Phylogeography of the supralittoral isopod Ligia occidentalis around the Point Conception marine biogeographical boundary
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AbstractAimThe 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 thePCboundary and other potential dispersal barriers on phylogeographical patterns in the rocky supralittoral isopodLigia occidentalis. This isopod has a limited dispersal potential, its distribution spans thePCregion, and it exhibits high levels of allopatric genetic differentiation south ofPC.LocationEasternPacific coastline between southernOregon and theBajaCaliforniaPeninsula.MethodsWe conducted a thorough sampling ofL.occidentalisfrom thePCarea to its northern range limit in southernOregon, and increased previous sampling coverage south ofPC. We obtained sequences of the mitochondrial genes cytochromecoxidase subunit I (COI) and 16SrDNAfor 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 sodiumpotassiumATPase subunit (NaK). We conducted phylogenetic and populationgenetic analyses.ResultsThe geographical limit between the two most divergent clades ofL.occidentalis(2025% divergence forCOI) distributed between southernOregon and theBajaCaliforniaPeninsula occurs at thePCbiogeographical boundary. Levels of allopatric genetic divergences were greatly reduced north ofPC(3.0% divergence forCOI).Main conclusionsThe geographical limit between the two mainL.occidentalisclades inCalifornia largely reflects the changes in sea surface temperature that define thePCbiogeographical boundary. Reduced levels of genetic divergence amongL.occidentalislineages found north ofPCsuggest a recent expansion in the northern range of this isopod.
