Potamopyrgus antipodarum presently is included in the family Hydrobiidae, the mudsnails (but see above). These are small, deposit-feeding prosobranchs, which means they have an operculum (contrary to most freshwater snails, which are pulmonates and therefore do not have an operculum). In Nordic marine and brackish waters there are 4 species of hydrobiids, which occupy different, though partly overlapping, habitats, depending on salinity (Fenchel, 1975a; Hylleberg & Siegismund, 1987; Brattegard and Holthe, 1997; Främmande Arter, 2006).