Intra-specific variation of ocean acidification effects in marine mussels and oysters: integrative physiological studies on tissue and organism responses

Ocean acidification (OA), caused by the oceanic uptake of anthropogenic CO2, is predicted to negatively affect marine mussels and oysters. In addition, the rapid rate at which OA occurs may outpace species’ ability to genetically adapt, leaving pre-existing genetic variation as a potential key to species resilience under OA. Against this backdrop, this thesis investigated the physiological mechanisms underlying intra-specific variation of OA sensitivity of Kiel Fjord blue mussels (Mytilus edulis) and Sydney rock oysters (Saccostrea glomerata). A long-term CO2 acclimation experiment with different family lines of blue mussel revealed that families whose offspring successfully settled at all experimental PCO2 levels (control, intermediate and high PCO2 level) were characterised by an inherently higher metabolic capacity at the whole animal and the cellular level compared to more sensitive family lines, whose offspring failed to survive at the highest experimental PCO2. This increased metabolic scope of tolerant family lines seems to cover elevated metabolic costs at the intermediate PCO2, however; at the highest PCO2, filtration rates and gill aerobic capacity declined, indicating an unfavourable shift in energy demand and supply. A second comparative CO2 acclimation study between a wild population of Sydney rock oysters and a more CO2 tolerant aquaculture line (selected for faster growth) showed that, in contrast to wild oysters, selected oysters were able to avoid a CO2-induced drop of extracellular pH, likely facilitated by an increased capacity for systemic CO2 release due to higher and more energetically efficient filtration rates. In conclusion, the observed pre-existing intra-specific variation in both species suggests potential adaptive capacities. However, as the physiology of marine bivalves is tightly linked with their functions within ecosystems, observed negative OA effects could have far reaching consequences at an ecosystem scale.