71.7 Friday, Jan. 6 Is the southern range boundary of the northern blue mussel, Mytilus trossulus, determined by constraints on larval dispersal? YUND, P.O.*; TILBURG, C.E.; MCCARTNEY, M.A.; University of New England; University of New England; University of North Carolina - Wilmington email@example.com
Constraints on larval dispersal can theoretically limit the range of marine invertebrates, but there are few concrete examples. The northern blue mussel, Mytilus trossulus, co-occurs with its congener, M. edulis, throughout the Canadian maritime provinces but abruptly decreases in abundance just south of the Bay of Fundy. The prevailing current in this area, the Eastern Maine Coastal Current (EMCC), flows from northeast to southwest, so upstream source populations should be plentiful and larval abundance high. However, the EMCC diverges from shore where M. trossulus abundance decreases, suggesting that limited mixing between the EMCC and nearshore waters may prevent larvae from returning to the coast. We tested this hypothesis by regularly collecting hydrographic and larval density data along a series of three transects, each of which extended from the nearshore waters out into the EMCC. Data from temperature loggers and an oceanographic buoy provided continuous time series to supplement the snapshot data from our cruises. Analysis of the hydrographic data indicates limited wind-driven across-shelf mixing in the northeast portion of our study region, but virtually no mixing to the southwest. Mussel larval densities were largely consistent with predictions from the hydrographic study, suggesting that a diverging coastal current can limit across-shelf larval dispersal. However, because these water masses differ in temperature, we are also considering the alternative hypothesis that the range boundary is set by thermal tolerance. We are currently testing this hypothesis via transplant experiments with juveniles and in the future will be conducting laboratory temperature experiments with larvae.