SCHNEIDER, Kimberly R.; HILBISH, Thomas J.; HELMUTH, Brian S.T.: Differential movement as a mechanism of selection in Mytilus spp.

Two predominantly intertidal species of mussel, Mytilus edulis and Mytilus galloprovincialis, successfully hybridize in southwest England. In this hybrid zone directional selection favors M. galloprovincialis and closely related hybrids and wave exposure and temperature have been implicated as potential selective factors. The objective of this study was to determine if M.edulis, M. galloprovincialis or hybrid genotypes differ in their movement patterns within mussel beds and, in doing so, are exposed to microhabitats with different levels of wave shock and/or temperature. Specifically we hypothesized that M.edulis genotypes are more likely to move to the tops of beds where their risk of dislodgement or extreme thermal stress is likely to be highest. Mixtures of M.edulis and M.galloprovincialis were covered with gravel substrate to simulate smothering by other mussels and the time to emergence was monitored over a six -day period. Differences in emergence ability between M.edulis and M.galloprovincialis were seen with M.edulis emerging more rapidly and in higher quantities than M.galloprovincialis. Mytilus edulis is thought to be more sensitive to disturbance events caused by wave shock and to warm temperatures than M. galloprovincialis. This differential movement could strongly contribute to the directional selection against M.edulis genotypes in the hybrid zone. Supported by NSF IBN-9708074 to T.J.H. and IBN-9985878 to B.S.H.