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 km 2 ) 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.

Determinants of species richness and similarity of species composition of land snail communities on Kimberley islands

“…Studies of land snails elsewhere have shown that even within a genus, shell morphology of the species has a clear relationship with habitat use (Cameron and Cook 1989;Chiba 2004;Stankowski 2011). For instance, high-spired shells are generally associated with vertical services and flat shells with horizontal surfaces (Barker 2001).…”

Rhagadarevisited: on the taxonomy of species from the Kimberley and Dampierland, Western Australia (Pulmonata, Camaenidae)

“…This variation has evolved within a very short period of time on Rosemary Island; probably post-Pleistocene. Variation between the two morphological extremes occurs over a distance of less than 200 m and is strongly associated with changes in the habitat (from rocky to sandy), suggesting that the shell shape is of high adaptive significance (Stankowski 2011). The Rosemary Island case proves that dramatic changes in shell morphology may occur in short evolutionary time spans if selection is strong.…”

“…In accordance with their extensive ranges, Rhagada species may be associated with a large variety of habitats, from monsoonal vine thickets to xeric habitats, such as sandy plains dominated by spinifex grasses (Triodia spp.) or more complex, rocky outcrops with shrubby vegetation (Stankowski 2011;Johnson et al 2012).…”

“…Molecular phylogenetic studies suggested that the morphological radiation in this archipelago evolved locally, rather than through multiple introductions of relictual mainland lineages. There seems to be a general disconnection between morphological diversification and levels of molecular genetic divergence within this radiation (Johnson et al 2004;Stankowski 2011;Johnson et al 2012;Stankowski and Johnson 2014 represent another undescribed species ('R. cf.…”

Camaenid land snails on Barrow Island: distributions, molecular phylogenetics and taxonomic revision

“…However, these differences are well within the range of variation seen within other species. For example, detailed study of Rhagada in the Dampier Archipelago showed that much larger differences in shell form need not indicate reproductively isolated species (Stankowski 2011). The high genetic similarities and the phylogenetic interspersion of COI haplotypes indicate that Q. barrowensis is the same species as Q. montebelloensis.…”