Maximum cardiac performance of Antarctic fishes that lack haemoglobin and myoglobin: exploring the effect of warming on nature’s natural knockouts

Antarctic notothenioids, some of which lack myoglobin (Mb) and/or haemoglobin (Hb), are considered extremely stenothermal, which raises conservation concerns since Polar regions are warming at unprecedented rates. Without reliable estimates of maximum cardiac output ([Formula: see text]), it is impossible to assess their physiological scope in response to warming seas. Therefore, we compared cardiac performance of two icefish species, Chionodraco rastrospinosus (Hb(−)Mb(+)) and Chaenocephalus aceratus (Hb(−)Mb(−)), with a related notothenioid, Notothenia coriiceps (Hb(+)Mb(+)) using an in situ perfused heart preparation. The maximum [Formula: see text] , heart rate (f(H)), maximum cardiac work (W(C)) and relative ventricular mass of N. coriiceps at 1°C were comparable to temperate-water teleosts, and acute warming to 4°C increased f(H) and W(C), as expected. In contrast, icefish hearts accommodated a higher maximum stroke volume (V(S)) and maximum [Formula: see text] at 1°C, but their unusually large hearts had a lower f(H) and maximum afterload tolerance than N. coriiceps at 1°C. Furthermore, maximum V(S), maximum [Formula: see text] and f(H) were all significantly higher for the Hb(−)Mb(+) condition compared with the Hb(−)Mb(−) condition, a potential selective advantage when coping with environmental warming. Like N. coriiceps, both icefish species increased f(H) at 4°C. Acutely warming C. aceratus increased maximum [Formula: see text] , while C. rastrospinosus (like N. coriiceps) held at 4°C for 1 week maintained maximum [Formula: see text] when tested at 4°C. These experiments involving short-term warming should be followed up with long-term acclimation studies, since the maximum cardiac performance of these three Antarctic species studied seem to be tolerant of temperatures in excess of predictions associated with global warming.