Phylogeography of the high intertidal isopod Ligia pallasii Brandt, 1833 (Isopoda: Oniscidea) from the Aleutian Islands to Monterey Bay

The genetic diversity and historical demography of the high intertidal isopod Ligia pallasii Brandt, 1833 from the eastern Pacific were assessed using mitochondrial DNA (mtDNA) sequences from portions of the cytochrome c oxidase subunit I (COI) gene collected at 21 sites spanning more than 4000 km of coastline from the Aleutian Islands, Alaska to Central California. Most of the 86 haplotypes (92%) were unique to a sampling location, but the two most common haplotypes were shared across 1000's of kilometers in the northern-most part of the distribution, where the lowest level of genetic diversity was found consistent with postglacial range expansion. Phylogenetic analysis revealed three moderately well-supported clades (N, C, S) separated by Cape Mendocino (40.0 degrees N) and Point Arena (38.9 degrees N). The three clades in Ligia pallasii were shallow compared to the deep divergences found in the congener "Ligia occidentalis" that is sympatric for part of the range, and maybe a cryptic species complex. The distribution of genetic diversity within L. pallasii as well as in all North American Ligia spp. examined to date follows a pattern of lower genetic divergence at higher latitudes. The observed pattern suggests that processes (gene flow, selection, and drift) that lead to genetic divergence may vary as a function of latitude.