The effect of heated effluent on the occurrence and the reproduction of the freshwater limpetsAncylus fluviatilis Müller, 1774,Ferrissia wautieri (Mirolli, 1960) andAcroloxus lacustris (L., 1758) in two Dutch water bodies

The discharge of cooling water favours the occurrence of freshwater limpets. Reproduction ofAncylus fluviatilis is stimulated by the higher temperature of the cooling water; the life cycle ofFerrissia wautieri (nearly exclusively found in the outlet areas) does not, however, differ from that under natural temperature regimes. ForAcroloxus lacustris this is not clear as detailed studies on the life cycle under natural temperature circumstances are lacking. Acroloxus lacustris andFerrissia wautieri could be found in large numbers on hard substrates (stones), as well as on plant material in sheltered as well as in exposed situations. In this respect they differ fromAncylus fluviatilis, which is restricted in its occurrence to hard substrates and exposed situations.It is not very clear whether the occurrence ofAncylus fluviatilis is also influenced by some form of competition for food byBithynia tentaculata; both feed on diatoms. Further experimental studies of this question should yield interesting results.The changes in numbers of the three freshwater limpet species are mainly due to reproduction and migration, both largely governed by temperature and day length.It was possible to reconstruct the influence of cooling water in the Bergumermeer area by means of the ratio between the numbers ofAcroloxus lacustris andFerrissia wautieri. The share ofFerrissia was highest near the outiet and decreased with the distance from this station.