Understanding thermal limits and the ability of species to cope with changing temperatures is crucial for a cause and effect understanding of climate effects on organisms and ecosystems. Data available for marine species from various phyla and climates led to the hypothesis that a mismatch between oxygen demand and limited capacity of oxygen supply to tissues is the first mechanism to restrict survival at thermal extremes. Here we show that doubling the oxygen content of the ambient seawater from 160 mmHg partial pressure to 350 mmHg raised the upper temperature limits of the Antarctic marine bivalve Laternula elliptica by about 2.5°C. It reduced the accumulation of the anaerobic end product succinate or of total CO2 as a sign of respiratory distress. These findings provide further evidence that oxygen supply does limit thermal tolerance in marine animals. As water temperatures rise animals will face a double problem of progressively reduced oxygen solubility in the water and enhanced costs reflected in increased metabolic rates. © Springer-Verlag 2006.