Phylogeography of the cold-water barnacle Chthamalus challengeri in the north-western Pacific: effect of past population expansion and contemporary gene flow

Aim Phylogeographical patterns of marine organisms in the north-western Pacific are shaped by the interaction of past sea-level fluctuations during glacial maxima and present-day gene flow. This study examines whether observed population differentiation in the barnacle Chthamalus challengeri, which is endemic to the north-western Pacific, can be explained by the interactions between historical glacial events and patterns of contemporary gene flow. Location Eleven locations in the north-western Pacific. Methods Partial sequences of mitochondrial cytochrome c oxidase subunit I (COI), 12S, 16S and nuclear internal transcribed spacer 1 (ITS1) were obtained from 312 individuals. Parsimony haplotype networks and analysis of molecular variance (AMOVA) were used to determine whether the observed genetic structure corresponds to marine provinces (Kuroshio Current and China Sea Coastal Provinces), zoogeographical zones (oriental and Japan warm-temperate zones) and/or potential refugial areas (Sea of Japan and East China Sea) in the north-western Pacific. Neutrality tests, mismatch distribution analysis and Bayesian skyline plots were used to infer the demographic history of C. challengeri. Results In total, 312, 117, 182 and 250 sequences were obtained for COI, 12S, 16S and ITS1, respectively. A panmictic population was revealed, which did not conform to the isolation by distance model. None of the a priori population groupings based on marine provinces, zoogeographical zones or potential refugial areas was associated with observed genetic patterns. Significant negative values from neutrality tests and the unimodal mismatch distribution and expansion patterns in Bayesian skyline plots for the COI and 16S data sets indicate a population expansion in the mid-Pleistocene (c. 200 ka). Information from the fossil record suggests that there has been a northward range shift of this species from the East China Sea or the Palaeo-Pacific coast of Japan to the Sea of Japan since the mid-Pleistocene. Main conclusions Chthamalus challengeri has experienced a population expansion and range shift since the mid-Pleistocene. The observed lack of population differentiation can be explained by this past population expansion and present-day wide-scale larval dispersal (owing to the long planktonic larval duration) across marine provinces, which have led to the successful establishment of the species in different zoogeographical zones and habitats.