This project seeks to quantify impacts of ocean acidification (OA) and warming on extrapallial fluid pH (EPF-pH) of three commercially important mollusk species: eastern oyster, Atlantic sea scallop, and blue mussel. Previous research by Ries has shown that mussels exhibit greater resilience to OA than oysters and scallops and that control over calcification site pH can be an important factor controlling a speciesí response to OA. Ries and et al. propose to apply a recently developed pH-microelectrode approach to quantify mollusk EPF-pH response to combined OA and warming. Three hypotheses will be tested: are more OA-resilient mussels able to maintain baseline EPF-pH at levels above those of less OA-resilient oysters and scallops; is the EPF-pH of mussels less impacted by reductions in seawater pH than EPF-pH of scallops and oysters; and, lastly, do temperature and , and OA stress synergistically impact calcification in all three species. Controlled 90-day laboratory experiments will be conducted to quantify effects of ocean acidification (pCO2 = 400, 900, 1800 µatm) and warming (optimal + 5 ?C) on EPF-pH, calcification rate, and physiological conditions of three commercially important mollusk species. Linking EPF-pH-control to calcification response to combined OA and warming may enable rapid assessment of relative vulnerability/resilience to these stressors at species, population, and individual levels via relatively easy-to-execute EPF-pH measurements. Likewise, results from this study could be used by shellfish hatcheries to identify high-EPF-pH individuals to selectively breed for OA resistance.