Diversification of carnivorous marine snails (Muricidae: Rapaninae and Ergalataxinae): phylogeny, biogeography and dietary specialization

Abstract

Diversification in the marine realm is driven by poorly understood processes including vicariance, dispersal and ecological specialization. To test the role of these processes, I construct the first comprehensive molecular phylogenies of the Rapaninae and Ergalataxinae, two cosmopolitan, ecologically important but taxonomically complex subfamilies of carnivorous neogastropods (four genes: 12S rRNA, 28S rRNA, 16S rRNA, cytochrome c oxidase I). I sampled more than 50% of the described species in both subfamilies: 66 species and 19 genera of Ergalataxinae, 82 species and 26 genera of Rapaninae, and used fossils to calibrate the phylogenies. Unlike previously studied gastropod groups, the rapanine genus Stramonita has long lived pelagic larvae with a potentially trans-oceanic scale of dispersal, raising the question of how allopatric speciation has proceeded. To delimit species, I use statistical methods of phylogenetic tree analysis and show that the S. haemastoma species complex diversified in response to barriers operating at both large and small geographic scales; habitat specialization may also have played a role in this radiation. Dietary specialization has also been implicated in marine radiations. The ergalataxine genus Drupella was previously thought to consist of five species of obligate corallivores. I show that this monophyletic genus includes the facultative coral-feeder ‘Ergalatax margariticola’, which is itself composed of two cryptic species. Genetic structure within this species complex does not relate to feeding mode, but instead seems to correspond to broad patterns of habitat ecology found in other gastropod taxa. Drupella diversified following the major expansion of coral reefs in the early Miocene. If dietary specialization has permitted sympatric speciation, it is predicted that sympatric sister species should be specialized on different food resources. I test this prediction in the rapanine genus Drupa. Contrary to expectation, the sympatric sister species D. ricinus and D. arachnoides have similarly generalist diets.