Temperature is one of the most important abiotic factors affecting the performance of ectotherms, and in the context of climate change, there has been renewed interest in thermal physiology. Climate change has already resulted in range shifts of many marine and terrestrial species, including the blue mussel (genus Mytilus). The blue mussel species complex is comprised of the congeners Mytilus trossulus, M. edulis, and M. galloprovincialis, which replace each other as temperature increases. Here we consider the cold-temperate M. edulis and the warm-temperate M. galloprovincialis and their hybrids. The two parent species are physiologically and genetically distinct yet retain the ability to produce viable hybrid offspring. In this study we sampled individuals in and around a hybrid zone in southwest England and acclimated them to either 15°C or 23°C for 14 days in order to evaluate their response to persistent, lower level warming. Physiological results support previous studies suggesting feeding rate and energy limitation as the mechanisms for lower heat tolerance in M. edulis. In both respiration rate and feeding rate, there is a significant interaction between genotype and temperature. Initial transcriptomic results, however, suggest that genotype, rather than temperature, is the main source of variance among the individuals. In addition, very few genes responded differently to temperature depending on genotype, suggesting that there may be limited intra-specific plasticity in gene expression when it comes to thermal tolerance genes. Energetics analyses are currently underway to examine energy stores and other metabolic indicators in these samples.