The effect of wintering sites on the survival and reproduction of Gyraulus acronicus (Gastropoda) in a partly frozen river.

In a boreal river about 95% of the individuals of Gyraulus acronicus overwinter in the littoral zone which freezes solid each year. These snails were compared with those overwintering in the unfrozen sublittoral area: The littoral snails had a higher survival rate, a higher tissue dry mass/CaCO3 ratio, and they deposited a higher number of eggs. Littoral snails had a more pronounced tissue degrowth during winter. High winter survival in the frozen littoral zone, a refuge totaly free from predation, indicates that overwintering here is advantageous. However, during frozen periods of short duration (<1 month) the high initial mortality to which the snails were exposed when freezing into the ice was not compensated for by higher survival after the initial phase. Under such conditions when the frozen period is very short the snails would have higher survival in unfrozen parts and are thus expected to avoid the ice.