Phylogeography of the supralittoral isopod Ligia occidentalis around the Point Conception marine biogeographical boundary
The Point Conception (PC) biogeographical boundary is defined by a transition between cold northern and warm southern water masses, accompanied by shifts in numerous ecological and environmental parameters. While these factors can potentially contribute to the genetic differentiation of lineages whose distributions span this boundary, few organisms exhibit genetic signatures of restricted gene flow across this region. We examine the effects of the PC boundary and other potential dispersal barriers on phylogeographical patterns in the rocky supralittoral isopod Ligia occidentalis. This isopod has a limited dispersal potential, its distribution spans the PC region, and it exhibits high levels of allopatric genetic differentiation south of PC.
Eastern Pacific coastline between southern Oregon and the Baja California Peninsula.
We conducted a thorough sampling of L. occidentalis from the PC area to its northern range limit in southern Oregon, and increased previous sampling coverage south of PC. We obtained sequences of the mitochondrial genes cytochrome c oxidase subunit I (COI) and 16S rDNA for the new localities and combined them with a previously published data set, resulting in a total of 58 sampling locations. We also obtained sequences of the nuclear gene sodium–potassium ATPase α-subunit (NaK). We conducted phylogenetic and population-genetic analyses.
The geographical limit between the two most divergent clades of L. occidentalis (20–25% divergence for COI) distributed between southern Oregon and the Baja California Peninsula occurs at the PC biogeographical boundary. Levels of allopatric genetic divergences were greatly reduced north of PC (≤ 3.0% divergence for COI).
The geographical limit between the two main L. occidentalis clades in California largely reflects the changes in sea surface temperature that define the PC biogeographical boundary. Reduced levels of genetic divergence among L. occidentalis lineages found north of PC suggest a recent expansion in the northern range of this isopod.