BRABY, C.E.*; SOMERO, G.N.: Larval settlement success and physiological adaptation in the patchwork distribution of introduced and native bay mussels (Mytilus spp.) in the central CA hybrid zone

California is home to two bay mussel species: the native Mytilus trossulus and the invasive M. galloprovincialis, which was introduced to southern CA in the early 1900’s and has since extended its range to the north. Currently, these two species overlap and hybridize from Monterey in the south to Cape Mendocino in the north. Within the hybrid zone, we find a mosaic or patchwork distribution of adults, with a different species composition at each site – not the smooth latitudinal gradient expected from a radiating invasion. What mechanisms drive the observed distribution and what do they suggest about the future spread of the invasive, M. galloprovincialis? We hypothesize that physiological adaptation of the adults to small-scale variation in temperature and salinity contributes to the observed distribution. Our results from a genetic survey within the hybrid zone show that adult distribution is highly correlated with both of these physical factors. In addition, laboratory-based comparative studies on adults show that the two species have different thermal limits to protein synthesis. To complement the studies on adults, we are testing whether the observed distribution of species is established by patterns of larval settlement or subsequent post-settlement mortality. Since December 2000, we have deployed larval collectors on a monthly basis at field sites in San Francisco and Monterey Bays. We will describe the abundance and species composition of settlers in these collectors and discuss whether spatio-temporal patterns of settlement influence the complex distribution observed in adult Mytilus within this mosaic hybrid zone.