to be absent in Finnish lakes and P. antipodarum has previously not been encountered in Finnish lakes. The absence of the species in Finnish mainland lakes may be due to too high contents of humic substances and too low calcium values.