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, (HbMb) and (HbMb), with a related notothenioid, (HbMb) using an perfused heart preparation. The maximum [Formula: see text], heart rate (), maximum cardiac work () and relative ventricular mass of at 1°C were comparable to temperate-water teleosts, and acute warming to 4°C increased and , as expected. In contrast, icefish hearts accommodated a higher maximum stroke volume () and maximum [Formula: see text] at 1°C, but their unusually large hearts had a lower and maximum afterload tolerance than at 1°C. Furthermore, maximum , maximum [Formula: see text] and were all significantly higher for the HbMb condition compared with the HbMb condition, a potential selective advantage when coping with environmental warming. Like , both icefish species increased at 4°C. Acutely warming increased maximum [Formula: see text], while (like ) 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.