Cardiorespiratory responses in an Antarctic fish suggest limited capacity for thermal acclimation

Polar fishes are at high risk from increasing seawater temperatures. Characterising the physiological responses to such changes may both clarify mechanisms that permit life under extreme conditions and identify limitations in the response to continued global warming. We hypothesised that Notothenia coriiceps would show physiological compensation after an acute exposure to 5°C, and following 6 weeks warm acclimation, compared with ambient temperature (0°C). However, initial tachycardia (22.4±2.8 versus 12.8±1.1 min^-1; P < 0.01) was not reversed by acclimation (21.0±1.9 min^-1). Hyperventilation (45.5±3.1 versus 21.4±2.4 breaths min^-1; P < 0.001) showed a modest reduction following acclimation (38.0±2.9 min^-1; P < 0.05), while resting oxygen consumption (0.52±0.08 mmol kg^-1 h^-1) was acutely increased at 5°C (1.07±0.10 mmol kg^-1 h^-1; P < 0.001) but unchanged with acclimation. Autonomic blockade showed initial responses were mainly of vagal origin, with little subsequent withdrawal or recovery in long-term heart rate variability after 6 weeks. Given the limited cardiorespiratory capacity to withstand sustained warming, effective physiological compensation probably requires a more prolonged acclimation period.