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 ( \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} }{}$\dot{Q}$\end{document} ), 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 \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} }{}$\dot{Q}$\end{document} , 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 \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} }{}$\dot{Q}$\end{document} 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 \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} }{}$\dot{Q}$\end{document} 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 \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} }{}$\dot{Q}$\end{document} , while C. rastrospinosus (like N. coriiceps ) held at 4°C for 1 week maintained maximum \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} }{}$\dot{Q}$\end{document} 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.