A Comparative Study of Asymmetric Migration Events Across a Marine Biogeographic Boundary

Abstract

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 wellknown 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.

Evolutionary Consequences of Asymmetric Dispersal Rates

“…For instance, asymmetric dispersal due to ocean currents has long been recognized as an important factor that affects the distribution, abundance, and genetic variation of marine benthic invertebrates with planktonic larvae (e.g., Scheltema 1986). A recent study of mtDNA in two barnacle and one sea urchin species off California found a signature of an excess of southward over northward dispersal, consistent with the pattern of currents (Wares et al 2001). Similar patterns should be expected in river invertebrates.…”

Environmental Correlates of Population Differentiation in Atlantic Herring

“…The barriers to gene flow detected between spawning components could presumably result from distinct hydrographic circulation systems that retain larvae and/or adults within local areas, as has been suggested for herring (Iles and Sinclair 1982) and other species including cod, Gadus morhua (Ruzzante et al 1998); sea bass, Dicentrarchus labrax (Bahri-Sfar et al 2000); and marine invertebrates with pelagic larvae (Wares et al 2001). However, in the herring populations studied here, juveniles and adults are not retained within natal areas.…”

Biogeographic structure of the northeastern Pacific rocky intertidal: the role of upwelling and dispersal to drive patterns

“…Compared to overall structure, biogeographic analyses of these groups reveal important diff erences (Supplementary material Appendix 1, Table A3 and A4). Each of these breaks occur when the three groups are combined, but some disappear when examining the groups separately, indicating that some locations are barriers only for certain dispersal types (Wares et al 2001). Th ese diff erences were spatially evident by the lack of a break at Point Conception ( ∼ 34.5 ° N) for planktotrophic species, the region between Punta Eugenia ( ∼ 28 ° N) and Punta Baja ( ∼ 30 ° N) for non-feeding planktonic species, and the vicinity around Cape Mendocino ( ∼ 41 ° N) for direct developers (Fig.…”