Across-shelf distribution of blue mussel larvae in the northern Gulf of Maine: Consequences for population connectivity and a species range boundary

Studies of population connectivity have largely focused on along-shelf, as opposed to across-shelf, processes. We hypothesized that a discontinuity in across-shelf mixing caused by the divergence of the Eastern Maine Coastal Current (EMCC) from shore acts as an ecological barrier to the supply of mussel larvae to the coast. Existing data on the relative abundance of two congeneric blue mussels,Mytilus edulisandM. trossulus, were analysed to quantify the association ofM. trossuluswith the colder temperature signal of the EMCC and generate larval distribution predictions. We then sampled the across-shelf distribution of larvae along two transects during 2011. Larvae were identified using restriction digests of PCR amplicons from the mitochondrial 16S rDNA.Mytilus edulislarvae were consistently abundant on either the inshore and offshore transect ends, but not homogeneously distributed across the shelf, whileM. trossuluslarvae were less common throughout the study area. The divergence of the EMCC from shore appears to create a break in the connectivity ofM. edulispopulations by isolating those inshore of the EMCC from upstream larval sources. Across-shelf transport processes can thus produce connectivity patterns that would not be predicted solely on the basis of along-shelf processes.