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

Polar fishes are at high risk from increasing seawater temperatures. Characterising the 30 physiological responses to such changes may both clarify mechanisms that permit life 31 under extreme conditions, and identify limitations in the response to continued global 32 warming. We hypothesised that Notothenia coriiceps would show physiological 33 compensation after an acute exposure to 5oC, and following 6wk warm acclimation, 34 compared to ambient temperature (0oC). However, initial tachycardia (22.4±2.8 vs. 35 12.8±1.1min-1; P<0.01) was not reversed by acclimation (21.0±1.9min-1). 36 Hyperventilation (45.5±3.1 vs. 21.4±2.4min-1; P<0.001) showed a modest reduction 37 (38.0±2.9min-1; P<0.05), while resting oxygen consumption (0.52±0.08mmol.kg–1.h– 38 1) was acutely increased at 5oC (1.07±0.10mmol.kg–1.h–1; P<0.001) but unchanged 39 with acclimation. Autonomic blockade showed initial responses were mainly of vagal 40 origin, with little subsequent withdrawal or recovery in long-term heart rate variability 41 after 6wk. Given the limited cardiorespiratory capacity to withstand sustained 42 warming, effective physiological compensation likely requires a more prolonged 43 acclimation period.