Heart rate and ventilation in Antarctic fishes are largely determined by ecotype

Acute and chronic cold exposure differentially affect cardiac control, but not cardiorespiratory function, in resting Atlantic salmon (Salmo salar)

No studies have examined the effects of cold temperatures (∼0-1 °C) on in vivo cardiac function and control, and metabolism, in salmonids. Thus, we examined: 1) how acclimation to 8 °C vs. acclimation (>3 weeks) or acute exposure (8-1 °C at 1 °C h-1) to 1 °C influenced cardiorespiratory parameters in resting Atlantic salmon; and 2) if/how the control of cardiac function was affected. Oxygen consumption ( M ˙ O 2 ) and cardiac function [i.e., heart rate (f H) and cardiac output (Q ˙ ) ] were 50% lower in the acutely cooled and 1oC-acclimated salmon as compared to 8 °C fish, whereas stroke volume (VS) was unchanged. Intrinsic f H was not affected by whether the fish were acutely exposed or acclimated to 1 °C (values ∼51, 24 and 21 beats min-1 in 8 and 1 °C-acclimated fish, and 8-1 °C fish, respectively), and in all groups f H was primarily under adrenergic control/tone (cholinergic tone 13-18%; adrenergic tone 37-70%). However, β-adrenergic blockade resulted in a 50% increase in VS in the 1oC-acclimated group, and this was surprising as circulating catecholamine levels were ∼1-3 nM in all groups. Overall, the data suggest that this species has a limited capacity to acclimate to temperatures approaching 0 °C. However, we cannot exclude the possibility that cardiac and metabolic responses are evoked when salmon are cooled to ∼ 0-1 °C, and that this prevented further declines in these parameters (i.e., they 'reset' quickly). Our data also provide further evidence that VS is temperature insensitive, and strongly suggest that changes in adrenoreceptor mediated control of venous pressure/capacitance occur when salmon are acclimated to 1 °C.

Understanding the Metabolic Capacity of Antarctic Fishes to Acclimate to Future Ocean Conditions

Antarctic fishes have evolved under stable, extreme cold temperatures for millions of years. Adapted to thrive in the cold environment, their specialized phenotypes will likely render them particularly susceptible to future ocean warming and acidification as a result of climate change. Moving from a period of stability to one of environmental change, species persistence will depend on maintaining energetic equilibrium, or sustaining the increased energy demand without compromising important biological functions such as growth and reproduction. Metabolic capacity to acclimate, marked by a return to metabolic equilibrium through physiological compensation of routine metabolic rate (RMR), will likely determine which species will be better poised to cope with shifts in environmental conditions. Focusing on the suborder Notothenioidei, a dominant group of Antarctic fishes, and in particular four well-studied species, Trematomus bernacchii, Pagothenia borchgrevinki, Notothenia rossii, and N. coriiceps, we discuss metabolic acclimation potential to warming and CO2-acidification using an integrative and comparative framework. There are species-specific differences in the physiological compensation of RMR during warming and the duration of acclimation time required to achieve compensation; for some species, RMR fully recovered within 3.5 weeks of exposure, such as P. borchgrevinki, while for other species, such as N. coriiceps, RMR remained significantly elevated past 9 weeks of exposure. In all instances, added exposure to increased PCO2, further compromised the ability of species to return RMR to pre-exposure levels. The period of metabolic imbalance, marked by elevated RMR, was underlined by energetic disturbance and elevated energetic costs, which shifted energy away from fitness-related functions, such as growth. In T. bernacchii and N. coriiceps, long duration of elevated RMR impacted condition factor and/or growth rate. Low growth rate can affect development and ultimately the timing of reproduction, severely compromising the species' survival potential and the biodiversity of the notothenioid lineage. Therefore, the ability to achieve full compensation of RMR, and in a short-time frame, in order to avoid long term consequences of metabolic imbalance, will likely be an important determinant in a species' capacity to persist in a changing environment. Much work is still required to develop our understanding of the bioenergetics of Antarctic fishes in the face of environmental change, and a targeted approach of nesting a mechanistic focus in an ecological and comparative framework will better aid our predictions on the effect of global climate change on species persistence in the polar regions.

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.

Exploring nature's natural knockouts: in vivo cardiorespiratory performance of Antarctic fishes during acute warming

We tested the hypothesis that blackfin icefish (Chaenocephalus aceratus), one of the six species in the family Channichthyidae (the icefishes) that do not express haemoglobin and myoglobin, lack regulatory cardiovascular flexibility during acute warming and activity. The experimental protocols were designed to optimize the surgical protocol and minimize stress. First, minimally invasive heart rate (f_H) measurements were made during a thermal ramp until cardiac failure in C. aceratus and compared with those from the closely related red-blooded black rockcod (Notothenia coriiceps). Then, integrative cardiovascular adjustments were more extensively studied using flow probes and intravascular catheters in C. aceratus during acute warming (from 0 to 8°C) at rest and after imposed activity. Chaenocephalus aceratus had a lower routine f_H than N. coriiceps (9 beats min^-1 versus 14 beats min^-1) and a lower peak f_H during acute warming (38 beats min^-1 versus 55 beats min^-1) with a similar cardiac breakpoint temperature (13 and 14°C, respectively). Routine cardiac output (Q̇) for C. aceratus at ∼0°C was much lower (26.6 ml min^-1 kg^-1) than previously reported, probably because fish in the present study had a low f_H (12 beats min^-1) indicative of a high routine vagal tone and low stress. Chaenocephalus aceratus increased oxygen consumption during acute warming and with activity. Correspondingly, Q̇ increased considerably (maximally 86.3 ml min^-1 kg^-1), as did vascular conductance (5-fold). Thus, unlike earlier suggestions, these data provide convincing evidence that icefish can mount a well-developed cardiovascular regulation of heart rate, cardiac output and vascular conductance, and this regulatory capacity provides flexibility during acute warming.

Acclimation potential of Arctic cod (Boreogadus saida) from the rapidly warming Arctic Ocean

As a consequence of the growing concern about warming of the Arctic Ocean, this study quantified the thermal acclimation responses of Boreogadus saida, a key Arctic food web fish. Physiological rates for cardio-respiratory functions as well as critical maximum temperature (T_c,max) for loss of equilibrium (LOE) were measured. The transition temperatures for these events (LOE, the rate of oxygen uptake and maximum heart rate) during acute warming were used to gauge phenotypic plasticity after thermal acclimation from 0.5°C up to 6.5°C for 1 month (respiratory and T_c,max measurements) and 6 months (cardiac measurements). T_c,max increased significantly by 2.3°C from 14.9°C to 17.1°C with thermal acclimation, while the optimum temperature for absolute aerobic scope increased by 4.5°C over the same range of thermal acclimation. Warm acclimation reset the maximum heart rate to a statistically lower rate, but the first Arrhenius breakpoint temperature during acute warming was unchanged. The hierarchy of transition temperatures was quantified at three acclimation temperatures and was fitted inside a Fry temperature tolerance polygon to better define ecologically relevant thermal limits to performance of B. saida We conclude that B. saida can acclimate to 6.5°C water temperatures in the laboratory. However, at this acclimation temperature 50% of the fish were unable to recover from maximum swimming at the 8.5°C test temperature and their cardio-respiratory performance started to decline at water temperatures greater than 5.4°C. Such costs in performance may limit the ecological significance of B. saida acclimation potential.

Cardiorespiratory responses in an Antarctic fish suggest limited capacity for thermal acclimation

Polar fishes are at high risk from increasing seawater temperatures. Characterising the physiological responses to such changes may both clarify mechanisms that permit life under extreme conditions and identify limitations in the response to continued global warming. We hypothesised that Notothenia coriiceps would show physiological compensation after an acute exposure to 5°C, and following 6 weeks warm acclimation, compared with ambient temperature (0°C). However, initial tachycardia (22.4±2.8 versus 12.8±1.1 min(-1); P<0.01) was not reversed by acclimation (21.0±1.9 min(-1)). Hyperventilation (45.5±3.1 versus 21.4±2.4 breaths min(-1); P<0.001) showed a modest reduction following acclimation (38.0±2.9 min(-1); P<0.05), while resting oxygen consumption (0.52±0.08 mmol kg(-1) h(-1)) was acutely increased at 5°C (1.07±0.10 mmol kg(-1) h(-1); P<0.001) but unchanged with acclimation. Autonomic blockade showed initial responses were mainly of vagal origin, with little subsequent withdrawal or recovery in long-term heart rate variability after 6 weeks. Given the limited cardiorespiratory capacity to withstand sustained warming, effective physiological compensation probably requires a more prolonged acclimation period.

Juvenile Antarctic rockcod (Trematomus bernacchii) are physiologically robust to CO2-acidified seawater

To date, numerous studies have shown negative impacts of CO2-acidified seawater (i.e. ocean acidification, OA) on marine organisms, including calcifying invertebrates and fishes; however, limited research has been conducted on the physiological effects of OA on polar fishes and even less on the impact of OA on early developmental stages of polar fishes. We evaluated aspects of aerobic metabolism and cardiorespiratory physiology of juvenile emerald rockcod, ITALIC! Trematomus bernacchii, an abundant fish in the Ross Sea, Antarctica, to elevated partial pressure of carbon dioxide ( ITALIC! PCO2 ) [420 (ambient), 650 (moderate) and 1050 (high) μatm ITALIC! PCO2 ] over a 1 month period. We examined cardiorespiratory physiology, including heart rate, stroke volume, cardiac output and ventilation rate, whole organism metabolism via oxygen consumption rate and sub-organismal aerobic capacity by citrate synthase enzyme activity. Juvenile fish showed an increase in ventilation rate under high ITALIC! PCO2 compared with ambient ITALIC! PCO2 , whereas cardiac performance, oxygen consumption and citrate synthase activity were not significantly affected by elevated ITALIC! PCO2 Acclimation time had a significant effect on ventilation rate, stroke volume, cardiac output and citrate synthase activity, such that all metrics increased over the 4 week exposure period. These results suggest that juvenile emerald rockcod are robust to near-future increases in OA and may have the capacity to adjust for future increases in ITALIC! PCO2  by increasing acid-base compensation through increased ventilation.