Polymorphism in Velacumantus australis (Gastropoda : Potamididae) and its relationship to parasitism

Abstract

Velacumantus australis is a common gastropod along the Australian coast. Samples of four populations were examined and the frequency of two banded forms was recorded. All brown-banded snails were small juveniles and these apparently die before reaching adolescence. White-banded snails occurred in all age groups and their frequency tends to be highest in juveniles and lowest in the oldest adults. White-banded snails have a much lower incidence of larval trematode infection than unbanded snails and also differ slightly in weight and in the onset of sexual maturity. It is suggested that these forms are maintained as a selectively balanced polymorphism.