Marine ectotherms, including oysters are exposed to variable environmentalconditions in coastal shallow waters and estuaries. In the light ofglobal climate change, additional stressors like pollution might posehigher risk to populations. On the basis of the concept of oxygen- andcapacity-limited thermal tolerance in aquatic ectotherms (40), weshow that a persistent pollutant, cadmium, can have detrimentaleffects on oysters (Crassostrea virginica). During acute warmingfrom 20 to 28°C (4°C/48 h) standard metabolic rate (SMR) rose incontrol and cadmium-exposed (50 g Cd2 /l) animals, with a consistentlyhigher SMR in Cd-exposed oysters. Additionally, Cd-exposedoysters showed a stronger temperature-dependent decrease in hemolymphoxygen partial pressures. This observation indicates that theeffect of temperature on aerobic metabolism was exacerbated due tothe additional Cd stress. The oxygen delivery systems could notprovide enough oxygen to cover Cd-induced elevated metabolicdemands at high temperatures. Interestingly, cardiac performance(measured as the heart rate and hemolymph supply to tissues) rose toa similar extent in control and Cd-exposed oysters with warmingindicating that cardiac output was unable to compensate for elevatedenergy demand in Cd-exposed oysters. Together with the literaturedata on metal-induced reduction of ventilatory capacity, these findingssuggest that synergistic effects of elevated temperatures and cadmiumexposure led to oxygen limitation by impaired performance in oxygensupply through ventilation and circulation. Overall, cadmium exposureresulted in progressive hypoxemia in oysters at high temperatures,suggesting that the thermal tolerance window is narrowed inmarine ectotherms inhabiting polluted areas compared with pristineenvironments.