A matter of persistence: differential Late Pleistocene survival of two rocky-shore idoteid isopod species in northern Japan

The expansion-contraction (EC) model of Pleistocene biogeography and the concept of refugia are of dubious applicability for rocky-shore marine species along the northwest Pacific coast, which was not glaciated at the last glacial maximum (LGM) and likely remained largely habitable by marine communities. We examined the population structure and historical demography of two ecologically similar rocky-intertidal idoteid isopods, Idotea ochotensis and Cleantiella isopus, in northern Japan based on mitochondrial COI and (for I. ochotensis) nuclear ITS nucleotide sequences. We concluded that I. ochotensis persisted in northern Japan across one or more glacial cycles, whereas C. isopus recolonized northern Japan after the LGM. We present an alternative general model for Pleistocene biogeography in temperate to subtropical, non-glaciated coastal regions, wherein species tend to retain large population size and high genetic diversity across glacial cycles in a zone of persistence, with flanking zones of expansion and contraction where the geographical range cyclically expands and contracts (i.e., undergoes latitudinal displacements) with climatic oscillations. We also found that local sea straits had less effect in determining phylogeographic boundaries than a long stretch of unfavorable shore habitat, and that late Holocene sea current patterns appear to have affected fine-scale phylogeographic patterns.