Geographical variation in thermal tolerance within Southern Ocean marine ectotherms

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.