Repeated evolution of an undescribed morphotype of Rhagada (Gastropoda : Camaenidae) from the inland Pilbara, Western Australia

Abstract

An undescribed small, banded morphotype of Rhagada land snails occurs widely in the rocky inland Pilbara region, Western Australia. Phylogenetic analysis of mitochondrial COI and 16S rRNA genes revealed that this novel morphotype is polyphyletic, comprising four distinct major clades, with divergences up to 21.4% at COI. These clades are apparently morphologically cryptic, with no obvious shell differences. Two of these species are associated with the major clade of Rhagada in the Pilbara mainland, one of which appears to be a variant of the larger, more globose species R. pilbarana, which occurs within 20km proximity. The other two small, banded species are phylogenetically distinct from each other and all other known Rhagada. This small, banded morphotype shows evidence for both plesiomorphy and homoplasy. The morphotype has evolved independently at least twice, and is associated with the reasonably uniform habitat and harsh conditions in the elevated hinterland of the inland Pilbara. The broad distribution of the inland, small, banded morphotype conforms to the pattern of broad-scale uniformity of shells of the more coastal species of Rhagada. Its repeated evolution, however, confirms that the morphological uniformity is not simply because of common ancestry, supporting the theory that shell form in Rhagada is adapted to a broadly homogenous environment. Shell morphology in this genus has been demonstrated on more than one occasion to have the potential to adapt to different available environments, and hence shells should be used with a degree of caution for taxonomic interpretation.