Latitudinal comparisons of the Southern Ocean limpet, Nacella concinna, and clam, Laternula elliptica, acclimated to 0.0 °C, were used to assess differences in thermal response to two regimes, 0.0, 5.1 to 10.0 °C and 2.5, 7.5 to 12.5 °C, raised at 5.0 °C per week. At each temperature, tissue energy status was measured through a combination of O2 consumption, intracellular pH, cCO2, citrate synthase (CS) activity, organic acids (succinate, acetate, propionate), adenylates (ATP, ADP, AMP, ITP, PLA (phospho-L-arginine)) and heart rate. L. elliptica from Signy (60°S) and Rothera (67°S), which experience a similar thermal regime (- 2 to + 1 °C) had the same lethal (7.5-10.0 °C), critical (5.1-7.5 °C) and pejus (< 5.1 °C; = getting worse) limits with only small differences in biochemical response. N. concinna, which experiences a wider thermal regime (- 2 to + 15.8 °C), had higher lethal limits (10.0-12.5 °C). However, at their Northern geographic limit N. concinna, which live in a warmer environment (South Georgia, 54°S), had a lower critical limit (5.1-10.0 °C; O2, PLA and organic acids) than Rothera and Signy N. concinna (10.0-12.5 °C). This lower limit indicates that South Georgia N. concinna have different biochemical responses to temperatures close to their thermal limit, which may make them more vulnerable to future warming trends. Crown Copyright © 2009.