An overview of Australia’s temperate marine phylogeography, with new evidence from high-dispersal gastropods.

Aim: We provide an overview of the location and ages of coastal phylogeographical breaks in southern Australian planktonic dispersers, and test the hypothesis that the absence of such breaks in some species is an artefact of insufficient resolution of genetic markers when such breaks evolved comparatively recently. Location: Temperate coastal Australia. Methods: We generated a large (> 1500 individuals) data set from rapidly evolving microsatellite markers for two codistributed Australian coastal gastropods, and compared it with mitochondrial DNA data. Both study species, the snail Nerita atramentosa and the limpet Siphonaria diemenensis, have long planktonic dispersal phases, and neither taxon exhibits substantial regional genetic structure on the basis of mitochondrial DNA. We tested for the presence of genetic structure by means of AMOVA, Bayesian clustering (structure) and iterated realloction (flock). Results: There was no compelling evidence for the existence of more than one evolutionary lineage in either species. Main conclusions: Discrepancies in the phylogeographical structuring of co-distributed intertidal taxa cannot be attributed to insufficient marker resolution for the two species considered here, and likely reflect a combination of abiotic and biotic factors that include porous dispersal barriers, life history and species age/history. It appears that contemporary oceanography does not explain the presence of phylogeographical breaks, but may serve to maintain breaks that evolved earlier. Deep genetic divergence in some of the previously studied coastal invertebrates suggests that these could be cryptic species, in which case competitive exclusion may play a role in constraining species biogeography.