Extreme, continuous variation in an island snail: local diversification and association of shell form with the current environment

Abstract

On Rosemary Island, a small continental island (11 km2) in the Dampier Archipelago, Western Australia, snails of the genus Rhagada have extremely diverse morphologies. Their shells vary remarkably in size and shape, with the latter ranging from globose to keeled-flat, spanning the range of variation in the entire genus. Based primarily on variation in shell morphology, five distinct species are currently recognized. However, a study of 103 populations has revealed continuity of shell form within a very closely-related group. A phylogenetic analysis of specimens from Rosemary Island, and other islands in the Dampier Archipelago, indicates that much of the morphological variation has evolved on the island, from within a monophyletic group. Within the island, snails with distinct shell morphologies could not be distinguished based on variation in mitochondrial DNA or their reproductive anatomy. The shell variation is geographically structured over a very fine scale, with clines linking the extreme forms over distances less than 200 m. Although there is no evidence that the different forms have evolved in isolation or as a consequence of drift, there is a strong association between keeled-flat shells and rocky habitats, suggesting that shell shape may be of adaptive significance.