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Marine species with ranges that span the Indo-Australian Archipelago (IAA) exhibit a range of phylogeographical patterns, most of which are interpreted in the context of vicariance between Indian and Pacific Ocean populations during Pliocene and Pleistocene low sea-level stands. However, patterns often vary among ecologically similar taxa, sometimes even within genera. This study compares phylogeographical patterns in two species of highly dispersive neritid gastropod, Nerita albicilla and Nerita plicata, with nearly sympatric ranges that span the Indo-Pacific. Mitochondrial COI sequences from >1000 individuals from 97 sites reveal similar phylogenies in both species (two divergent clades differing by 3.2% and 2.3%, for N. albicilla and N. plicata, respectively). However, despite ecological similarity and congeneric status, the two species exhibit phylogeographical discordance. N. albicilla has maintained reciprocal monophyly of Indian and Pacific Ocean populations, while N. plicata is panmictic between oceans, but displays a genetic cline in the Central Pacific. Although this difference might be explained by qualitatively different demographic histories, parameter estimates from three coalescent models indicate that both species have high levels of gene flow between demes (2Nem>75), and share a common history of population expansion that is likely associated with cyclical flooding of continental shelves and island lagoons following low sea-level stands. Results indicate that ecologically similar, codistributed species may respond very differently to shared environmental processes, suggesting that relatively minor differences in traits such as pelagic larval duration or microhabitat association may profoundly impact phylogeographical structure.

Phylogeographical disjunctions in high-dispersal marine taxa are variously ascribed to palaeogeographical conditions or contemporary ecological factors. Associated biogeographical studies, however, seldom incorporate the sampling design required to confidently discriminate among such competing hypotheses. In the current study, over 7800 gastropod specimens were examined for operculum colour, and 129 specimens genetically, to test ecological and historical biogeographical hypotheses relating to biogeographical disjunction in the Southern Hemisphere, and to southern Australia in particular. Mitochondrial DNA sequence analysis of the high-dispersal intertidal gastropod Nerita atramentosa in southern Australia (88 specimens; 18 localities) revealed an east-west phylogeographical split involving two highly divergent clades (26.0 +/- 1.9%) exhibiting minimal geographical overlap in the southeast. The eastern clade of Nerita atramentosa is also widespread in northern New Zealand (43 specimens, 10 localities), but no significant genetic differentiation is explained by the Tasman Sea, a 2000-km-wide oceanic barrier. Spatial genetic structure was not detected within either clade, consistent with the species' dispersive planktotrophic phase lasting for 5-6 months. Digital analysis of operculum colouration revealed substantial differences between eastern (tan) and western (black) specimens. Genetic analysis and visual inspection of 88 Australian specimens revealed a completely nonrandom association between mtDNA data and operculum colouration. Independent examination of a further 7822 specimens from 14 sites in southern Australia revealed both colour morphs at all localities, but reinforced the phylogeographical data by indicating a marked turnover in colour morph abundance associated with a palaeogeographical barrier: Wilsons Promontory. This sharp biogeographical disjunction is in marked contrast to the species' high dispersal abilities. The genetic similarity of Nerita morio (Easter Island) and the eastern Australian + New Zealand lineage (1.1 +/- 0.3%) provides further evidence of long-distance dispersal in southern Nerita. Phylogenetic relationships of nine species (four genera) of Neritidae, an almost exclusively tropical gastropod family, are consistent with the hypothesis that southern temperate black nerites comprise a monophyletic radiation.