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 (Q), 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.sup.-][Mb.sup.+]) and Chaenocephalus aceratus ([Hb.sup.-][Mb.sup.-]), with a related notothenioid, Notothenia coriiceps ([Hb.sup.+][Mb.sup.+]) using an in situ perfused heart preparation. The maximum Q, heart rate ([f.sub.H]), maximum cardiac work ([W.sub.C]) and relative ventricular mass of N. coriiceps at 1 [degrees]C were comparable to temperate-water teleosts, and acute warming to 4[degrees]C increased [f.sub.H] and [W.sub.C], as expected. In contrast, icefish hearts accommodated a higher maximum stroke volume ([V.sub.S]) and maximum Q at 1[degrees]C, but their unusually large hearts had a lower [f.sub.H] and maximum afterload tolerance than N. coriiceps at 1[degrees]C. Furthermore, maximum [V.sub.S], maximum Q and [f.sub.H] were all significantly higher for the [Hb.sup.-][Mb.sup.+] condition compared with the [Hb.sup.-][Mb.sup.-] condition, a potential selective advantage when coping with environmental warming. Like N. coriiceps, both icefish species increased [f.sub.H] at 4[degrees]C. Acutely warming C. aceratus increased maximum Q, while C. rastrospinosus (like N. coriiceps) held at 4[degrees]C for 1 week maintained maximum Q when tested at 4[degrees]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.