BRABY, C.E.*; LOPEZ, J.P.; CHANEY, M.L.; SOMERO, G.N.; Hopkins Marine Station, Stanford University; Hopkins Marine Station, Stanford University; Hopkins Marine Station, Stanford University; Hopkins Marine Station, Stanford University: Relative importance of stress physiology and recruitment pressure in determining local distribution patterns

Recruitment patterns are known to play an important role in the population structure of marine invertebrates and are assumed to be important in establishing invasive species at new sites. We are interested in the contribution of both physiological tolerance and settlement intensity in determining the distribution of 2 species of Mytilus blue mussels – one of which is an invasive species – in their zone of overlap and hybridization at protected sites in northern California. Physiological tolerance to temperature and salinity stress is different between the congeners but does not appear to explain adult distribution within this geographical region. We have also monitored settlement of M. trossulus (native), M. galloprovincialis (invasive) and their hybrids at 9 sites in San Francisco and Monterey Bays from Dec’00-Aug’03. We find that the timing, abundance and species composition of settlers differs between sites, genotypes and bays. During the monitoring period, San Francisco Bay had a small settlement pulse in the early spring, while Monterey Bay had persistent and more abundant settlement in the later spring and summer. Additionally, the species composition of settlers relative to the species composition of adults at a given site has a stronger correlation in San Francisco Bay than in Monterey Bay. We therefore suggest that there are different selective processes acting on the two populations, which shape the distribution of Mytilus in this hybrid zone, and which will be important in understanding the future dynamics of this species invasion.