Discordant population expansions in four species of coral-associated Pacific hermit crabs (Anomura: Diogenidae) linked to habitat availability resulting from sea-level change

AimTo explore the spatial and temporal genetic structure of four tropical hermit crabs in the genus Calcinus with varying habitat use in order to test for impacts of sea-level change and demography in shaping contemporary population structure and to understand how this structure relates to the process of speciation in the genus. LocationCentral Pacific. MethodsWe sequenced 586bp of the cytochrome c oxidase subunit I gene (COI) (n=702) and analysed the data for population differentiation and demographic changes. We complemented the sequences with abundance surveys of targeted species and modelled potential habitat availability in response to sea-level change. ResultsSequence data revealed high haplotype diversity (h=0.685-0.983) and low nucleotide diversity (=0.002-0.018) for all species. There was little population differentiation and extensive geographical haplotype sharing within all species, despite dramatic shifts in the abundance of species among locations, which appear to result from ecological interactions rather than limits to contemporary larval dispersal. Population expansion occurred most recently in the subtidal species Calcinushaigae (30-70ka), whereas the intertidal species showed earlier expansions: Calcinusseurati: 110-140ka, and Calcinuslaevimanus: 220-250ka. Differences in timing can be linked to the prediction that subtidal habitat area is strongly affected by changing sea level, while intertidal habitat area is affected less strongly. Interestingly, unlike the West Pacific, habitat in the Central Pacific remains relatively equidistant. Main conclusionsHabitat availability during sea-level fluctuations, rather than dispersal limitation per se, appears to be a potential force shaping the population genetic patterns of subtidal species, and perhaps speciation of tropical Calcinus hermit crabs. Insofar as this represents a general pattern, historical sea-level change may play a role in structuring Central Pacific reef species through alteration of habitat availability rather than isolation and vicariance.