Aerobic scope does not predict the performance of a tropical eurythermal fish at elevated temperatures

Ecological features of upriver migration in Kitakami River chum salmon and their connection to aerobic thermal performance

The physiological performance of ectotherms is influenced by temperature, raising concerns about the impact of global warming on ectotherms. Understanding the relationship between ecologically relevant temperatures and the physiological performance of ectotherms provides a basis for assessing their resilience to changing environments. Absolute aerobic scope (AAS) is a functional metric of the thermal performance of aquatic ectotherms. The thermal profile of chum salmon (Oncorhynchus keta) returning to the Kitakami River, Japan, in early October has already been explored in a previous study; however, the ecological aspects of their upriver migration (e.g. spawning site, migratory duration and experienced temperature) and their connection to AAS thermal profiles are not fully understood. To address this gap, we released 53 marked chum salmon throughout the spawning season (October-November), of which 48 were tracked using radio telemetry. Over 3 years, 18 were successfully tracked to their spawning sites, and 13 were tracked partway. The longest track was 93 km. The spawning sites of Kitakami River chum salmon depended on migration timing, with earlier run salmon tending towards upriver sites. Chum salmon returning in October spawned in the middle basin, typically requiring >5 days to reach the spawning sites, whereas those returning in November spawned in the lower sections in 1-3 days. Comparing the estimated thermal occupancy of migrating salmon with the published AAS profile, we found that Kitakami River chum salmon in early October spent almost all of their time within the optimal temperature window for AAS and tended to be below the peak temperature of AAS. Our findings provide a basis for the ecological features of migrating chum salmon in rivers and shed light on their aerobic thermal performance in the natural environment.

The Impact of Acclimation on Standard and Maximum Metabolic Rate in a Small Freshwater Fish

AbstractThe ability of freshwater fish to acclimate quickly to water temperature variation is imperative when living in shallow changeable environments. However, while it has often been assumed that maximum metabolic rate is constant and therefore that metabolic scope (the difference between maximum and standard metabolic rates) decreases with ambient temperature, this assumption is weakly supported and remains controversial. We investigated acclimation in a temperate, shallow-dwelling Australian freshwater fish, the Pacific blue-eye (Pseudomugil signifer), to rising water temperatures. We placed wild-caught fish into three acclimation treatments (24°C, 28°C, and 30°C) and measured metabolic rate at three test temperatures (24°C, 28°C, and 30°C). We found that fish acclimated (recovered standard metabolic rate) to housing temperatures before the first measurement at 10 d. Moreover, we found that regardless of acclimation temperature, standard metabolic rate, maximum metabolic rate, and aerobic scope all increased with test temperature. Our findings suggest that maximum metabolic rate and metabolic scope can adjust rapidly to ambient temperature. More research is needed to understand the generality of these effects, as well as their consequences for fitness.

Hyperoxia disproportionally benefits the aerobic performance of large fish at elevated temperature

Increasing evidence shows that larger fish are more vulnerable to acute warming than smaller individuals of the same species. This size-dependency of thermal tolerance has been ascribed to differences in aerobic performance, largely owing to a decline in oxygen supply relative to demand. To shed light on these ideas, we examined metabolic allometry in 130 rainbow trout ranging from 12 to 358 g under control conditions (17°C) and in response to acute heating (to 25°C), with and without supplemental oxygen (100% versus 150% air saturation). Under normoxia, high temperature caused an average 17% reduction in aerobic scope compared with 17°C. Aerobic performance disproportionally deteriorated in bigger fish as the scaling exponent (b) for aerobic scope declined from b=0.87 at 17°C to b=0.74 at 25°C. Hyperoxia increased maximum metabolic rate and aerobic scope at both temperatures and disproportionally benefited larger fish at 25°C as the scaling exponent for aerobic scope was reestablished to the same level as at 17°C (b=0.86). This suggests that hyperoxia may provide metabolic refuge for larger individuals, allowing them to sustain aerobic activities when facing acute warming. Notably, the elevated aerobic capacity afforded by hyperoxia did not appear to improve thermal resilience, as mortality in 25°C hyperoxia (13.8%, n=4) was similar to that in normoxia (12.1%, n=4), although we caution that this topic warrants more targeted research. We highlight the need for mechanistic investigations of the oxygen transport system to determine the consequences of differential metabolic scaling across temperature in a climate warming context.

The upper temperature and hypoxia limits of Atlantic salmon (Salmo salar) depend greatly on the method utilized

In this study, Atlantic salmon were: (i) implanted with heart rate (fH) data storage tags (DSTs), pharmacologically stimulated to maximum fH, and warmed at 10°C h-1 (i.e. tested using a 'rapid screening protocol'); (ii) fitted with Doppler® flow probes, recovered in respirometers and given a critical thermal maximum (CTmax) test at 2°C h-1; and (iii) implanted with fH DSTs, recovered in a tank with conspecifics for 4 weeks, and had their CTmax determined at 2°C h-1. Fish in respirometers and those free-swimming were also exposed to a stepwise decrease in water oxygen level (100% to 30% air saturation) to determine the oxygen level at which bradycardia occurred. Resting fH was much lower in free-swimming fish than in those in respirometers (∼49 versus 69 beats min-1) and this was reflected in their scope for fH (∼104 versus 71 beats min-1) and CTmax (27.7 versus 25.9°C). Further, the Arrhenius breakpoint temperature and temperature at peak fH for free-swimming fish were considerably greater than for those tested in the respirometers and given a rapid screening protocol (18.4, 18.1 and 14.6°C; and 26.5, 23.2 and 20.2°C, respectively). Finally, the oxygen level at which bradycardia occurred was significantly higher in free-swimming salmon than in those in respirometers (∼62% versus 53% air saturation). These results: highlight the limitations of some lab-based methods of determining fH parameters and thermal tolerance in fishes; and suggest that scope for fH may be a more reliable and predictive measure of a fish's upper thermal tolerance than their peak fH.

Metabolic resilience of the Australasian snapper (Chrysophrys auratus) to marine heatwaves and hypoxia

Marine organisms are under threat from a simultaneous combination of climate change stressors, including warming sea surface temperatures (SST), marine heatwave (MHW) episodes, and hypoxic events. This study sought to investigate the impacts of these stressors on the Australasian snapper (C. auratus) - a finfish species of high commercial and recreational importance, from the largest snapper fishery in Aotearoa New Zealand (SNA1). A MHW scenario was simulated from 21°C (current February SST average for north-eastern New Zealand) to a future predicted level of 25°C, with the whole-animal and mitochondrial metabolic performance of snapper in response to hypoxia and elevated temperature tested after 1-, 10-, and 30-days of thermal challenge. It was hypothesised that key indicators of snapper metabolic performance would decline after 1-day of MHW stress, but that partial recovery might arise as result of thermal plasticity after chronic (e.g., 30-day) exposures. In contrast to this hypothesis, snapper performance remained high throughout the MHW: 1) Aerobic metabolic scope increased after 1-day of 25°C exposure and remained high. 2) Hypoxia tolerance, measured as the critical O₂ pressure and O₂ pressure where loss of equilibrium occurred, declined after 1-day of warm-acclimation, but recovered quickly with no observable difference from the 21°C control following 30-days at 25°C. 3) The performance of snapper mitochondria was also maintained, with oxidative phosphorylation respiration and proton leak flux across the inner mitochondrial membrane of the heart remaining mostly unaffected. Collectively, the results suggest that heart mitochondria displayed resilience, or plasticity, in snapper chronically exposed to 25°C. Therefore, contrary to the notion of climate change having adverse metabolic effects, future temperatures approaching 25°C may be tolerated by C. auratus in Northern New Zealand. Even in conjunction with supplementary hypoxia, 25°C appears to represent a metabolically optimal temperature for this species.

Diet changes thermal acclimation capacity, but not acclimation rate, in a marine ectotherm (Girella nigricans) during warming

Global climate change is increasing thermal variability in coastal marine environments and the frequency, intensity and duration of marine heatwaves. At the same time, food availability and quality are being altered by anthropogenic environmental changes. Marine ectotherms often cope with changes in temperature through physiological acclimation, which can take several weeks and is a nutritionally demanding process. Here, we tested the hypothesis that different ecologically relevant diets (omnivorous, herbivorous, carnivorous) impact thermal acclimation rate and capacity, using a temperate omnivorous fish as a model (opaleye, Girella nigricans). We measured acute thermal performance curves for maximum heart rate because cardiac function has been observed to set upper thermal limits in ectotherms. Opaleye acclimated rapidly after raising water temperatures, but their thermal limits and acclimation rate were not affected by their diet. However, the fish's acclimation capacity for maximum heart rate was sensitive to diet, with fish in the herbivorous treatment displaying the smallest change in heart rate throughout acclimation. Mechanistically, ventricle fatty acid composition differed with diet treatment and was related to cardiac performance in ways consistent with homoviscous adaptation. Our results suggest that diet is an important, but often overlooked, determinant of thermal performance in ectotherms on environmentally relevant time scales.

Chronic experimental hyperoxia elevates aerobic scope: a valid method to test for physiological oxygen limitations in fish

Experimental hyperoxia has been shown to enhance the maximum oxygen uptake capacity of fishes under acute conditions, potentially offering an avenue to test prominent physiological hypotheses attempting to explain impacts of climate warming on fish populations (e.g., gill-oxygen limitation driving declines in fish size). Such benefits of experimental hyperoxia must persist under chronic conditions if it is to provide a valid manipulation to test the relevant hypotheses, yet the long-term benefits of experimental hyperoxia to oxygen uptake capacity have not been examined. Here, the authors measured aerobic metabolic performance of Galaxias maculatus upon acute exposure to hyperoxia (150% air saturation) and after 5 months of acclimation, at both 15°C and 20°C. Acute hyperoxia elevated aerobic scope by 74%-94% relative to normoxic controls, and an elevation of 58%-73% persisted after 5 months of hyperoxia acclimation. When hyperoxia-acclimated fish were acutely transitioned back to normoxia, they maintained superior aerobic performance compared with normoxic controls, suggesting an acclimation of the underlying metabolic structures/processes. In demonstrating the long-term benefits of experimental hyperoxia on the aerobic performance of a fish, the authors encourage the use of such approaches to disentangle the role of oxygen in driving the responses of fish populations to climate warming.

Thermal preference does not align with optimal temperature for aerobic scope in zebrafish (Danio rerio)

Warming is predicted to have negative consequences for fishes by causing a mismatch between oxygen demand and supply, and a consequent reduction in aerobic scope (AS) and performance. This oxygen and capacity limited thermal tolerance (OCLTT) hypothesis features prominently in the literature but remains controversial. Within the OCLTT framework, we hypothesised that fish would select temperatures that maximise their AS, and thus their performance. We tested this hypothesis using intermittent flow respirometry to measure AS at, above (+2.5°C) and below (-2.5°C) the self-selected, preferred temperature (Tpref) of individual zebrafish (Danio rerio). AS was greatest 2.5°C above Tpref, which was driven by an increase in maximal metabolic rate. This mismatch between Tpref and the optimal temperature for AS suggests that factor(s) aside from AS maximisation influence the thermal preference of zebrafish.

An energetics-performance framework for wild fishes

There is growing evidence that bioenergetics can explain relationships between environmental conditions and fish behaviour, distribution and fitness. Fish energetic needs increase predictably with water temperature, but metabolic performance (i.e., aerobic scope) exhibits varied relationships, and there is debate about its role in shaping fish ecology. Here we present an energetics-performance framework, which posits that ecological context determines whether energy expenditure or metabolic performance influence fish behaviour and fitness. From this framework, we present testable predictions about how temperature-driven variability in energetic demands and metabolic performance interact with ecological conditions to influence fish behaviour, distribution and fitness. Specifically, factors such as prey availability and the spatial distributions of prey and predators may alter fish temperature selection relative to metabolic and energetic optima. Furthermore, metabolic flexibility is a key determinant of how fish will respond to changing conditions, such as those predicted with climate change. With few exceptions, these predictions have rarely been tested in the wild due partly to difficulties in remotely measuring aspects of fish energetics. However, with recent advances in technology and measurement techniques, we now have a better capacity to measure bioenergetics parameters in the wild. Testing these predictions will provide a more mechanistic understanding of how ecological factors affect fish fitness and population dynamics, advancing our knowledge of how species and ecosystems will respond to rapidly changing environments.

Influence of photoperiod and protocol length on metabolic rate traits in ballan wrasse Labrus bergylta

In this study, ballan wrasse Labrus bergylta were subjected to either a conventional 1-day or an extended 5-day respirometry protocol. Additionally, in the 5-day protocol the fish were subjected to a 12 h light-dark cycle to assess the effects of photoperiods on metabolic rates (ṀO₂ ). Diurnal patterns in routine and resting ṀO₂ were not observed, suggesting that circadian rhythms in metabolism largely are driven by activity patterns rather than being of endogenous origin. Moreover, lack of a detectable circadian ṀO₂ may be an adaptation to lower costs of living in ballan wrasse. Protocol length influenced standard metabolic rates (SMR) where estimates decreased by 13% and 17% when using 48 h and 5 days, respectively, compared to 24 h. The maximum metabolic rate (MMR) and the derived absolute aerobic scope (MMR-SMR) were unaffected by protocol length. However, factorial scopes (MMR/SMR) were reduced from 8.5 to 6.4 in the 5-day protocol, showing that factorial scopes are more sensitive to how SMR are obtained. The critical oxygen tension (P_crit ) was reduced from 15% PO₂ in the 1-day group to 11% PO₂ in the 5-day group. However, ṀO₂ in response to decreasing PO₂ was similar, which together with a similar oxygen extraction coefficient, α (ṀO₂ /PO₂ ), suggested that the higher P_crit in the 1-day group was an artefact of overestimating SMR. Finally, α was 12% lower at MMR compared to at P_crit , which either means that MMR was underestimated in proportion to this difference or that α is not constant in the entire PO₂ range. In summary, this study found that a conventional 1-day respirometry protocol may overestimate SMR and thereby alter the derived P_crit and aerobic scope, while α is unaffected by protocol length. Moreover, alternating light conditions in the absence of other stressors did not influence ṀO₂ in ballan wrasse.

Effects of Dietary Shifts on Ontogenetic Development of Metabolic Rates in Age 0 Lake Sturgeon (Acipenser fulvescens)

AbstractIn many fish species, ontogenetic dietary shifts cause changes in both quantitative and qualitative intake of energy, and these transitions can act as significant bottlenecks in survival within a given year class. In the present study, we estimated routine metabolic rate (RMR) and forced maximum metabolic rate (FMR) in age 0 lake sturgeon (Acipenser fulvescens) on a weekly basis from 6 to 76 days posthatch (dph) within the same cohort of fish. We were particularly interested in the period of dietary transition from yolk to exogenous feeding between 6 and 17 dph and as the fish transitioned from an artemia-based diet to a predominantly bloodworm diet between 49 and 67 dph. Measurement of growth rate and energy density throughout indicated that there was a brief period of growth arrest during the transition from artemia to bloodworm. The highest mass-specific RMR (mg O₂ kg^-1 h^-1) recorded throughout the first 76 d of development occurred during the yolk sac phase and during transition from artemia to bloodworm. Similarly, diet transition from artemia to bloodworm-when growth arrest was observed-increased scaled RMR (i.e., mg O₂ kg^-0.89 h^-1), and it did not significantly differ from scaled FMR. Log-log relationships between non-mass-specific RMR or FMR (i.e., mg O₂ h^-1) and body mass significantly changed as the growing fish adapted to the nutritional differences of their primary diet. We demonstrate that dietary change during early ontogeny has consequences for growth that may reflect altered metabolic performance. Results have implications for understanding cohort and population dynamics during early life and effective management for conservation fish hatcheries.

Thermal performance curves for aerobic scope in a tropical fish (Lates calcarifer): flexible in amplitude but not breadth

Aerobic metabolic scope is a popular metric to estimate the capacity for temperature-dependent performance in aquatic animals. Despite this popularity, little is known of the role of temperature acclimation and variability in shaping the breadth and amplitude of the thermal performance curve for aerobic scope. If daily thermal experience can modify the characteristics of the thermal performance curve, interpretations of aerobic scope data from the literature may be misguided. Here, tropical barramundi (Lates calcarifer) were acclimated for ∼4 months to cold (23°C), optimal (29°C) or warm (35°C) conditions, or to a daily temperature cycle between 23 and 35°C (with a mean of 29°C). Measurements of aerobic scope were conducted every 3-4 weeks at three temperatures (23, 29 and 35°C), and growth rates were monitored. Acclimation to constant temperatures caused some changes in aerobic scope at the three measurement temperatures via adjustments in standard and maximum metabolic rates, and growth rates were lower in the 23°C-acclimated group than in all other groups. The metabolic parameters and growth rates of the thermally variable group remained similar to those of the 29°C-acclimated group. Thus, acclimation to a variable temperature regime did not broaden the thermal performance curve for aerobic scope. We propose that thermal performance curves for aerobic scope are more plastic in amplitude than in breadth, and that the metabolic phenotype of at least some fish may be more dependent on the mean daily temperature than on the daily temperature range.

Normoxic limitation of maximal oxygen consumption rate, aerobic scope and cardiac performance in exhaustively exercised rainbow trout (Oncorhynchus mykiss)

In fish, maximum O2 consumption rate (ṀO2,max) and aerobic scope can be expanded following exhaustive exercise in hyperoxia; however, the mechanisms explaining this are yet to be identified. Here, in exhaustively exercised rainbow trout (Oncorhynchus mykiss), we assessed the influence of hyperoxia on ṀO2,max, aerobic scope, cardiac function and blood parameters to address this knowledge gap. Relative to normoxia, ṀO2,max was 33% higher under hyperoxia, and this drove a similar increase in aerobic scope. Cardiac output was significantly elevated under hyperoxia at ṀO2,max because of increased stroke volume, indicating that hyperoxia released a constraint on cardiac contractility apparent with normoxia. Thus, hyperoxia improved maximal cardiac performance, thereby enhancing tissue O2 delivery and allowing a higher ṀO2,max. Venous blood O2 partial pressure (PvO2) was elevated in hyperoxia at ṀO2,max, suggesting a contribution of improved luminal O2 supply in enhanced cardiac contractility. Additionally, despite reduced haemoglobin and higher PvO2, hyperoxia treated fish retained a higher arterio-venous O2 content difference at ṀO2,max. This may have been possible because of hyperoxia offsetting declines in arterial oxygenation that are known to occur following exhaustive exercise in normoxia. If this occurs, increased contractility at ṀO2,max with hyperoxia may also relate to an improved O2 supply to the compact myocardium via the coronary artery. Our findings show ṀO2,max and aerobic scope may be limited in normoxia following exhaustive exercise as a result of constrained maximal cardiac performance and highlight the need to further examine whether or not exhaustive exercise protocols are suitable for eliciting ṀO2,max and estimating aerobic scope in rainbow trout.

Is hypoxia vulnerability in fishes a by-product of maximum metabolic rate?

The metabolic index concept combines metabolic data and known thermal sensitivities to estimate the factorial aerobic scope of animals in different habitats, which is valuable for understanding the metabolic demands that constrain species' geographical distributions. An important assumption of this concept is that the O2 supply capacity (which is equivalent to the rate of oxygen consumption divided by the environmental partial pressure of oxygen: ) is constant at O2 tensions above the critical O2 threshold (i.e. the where O2 uptake can no longer meet metabolic demand). This has led to the notion that hypoxia vulnerability is not a selected trait, but a by-product of selection on maximum metabolic rate. In this Commentary, we explore whether this fundamental assumption is supported among fishes. We provide evidence that O2 supply capacity is not constant in all fishes, with some species exhibiting an elevated O2 supply capacity in hypoxic environments. We further discuss the divergent selective pressures on hypoxia- and exercise-based cardiorespiratory adaptations in fishes, while also considering the implications of a hypoxia-optimized O2 supply capacity for the metabolic index concept.

Thermal variation near the thermal optimum does not affect the growth, metabolism or swimming performance in wild Atlantic salmon Salmo salar

Typically, laboratory studies on the physiological effects of temperature are conducted using stable acclimation temperatures. Nonetheless, information extrapolated from these studies may not accurately represent wild populations living in thermally variable environments. The aim of this study was to compare the growth rate, metabolism and swimming performance of wild Atlantic salmon exposed to cycling temperatures, 16-21°C, and stable acclimation temperatures, 16, 18.5, 21°C. Growth rate, metabolic rate, swimming performance and anaerobic metabolites did not change among acclimation groups, suggesting that within Atlantic salmon's thermal optimum range, temperature variation has no effect on these physiological properties.

Intraspecific variation in tolerance of warming in fishes

Intraspecific variation in key traits such as tolerance of warming can have profound effects on ecological and evolutionary processes, notably responses to climate change. The empirical evidence for three primary elements of intraspecific variation in tolerance of warming in fishes is reviewed. The first is purely mechanistic that tolerance varies across life stages and as fishes become mature. The limited evidence indicates strongly that this is the case, possibly because of universal physiological principles. The second is intraspecific variation that is because of phenotypic plasticity, also a mechanistic phenomenon that buffers individuals' sensitivity to negative impacts of global warming in their lifetime, or to some extent through epigenetic effects over successive generations. Although the evidence for plasticity in tolerance to warming is extensive, more work is required to understand underlying mechanisms and to reveal whether there are general patterns. The third element is intraspecific variation based on heritable genetic differences in tolerance, which underlies local adaptation and may define long-term adaptability of a species in the face of ongoing global change. There is clear evidence of local adaptation and some evidence of heritability of tolerance to warming, but the knowledge base is limited with detailed information for only a few model or emblematic species. There is also strong evidence of structured variation in tolerance of warming within species, which may have ecological and evolutionary significance irrespective of whether it reflects plasticity or adaptation. Although the overwhelming consensus is that having broader intraspecific variation in tolerance should reduce species vulnerability to impacts of global warming, there are no sufficient data on fishes to provide insights into particular mechanisms by which this may occur.

High temperature impairs mitochondrial function in rainbow trout cardiac mitochondria

Mitochondria provide cellular energy through oxidative phosphorylation, and thus temperature-induced constraints on mitochondrial function may be crucial to animal aerobic scope and thermal tolerance. Here, we report the effect of temperature in the range 5–30°C on respiration rates of isolated cardiac mitochondria from rainbow trout (Oncorhynchus mykiss) studied by high-resolution respirometry and spectrophotometric enzyme activity assays. Arrhenius breakpoint temperature analysis indicated that mitochondrial respiration rates under phosphorylating and fully uncoupled conditions increased exponentially up to 20°C, but stopped increasing at higher temperatures. In contrast, respiration rates measured under non-phosphorylating leak conditions continued to increase up to 30°C. The decrease in the ratio between phosphorylating and uncoupled respiration at high temperature indicated that phosphorylation was gradually impaired with increasing temperature, possibly because of the steadily increasing proton leak across the membrane. In addition, we found that complex I (NADH dehydrogenase) activity decreased above 20°C, similarly to mitochondrial respiration, and that complex I was unstable in the presence of detergents, suggesting that it may be particularly sensitive to changes in its interaction with membrane phospholipids. In contrast, complex II (succinate dehydrogenase) maintained activity at temperatures above 20°C, although succinate oxidation was insufficient to compensate for the loss of complex I activity in intact mitochondria. Together, these results indicate that the temperature-induced decrease in cardiac mitochondrial function coincides with the temperature at which trout aerobic scope peaks, and is largely due to impaired phosphorylation and complex I activity.

Fish heating tolerance scales similarly across individual physiology and populations

Extrapolating patterns from individuals to populations informs climate vulnerability models, yet biological responses to warming are uncertain at both levels. Here we contrast data on the heating tolerances of fishes from laboratory experiments with abundance patterns of wild populations. We find that heating tolerances in terms of individual physiologies in the lab and abundance in the wild decline with increasing temperature at the same rate. However, at a given acclimation temperature or optimum temperature, tropical individuals and populations have broader heating tolerances than temperate ones. These congruent relationships implicate a tight coupling between physiological and demographic processes underpinning macroecological patterns, and identify vulnerability in both temperate and tropical species.

Nicholas Payne et al. use physiological and population-level abundance data from 823 fish species to examine how heating tolerance scales at both the individual and population level. This study shows that heating tolerance declines in the lab and the wild at the same rate, and for a given temperature, individuals and populations from tropical areas have broader heating tolerances than temperate species.

The role of mechanistic physiology in investigating impacts of global warming on fishes

Warming of aquatic environments as a result of climate change is already having measurable impacts on fishes, manifested as changes in phenology, range shifts and reductions in body size. Understanding the physiological mechanisms underlying these seemingly universal patterns is crucial if we are to reliably predict the fate of fish populations with future warming. This includes an understanding of mechanisms for acute thermal tolerance, as extreme heatwaves may be a major driver of observed effects. The hypothesis of gill oxygen limitation (GOL) is claimed to explain asymptotic fish growth, and why some fish species are decreasing in size with warming; but its underlying assumptions conflict with established knowledge and direct mechanistic evidence is lacking. The hypothesis of oxygen- and capacity-limited thermal tolerance (OCLTT) has stimulated a wave of research into the role of oxygen supply capacity and thermal performance curves for aerobic scope, but results vary greatly between species, indicating that it is unlikely to be a universal mechanism. As thermal performance curves remain important for incorporating physiological tolerance into models, we discuss potentially fruitful alternatives to aerobic scope, notably specific dynamic action and growth rate. We consider the limitations of estimating acute thermal tolerance by a single rapid measure whose mechanism of action is not known. We emphasise the continued importance of experimental physiology, particularly in advancing our understanding of underlying mechanisms, but also the challenge of making this knowledge relevant to the more complex reality.

An acute increase in water temperature can decrease the swimming performance and energy utilization efficiency in rainbow trout (Oncorhynchus mykiss)

In order to evaluate the effects of acute temperature exposure on the swimming performance of rainbow trout (Oncorhynchus mykiss), the critical swimming speed (U_crit) and oxygen consumption rates (MO₂) were determined at different temperatures (13.2, 18.4, and 22.5 °C). The U_crit and MO₂ of different body mass (109.44, 175.74, and 249.42 g) fish were also obtained at 13.4 °C. The U_crit first increased as the temperature increased from 13.2 to 15.2 °C, which was calculated to be the optimal temperature for the U_crit, and then decreased with increasing temperature. The optimal swimming speed (U_opt) showed a similar trend to the U_crit. At a given swimming speed, the MO₂ and cost of transport (COT) were significantly higher at 22.5 than at 13.2 °C, suggesting the energy utilization efficiency decreased with increasing temperature. The absolute values of U_crit and U_opt increased as the body mass increased from 109.44 to 249.42 g, whereas the relative values decreased. Although not statistically significant, the maximum metabolic rate (MMR) tended to increase with temperature but decrease with body mass. Results can be of value in understanding the behavioral and physiological response of rainbow trout to acute temperature change.

Effects of temperature on physiological performance and behavioral thermoregulation in an invasive fish, the round goby

Invasive species exert negative impacts on biodiversity and ecosystems on a global scale, which may be enhanced in the future by climate change. Knowledge of how invasive species respond physiologically and behaviorally to novel and changing environments can improve our understanding of which traits enable the ecological success of these species, and potentially facilitate mitigation efforts. We examined the effects of acclimation to temperatures ranging from 5 to 28°C on aerobic metabolic rates, upper temperature tolerance (critical thermal maximum, CTmax), as well as temperature preference (Tpref) and avoidance (Tavoid) of the round goby (Neogobius melanostomus), one of the most impactful invasive species in the world. We show that round goby maintained a high aerobic scope from 15 to 28°C; that is, the capacity to increase its aerobic metabolic rate above that of its maintenance metabolism remained high across a broad thermal range. Although CTmax increased relatively little with acclimation temperature compared with other species, Tpref and Tavoid were not affected by acclimation temperature at all, meaning that round goby maintained a large thermal safety margin (CTmax-Tavoid) across acclimation temperatures, indicating a high level of thermal resilience in this species. The unperturbed physiological performance and high thermal resilience were probably facilitated by high levels of phenotypic buffering, which can make species readily adaptable and ecologically competitive in novel and changing environments. We suggest that these physiological and behavioral traits could be common for invasive species, which would only increase their success under continued climate change.

Effects of temperature on physiological performance and behavioral thermoregulation in an invasive fish, the round goby

Invasive species exert negative impacts on biodiversity and ecosystems on a global scale, which may be enhanced in the future by climate change. Knowledge of how invasive species respond physiologically and behaviorally to novel and changing environments can improve our understanding of which traits enable the ecological success of these species, and potentially facilitate mitigation efforts. We examined the effects of acclimation to temperatures ranging from 5 to 28°C on aerobic metabolic rates, upper temperature tolerance (critical thermal maximum, CT_max), as well as temperature preference (T _pref) and avoidance (T _avoid) of the round goby (Neogobius melanostomus), one of the most impactful invasive species in the world. We show that round goby maintained a high aerobic scope from 15 to 28°C; that is, the capacity to increase its aerobic metabolic rate above that of its maintenance metabolism remained high across a broad thermal range. Although CT_max increased relatively little with acclimation temperature compared with other species, T _pref and T _avoid were not affected by acclimation temperature at all, meaning that round goby maintained a large thermal safety margin (CT_max-T _avoid) across acclimation temperatures, indicating a high level of thermal resilience in this species. The unperturbed physiological performance and high thermal resilience were probably facilitated by high levels of phenotypic buffering, which can make species readily adaptable and ecologically competitive in novel and changing environments. We suggest that these physiological and behavioral traits could be common for invasive species, which would only increase their success under continued climate change.

Mismatch of thermal optima between performance measures, life stages and species of spiny lobster

In an ocean warming hotspot off south-east Australia, many species have expanded their ranges polewards, including the eastern rock lobster, Sagmariasus verreauxi. This species is likely extending its range via larval advection into Tasmanian coastal waters, which are occupied by the more commercially important southern rock lobster, Jasus edwardsii. Here, thermal tolerances of these lobster species at two life stages were investigated to assess how they may respond to warming ocean temperatures. We found that the pattern, optimum and magnitude of thermal responses differed between performance measures, life stages and species. Sagmariasus verreauxi had a warmer optimal temperature for aerobic scope and escape speed than J. edwardsii. However, J. edwardsii had a higher magnitude of escape speed, indicating higher capacity for escape performance. There were also differences between life stages within species, with the larval stage having higher variation in optimal temperatures between measures than juveniles. This inconsistency in performance optima and magnitude indicates that single performance measures at single life stages are unlikely to accurately predict whole animal performance in terms of life-time survival and fitness. However, combined results of this study suggest that with continued ocean warming, S. verreauxi is likely to continue to extend its distribution polewards and increase in abundance in Tasmania.

Oxygen-dependence of upper thermal limits in crustaceans from different thermal habitats

The critical thermal maximum (CT_MAX) is the temperature at which animals exhibit loss of motor response because of a temperature-induced collapse of vital physiological systems. A central mechanism hypothesised to underlie the CT_MAX of water-breathing ectotherms is insufficient tissue oxygen supply for vital maintenance functions because of a temperature-induced collapse of the cardiorespiratory system. The CT_MAX of species conforming to this hypothesis should decrease with declining water oxygen tension (PO₂) because they have oxygen-dependent upper thermal limits. However, recent studies have identified a number of fishes and crustaceans with oxygen-independent upper thermal limits, their CT_MAX unchanged in progressive aquatic hypoxia. The previous studies, which were performed separately on cold-water, temperate and tropical species, suggest the oxygen-dependence of upper thermal limits and the acute thermal sensitivity of the cardiorespiratory system increases with decreasing habitat temperature. Here we directly test this hypothesis by assessing the oxygen-dependence of CT_MAX in the polar Antarctic krill (Euphausia superba), as well as the temperate Baltic prawn (Palaemon adspersus) and brown shrimp (Crangon crangon). We found that P. adspersus and C. crangon maintain CT_MAX in progressive hypoxia down to 40 mmHg, and that only E. superba have oxygen-dependent upper thermal limits at normoxia. In E. superba, the observed decline in CT_MAX with water PO₂ is further supported by heart-rate measurements showing a plateauing, and subsequent decline and collapse of heart performance at CT_MAX. Our results support the hypothesis that the oxygen-dependence of upper thermal limits in water-breathing ectotherms and the acute thermal sensitivity of their cardiorespiratory system increases with decreasing habitat temperature.

Direct and indirect effects of climate change-amplified pulse heat stress events on coral reef fish communities

Climate change-amplified temperature anomalies pose an imminent threat to coral reef ecosystems. While much focus has been placed on the effects of heat stress on scleractinian corals-including bleaching, mortality, and loss of reef structural complexity-and many studies have documented changes to reef fish communities arising indirectly from shifts in benthic composition, the direct impacts of heat stress on reef fish are much less well understood. Here, we quantify the direct and indirect effects of heat stress on reef fishes, using underwater visual censuses of coral reef fish communities conducted before, during, and after the 2015-2016 El Niño-induced global coral bleaching event. Surveys took place at the epicenter of this event, at 16 sites on Kiritimati (Republic of Kiribati; central equatorial Pacific) spanning across a gradient of local human disturbance. We expected that heat stress would have both direct and indirect negative effects on the reef fish community, with direct effects resulting from physiological stress during the event and indirect effects manifesting afterward as a consequence of coral mortality, and that the ability of fish communities to recover following the heat stress would depend on levels of local human disturbance. We found that total reef fish biomass and abundance declined by >50% during heat stress, likely as a result of vertical migration of fish to cooler waters. One year after the cessation of heat stress, however, total biomass, abundance, and species richness had recovered to, or even exceeded, pre-heat stress levels. However, the biomass of corallivores declined by over 70% following severe coral loss, and reefs exposed to higher levels of local human disturbance showed impaired recovery following the heat stress. These findings enhance understanding of the projected impacts of climate change-associated marine heatwaves on reef fishes, and highlight the interacting effects of local and global stressors on this vital component of coral reef ecosystems.

The combined effect of body size and temperature on oxygen consumption rates and the size-dependency of preferred temperature in European perch Perca fluviatilis

The present study determined the effect of body mass and acclimation temperature (15-28°C) on oxygen consumption rate (ṀO₂ ) and the size dependency of preferred temperature in European perch Perca fluviatilis. Standard metabolic rate (SMR) scaled allometrically with body mass by an exponent of 0.86, and temperature influenced SMR with a Q₁₀ of 1.9 regardless of size. Maximum metabolic rate (MMR) and aerobic scope (MMR-SMR) scaled allometrically with body mass by exponents of 0.75-0.88. The mass scaling exponents of MMR and aerobic scope changed with temperature and were lowest at the highest temperature. Consequently, the optimal temperature for aerobic scope decreased with increasing body mass. Notably, fish <40 g did not show a decrease aerobic scope with increasing temperature. Factorial aerobic scope (MMR × SMR^-1 ) generally decreased with increasing temperatures, was unaffected by size at the lower temperatures, and scaled negatively with body mass at the highest temperature. Similar to the optimal temperature for aerobic scope, preferred temperature declined with increasing body mass, unaffectedly by acclimation temperature. The present study indicates a limitation in the capacity for oxygen uptake in larger fish at high temperatures. A constraint in oxygen uptake at high temperature may restrict the growth of larger fish with environmental warming, at least if food availability is not limited. Furthermore, behavioural thermoregulation may be contributing to regional changes in the size distribution of fish in the wild caused by global warming as larger individuals will prefer colder water at higher latitudes and at larger depths than smaller conspecifics with increasing environmental temperatures.

Synergism between elevated temperature and nitrate: Impact on aerobic capacity of European grayling, Thymallus thymallus in warm, eutrophic waters

Climate warming and nitrate pollution are pervasive aquatic stressors that endanger the persistence of fishes prevailing in anthropogenically disturbed habitats. Individually, elevated nitrate and temperature can influence fish energy homeostasis by increasing maintenance costs and impairing oxygen transport capacity. However, it remains unknown how fish respond to simultaneous exposure to elevated temperature and nitrate pollution. Hence, we examined the combined effects of nitrate and elevated temperatures on aerobic scope (AS, maximum-standard metabolic rates) and cardiorespiratory attributes (haemoglobin H_B, haematocrit H_CT, relative ventricle mass RVM, and somatic spleen index SSI) in a freshwater salmonid, Thymallus thymallus. A 3 × 2 factorial design was used, where fish were exposed to one of three ecologically relevant levels of nitrate (0, 50, or 200 mg NO₃^- l^-1) and one of two temperatures (18 °C or 22 °C) for 6 weeks. Elevated temperature increased AS by 36 % and the improvement was stronger when coupled with nitrate exposure, indicating a positive synergistic interaction. H_B was reduced by nitrate exposure, while H_CT was independent of nitrate pollution and temperature. Stressor exposure induced remodeling of key elements of the cardiorespiratory system. RVM was 39 % higher in fish exposed to 22 °C compared to 18 °C but was independent of nitrate exposure. SSI was independent of temperature but was 85 % and 57 % higher in fish exposed to 50 and 200 mg NO₃^- l^-1, respectively. Taken together, these results highlight that simultaneous exposure to elevated temperatures and nitrate pollution offers cross-tolerance benefits, which may be underscored by cardiorespiratory remodeling.

Thermal acclimation offsets the negative effects of nitrate on aerobic scope and performance

Rising temperatures are set to imperil freshwater fishes as climate change ensues unless compensatory strategies are employed. However, the presence of additional stressors, such as elevated nitrate concentrations, may affect the efficacy of compensatory responses. Here, juvenile silver perch (Bidyanus bidyanus) were exposed to current-day summer temperatures (28°C) or a future climate-warming scenario (32°C) and simultaneously exposed to one of three ecologically relevant nitrate concentrations (0, 50 or 100 mg l-1). We measured indicators of fish performance (growth, swimming), aerobic scope (AS) and upper thermal tolerance (CTmax) to test the hypothesis that nitrate exposure would increase susceptibility to elevated temperatures and limit thermal compensatory responses. After 8 weeks of acclimation, the thermal sensitivity and plasticity of AS and swimming performance were tested at three test temperatures (28, 32, 36°C). The AS of 28°C-acclimated fish declined with increasing temperature, and the effect was more pronounced in nitrate-exposed individuals. In these fish, declines in AS corresponded with poorer swimming performance and a 0.8°C decrease in CTmax compared with unexposed fish. In contrast, acclimation to 32°C masked the effects of nitrate; fish acclimated to 32°C displayed a thermally insensitive phenotype whereby locomotor performance remained unchanged, AS was maintained and CTmax was increased by ∼1°C irrespective of nitrate treatment compared with fish acclimated to 28°C. However, growth was markedly reduced in 32°C-acclimated compared with 28°C-acclimated fish. Our results indicate that nitrate exposure increases the susceptibility of fish to acute high temperatures, but thermal compensation can override some of these potentially detrimental effects.

Litopenaeus vannamei oxygen consumption and HSP gene expression at cyclic conditions of hyperthermia and hypoxia

Although Litopenaeus vannamei is a widely studied species, the information on how the organisms respond to natural daily variations of environmental conditions such as temperature and dissolved oxygen, and how such conditions alter the physiological responses, is scarce. In the present work, the strategies used by shrimps to cope with temperature and dissolved oxygen fluctuations during 24 days were investigated through the evaluation of oxygen consumption and heat shock proteins (HSP) gene expression. During daily fluctuations, no change in oxygen consumption in the short-term, but a significant increase in the long-term during hyperthermia conditions was registered, whereas a significant decrease during hypoxia was observed during all the bioassay. On the other hand, HSP70 and HSP90 gene expression increased in gills under thermal stress but was down-regulated under hypoxia, in both the short- and the long-term. This study highlights that to counteract environmental variations of temperature and dissolved oxygen, the shrimps use molecular compensatory mechanisms (HSP gene expression) that are different to those used under constant hypoxic conditions, suggesting that hypoxia can compromise physiological cytoprotection.

Metabolic adjustment of Pyrrhulina aff. brevis exposed to different climate change scenarios

The increases in CO₂ concentrations and, consequently, temperature due to climate change are predicted to intensify. Understanding the physiological responses of Pyrrhulina aff. brevis to the climatic scenarios proposed by the IPCC (2014) for the next 100 years is of fundamental importance to determine its susceptibility. Thus, the present study aimed to evaluate the effects of the predicted climatic scenarios for the year 2100 on the metabolic adjustments of P. aff. brevis . Specifically, the rate of oxygen uptake, electron transport system capacity, glycogen and lactate content and the role of Na⁺K⁺-ATPases and H⁺-ATPase were evaluated. P. aff. brevis individuals were exposed for 15 days to the simulated climatic scenarios in climate scenario rooms, where temperature and CO₂ in the air were controlled. Two rooms were used to simulate the climatic scenarios predicted by the IPCC (2014): moderate (RCP 6; 2.5 °C and 400 μatm CO₂ above current levels) and extreme (RCP 8.5; 4.5 °C and 900 μatm CO₂ above current levels), in addition to the "control room" that represents the current scenario. There was an increase in the metabolic rate (MO₂) in the animals acclimated to the climate change scenarios (RCP 6 and RCP 8.5) compared to the current scenario. These responses showed a typical effect of temperature on energy demand in relation to the increase in temperature and CO₂. Our data showed an increase in O₂ consumption (MO₂), lactate levels and H⁺-ATPase activity of the animals acclimated to the moderate and extreme climate change scenarios. Such adjustments presented a clear metabolic imbalance, an alteration that may imply challenges for survival, growth, distribution and reproduction in the face of the expected environmental changes for the year 2100.

The metabolism and swimming performance of sheepshead minnows (Cyprinodon variegatus) following thermal acclimation or acute thermal exposure

Ectothermic animals are especially susceptible to temperature change, considering that their metabolism and core temperature are linked to the environmental temperature. As global water temperatures continue to increase, so does the need to understand the capacity of organisms to tolerate change. Sheepshead minnows (Cyprinodon variegatus) are the most eurythermic fish species known to date and can tolerate a wide range of environmental temperatures from - 1.9 to 43.0 °C. But little is known about the physiological adjustments that occur when these fish are subjected to acute thermal challenges and long-term thermal acclimation. Minnows were acclimated to 10, 21, or 32 °C for 4 weeks or acutely exposed to 10 and 32 °C and then assessed for swimming performance [maximum sustained swimming velocity (U_crit), optimum swimming velocity (U_opt)] and metabolic endpoints (extrapolated standard and maximum metabolic rate [SMR, MMR), absolute aerobic scope (AS), and cost of transport (COT)]. Our findings show that the duration of thermal exposure (acute vs. acclimation) did not influence swimming performance. Rather, swimming performance was influenced by the exposure temperature. Swimming performance was statistically similar in fish exposed to 21 or 32 °C (approximately 7.0 BL s^-1), but was drastically reduced in fish exposed to 10 °C (approximately 2.0 BL s^-1), resulting in a left-skewed performance curve. There was no difference in metabolic end points between fish acutely exposed or acclimated to 10 °C. However, a different pattern was observed in fish exposed to 32 °C. MMR was similar between acutely exposed or acclimated fish, but acclimated fish had a 50% reduction in extrapolated SMR, which increased AS by 25%. However, this enhanced AS was not associated with changes in swimming performance, which opposes the oxygen-capacity limited thermal tolerance concept. Our findings suggest that sheepshead minnows may utilize two distinct acclimation strategies, resulting in different swimming performance and metabolic patterns observed between 10 and 32 °C exposures.

Measuring maximum oxygen uptake with an incremental swimming test and by chasing rainbow trout to exhaustion inside a respirometry chamber yields the same results

This study hypothesized that oxygen uptake (ṀO₂ ) measured with a novel protocol of chasing rainbow trout Oncorhynchus mykiss to exhaustion inside a static respirometer while simultaneously monitoring ṀO₂ (ṀO_2chase ) would generate the same and repeatable peak value as when peak active ṀO₂ (ṀO_2active ) is measured in a critical swimming speed protocol. To reliably determine peak ṀO_2chase , and compare to the peak during recovery of ṀO₂ after a conventional chase protocol outside the respirometer (ṀO_2rec ), this study applied an iterative algorithm and a minimum sampling window duration (i.e., 1 min based on an analysis of the variance in background and exercise ṀO₂ ) to account for ṀO₂ dynamics. In support of this hypothesis, peak ṀO_2active (707 ± 33 mg O₂ h^-1 kg^-1 ) and peak ṀO_2chase (663 ± 43 mg O₂ h^-1 kg^-1 ) were similar (P = 0.49) and repeatable (Pearson's and Spearman's correlation test; r ≥ 0.77; P < 0.05) when measured in the same fish. Therefore, estimates of ṀO_2max can be independent of whether a fish is exhaustively chased inside a respirometer or swum to fatigue in a swim tunnel, provided ṀO₂ is analysed with an iterative algorithm and a minimum but reliable sampling window. The importance of using this analytical approach was illustrated by peak ṀO_2chase being 23% higher (P < 0.05) when compared with a conventional sequential interval regression analysis, whereas using the conventional chase protocol (1-min window) outside the respirometer increased this difference to 31% (P < 0.01). Moreover, because peak ṀO_2chase was 18% higher (P < 0.05) than peak ṀO_2rec , chasing a fish inside a static respirometer may be a better protocol for obtaining maximum ṀO₂ .

Divergence in aerobic scope and thermal tolerance is related to local thermal regime in two populations of introduced Nile perch (Lates niloticus)

We tested whether thermal tolerance and aerobic performance differed between two populations of Nile perch (Lates niloticus) originating from the same source population six decades after their introduction into two lakes in the Lake Victoria basin in East Africa. We used short-term acclimation of juvenile fish to a range of temperatures from ambient to +6°C, and performed critical thermal maximum (CT_max ) and respirometry tests to measure upper thermal tolerance, resting and maximum metabolic rates, and aerobic scope (AS). Across acclimation temperatures, Nile perch from the cooler lake (Lake Nabugabo, Uganda) tended to have lower thermal tolerance (i.e., CT_max ) and lower aerobic performance (i.e., AS) than Nile perch from the warmer waters of Lake Victoria (Bugonga region, Uganda). Effects of temperature acclimation were more pronounced in the Lake Victoria population, with the Lake Nabugabo fish showing less thermal plasticity in most metabolic traits. Our results suggest phenotypic divergence in thermal tolerance between these two introduced populations in a direction consistent with an adaptive response to local thermal regimes.

Oxygen supply capacity in animals evolves to meet maximum demand at the current oxygen partial pressure regardless of size or temperature

The capacity to extract oxygen from the environment and transport it to respiring tissues in support of metabolic demand reportedly has implications for species' thermal tolerance, body size, diversity and biogeography. Here, we derived a quantifiable linkage between maximum and basal metabolic rate and their oxygen, temperature and size dependencies. We show that, regardless of size or temperature, the physiological capacity for oxygen supply precisely matches the maximum evolved demand at the highest persistently available oxygen pressure and this is the critical PO2  for the maximum metabolic rate, Pcrit-max For most terrestrial and shallow-living marine species, Pcrit-max is the current atmospheric pressure, 21 kPa. Any reduction in oxygen partial pressure from current values will result in a calculable decrement in maximum metabolic performance. However, oxygen supply capacity has evolved to match demand across temperatures and body sizes and so does not constrain thermal tolerance or cause the well-known reduction in mass-specific metabolic rate with increasing body mass. The critical oxygen pressure for resting metabolic rate, typically viewed as an indicator of hypoxia tolerance, is, instead, simply a rate-specific reflection of the oxygen supply capacity. A compensatory reduction in maintenance metabolic costs in warm-adapted species constrains factorial aerobic scope and the critical PO2  to a similar range, between ∼2 and 6, across each species' natural temperature range. The simple new relationship described here redefines many important physiological concepts and alters their ecological interpretation.

The effect of ambient oxygen on the thermal performance of a cockroach, Nauphoeta cinerea

The oxygen and capacity-limited thermal tolerance (OCLTT) hypothesis proposes that the thermal tolerance of an animal is shaped by its capacity to deliver oxygen in relation to oxygen demand. Studies testing this hypothesis have largely focused on measuring short-term performance responses in animals under acute exposure to critical thermal maximums. The OCLTT hypothesis, however, emphasises the importance of sustained animal performance over acute tolerance. The present study tested the effect of chronic hypoxia and hyperoxia during development on moderate to long-term performance indicators at temperatures spanning the optimal temperature for growth in the speckled cockroach, Nauphoeta cinerea In contrast to the predictions of the OCLTT hypothesis, development under hypoxia did not significantly reduce growth rate or running performance, and development under hyperoxia did not significantly increase growth rate or running performance. The effects of developmental temperature and oxygen on tracheal morphology and metabolic rate were also not consistent with OCLTT predictions, suggesting that oxygen delivery capacity is not the primary driver shaping thermal tolerance in this species. Collectively, these findings suggest that the OCLTT hypothesis does not explain moderate to long-term thermal performance in N.cinerea, which raises further questions about the generality of the hypothesis.

Physiological mechanisms linking cold acclimation and the poleward distribution limit of a range-extending marine fish

Extensions of species' geographical distributions, or range extensions, are among the primary ecological responses to climate change in the oceans. Considerable variation across the rates at which species' ranges change with temperature hinders our ability to forecast range extensions based on climate data alone. To better manage the consequences of ongoing and future range extensions for global marine biodiversity, more information is needed on the biological mechanisms that link temperatures to range limits. This is especially important at understudied, low relative temperatures relevant to poleward range extensions, which appear to outpace warm range edge contractions four times over. Here, we capitalized on the ongoing range extension of a teleost predator, the Australasian snapper Chrysophrys auratus, to examine multiple measures of ecologically relevant physiological performance at the population's poleward range extension front. Swim tunnel respirometry was used to determine how mid-range and poleward range edge winter acclimation temperatures affect metabolic rate, aerobic scope, swimming performance and efficiency and recovery from exercise. Relative to 'optimal' mid-range temperature acclimation, subsequent range edge minimum temperature acclimation resulted in absolute aerobic scope decreasing while factorial aerobic scope increased; efficiency of swimming increased while maximum sustainable swimming speed decreased; and recovery from exercise required a longer duration despite lower oxygen payback. Cold-acclimated swimming faster than 0.9 body lengths sec-1 required a greater proportion of aerobic scope despite decreased cost of transport. Reduced aerobic scope did not account for declines in recovery and lower maximum sustainable swimming speed. These results suggest that while performances decline at range edge minimum temperatures, cold-acclimated snapper are optimized for energy savings and range edge limitation may arise from suboptimal temperature exposure throughout the year rather than acute minimum temperature exposure. We propose incorporating performance data with in situ behaviour and environmental data in bioenergetic models to better understand how thermal tolerance determines range limits.

Acute high temperature exposure impairs hypoxia tolerance in an intertidal fish

Acute heat shock has previously been shown to improve subsequent low O₂ (hypoxia) tolerance in an intertidal fish species, a process known as cross-tolerance, but it is not known whether this is a widespread phenomenon. This study examined whether a rock pool specialist, the triplefin fish Bellapiscis medius, exhibits heat shock induced cross-tolerance to hypoxia, i.e., longer time to loss of equilibrium (LOE) and lower critical O₂ saturation (S_crit) after recovering from an acute heat challenge. Non-heat shock controls had a median time to loss of equilibrium (LOE₅₀) of 54.4 min under severe hypoxia (7% of air saturation) and a S_crit of 15.8% air saturation. Contrary to expectations, however, treatments that received an 8 or 10°C heat shock showed a significantly shorter LOE₅₀ in hypoxia (+8°C = 41.5 min; +10°C = 28.7 min) and no significant change in S_crit (+8°C = 17.0% air saturation; +10°C = 18.3% of air saturation). Thus, there was no evidence of heat shock induced cross-tolerance to hypoxia in B. medius because exposure to acute heat shock impaired hypoxia tolerance.

In the face of climate change and exhaustive exercise: the physiological response of an important recreational fish species

Cobia (Rachycentron canadum) support recreational fisheries along the US mid- and south-Atlantic states and have been recently subjected to increased fishing effort, primarily during their spawning season in coastal habitats where increasing temperatures and expanding hypoxic zones are occurring due to climate change. We therefore undertook a study to quantify the physiological abilities of cobia to withstand increases in temperature and hypoxia, including their ability to recover from exhaustive exercise. Respirometry was conducted on cobia from Chesapeake Bay to determine aerobic scope, critical oxygen saturation, ventilation volume and the time to recover from exhaustive exercise under temperature and oxygen conditions projected to be more common in inshore areas by the middle and end of this century. Cobia physiologically tolerated predicted mid- and end-of-century temperatures (28-32°C) and oxygen concentrations as low as 1.7-2.4 mg l-1. Our results indicated cobia can withstand environmental fluctuations that occur in coastal habitats and the broad environmental conditions their prey items can tolerate. However, at these high temperatures, some cobia did suffer post-exercise mortality. It appears cobia will be able to withstand near-future climate impacts in coastal habitats like Chesapeake Bay, but as conditions worsen, catch-and-release fishing may result in higher mortality than under present conditions.

Ecological and evolutionary consequences of metabolic rate plasticity in response to environmental change

Basal or standard metabolic rate reflects the minimum amount of energy required to maintain body processes, while the maximum metabolic rate sets the ceiling for aerobic work. There is typically up to three-fold intraspecific variation in both minimal and maximal rates of metabolism, even after controlling for size, sex and age; these differences are consistent over time within a given context, but both minimal and maximal metabolic rates are plastic and can vary in response to changing environments. Here we explore the causes of intraspecific and phenotypic variation at the organ, tissue and mitochondrial levels. We highlight the growing evidence that individuals differ predictably in the flexibility of their metabolic rates and in the extent to which they can suppress minimal metabolism when food is limiting but increase the capacity for aerobic metabolism when a high work rate is beneficial. It is unclear why this intraspecific variation in metabolic flexibility persists-possibly because of trade-offs with the flexibility of other traits-but it has consequences for the ability of populations to respond to a changing world. It is clear that metabolic rates are targets of selection, but more research is needed on the fitness consequences of rates of metabolism and their plasticity at different life stages, especially in natural conditions. This article is part of the theme issue 'The role of plasticity in phenotypic adaptation to rapid environmental change'.

Reverse translation: effects of acclimation temperature and acute temperature challenges on oxygen consumption, diffusive water flux, net sodium loss rates, Q10 values and mass scaling coefficients in the rainbow trout (Oncorhynchus mykiss)

Our understanding is limited on how fish adjust the effective permeability of their branchial epithelium to ions and water while altering O₂ uptake rate (MO₂) with acute and chronic changes in temperature. We investigated the effects of acclimation temperature (8 °C, 13 °C and 18 °C) and acute temperature challenges [acute rise (acclimated at 8 °C, measured at 13 °C and 18 °C), acute drop (acclimated at 18 °C, measured at 8 °C and 13 °C) and intermediate (acclimated at 13 °C, measured at 8 °C and 18 °C)] on routine MO₂, diffusive water flux, and net sodium loss rates in 24-h fasted rainbow trout (Oncorhynchus mykiss). In the temperature challenge tests, measurements were made during the first hour. In acclimated trout at all temperatures, allometric mass scaling coefficients were much higher for diffusive water flux than for MO₂. Furthermore, the diffusive water flux rate was more responsive (overall Q₁₀ = 2.75) compared to MO₂ (Q₁₀ = 1.80) over the 8-18 °C range, and for both, Q₁₀ values were greater at 8-13 °C than at 13-18 °C. The net Na⁺ flux rates were highly sensitive to acclimation temperature with an overall Q₁₀ of 3.01 for 8-18 °C. In contrast, very different patterns occurred in trout subjected to acute temperature challenges. The net Na⁺ flux rate was temperature-insensitive with a Q₁₀ around 1.0. Both MO₂ and diffusive water flux rates exhibited lower Q₁₀ values than for the acclimated rates in response to either acute increases or decreases in temperature. These results fit Pattern 5 of Precht (undercompensation, reverse effect) and more precisely Pattern IIB of Prosser (reverse translation). These inverse compensatory patterns suggest that trout do not alter their rates very much when undergoing acute thermal challenges (diurnal fluctuations, migration through the thermocline). The greater changes seen with acclimation may be adaptive to long-term seasonal changes in temperature. We discuss the roles of aquaporins, spontaneous activity, and recent feeding in these responses.

Different drivers, common mechanism; the distribution of a reef fish is restricted by local-scale oxygen and temperature constraints on aerobic metabolism

The distributions of ectothermic marine organisms are limited to temperature ranges and oxygen conditions that support aerobic respiration, quantified within the metabolic index (ϕ) as the ratio of oxygen supply to metabolic oxygen demand. However, the utility of ϕ at local scales and across heterogenous environments is unknown; yet, these scales are often where actionable management decisions are made. Here, we test if ϕ can delimit the entire distribution of marine organisms when calibrated across an appropriate temperature range and at local scales (~10 km) using the endemic reef fish, Chrysoblephus laticeps, which is found in the highly heterogenous temperature and oxygen environment along the South African coastal zone, as a model species. In laboratory experiments, we find a bidirectional (at 12°C) hypoxia tolerance response across the temperature range tested (8 to 24°C), permitting a piecewise calibration of ϕ. We then project this calibrated ϕ model through temperature and oxygen data from a high spatial resolution (11 to 13 km) ocean model for the periods 2005 to 2009 and 2095 to 2099 to quantify various magnitudes of ϕ across space and time paired with complementary C. laticeps occurrence points. Using random forest species distribution models, we quantify a critical ϕ value of 2.78 below which C. laticeps cannot persist and predict current and future distributions of C. laticeps in line with already observed distribution shifts of other South African marine species. Overall, we find that C. laticeps' distribution is limited by increasing temperatures towards its warm edge but by low oxygen availability towards its cool edge, which is captured within ϕ at fine scales and across heterogenous oxygen and temperature combinations. Our results support the application of ϕ for generating local- and regional-scale predictions of climate change effects on organisms that can inform local conservation management decisions.

Cortisol modulates metabolism and energy mobilization in wild-caught pumpkinseed (Lepomis gibbosus)

Acute elevation of cortisol via activation of the hypothalamic-pituitary-interrenal (HPI) axis aids the fish in dealing with a stressor. However, chronic elevation of cortisol has detrimental effects and has been studied extensively in lab settings. However, data pertaining to wild teleosts are lacking. Here, we characterized the metabolic consequences of prolonged cortisol elevation (96 h) in wild-caught pumpkinseed (Lepomis gibbosus). Pumpkinseed were implanted with cocoa butter alone (sham) or containing cortisol (25 mg kg^-1 body weight), and at 24, 48, 72, and 96 h, tissue samples were collected, whole-body ammonia excretion was determined, and whole-organism metabolism was assessed using intermittent flow respirometry. Cortisol-treated pumpkinseed exhibited the highest plasma cortisol concentration at 24 h post-implantation, with levels decreasing over the subsequent time points although remaining higher than in sham-treated fish. Cortisol-treated fish exhibited higher standard and maximal metabolic rates than sham-treated fish, but the effect of cortisol treatment on aerobic scope was negligible. Indices of energy synthesis/mobilization, including blood glucose concentrations, hepatosomatic index, hepatic glycogen concentrations, and ammonia excretion rates, were higher in cortisol-treated fish compared with controls. Our work suggests that although aerobic scope was not diminished by prolonged elevation of cortisol levels, higher metabolic expenditures may be of detriment to the animal's performance in the longer term.

An appetite for invasion: digestive physiology, thermal performance and food intake in lionfish (Pterois spp.)

Species invasions threaten global biodiversity, and physiological characteristics may determine their impact. Specific dynamic action (SDA; the increase in metabolic rate associated with feeding and digestion) is one such characteristic, strongly influencing an animal's energy budget and feeding ecology. We investigated the relationship between SDA, scope for activity, metabolic phenotype, temperature and feeding frequency in lionfish (Pterois spp.), which are invasive to western Atlantic marine ecosystems. Intermittent-flow respirometry was used to determine SDA, scope for activity and metabolic phenotype at 26°C and 32°C. Maximum metabolic rate occurred during digestion, as opposed to exhaustive exercise, as in more athletic species. SDA and its duration (SDA_dur) were 30% and 45% lower at 32°C than at 26°C, respectively, and lionfish ate 42% more at 32°C. Despite a 32% decline in scope for activity from 26°C to 32°C, aerobic scope may have increased by 24%, as there was a higher range between standard metabolic rate (SMR) and peak SDA (SDA_peak; the maximum postprandial metabolic rate). Individuals with high SMR and low scope for activity phenotypes had a less costly SDA and shorter SDA_dur but a higher SDA_peak Feeding frequently had a lower and more consistent cost than consuming a single meal, but increased SDA_peak These findings demonstrate that: (1) lionfish are robust physiological performers in terms of SDA and possibly aerobic scope at temperatures approaching their thermal maximum, (2) lionfish may consume more prey as oceans warm with climate change, and (3) metabolic phenotype and feeding frequency may be important mediators of feeding ecology in fish.

Thermal biology and swimming performance of Atlantic cod (Gadus morhua) and haddock (Melanogrammus aeglefinus)

Atlantic cod (Gadus morhua) and haddock (Melanogrammus aeglefinus) are two commercially important marine fishes impacted by both overfishing and climate change. Increasing ocean temperatures are affecting the physiology of these species and causing changes in distribution, growth, and maturity. While the physiology of cod has been well investigated, that of haddock has received very little attention. Here, we measured the metabolic response to increasing temperatures, as well as the critical thermal maximum (CT_max), of cod acclimated to 8 and 12 °C and haddock acclimated to 12 °C. We also compared the swimming performance (critical swimming speed, U_crit) of cod and haddock at 12 °C, as well as the U_crit of 12 °C-acclimated cod acutely exposed to a higher-than-optimal temperature (16 °C). The CT_max for cod was 21.4 and 23.0 °C for 8- and 12 °C-acclimated fish, respectively, whereas that for the 12 °C-acclimated haddock was 23.9 °C. These values were all significantly different and show that haddock are more tolerant of high temperatures. The aerobic maximum metabolic rate (MMR) of swimming cod remained high at 16 °C, suggesting that maximum oxygen transport capacity was not limited at a temperature above optimal in this species. However, signs of impaired swimming (struggling) were becoming evident at 16 °C. Haddock were found to reach a higher U_crit than cod at 12 °C (3.02 vs. 2.62 body lengths s⁻¹, respectively), and at a lower MMR. Taken together, these results suggest that haddock perform better than cod in warmer conditions, and that haddock are the superior swimmer amongst the two species.