Award: OCE-1041267

The overall purpose and approach of this project was to understand the mechanisms of larval bivalve sensitivity to ocean acidification by experimentally evaluating what the early life stages of these organisms were responding to. We conducted a series of experiments using a suite of 16 different chemical treatments that allowed us to evaluate what was most important to developing bivalve larvae, the partial pressure of carbon dioxide, the mineral saturation state, or pH. We measured the proportion of normally developed larvae, the size of normally developed larvae, and in a subset of experiments we also measured feeding responses and respiration rate. The experiments were conducted on native and non-native species of oysters, clams, and mussels and repeated on several of our test species to confirm robust findings. In addition to these short-term experiments we also carried out several novel measurements on developing bivalve larvae, and have designed, built, and tested a flow-through manipulation system capable of creating unique carbonate chemistry conditions so we may conduct long-term experiments tracking larvae over the entire larval period. Over the course of the project we made several interesting discoveries in larval bivalve responses to ocean acidification. First and foremost we identified a new fundamental mechanism for the sensitivity of early bivalve larvae to calcium carbonate saturation state. During the development and calcification of the first shell the larvae build, there is an acute sensitivity to saturation state due to very rapid calcification, limited maternal energy, and an apparently greater exposure to the external environment. We have termed this a kinetic based sensitivity to ocean acidification. Our experimental data support this as we have found in three of the four species (mussels and oysters) that we were able to run experiments on, that saturation state was the only variable that had any significant impact on larval development and growth. Unfortunately our multiple attempts at culturing and experiments with larval clams failed, on three different species. Interestingly, the forth species, the native Olympia oyster showed no acute response to any of our acidification treatments during early shell formation. Further measurements allowed us to quantify the actual calcification rate for this species and the non-native Pacific oyster, and we found a significantly slower calcification in the Olympia oyster larvae, as well as large differences in energy utilization, suggesting that the slower shell formation required less energy, and thus supported our hypothesis. The original hypothesis of nativity providing resistance to ocean acidification proved to be untrue, as the native California mussel responded identically to the treatments as the introduced Mediterranean mussel. Our work strongly supports a trait-based sensitivity of bivalves (and likely other mollusks), that early shell development rate provides a strong predictor of response. We are however conducting the long-term experiments at this time, and will then be able to evaluate whether chronic exposure to low pH or acute low saturation state exposure appears more important to survival of larvae. A kinetic based sensitivity to ocean acidification is significant for several reasons; first it differs from what has been the generally accepted role of pH driving organismal responses to ocean acidification. Second, it allows us to identify what variable matters most at this fundamentally sensitive life-history stage, as in coastal zones, the carbonate system variables can decouple and predicting outcomes for organisms becomes far more complicated without intimate knowledge of whatÆs important. Third, we have shown experimentally, that ocean acidification itself is a multiple stressor, helping to frame the emerging interest in multiple stressors in marine systems. And finally, it adds a significant amount of support and underlying mechanisms ...