Effects of mode of reproduction on genetic divergence over large spatial and temporal scales in intertidal snails of the genus Bembicium Philippi (Gastropoda: Littorinidae)

Abstract

The littorine genus Bembicium, restricted to Australia and Lord Howe and Norfolk Islands, provides special opportunities to examine the effects of contrasting modes of development on genetic divergence over large spatial and temporal scales. Two species, Bembicium auratum and Bembicium nanum, have planktotrophic larvae, and large, overlapping geographical distributions, whereas the three direct developers, Bembicium vittatum, Bembicium melanostoma, and Bembicium flavescens, are geographical replacements. Phylogenetic analysis of genetic distances at 28 allozyme loci supported the current taxonomic treatment of the genus and monophyly of the direct developers. Both individually and as a clade, the direct developers showed substantially greater divergence than the species with planktotrophic larvae. Controlling for geographical distance and for particular sections of coastline, genetic subdivision within the direct developers was shown to be much higher than in the planktotrophs. Low levels of subdivision in B. auratum and B. nanum over distances of 4000-6800 km indicate maintenance of substantial genetic connectivity, independent of habitat and biogeographical history. By contrast, the direct developers show clear genetic impacts of their geographical histories. Despite extreme genetic subdivision within species, the direct developers B. vittatum and B. melanostoma have geographically coherent and complementary distributions, associated with biogeographical provinces. Thus, speciation appears to be driven by special biogeographical circumstances, rather than simply by genetic divergence of locally isolated populations. © 2006 The Linnean Society of London.