Demographic history, not larval dispersal potential, explains differences in population structure of two New Zealand intertidal species

Two unresolved questions in marine population connectivity are: (1) the relative importance of contemporary and historical factors (e.g., ongoing gene flow versus past demographic events), and (2) to what extent species subject to the same evolutionary forces exhibit similar phylogeographic patterns. Here, we address these questions using two species from New Zealand's rocky shore that have very similar distributions and life histories, but very different larval dispersal abilities: the cat's eye snail Lunella smaragda has short-lived pelagic larvae (3-4 days) while the half-crab Petrolisthes elongatus has a longer pelagic larval duration (3-4 weeks). A large number of individuals of these species were collected (n = 727 and 440) at different locations (31 and 20) throughout their wide New Zealand distribution. These species were analysed for both mitochondrial DNA cytochrome oxidase I (COI) and nuclear ribosomal internal transcribed spacer (ITS-1) variation. Contrary to expectations, the species with much greater dispersal potential, P. elongatus, exhibited much greater population differentiation (> fivefold for mtDNA Phi(ST), > 50-fold for nDNA Phi(ST)). This study highlights that species along the same coast can show remarkably different patterns of population structure, and that although there appear to be some common geographic discontinuities in New Zealand, there are few common overall patterns that apply to many species. The study reinforces the observation that predictions of population structure based on life history are often not upheld, and shows that differences in demographic history may be an important factor in driving contemporary patterns of genetic diversity.