A comparative study of asymmetric migration events across a marine biogeographic boundary.

In many nonclonal, benthic marine species, geographic distribution is mediated by the dispersal of their larvae. The dispersal and recruitment of marine larvae may be limited by temperature gradients that can affect mortality or by ocean currents that can directly affect the movements of pelagic larvae. We focus on Point Conception, a well-known biogeographic boundary between the Californian and Oregonian biogeographic provinces, to investigate whether ocean currents affect patterns of gene flow in intertidal marine invertebrates. The predominance of pelagically dispersing species with northern range limits at Point Conception suggests that ocean currents can affect species distributions by erecting barriers to the dispersal of planktonic larvae. In this paper, we investigate whether the predominantly southward currents have left a recognizable genetic signature in species with pelagically dispersing larvae whose ranges span Point Conception. We use patterns of genetic diversity and a new method for inferring cladistic migration events to test the hypothesis that southward currents increase southward gene flow for species with pelagically dispersing larvae. We collected mitochondrial DNA (mtDNA) sequence data for the barnacles Balanus glandula and Chthamalus fissus and also reanalyzed a previously published mtDNA dataset (Strongylocentrotus purpuratus, Edmands et al. 1996). For all three species, our cladistic approach identified an excess of southward migration events across Point Conception. In data from a fourth species with nondispersing larvae (Nucella emarginata, Marko 1998), our method suggests that ocean currents have not played a role in generating genetic structure.