Molluscs of the middle and lower Odra: the role of the river in the expansion of alien species in Poland

The malacofauna of the middle and lower Odra (Oder River), a large river degraded by human ac- tivities, is still rich and diversified. Fifty five mollusc species were recorded from 31 studied localities: 32 Gastropoda and 23 Bivalvia. The highest number of species (47) inhabited the areas between groynes, whereas the fewest species (20) were found in the river current outside groynes. As many as 14.5% of the re- corded species were alien to the Polish fauna: Lithoglyphus naticoides, Potamopyrgus antipodarum, Physella acuta, Ferrissia fragilis, Menetus dilatatus, Corbicula fluminalis, C. fluminea, and Dreissena polymorpha. The presence of so many alien species shows that the Odra, together with the system of canals (which through the Spree and the Havel connect it to the drainage basin of the Elbe), is a major route along which alien molluscs migrate to Poland.