Understanding the extent to which populations are locally adapted is important for predicting how species will respond to climate change and in conservation and restoration planning. As a consequence of local adaptation, populations of the same species may respond differently to changing conditions. In terrestrial ecosystem restoration the selection of source material is often an important consideration, but it has seldom been considered in marine systems where local adaptation is less commonly studied. In this study, we examined whether populations of native Olympia oysters (Ostrea lurida) in northern California are locally adapted to salinity, a critical environmental factor predicted to shift with climate change. First, we spawned oysters from three sites in San Francisco Bay, and raised their offspring in common conditions in the lab. We conducted reciprocal transplants with first generation, lab-reared oysters to assess local adaptation within this large estuary. Second, we raised oysters from two sites in San Francisco Bay and one site in nearby Tomales Bay through two generations under common conditions in the lab and then subjected second generation, lab-reared oysters to different salinity regimes in the lab and assessed their survival rates.
Results/Conclusions
Survival of oysters reciprocally transplanted among sites within San Francisco Bay varied with source population and the site to which oysters were outplanted. Additionally, there was suggestive evidence that oysters transplanted to their site of origin survived better than oysters from other source populations. These data suggest possible local adaptation within an estuary, but are inconclusive. In the lab, salinity tolerances differed significantly among oyster populations. Oysters from northern San Francisco Bay, the site with the lowest recorded salinity, had significantly higher survival rates than the other two populations when challenged with low salinity. This shows that some populations of Olympia oysters, even within the same estuary, are more tolerant of extreme low salinity events, which are predicted to increase in frequency with climate change. These results have important implications for Olympia oyster restoration. Populations of Olympia oysters, the only native oyster along the west coast of North America, plummeted in the late 1800s due to overfishing and pollution. Oyster restoration is underway along the west coast. Understanding the extent and spatial scale of local adaptation is important in predicting how this foundation species will respond to changing conditions and in deciding how and where to conserve and restore oysters.