NYLUND, K.L.*; SCHNEIDER, K.R.; HELMUTH, B.S.T.; University of South Carolina: A fine scale study of thermal heterogeneity and mussel (Mytilus spp.) species distribution in San Francisco Bay

Thermal stress is considered a primary abiotic factor setting geographic species distribution limits. If temperature plays a prominent role in setting large scale distribution patterns, then this pattern should be reflected at the microscale in areas where species� distribution overlap. Common blue mussels of the genus Mytilus form a hybrid zone off the coast of California. North of the hybrid zone is predominately Mytilus trossulus and south of the hybrid zone is predominately M. galloprovincialis. Previous studies have suggested that M. galloprovincialis is better adapted to tolerate warmer climatic conditions than M. trossulus. Within the hybrid zone, body temperatures during aerial exposure at low tide can vary significantly over the space of a few meters. To explore how thermal heterogeneity at the local level affects species distribution, we quantified distribution of M. galloprovincialis, M. trossulus and their hybrids in shaded and unshaded microsites in the San Francisco Bay (San Bruno). We hypothesized that if M. trossulus is less tolerant of thermal stress than M. galloprovincialis, then shaded microsites should have a higher proportion of M. trossulus. We extensively sampled the San Bruno site and categorized microsites as shaded or unshaded. Preliminary data suggest different genotype frequencies between warmer (unshaded) and cooler (shaded) micrositess, with a decrease of M. trossulus alleles in unshaded areas. This correlation supports previous work that suggests that M. galloprovincialis may be better adapted to tolerate thermal stress than M. trossulus.