BackgroundAlthough polychaetes are one of the dominant taxa in marine communities, their distributions and taxonomic diversity are poorly understood. Recent studies have shown that many species thought to have broad distributions are actually a complex of allied species. In Canada, 12% of polychaete species are thought to occur in Atlantic, Arctic, and Pacific Oceans, but the extent of gene flow among their populations has not been tested.Methodology/Principal FindingsSequence variation in a segment of the mitochondrial cytochrome c oxidase I (COI) gene was employed to compare morphological versus molecular diversity estimates, to examine gene flow among populations of widespread species, and to explore connectivity patterns among Canada's three oceans. Analysis of 1876 specimens, representing 333 provisional species, revealed 40 times more sequence divergence between than within species (16.5% versus 0.38%). Genetic data suggest that one quarter of previously recognized species actually include two or more divergent lineages, indicating that richness in this region is currently underestimated. Few species with a tri-oceanic distribution showed genetic cohesion. Instead, large genetic breaks occur between Pacific and Atlantic-Arctic lineages, suggesting their long-term separation. High connectivity among Arctic and Atlantic regions and low connectivity with the Pacific further supports the conclusion that Canadian polychaetes are partitioned into two distinct faunas.Conclusions/SignificanceResults of this study confirm that COI sequences are an effective tool for species identification in polychaetes, and suggest that DNA barcoding will aid the recognition of species overlooked by the current taxonomic system. The consistent geographic structuring within presumed widespread species suggests that historical range fragmentation during the Pleistocene ultimately increased Canadian polychaete diversity and that the coastal British Columbia fauna played a minor role in Arctic recolonization following deglaciation. This study highlights the value of DNA barcoding for providing rapid insights into species distributions and biogeographic patterns in understudied groups.

Theoretical development in the field of community ecology needs ground proofing with empirical tests. In addition, these tests need to be continuously updated. Cottenie (2005) linked observed metacommunities to theoretical models based on whether environmental and/or spatial effects in the observed metacommunity significantly explain community structure. However, a species-sorting metacommunity with high dispersal and one with limited dispersal cannot be distinguished in this manner; both produce significant environmental and spatial effects. In the present study, we demonstrate a solution to this problem using a zooplankton rock pool metacommunity in Churchill, Manitoba, sampled in August 2006. We established a hierarchy of metacommunities in the Churchill rock bluff system-a large, across-bluff metacommunity, and small, within-bluff metacommunities. Using this spatial hierarchy, it is possible to determine the zooplankton dispersal capability in the rock bluff system and hence to link the metacommunity to its corresponding model. We found the zooplankton rock bluff system to exhibit limited dispersal, meaning that spatial effects were significant at the acrossbluff scale, but depending on the bluff, were significant or insignificant at the within-bluff scale. Environmental effects were significant at both scales. This study demonstrates a novel way to determine dispersal capabilities in species that are cryptic dispersers, and to successfully link observed metacommunities with theoretical models.Keywords Metacommunity Á Dispersal Á Spatial scales Á Rock pools Á Sub-arctic Á Zooplankton Handling editor: S. I. DodsonElectronic supplementary material The online version of this article (

The last decade has seen an increase in the frequency and breadth of application of molecular tools, many of which are beginning to shed light on long-standing questions in biogeography and evolutionary history of marine fauna. We explore new developments with respect to Arctic marine invertebrates, focusing on molecular taxonomy and phylogeography-two areas that have seen the most progress in the time-frame of the Census of Marine Life. International efforts to generate genetic 'barcodes' have yielded new taxonomic insights and applications ranging from diet analysis to identification of larval forms. Increasing availability of genetic data in public databases is also facilitating exploration of largescale patterns in Arctic marine populations. We present new case-studies in meta-population analysis of barcode data from polychaetes and echinoderms that demonstrate such phylogeographic applications. Emerging patterns from ours and other published studies include influences of a complex climatic and glacial history on genetic diversity and evolution in the Arctic, and contrasting patterns of both high gene flow and persistent biogeographic boundaries in contemporary populations.

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Divergence times for species assemblages of Arctic marine invertebrates have often been estimated using a standard rate (1.4%/MY) of molecular evolution calibrated using a single sister pair of tropical crustaceans. Because rates of molecular evolution vary among taxa and environments, it is essential to obtain clock calibrations from northern lineages. The recurrent opening and closure of the Bering Strait provide an exceptional opportunity for clock calibration. Here, we apply the iterative calibration approach to investigate patterns of molecular divergence among lineages of northern marine molluscs and arthropods using publicly available sequences of the cytochrome c oxidase subunit I (COI) gene and compare these results with previous estimates of trans-Bering divergences for echinoderms and polychaetes. The wide range of Kimura two-parameter (K2P) divergences among 73 trans-Bering sister pairs (0.12%–16.89%) supports multiple pulses of migration through the Strait. Overall, the results indicate a rate of K2P divergence of 3.2%/MY in molluscs, 5%–5.2%/MY in arthropods, and 3.5%–4.7%/MY in polychaetes. While these rates are considerably higher than the often-adopted 1.4%/MY rate, they are similar to calibrations (3%–5%/MY) in several other studies of marine invertebrates. This upward revision in rates means there is a need both to reevaluate the evolutionary history of marine lineages and to reexamine the impact of prior climatic changes upon the diversification of marine life.