Dissertation: Physiological ecology of native and invasive blue mussels (genus Mytilus) in Central California

Caren Braby, caren.braby@gmail.com

Abstract: Marine species invasions pose serious ecological and economic threats to coastal areas world-wide, however, we know little about why certain species are able to succeed in a novel habitat. In this study, I used congeners in the blue mussel species complex, Mytilus trossulus (non-invasive) and M. galloprovincialis (invasive from the Mediterranean), as a model system to address the role ecological physiology plays in determining which species succeed. It has been suggested that, within the California hybrid zone, the distribution of these species is dependent on salinity and temperature gradients and that salinity is the more important of the two physical variables. In this study, I focus on characterizing 8 sites in San Francisco and Monterey Bays by quantifying: 1) temperature and salinity variability, 2) adult distribution, and 3) recruitment patterns. The temperature and salinity characteristics of each site fall along an estuarine-oceanic gradient, with estuarine sites exhibiting greater temperature variability and lower salinity. Using genetic identification to map adult populations, I find that M. trossulus increases in abundance as one moves up this estuarine gradient. Physiological tolerances of each species in response to acute temperature and salinity stress indicate that there is a characteristic break in heart activity during hyperthermal, hypothermal and hypohaline stress. This break in heart activity is associated with valve closure and indicates a non-lethal event, evidenced by consistent recovery of heart activity upon return to non-stressful conditions. Using acclimation mortality, heat shock protein expression and heart activity parameters, it is clear that M. trossulus is better suited to cooler conditions than the invasive M. galloprovincialis and that the two species are equal in their tolerances to low salinity. The latter finding argues against the importance of salinity in driving adult distribution. Recruitment patterns indicate that there is great spatiotemporal variability in the seasonality and magnitude of recruitment events and that these patterns are species-specific. The genetic techniques used here to differentiate among Mytilus genotypes are imperative for understanding specieslevel recruitment patterns, which have not been rigorously established in any other large-scale study on Mytilus recruitment. In Monterey Bay, recruitment correlates with upwelling and chlorophyll abundance, as has been observed in other coastal species. Small-scale recruitment patterns show that larval retention and tolerance of hyposalinity may play a role in recruitment patterns deep in the estuary. At more oceanic sites, recruitment patterns indicate that a well-mixed pool supplies larvae to all sites. This work suggests that physiological tolerance may play a role in this species invasion, but that it is most likely going to play a role at the larval or juvenile stage rather than at the adult life stage.