Within the second phase of the German ocean acidification research programme BIOACID (2012-2015) we investigate how the combined effects of ocean acidification and warming (OAW) affect different life stages and interactions between Atlantic and Polar cod and their prey. Objectives include addressing the question whether OAW affects interacting species differently due to divergent physiological optima and ranges, expressed in thermal tolerance windows and associated performance capacities and phenologies of specific life stages. We aim to identify fundamental mechanisms by unravelling the connections between levels of biological organisation, from genomic, molecular to cellular, individual and population level. Scopes for acclimation (physiology and behaviour) and adaptation (evolution) that together define species resilience are studied in various life stages (eggs, larvae, juveniles, adults) to identify the most sensitive one(s). Functional determinants of individual fitness such as ion and acid-base regulation, mitochondrial energy metabolism, and immune response, as well as their dependence on food quality and availability are also examined.