Selection pressures and polymorphisms in Velacumantus australis (Quoy & Gaimard)

Populations of Velacumantus australis from the Australian mainland are polymorphic for white-bandedness. The frequency of banded snails in a number of different populations is higher in small juveniles than in older snails, which suggests that the same kinds of selection pressures may be maintaining the polymorphism in different populations. Selection pressures due to parasitism and predation by boring gastropods and fish act differentially on banded and unbanded snails. Banded snails are less often infected with larval trematodes than unbanded snails. These infections cause either complete castration or considerable degeneration of the gonads. The significance of this resistance to infection is discussed. On theoretical grounds there should be a relationship between the incidence of larval trematode infection and the frequency of banded snails. Data from a number of populations suggest that there is such a relationship. Selection pressures other than those due to parasitism and predation probably play a part in the maintenance of the polymorphism.