Larval development has strong impacts on dispersal potential and gene flow among populations of marine invertebrates. However, Pleistocene glaciations have also played an important role in shaping population structure in benthic taxa in the Northern Hemisphere, even those with planktotrophic larvae. Each glacial advance tended to fragment species distributions, often separating populations for long periods and setting the stage for their differentiation. This study examines patterns of sequence divergence of the mitochondrial cytochrome c oxidase subunit I gene in North American populations of two bivalve species complexes, Hiatella arctica s. l. and Macoma balthica s. l., with complementary data from the nuclear internal transcribed spacer-2 (ITS2) gene for the latter. Our results confirm the presence of two known species from the M. balthica complex in Canada, but also provide evidence for a third clade in Atlantic Canada. Our study confirms that the H. arctica complex in Canada contains at least four species, with support for a novel clade (Hiatella N) in the northeastern Pacific. Our results extend the range of a previously identified Hiatella clade (K) to include the northwestern Atlantic. Both M. balthica s. l. and H. arctica s. l. have broad Holarctic distributions and planktotrophic larvae, but this work reveals differences in phylogeographic structure and genetic diversity.
We thank Barry McDonald, Christina Carr, Katrin Iken, Melissa Frey, Paolo Pierossi, Rick Harbo, Sarah Hardy and Suzanne Dufour for aid in specimen collection, and staff at the CCDB for support in sequence acquisition. We also thank Elizabeth Boulding and Sean Prosser for advice on molecular techniques
and feedback on data analysis. Fieldwork in Churchill, Manitoba was conducted under permits issued by Manitoba Conservation Wildlife and Ecosystem Protection to the Churchill Northern Studies Centre (CNSC) for research in the Churchill Wildlife Management Area. Collections in Alaska were conducted under
a fish resource permit granted to Sarah Hardy by the State of Alaska Department of Fish and Game for scientific/educational purposes. Collections in New Brunswick and Labrador were conducted under experimental licenses from Fisheries and Oceans Canada. This research was funded, in part, by the NSERC
Canadian Healthy Oceans Network and by a NSERC Discovery Grant to PDNH. Field work was aided by a Northern Scientific Training Program grant to KKSL from Indian and Northern Affairs Canada. Sequence analysis was enabled by funding from the government of Canada through Genome Canada and
the Ontario Genomics Institute in support of the International Barcode of Life Project.