Divergent shell shape as an antipredator adaptation in tropical land snails

Although many land snails exhibit amazingly divergent shell shapes in the tropics, the functions of these remain obscure. Here we show that a modified aperture shape acts as an impediment specifically to predation by a snail-eating snake. Pareas iwasakii (Colubridae: Pareatinae) uses a unique method to feed on land snails: the snake extracts the soft body from the shell through the aperture by alternately retracting its mandibles. The snail Satsuma caliginosa (Camaenidae: Camaeninae) has apertural variation in regard to the presence of snail-eating snakes. Our experiments demonstrated that the distorted aperture mechanically impeded predation by this gape-limited predator, interrupting the mandibular movements. In contrast, congeneric snails with round apertures did not escape predation by snakes. The paleobiogeography of the focal area indicates that the subspecies Satsuma caliginosa picta, which does not have apertural modification, was derived from a defensive ancestor after the extinction of snail-eating snakes. Our study suggests a possibility that snail-eating snakes are responsible for divergent shell shapes in a variety of tropical land snails.