SCHNEIDER, K.R.; NYLUND, K; HELMUTH, B.S.T.; Univ. of South Carolina; Univ. of South Carolina; Univ. of South Carolina: Does temperature regulate intertidal microhabitat distributions of Mytilus trossulus and M. galloprovincialis within San Francisco Bay?

The rocky intertidal is considered one of the most physiological stressful environments and is, therefore, a model system to study species distribution patterns. The Mytilus edulis (common blue mussel) complex exhibits a worldwide distribution and several abiotic factors could contribute to Mytilus distribution patterns but why the current worldwide distribution patterns exist remains unclear. In southern California, M. galloprovincialis was introduced 100 years ago and has expanded northward, replacing the endemic mussel, M. trossulus . Currently, in central CA the two mussel species overlap in their range distribution. Previous studies have suggested that M. galloprovincialis has a higher tolerance to warmer temperatures than M. trossulus. Our objective was to determine if individuals with M. trossulus and M. galloprovincialis genotypes show thermal heterogeneity between and within four intertidal sites in the San Francisco Bay. Mussel temperature loggers were placed in shaded and unshaded microhabitats within each of the sites and mussels were collected in June and September 2003 for genetic identification. Our results show that each site has a unique genotype frequency. Results also suggest that at least two of the sites M. trossulus is occupying �cooler� areas than M. galloprovincialis shown by a shift in genotype frequency between shaded and unshaded microsites. Mussel temperature data confirm that temperatures in shaded habitats are often at least 10 &deg C cooler than unshaded areas. At a third site, results suggest that shading by macroalgae may be buffering temperatures so that selection against M. trossulus is not occurring. Mussels are also surviving in higher numbers in areas that are shaded and are unable to utilize available habitat possibly due to high temperatures.