Abstract
The Southern Ocean has experienced stable, cold temperatures for over 10 million years, yet particular regions are currently undergoing rapid warming. To investigate the impacts of warming on cardiovascular oxygen transport, we compared the cardio-respiratory performance in an Antarctic notothenioid (Notothenia coriiceps) that was maintained at 0 or 5°C for 6.0-9.5 weeks. When compared at the fish's respective acclimation temperature, the oxygen consumption rate and cardiac output were significantly higher in 5°C-acclimated than 0°C-acclimated fish. The 2.7-fold elevation in cardiac output in 5°C-acclimated fish (17.4 vs. 6.5 ml min-1 kg-1) was predominantly due to a doubling of stroke volume, likely in response to increased cardiac preload, as measured by higher central venous pressure (0.15 vs. 0.08 kPa); tachycardia was minor (29.5 vs. 25.2 beats min-1). When fish were acutely warmed, oxygen consumption rate increased by similar amounts in 0°C- and 5°C-acclimated fish at equivalent test temperatures. In both acclimation groups, the increases in oxygen consumption rate during acute heating were supported by increased cardiac output achieved by elevating heart rate, while stroke volume changed relatively little. Cardiac output was similar between both acclimation groups until 12°C when cardiac output became significantly higher in 5°C-acclimated fish, driven largely by their higher stroke volume. Although cardiac arrhythmias developed at a similar temperature (~14.5°C) in both acclimation groups, the hearts of 5°C-acclimated fish continued to pump until significantly higher temperatures (CTmax for cardiac function 17.7 vs. 15.0°C for 0°C-acclimated fish). These results demonstrate that N. coriiceps is capable of increasing routine cardiac output during both acute and chronic warming, although the mechanisms are different (heart rate-dependent versus primarily stroke volume-dependent regulation, respectively). Cardiac performance was enhanced at higher temperatures following 5°C acclimation, suggesting cardiovascular function may not constrain the capacity of N. coriiceps to withstand a warming climate.
Highlights
The Southern Ocean has been characterised by stable, frigid temperatures (−1.9 to +1.5°C; Littlepage, 1965; Zachos et al, 2001) for over 10 million years that may have resulted in the evolution of stenothermy in its ectothermic inhabitants (Peck et al, 2014; Beers and Jayasundara, 2015)
Cardiac arrhythmias developed at a similar temperature (~14.5°C) in both acclimation groups, the hearts of 5°C-acclimated fish continued to pump until significantly higher temperatures (CTmax for cardiac function 17.7 vs. 15.0°C for 0°C-acclimated fish)
In Trematomus bernacchii, 4.5°C acclimation did not alter the temperature at which cardiac arrhythmia develops (Tarr) during acute warming (Jayasundara et al, 2013), which is in contrast to similar studies in temperate fishes such as Gillichthys mirabilis (Jayasundara and Somero, 2013), Carassius auratus (Ferreira et al, 2014) and Rutilus rutilus (Badr et al, 2016), in which warm acclimation increased Tarr
Summary
The Southern Ocean has been characterised by stable, frigid temperatures (−1.9 to +1.5°C; Littlepage, 1965; Zachos et al, 2001) for over 10 million years that may have resulted in the evolution of stenothermy in its ectothermic inhabitants (Peck et al, 2014; Beers and Jayasundara, 2015). Some areas, the Western Antarctic Peninsula region, are currently experiencing some of the most dramatic effects of climate warming in which average surface water temperatures have risen by ~1°C over the past 50 years (Meredith and King, 2005; Steig et al, 2009). This rate is expected to continue over the century (IPCC, 2014; Ashton et al 2017). Recent studies have suggested upper thermal tolerance can be improved following a period (days to weeks) of acclimation to a 4°C elevation from ambient temperatures (Podrabsky and Somero, 2006; Bilyk and DeVries, 2011). In Trematomus bernacchii, 4.5°C acclimation did not alter the temperature at which cardiac arrhythmia develops (Tarr) during acute warming (Jayasundara et al, 2013), which is in contrast to similar studies in temperate fishes such as Gillichthys mirabilis (Jayasundara and Somero, 2013), Carassius auratus (Ferreira et al, 2014) and Rutilus rutilus (Badr et al, 2016), in which warm acclimation increased Tarr
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