Temperature acclimation rate of aerobic scope and feeding metabolism in fishes: implications in a thermally extreme future

Global warming affects foraging efficiency of fish by influencing mutual interference

Predator-prey interactions underpin ecological dynamics from population to ecosystem scales, affecting population growth and influencing community stability. One of the classic methods to study these relationships is the functional response (FR) approach, measuring resource use across resource densities. Global warming is known to strongly mediate consumer-resource interactions, but the relevance of prey and predator densities remains largely unknown. Elevated temperature could increase consumer energy expenditure, which needs to be compensated by greater foraging activity. However, such greater activity may concurrently result in a higher encounter rate with other consumers, which potentially affects their total pressure on resource populations because of synergistic or antagonistic effects among multiple predators. We performed a laboratory experiment using three densities of a fish predator (pumpkinseed, Lepomis gibbosus) (one, two and four specimens), two temperatures (25 and 28°C) and six prey densities. Using the FR approach, we investigated the combined effects of elevated temperature and predator and prey density on the consumer's foraging efficiency. We observed a reduced maximum feeding rate at the higher temperature for single predators. However, the foraging efficiency of predators in groups was negatively affected by antagonistic interactions between individuals and further mediated by the temperature. Specifically, we observed a general decrease in antagonistic interactions at elevated compared to the ambient water temperature for multiple predator groupings. Irrespective of temperature, antagonistic multiple predator effects increased with predator density and peaked unimodally at intermediate prey densities, indicating multiple dimensions of density-dependence that interact to supersede the effects of warming. This study shows that conspecific presence negatively affects the per capita performance of predators, but that this effect is dampened with increasing temperature. Their adaptive response to temperature consists of limited food intake and further reduced intraspecific interactions. Including intraspecific competition in study design may thus offer more realistic outcomes compared to widely used experiments with only single predator individuals, which could overestimate the effect of increasing temperature.

Beyond latitude: thermal tolerance and vulnerability of a broadly distributed salmonid across a habitat temperature gradient

Salmonid fishes are a focal point of conservation physiology due to their high value to humans and ecosystems, and their susceptibility to decline from climate change. A significant challenge in conserving these fishes is that populations of the same species can be locally adapted to vastly different habitats within their wild ranges and can therefore have unique tolerance or vulnerability to environmental stressors within those habitats. Within the state of Oregon, USA, summer steelhead (Oncorhynchus mykiss) inhabit both cool, coastal waters most typically associated with Pacific salmonids and arid, inland environments where temperatures are more extreme. Here, we utilized streamside physiological experiments paired with habitat temperature monitoring to assess the thermal tolerance and vulnerability of four populations of summer steelhead from distinct thermal habitats. All populations had unique responses of critical thermal maximum, aerobic scope and exercise recovery to temperature. Despite populations from warm habitats exhibiting higher thermal tolerance than populations from cooler habitats, summer steelhead from warm habitats appear to be more vulnerable to the physiological consequences of warming based on the extreme temperatures they already experience during the summer. These results demonstrate an example of thermal physiology varying between populations within the same portion of their latitudinal range and highlight the need for habitat-specific conservation strategies for this species.

Eat more, often: The capacity of piscivores to meet increased energy demands in warming oceans

Marine heatwaves (MHWs) profoundly disturb tropical coral reefs, imperilling species fitness and survival. Ectothermic piscivorous reef fishes are particularly vulnerable to MHWs since all aspects of their survival are dictated by ambient temperature. Severe +4 °C MHWs are projected to escalate daily energy demands by ~32-55 %, compelling piscivores to pursue larger or more frequent prey to survive. However, the feasibility of these responses have been questioned, as evolved predation and digestive strategies are constrained to specific prey types and sizes to safeguard residual aerobic scope (AS) during digestion for other vital processes. Instead, prevailing theory proposes appetite reductions at temperatures above optimal, preserving AS at the expense of growth and/or fitness. We investigated this dichotomy in the thermal foraging responses of Arc-eye hawkfish (Paracirrhites arcatus) and blacktail snapper (Lutjanus fulvus), evaluating energetic demand (standard metabolic rate, SMR), AS, appetite (meal mass intake), and capacity for digestion (specific dynamic action, SDA). Spanning a thermal gradient encompassing present-day winter (24.0 ± 0.1 °C), summer (27.5 ± 0.1 °C), and MHW (31.0 ± 0.1 °C), we show that SMR increased by ~65 % from winter to MHW for both species, while AS increased by ~31-67 %. Contrary to predictions of reduced appetite, both species consumed ~106 % larger meals, yielding a ~ 35-105 % greater SDA magnitude. Surprisingly, increased appetite did not encroach on residual AS as both species maintained the physiological flexibility to process larger meals while retaining ~45-60 % of AS at the post-prandial peak. Although larger meals take longer to digest, both species exhibited faster digestion with rising temperatures resulting in a maintained or shortened SDA duration during MHWs, simultaneously enabling increased feeding rates while preserving aerobic reserves to support heightened predation. Our findings underscore the physiological feasibility of increasing appetite for some piscivores, while highlighting the ecological challenge of increasing prey numbers and sizes amid declining prey densities and prey size-reductions caused by ocean warming.

The effects of warm thermal variability on metabolism and swimming performance in wild Atlantic salmon (Salmo salar)

Warmer and more variable temperatures have been implicated in the recent decline of Atlantic salmon (Salmo salar) in Eastern Canada. To date, we know little on how ecologically relevant thermal fluctuations affect swimming performance in fishes. The goal of this study is to determine the effects of warm versus cool diel thermal variability on swimming efficiency and the speed limit for sustainable aerobically fueled swimming. We acclimated wild S. salar juveniles to a cool and a warm ecologically realistic diel thermal profile (16-21 and 19-24°C), and then tested individuals over a common acute change in temperature (16-24°C). We measured metabolic rate and swimming kinematics at a range of swimming speeds, at five temperatures (16, 18, 20, 22, and 24°C) and calculated swimming efficiency. Our temperature acclimation did not appear to significantly affect energetic and kinematic swimming efficiency, but acute exposure to high temperature did increase overall metabolic rate. It appears that wild S. salar can swim efficiently and sustainably during both acute cool and warm exposures, and after acclimation to diel thermal variation of 16-21 or 19-24°C.

Differences in gene expression between high and low tolerance rainbow trout (Oncorhynchus mykiss) to acute thermal stress

Understanding the mechanisms that underlie the adaptive response of ectotherms to rising temperatures is key to mitigate the effects of climate change. We assessed the molecular and physiological processes that differentiate between rainbow trout (Oncorhynchus mykiss) with high and low tolerance to acute thermal stress. To achieve our goal, we used a critical thermal maximum trial in two strains of rainbow trout to elicit loss of equilibrium responses to identify high and low tolerance fish. We then compared the hepatic transcriptome profiles of high and low tolerance fish relative to untreated controls common to both strains to uncover patterns of differential gene expression and to gain a broad perspective on the interacting gene pathways and functional processes involved. We observed some of the classic responses to increased temperature (e.g., induction of heat shock proteins) but these responses were not the defining factors that differentiated high and low tolerance fish. Instead, high tolerance fish appeared to suppress growth-related functions, enhance certain autophagy components, better regulate neurodegenerative processes, and enhance stress-related protein synthesis, specifically spliceosomal complex activities, mRNA regulation, and protein processing through post-translational processes, relative to low tolerance fish. In contrast, low tolerance fish had higher transcript diversity and demonstrated elevated developmental, cytoskeletal, and morphogenic, as well as lipid and carbohydrate metabolic processes, relative to high tolerance fish. Our results suggest that high tolerance fish engaged in processes that supported the prevention of further damage by enhancing repair pathways, whereas low tolerance fish were more focused on replacing damaged cells and their structures.

Insights into thermal sensitivity: Effects of elevated temperature on growth, metabolic rate, and stress responses in Atlantic wolffish (Anarhichas lupus)

The Atlantic wolffish (Anarhichas lupus) is a cold-water fish with potential for aquaculture diversification. To unveil the mechanisms underlying the compromised growth in Atlantic wolffish when reared at higher temperatures, we investigated the relationship between temperature, growth rate, aerobic capacity, stress biomarkers, and gut barrier function. Juveniles acclimated to 10°C were maintained at 10°C (control) or exposed to 15°C for either 24 h (acute exposure) or 50 days (chronic exposure). Fish exposed to 15°C exhibited reduced growth, higher standard, and maximum metabolic rates compared to those at 10°C. In the chronically exposed group at 15°C, metabolic rates were lower than those of acutely exposed fish. The absolute aerobic scope exhibited no significant variation in temperatures; however, the factorial scope showed a notable reduction at 15°C in both acute and chronic exposed groups, aligning with a correlated decrease in individual growth rates. Chronic warming led to increased plasma glucose levels, indicating energy mobilization, but cortisol levels were unaffected. Furthermore, chronic warming resulted in reduced intestinal barrier function, as evidenced by increased ion permeability and a negative potential in the serosa layer. We conclude that warming elevates metabolic rates while reducing intestinal barrier function, thus increasing energy expenditure, collectively, limiting energy available for growth at this temperature from increased allostatic load. Thus, juvenile wolffish maintaining their aerobic scope under thermal stress experience slower growth. This research provides insights for improving the welfare and resilience of wolffish in aquaculture at elevated temperatures and understanding their response to increased environmental temperatures.

Lack of thermal acclimation in multiple indices of climate vulnerability in bumblebees

Indices of climate vulnerability are used to predict species' vulnerability to climate change based on intrinsic physiological traits, such as thermal tolerance, thermal sensitivity and thermal acclimation, but rarely is the consistency among indices evaluated simultaneously. We compared the thermal physiology of queen bumblebees between a species experiencing local declines (Bombus auricomus) and a species exhibiting continent-wide increases (B. impatiens). We conducted a multi-week acclimation experiment under simulated climate warming to measure critical thermal maximum (CTmax), critical thermal minimum (CTmin), the thermal sensitivity of metabolic rate and water loss rate and acclimation in each of these traits. We also measured survival throughout the experiment and after the thermal tolerance trials. Neither species acclimated to the temperature treatments by adjusting any physiological trait. We found conflicting patterns among indices of vulnerability within and between species. We also found that individuals with the highest CTmax exhibited the lowest survival following the thermal tolerance trial. Our study highlights inconsistent patterns across multiple indices of climate vulnerability within and between species, indicating that physiological studies measuring only one index of climate vulnerability may be limited in their ability to inform species' responses to environmental change.

Physiological response of longfin smelt to changing temperatures and turbidities

Coastal estuaries globally, including the San Francisco Estuary (SFE), are experiencing significant degradation, often resulting in fisheries collapses. The SFE has undergone profound modifications due to population growth, industrialization, urbanization and increasing water exports for human use. These changes have significantly altered the aquatic ecosystem, favouring invasive species and becoming less hospitable to native species such as the longfin smelt (Spirinchus thaleichthys). With longfin smelt abundance declining to <1% of historical numbers, there is a pressing need for laboratory-based experiments aimed at investigating the effects of varying environmental conditions on their stress response and physiology. This study explored the impact of temperature (11 and 14°C) and turbidity maintained with algae (1, 4 and 11 nephelometric turbidity units (NTU)) on the physiological condition of juvenile longfin smelt. Fish were sampled after 2 and 4 weeks in experimental conditions and analysed for whole-body cortisol, glucose, lactate and protein. Condition factor was calculated using length and weight measurements. Critical thermal maximum trials were conducted to assess how prior rearing conditions affected upper thermal tolerance. Cortisol levels were significantly higher in fish held in low-turbidity conditions, whilst glucose levels were significantly greater at lower temperatures and higher turbidities. Protein-to-mass ratios were significantly greater in higher turbidity conditions, with a significant interaction between temperature and turbidity further influencing these ratios. Moreover, 14°C led to diminished condition factors but increased upper thermal tolerances (26.3 ± 0.05 vs 24.6 ± 0.18) compared to longfin smelt at 11°C, highlighting a potential trade-off between the induction of defense mechanisms and subsequent reductions in energy and growth. Data suggest that cooler temperatures (11°C) and elevated turbidities (11 NTU) can benefit juvenile longfin smelt by reducing stress and enhancing growth and energy. These findings hold significant implications for informing and optimizing future endeavours in the culturing and conservation of this species.

Swimming performance, but not metabolism, is related to a boldness-activity syndrome in schoolmaster snapper (Lutjanus apodus)

Commercial overexploitation and climate change can alter the physiology and behavior of marine organisms, although intraspecific phenotypic responses to such changes can vary greatly depending on the environment, species, and severity of the stressor. Under the pace-of-life syndrome (POLS) hypothesis, behavior, physiology, and life-history traits are linked, and thus, affected by selection targeting any aspect of organismal biology. However, these links are understudied in tropical marine fishes, and further work is needed to better understand the impacts of fisheries and climate change on wild stocks. Moreover, tropical regions have a greater reliance on fisheries; thus investigations should focus on species with substantial socioeconomic value to ensure benefits at the local level. This study aimed to address this need by measuring the behavior (boldness and activity), metabolism, and swimming performance (using a critical swim speed [Ucrit] test) of schoolmaster snapper Lutjanus apodus in Eleuthera, the Bahamas. We report a strong positive correlation between boldness and activity, high repeatability of these behavioral metrics, and two groupings that were consistent with "proactive" and "reactive" behavioral types. These behavioral types differed significantly in their swimming performance, with reactive individuals having a 13.1% higher mean Ucrit. In contrast, no significant differences were found in the measured metabolic parameters between behavioral types. This study is the first to investigate the intraspecific links between behavior and physiology in a snapper species, using the novel and ecologically relevant comparison of Ucrit with behavioral syndrome types. These data suggest that additional research is needed to better predict the success of proactive/reactive tropical fish if overexploited and as influenced by climate change.

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.

Reporting guidelines for terrestrial respirometry: Building openness, transparency of metabolic rate and evaporative water loss data

Respirometry is an important tool for understanding whole-animal energy and water balance in relation to the environment. Consequently, the growing number of studies using respirometry over the last decade warrants reliable reporting and data sharing for effective dissemination and research synthesis. We provide a checklist guideline on five key sections to facilitate the transparency, reproducibility, and replicability of respirometry studies: 1) materials, set up, plumbing, 2) subject conditions/maintenance, 3) measurement conditions, 4) data processing, and 5) data reporting and statistics, each with explanations and example studies. Transparency in reporting and data availability has benefits on multiple fronts. Authors can use this checklist to design and report on their study, and reviewers and editors can use the checklist to assess the reporting quality of the manuscripts they review. Improved standards for reporting will enhance the value of primary studies and will greatly facilitate the ability to carry out higher quality research syntheses to address ecological and evolutionary theories.

Otolith reliability is context-dependent for estimating warming and CO2 acidification impacts on fish growth

Otoliths are frequently used as proxies to examine the impacts of climate change on fish growth in marine and freshwater ecosystems worldwide. However, the large sensitivity differences in otolith growth responses to typical changing environmental factors (i.e., temperature and CO2 concentration), coupled with unclear drivers and potential inconsistencies with fish body growth, fundamentally challenge the reliability of such otolith applications. Here, we performed a global meta-analysis of experiments investigating the direct effects of warming (297 cases) and CO2 acidification (293 cases) on fish otolith growth and compared them with fish body growth responses. Hierarchical models were used to assess the overall effect and quantify the influence of nine explanatory factors (e.g., fish feeding habit, life history stage, habitat type, and experimental amplitude and duration). The overall effects of warming and acidification on otolith growth were positive and significant, and the effect size of warming (effect size = 0.4003, otolith size of the treatment group increased by 49.23% compared to that of the control group) was larger than that of acidification (0.0724, 7.51%). All factors examined contributed to the heterogeneity of effect sizes, with larger responses commonly observed in carnivorous fish, marine species, and young individuals. Warming amplitudes and durations and acidification amplitudes increased the effect sizes, while acidification durations decreased the effect sizes. Otolith growth responses were consistent with, but greater than, fish body growth responses under warming. In contrast, fish body growth responses were not significant under acidification (effect size = -0.0051, p = .6185) and thus cannot be estimated using otoliths. Therefore, our study highlights that the reliability of applying otoliths to examine climate change impacts is likely varied, as the sensitivity of otolith growth responses and the consistency between the growth responses of otoliths and fish bodies are context-dependent.

The effects of the predictability of acclimatory temperature on the growth and thermal tolerance of juvenile Spinibarbus sinensis

Heated effluent injection, cold hypolimnetic water inputs from dams, and extreme weather events can lead to unpredictable temperature fluctuations in natural waters, impacting fish performance and fitness. We hypothesized that fish exposed to such unpredictable fluctuations would exhibit weaker growth and enhanced thermal tolerance compared to predictable conditions. Qingbo (Spinibarbus sinensis) was selected as the experimental subject in this study. The qingbo were divided into a constant temperature group (C, 22 ± 0.5 °C), a predictable temperature fluctuation group (PF, 22 ± 4 °C, first warming, then cooling within a day) and an unpredictable temperature fluctuation group (UF, 22 ± 4 °C, the order of warming or cooling is random). After 40 days of temperature acclimation, the growth, metabolic rate, spontaneous activity, thermal tolerance, plasma cortisol concentration and liver hsp70 level of the fish were measured. Unexpectedly, neither the PF nor the UF group showed decreased growth compared to the C group. This could be attributed to the fact that temperature variation did not lead to a substantial increase in basic energy expenditure. Furthermore, feeding rates increased due to temperature fluctuations, although the difference was not significant. Both the PF and UF groups exhibited increased upper thermal tolerance, but only the UF group exhibited improved lower thermal tolerance and higher liver hsp70 levels compared to the C group. The qingbo that experienced unpredictable temperature fluctuations had the best thermal tolerance among the 3 groups, which might have occurred because they had the highest level of hsp70 expression. This may safeguard fish against the potential lethal consequences of extreme temperatures in the future. These findings suggested that qingbo exhibited excellent adaptability to both predictable and unpredictable temperature fluctuations, which may be associated with frequent temperature fluctuations in its natural habitat.

High water temperature significantly influences swimming performance of New Zealand migratory species

Anthropogenic structures in freshwater systems pose a significant threat by fragmenting habitats. Effective fish passage solutions must consider how environmental changes introduce variability into swimming performance. As temperature is considered the most important external factor influencing fish physiology, it is especially important to consider its effects on fish swimming performance. Even minor alterations in water properties, such as temperature and velocity, can profoundly affect fish metabolic demands, foraging behaviours, fitness and, consequently, swimming performance and passage success. In this study, we investigated the impact of varying water temperatures on the critical swimming speeds of four migratory New Zealand species. Our findings revealed a significant reduction in critical swimming speeds at higher water temperatures (26°C) compared to lower ones (8 and 15°C) for three out of four species (Galaxias maculatus, Galaxias brevipinnis and Gobiomorphus cotidianus). In contrast, Galaxias fasciatus exhibited no significant temperature-related changes in swimming performance, suggesting species-specific responses to temperature. The cold temperature treatment did not impact swimming performance for any of the studied species. As high water temperatures significantly reduce fish swimming performance, it is important to ensure that fish passage solutions are designed to accommodate a range of temperature changes, including spatial and temporal changes, ranging from diel to decadal fluctuations. Our research underscores the importance of incorporating temperature effects into fish passage models for habitat restoration, connectivity initiatives, and freshwater fish conservation. The influence of temperature on fish swimming performance can alter migration patterns and population dynamics, highlighting the need for adaptive conservation strategies. To ensure the resilience of freshwater ecosystems it is important to account for the impact of temperature on fish swimming performance, particularly in the context of a changing climate.

Despite plasticity, heatwaves are costly for a coral reef fish

Climate change is intensifying extreme weather events, including marine heatwaves, which are prolonged periods of anomalously high sea surface temperature that pose a novel threat to aquatic animals. Tropical animals may be especially vulnerable to marine heatwaves because they are adapted to a narrow temperature range. If these animals cannot acclimate to marine heatwaves, the extreme heat could impair their behavior and fitness. Here, we investigated how marine heatwave conditions affected the performance and thermal tolerance of a tropical predatory fish, arceye hawkfish (Paracirrhites arcatus), across two seasons in Moorea, French Polynesia. We found that the fish's daily activities, including recovery from burst swimming and digestion, were more energetically costly in fish exposed to marine heatwave conditions across both seasons, while their aerobic capacity remained the same. Given their constrained energy budget, these rising costs associated with warming may impact how hawkfish prioritize activities. Additionally, hawkfish that were exposed to hotter temperatures exhibited cardiac plasticity by increasing their maximum heart rate but were still operating within a few degrees of their thermal limits. With more frequent and intense heatwaves, hawkfish, and other tropical fishes must rapidly acclimate, or they may suffer physiological consequences that alter their role in the ecosystem.

Comparing methods for determining the metabolic capacity of lumpfish (Cyclopterus lumpus Linnaeus 1758)

Lumpfish (Cyclopterus lumpus) mortalities have been reported during the summer at some North Atlantic salmon cage-sites where they serve as "cleaner fish." To better understand this species' physiology and whether limitations in their metabolic capacity and thermal tolerance can explain this phenomenon, we compared the aerobic scope (AS) of 6°C-acclimated lumpfish (~50 g and 8.8 cm in length at the beginning of experiments) when all individuals (N = 12) were given a chase to exhaustion, a critical swim speed (Ucrit) test, and a critical thermal maximum (CTMax) test (rate of warming 2°C h-1). The Ucrit and CTMax of the lumpfish were 2.36 ± 0.08 body lengths per second and 20.6 ± 0.3°C. The AS of lumpfish was higher during the Ucrit test (206.4 ± 8.5 mg O2 kg-1 h-1) versus that measured in either the CTMax test or after the chase to exhaustion (141.0 ± 15.0 and 124.7 ± 15.5 mg O2 kg-1 h-1, respectively). Maximum metabolic rate (MMR), AS, and "realistic" AS (ASR) measured using the three different protocols were not significantly correlated, indicating that measurements of metabolic capacity using one of these methods cannot be used to estimate values that would be obtained using another method. Additional findings include that (1) the lumpfish's metabolic capacity is comparable to that of Atlantic cod, suggesting that they are not as "sluggish" as previously suggested in the literature, and (2) their CTMax (20.6°C when acclimated to 6°C), in combination with their recently determined ITMax (20.6°C when acclimated to 10°C), indicates that high sea-cage temperatures are unlikely to be the primary cause of lumpfish mortalities at salmon sea-cages during the summer.

Impacts of behaviour and acclimation of metabolic rate on energetics in sheltered ectotherms: a climate change perspective

Many ectothermic organisms counter harsh abiotic conditions by seeking refuge in underground retreats. Variations in soil hydrothermal properties within these retreats may impact their energy budget, survival and population dynamics. This makes retreat site choice a critical yet understudied component of their strategies for coping with climate change. We used a mechanistic modelling approach to explore the implications of behavioural adjustments and seasonal acclimation of metabolic rate on retreat depth and the energy budget of ectotherms, considering both current and future climate conditions. We used a temperate amphibian, the alpine newt (Ichthyosaura alpestris), as a model species. Our simulations predict an interactive influence of different thermo- and hydroregulatory strategies on the vertical positioning of individuals in underground refuges. The adoption of a particular strategy largely determines the impact of climate change on retreat site choice. Additionally, we found that, given the behavioural thermoregulation/hydroregulation and metabolic acclimation patterns considered, behaviour within the retreat has a greater impact on ectotherm energetics than acclimation of metabolic rate under different climate change scenarios. We conclude that further empirical research aimed at determining ectotherm behavioural strategies during both surface activity and inactivity is needed to understand their population dynamics and species viability under climate change.

Does size-selective harvesting erode adaptive potential to thermal stress?

Overharvesting is a serious threat to many fish populations. High mortality and directional selection on body size can cause evolutionary change in exploited populations via selection for a specific phenotype and a potential reduction in phenotypic diversity. Whether the loss of phenotypic diversity that accompanies directional selection impairs response to environmental stress is not known. To address this question, we exposed three zebrafish selection lines to thermal stress. Two lines had experienced directional selection for (1) large and (2) small body size, and one was (3) subject to random removal of individuals with respect to body size (i.e. line with no directional selection). Selection lines were exposed to three temperatures (elevated, 34°C; ambient, 28°C; low, 22°C) to determine the response to an environmental stressor (thermal stress). We assessed differences among selection lines in their life history (growth and reproduction), physiological traits (metabolic rate and critical thermal max) and behaviour (activity and feeding behaviour) when reared at different temperatures. Lines experiencing directional selection (i.e. size selected) showed reduced growth rate and a shift in average phenotype in response to lower or elevated thermal stress compared with fish from the random-selected line. Our data indicate that populations exposed to directional selection can have a more limited capacity to respond to thermal stress compared with fish that experience a comparable reduction in population size (but without directional selection). Future studies should aim to understand the impacts of environmental stressors on natural fish stocks.

Effects of Temperature Adaptation on the Metabolism and Physiological Properties of Sturgeon Fish Larvae Cell Line

This study investigated how Atlantic sturgeon cells respond to elevated temperatures, shedding light on the potential impacts of climate change on fish. Atlantic sturgeon (Acipenser oxyrinchus), an IUCN (International Union for Conservation of Nature) Red List species and evolutionarily related to paleonisiform species, may have considerable physiological adaptability, suggesting that this species may be able to cope with changing climatic conditions and higher temperatures. To test this hypothesis, the AOXlar7y cell line was examined at 20 °C (control) and at elevated temperatures of 25 °C and 28 °C. Parameters including proliferation, vitality, morphology, and gene expressions related to proliferation, stemness, and stress were evaluated. Additionally, to achieve a comprehensive understanding of cellular changes, mitochondrial and metabolic activities were assessed using Seahorse XF96. AOXlar7y cells adapted to 28 °C exhibited enhanced mitochondrial adaptability, plasticity, heightened cell proliferation, and increased hsp70 expression. Increased baseline respiration indicated elevated ATP demand, which is potentially linked to higher cell proliferation and heat stress defense. Cells at 28 °C also displayed elevated reserve respiration capacity, suggesting adaptation to energy demands. At 25 °C, AOXlar7y cells showed no changes in basal respiration or mitochondrial capacity, suggesting unchanged ATP demand compared to cells cultivated at 20 °C. Proliferation and glycolytic response to energy requirements were diminished, implying a connection between glycolysis inhibition and proliferation suppression. These research results indicate sturgeon cells are capable of withstanding and adapting to an 8 °C temperature increase. This cellular analysis lays a foundation for future studies aimed at a deeper understanding of fish cell physiological adaptations, which will contribute to a better knowledge of environmental threats facing Atlantic sturgeon and fish populations amid climate change.

Capture and discard practises associated with an ornamental fishery affect the metabolic rate and aerobic capacity of three-striped dwarf cichlids Apistogramma trifasciata

Fishing causes direct removal of individuals from wild populations but can also cause a physiological disturbance in fish that are released or discarded after capture. While sublethal physiological effects of fish capture have been well studied in commercial and recreational fisheries, this issue has been overlooked for the ornamental fish trade, where it is common to capture fish from the wild and discard non-target species. We examined metabolic responses to capture and discard procedures in the three-striped dwarf cichlid Apistogramma trifasciata, a popular Amazonian aquarium species that nonetheless may be discarded when not a target species. Individuals (n = 34) were tagged and exposed to each of four treatments designed to simulate procedures during the capture and discard process: 1) a non-handling control; 2) netting; 3) netting +30 seconds of air exposure; and 4) netting +60 seconds of air exposure. Metabolic rates were estimated using intermittent-flow respirometry, immediately following each treatment then throughout recovery overnight. Increasing amounts of netting and air exposure caused an acute increase in oxygen uptake and decrease in available aerobic scope. In general, recovery occurred quickly, with rapid decreases in oxygen uptake within the first 30 minutes post-handling. Notably, however, male fish exposed to netting +60 seconds of air exposure showed a delayed response whereby available aerobic scope was constrained <75% of maximum until ~4-6 hours post-stress. Larger fish showed a greater initial increase in oxygen uptake post-stress and slower rates of recovery. The results suggest that in the period following discard, this species may experience a reduced aerobic capacity for additional behavioural/physiological responses including feeding, territory defence and predator avoidance. These results are among the first to examine impacts of discard practises in the ornamental fishery and suggest ecophysiological research can provide valuable insight towards increasing sustainable practises in this global trade.

Beyond reaction norms: the temporal dynamics of phenotypic plasticity

Phenotypic plasticity can allow organisms to cope with environmental changes. Although reaction norms are commonly used to quantify plasticity along gradients of environmental conditions, they often miss the temporal dynamics of phenotypic change, especially the speed at which it occurs. Here, we argue that studying the rate of phenotypic plasticity is a crucial step to quantify and understand its adaptiveness. Iteratively measuring plastic traits allows us to describe the actual dynamics of phenotypic changes and avoid quantifying reaction norms at times that do not truly reflect the organism's capacity for plasticity. Integrating the temporal component in how we describe, quantify, and conceptualise phenotypic plasticity can change our understanding of its diversity, evolution, and consequences.

Cardiorespiratory physiology and swimming capacity of Atlantic salmon (Salmo salar) at cold temperatures

We investigated how acclimation to 8, 4 and 1°C, and acute cooling from 8  to 1°C, affected the Atlantic salmon's aerobic and anaerobic metabolism, and cardiac function, during a critical swim speed (Ucrit) test. This study revealed several interesting temperature-dependent effects. First, while differences in resting heart rate (fH) between groups were predictable based on previous research (range ∼28-65 beats  min-1), with values for 1°C-acclimated fish slightly higher than those of acutely exposed conspecifics, the resting cardiac output () of 1°C-acclimated fish was much lower and compensated for by a higher resting blood oxygen extraction (ṀO2/). In contrast, the acutely exposed fish had a ∼2-fold greater resting stroke volume (VS) compared with that of the other groups. Second, increases in fH (1.2- to 1.4-fold) contributed little to during the Ucrit test, and the contributions of (VS) versus ṀO2/ to aerobic scope (AS) were very different in the two groups tested at 1°C (1°C-acclimated and 8-1°C fish). Finally, Ucrit was 2.08 and 1.69 body lengths (BL) s-1 in the 8 and 4°C-acclimated groups, but only 1.27 and 1.44 BL s-1 in the 1°C-acclimated and 8-1°C fish, respectively - this lower value in 1°C versus 8-1°C fish despite higher values for maximum metabolic rate and AS. These data: support recent studies which suggest that the capacity to increase fH is constrained at low temperatures; show that cardiorespiratory function at cold temperatures, and its response to increased demands, depends on exposure duration; and suggest that AS does not constrain swimming capacity in salmon when chronically exposed to temperatures approaching their lower limit.

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.

Metabolic constraints and individual variation shape the trade-off between physiological recovery and anti-predator responses in adult sockeye salmon

Metabolic scope represents the aerobic energy budget available to an organism to perform non-maintenance activities (e.g., escape a predator, recover from a fisheries interaction, compete for a mate). Conflicting energetic requirements can give rise to ecologically relevant metabolic trade-offs when energy budgeting is constrained. The objective of this study was to investigate how aerobic energy is utilized when individual sockeye salmon (Oncorhynchus nerka) are exposed to multiple acute stressors. To indirectly assess metabolic changes in free-swimming individuals, salmon were implanted with heart rate biologgers. The animals were then exercised to exhaustion or briefly handled as a control, and allowed to recover from this stressor for 48 h. During the first 2 h of the recovery period, individual salmon were exposed to 90 ml of conspecific alarm cues or water as a control. Heart rate was recorded throughout the recovery period. Recovery effort and time was higher in exercised fish, relative to control fish, whereas exposure to an alarm cue had no effect on either of these metrics. Individual routine heart rate was negatively correlated with recovery time and effort. Together, these findings suggest that metabolic energy allocation towards exercise recovery (i.e., an acute stressor; handling, chase, etc.) trumps anti-predator responses in salmon, although individual variation may mediate this effect at the population level.

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.

Seasonal, environmental and individual effects on hypoxia tolerance of eastern sand darter (Ammocrypta pellucida)

Metabolic rate and hypoxia tolerance are highly variable among individual fish in a stable environment. Understanding the variability of these measures in wild fish populations is critical for assessing adaptive potential and determining local extinction risks as a result of climate-induced fluctuations in temperature and hypoxic conditions. We assessed the field metabolic rate (FMR) and two hypoxia tolerance metrics, oxygen pressure at loss of equilibrium (PO2 at LOE) and critical oxygen tolerance (Pcrit) of wild-captured eastern sand darter (Ammocrypta pellucida), a threatened species in Canada, using field trials (June to October) that encompassed ambient water temperatures and oxygen conditions typically experienced by the species. Temperature was significantly and positively related to hypoxia tolerance but not FMR. Temperature alone explained 1%, 31% and 7% of the variability observed in FMR, LOE, and Pcrit, respectively. Environmental and fish-specific factors such as reproductive season and condition explained much of the residual variation. Reproductive season significantly affected FMR by increasing it by 159-176% over the tested temperature range. Further understanding the impact of reproductive season on metabolic rate over a temperature range is crucial for understanding how climate change could impact species fitness. Among-individual variation in FMR significantly increased with temperature while among-individual variation in both hypoxia tolerance metrics did not. A large degree of variation in FMR in the summer might allow for evolutionary rescue with increasing mean and variance of global temperatures. Findings suggest that temperature may be a weak predictor in a field setting where biotic and abiotic factors can act concurrently on variables that affect physiological tolerance.

Tissue distribution of appetite regulation genes and their expression in the Amazon fish Colossoma macropomum exposed to climate change scenario

Climate change leads to an increase in water acidification and temperature, two environmental factors that can change fish appetite and metabolism, affecting fish population in both wild and aquaculture facilities. Therefore, our study tested if climate change affects gene expression levels of two appetite-regulating peptides - Neuropeptide Y (NPY) and Cholecystokinin (CCK) - in the brain of tambaqui, Colossoma macropomum. Additionally, we show the distribution of these genes throughout the body. Amino acid sequences of CCK and NPY of tambaqui showed high similarity with other Characiformes, with the closely related order Cypriniformes, and even with the more distantly related order Salmoniformes. High apparent levels of both peptides were expressed in all brain areas, while expression levels varied for peripheral tissues. NPY and CCK mRNA were detected in all peripheral tissues but cephalic kidney for CCK. As for the effects of climate change, we found that fish exposed to extreme climate scenario (800 ppm CO2 and 4.5 °C above current climate scenario) had higher expression levels of NPY and lower expression levels of CCK in the telencephalon. The extreme climate scenario also increased food intake, weight gain, and body length. These results suggest that the telencephalon is probably responsible for sensing the metabolic status of the organism and controlling feeding behavior through NPY, likely an orexigenic hormone, and CCK, which may act as an anorexigenic hormone. To our knowledge, this is the first study showing the effects of climate change on the endocrine regulation of appetite in an endemic and economically important fish from the Amazon. Our results can help us predict the impact of climate change on both wild and farmed fish populations, thus contributing to the elaboration of future policies regarding their conservation and sustainable use.

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.

Metabolic rate increases with acclimation temperature and is associated with mitochondrial function in some tissues of threespine stickleback

The metabolic rate (ṀO2) of eurythermal fishes changes in response to temperature, yet it is unclear how changes in mitochondrial function contribute to changes in ṀO2. We hypothesized that ṀO2 would increase with acclimation temperature in the threespine stickleback (Gasterosteus aculeatus) in parallel with metabolic remodeling at the cellular level but that changes in metabolism in some tissues, such as liver, would contribute more to changes in ṀO2 than others. Threespine stickleback were acclimated to 5, 12 and 20°C for 7 to 21 weeks. At each temperature, standard and maximum metabolic rate (SMR and MMR, respectively), and absolute aerobic scope (AAS) were quantified, along with mitochondrial respiration rates in liver, oxidative skeletal and cardiac muscles, and the maximal activity of citrate synthase (CS) and lactate dehydrogenase (LDH) in liver, and oxidative and glycolytic skeletal muscles. SMR, MMR and AAS increased with acclimation temperature, along with rates of mitochondrial phosphorylating respiration in all tissues. Low SMR and MMR at 5°C were associated with low or undetectable rates of mitochondrial complex II activity and a greater reliance on complex I activity in liver, oxidative skeletal muscle and heart. SMR was positively correlated with cytochrome c oxidase (CCO) activity in liver and oxidative muscle, but not mitochondrial proton leak, whereas MMR was positively correlated with CCO activity in liver. Overall, the results suggest that changes in ṀO2 in response to temperature are driven by changes in some aspects of mitochondrial function in some, but not all, tissues of threespine stickleback.

The effect of temperature on specific dynamic action of juvenile fall-run Chinook salmon, Oncorhynchus tshawytscha

Juvenile fall-run Chinook salmon (Oncorhynchus tshawytscha) in the Sacramento-San Joaquin River Basin experience temporally and spatially heterogenous temperature regimes, between cool upper tributaries and the warm channelized Delta, during freshwater rearing and outmigration. Limited water resources necessitate human management of dam releases, allowing temperature modifications. The objective of this study was to examine the effect of temperature on specific dynamic action (SDA), or the metabolic cost associated with feeding and digestion, which is thought to represent a substantial portion of fish energy budgets. Measuring SDA with respect to absolute aerobic scope (AAS), estimated by the difference between maximum metabolic rate (MMR) and standard metabolic rate (SMR), provides a snapshot of its respective energy allocation. Fish were acclimated to 16°C, raised or lowered to each acute temperature (13°C, 16°C, 19°C, 22°C or 24°C), then fed a meal of commercial pellets weighing 2% of their wet mass. We detected a significant positive effect of temperature on SMR and MMR, but not on AAS. As expected, there was no significant effect of temperature on the total O₂ cost of digestion, but unlike other studies, we did not see a significant difference in duration, peak metabolic rate standardized to SMR, time to peak, percent of meal energy utilized, nor the ratio of peak O₂ consumption to SMR. Peak O₂ consumption represented 10.4-14.5% of AAS leaving a large amount of aerobic capacity available for other activities, and meal energy utilized for digestion ranged from 5.7% to 7.2%, leaving substantial remaining energy to potentially assimilate for growth. Our juvenile fall-run Chinook salmon exhibited thermal stability in their SDA response, which may play a role in maintaining homeostasis of digestive capability in a highly heterogeneous thermal environment where rapid growth is important for successful competition with conspecifics and for avoiding predation.

Hypoxia and High Temperature as Interacting Stressors: Will Plasticity Promote Resilience of Fishes in a Changing World?

AbstractDetermining the resilience of a species or population to climate change stressors is an important but difficult task because resilience can be affected both by genetically based variation and by various types of phenotypic plasticity. In addition, most of what is known about organismal responses is for single stressors in isolation, but environmental change involves multiple environmental factors acting in combination. Here, our goal is to summarize what is known about phenotypic plasticity in fishes in response to high temperature and low oxygen (hypoxia) in combination across multiple timescales, to ask how much resilience plasticity may provide in the face of climate change. There are relatively few studies investigating plasticity in response to these environmental stressors in combination; but the available data suggest that although fish have some capacity to adjust their phenotype and compensate for the negative effects of acute exposure to high temperature and hypoxia through acclimation or developmental plasticity, compensation is generally only partial. There is very little known about intergenerational and transgenerational effects, although studies on each stressor in isolation suggest that both positive and negative impacts may occur. Overall, the capacity for phenotypic plasticity in response to these two stressors is highly variable among species and extremely dependent on the specific context of the experiment, including the extent and timing of stressor exposure. This variability in the nature and extent of plasticity suggests that existing phenotypic plasticity is unlikely to adequately buffer fishes against the combined stressors of high temperature and hypoxia as our climate warms.

Divergence in digestive and metabolic strategies matches habitat differentiation in juvenile salmonids

Divergent energy acquisition and processing strategies associated with using different microhabitats may allow phenotypes to specialize and coexist at small spatial scales. To understand how ecological specialization affects differentiation in energy acquisition and processing strategies, we examined relationships among digestive physiology, growth, and energetics by performing captive experiments on juveniles of wild coho salmon (Oncorhynchus kisutch) and steelhead trout (O. mykiss) that exploit adjacent habitats along natural low-to-high energy flux gradients (i.e., pools versus riffles) in coastal streams. We predicted that: (i) the specialization of steelhead trout to high-velocity, high-energy habitats would result in elevated food intake and growth at the cost of lower growth efficiency relative to coho salmon; (ii) the two species would differentiate along a rate-maximizing (steelhead trout) versus efficiency-maximizing (coho salmon) axis of digestive strategies matching their ecological lifestyle; and (iii) the higher postprandial metabolic demand (i.e., specific dynamic action, SDA) associated with elevated food intake would occupy a greater fraction of the steelhead trout aerobic budget. Relative to coho salmon, steelhead trout presented a pattern of faster growth and higher food intake but lower growth efficiency, supporting the existence of a major growth versus growth efficiency trade-off between species. After accounting for differences in ration size between species, steelhead trout also presented higher SDA than coho salmon, but similar intestinal transit time and lower assimilation efficiency. Both species presented similar aerobic budgets since the elevated SDA of steelhead trout was largely compensated by their higher aerobic scope relative to coho salmon. Our results illustrate the key contribution of digestive physiology to the adaptive differentiation of juvenile growth, energetics, and overall performance of taxa with divergent habitat specializations along a natural productivity gradient.

Environmental change and the rate of phenotypic plasticity

With rapid and less predictable environmental change emerging as the 'new norm', understanding how individuals tolerate environmental stress via plastic, often reversible changes to the phenotype (i.e., reversible phenotypic plasticity, RPP), remains a key issue in ecology. Here, we examine the potential for better understanding how organisms overcome environmental challenges within their own lifetimes by scrutinizing a somewhat overlooked aspect of RPP, namely the rate at which it can occur. Although recent advances in the field provide indication of the aspects of environmental change where RPP rates may be of particular ecological relevance, we observe that current theoretical models do not consider the evolutionary potential of the rate of RPP. Whilst recent theory underscores the importance of environmental predictability in determining the slope of the evolved reaction norm for a given trait (i.e., how much plasticity can occur), a hitherto neglected possibility is that the rate of plasticity might be a more dynamic component of this relationship than previously assumed. If the rate of plasticity itself can evolve, as empirical evidence foreshadows, rates of plasticity may have the potential to alter the level predictability in the environment as perceived by the organism and thus influence the slope of the evolved reaction norm. However, optimality in the rate of phenotypic plasticity, its evolutionary dynamics in different environments and influence of constraints imposed by associated costs remain unexplored and may represent fruitful avenues of exploration in future theoretical and empirical treatments of the topic. We conclude by reviewing published studies of RPP rates, providing suggestions for improving the measurement of RPP rates, both in terms of experimental design and in the statistical quantification of this component of plasticity.

The interaction between metabolic rate, habitat choice, and resource use in a polymorphic freshwater species

Resource polymorphism is common across taxa and can result in alternate ecotypes with specific morphologies, feeding modes, and behaviors that increase performance in a specific habitat. This can result in high intraspecific variation in the expression of specific traits and the extent to which these traits are correlated within a single population. Although metabolic rate influences resource acquisition and the overall pace of life of individuals it is not clear how metabolic rate interacts with the larger suite of traits to ultimately determine individual fitness.We examined the relationship between metabolic rates and the major differences (habitat use, morphology, and resource use) between littoral and pelagic ecotypes of European perch (Perca fluviatilis) from a single lake in Central Sweden.Standard metabolic rate (SMR) was significantly higher in pelagic perch but did not correlate with resource use or morphology. Maximum metabolic rate (MMR) was not correlated with any of our explanatory variables or with SMR. Aerobic scope (AS) showed the same pattern as SMR, differing across habitats, but contrary to expectations, was lower in pelagic perch.This study helps to establish a framework for future experiments further exploring the drivers of intraspecific differences in metabolism. In addition, since metabolic rates scale with temperature and determine predator energy requirements, our observed differences in SMR across habitats will help determine ecotype-specific vulnerabilities to climate change and differences in top-down predation pressure across habitats.

Aerobic scope in fishes with different lifestyles and across habitats: Trade-offs among hypoxia tolerance, swimming performance and digestion

Exercise and aerobic scope in fishes have attracted scientists' attention for several decades. While it has been suggested that aerobic scope may limit behavioral expression and tolerance to environmental stressors in fishes, the exact importance of aerobic scope in an ecological context remains poorly understood. In this review, we examine the ecological relevance of aerobic scope by reconsidering and reanalyzing the existing literature on Chinese freshwater fishes across a wide-range of habitats and lifestyles. The available evidence suggests that natural selection in fast-flowing aquatic habitats may favor species with a high aerobic scope and anaerobic capacity for locomotion, whereas in relatively slow-flowing habitats, hypoxia tolerance may be favored at the cost of reduced locomotor capacity. In addition, while physical activity can usually cause fishes from fast-flowing habitats to reach their aerobic metabolic ceiling (i.e., maximum metabolic rate), possibly due to selection pressure on locomotion, most species from slow-flowing habitats can only reach their metabolic ceiling during digestion, either alone or in combination with physical activity. Overall, we suggest that fish exhibit a continuum of metabolic types, from a 'visceral metabolic type' with a higher digestive performance to a 'locomotion metabolic type' which appears to have reduced capacity for digestion but enhanced locomotor performance. Generally, locomotor-type species can either satisfy the demands of their high swimming capacity with a high oxygen uptake capacity or sacrifice digestion while swimming. In contrast, most visceral-type species show a pronounced decrease in swimming performance while digesting, probably owing to conflicts within their aerobic scope. In conclusion, the ecological relevance of aerobic scope and the consequent effects on other physiological functions are closely related to habitat and the lifestyle of a given species. These results suggest that swimming performance, digestion and hypoxia tolerance might coevolve due to dependence on metabolic traits such as aerobic scope.

The time course of metabolic plasticity and its consequences for growth performance under variable food supply in the northern pike

Many species up- or downregulate their resting metabolic rate (RMR) when they encounter favourable or unfavourable feeding conditions, respectively. This is thought to promote faster growth when food is abundant and conserve energy reserves when food is scarce. The time it takes to express metabolic plasticity remain little studied. Here, we develop a conceptual model showing how rapid or slow metabolic plasticity alter growth trajectories in response to changes in food supply. We test predictions from the model in a food manipulation experiment with young-of-the-year northern pike, Esox lucius, a species that experience drastic changes in food supply in nature. We find that metabolic plasticity is expressed gradually over several weeks in this species. Rapid changes in food supply thus caused apparent trait-environment mismatches that persisted for at least five weeks. Contrary to predictions, pike grew faster at high food levels when they had previously experienced low food levels and downregulated their RMR. This was not owing to increases in food intake but probably reflected that low RMRs increased the energetic scope for growth when feeding conditions improved. This highlights the important but complex effects of metabolic plasticity on growth dynamics under variable resource levels on ecologically relevant time scales.

Temperature and Diet Acclimation Modify the Acute Thermal Performance of the Largest Extant Amphibian

The Chinese giant salamander (Andrias davidianus), one of the largest extant amphibian species, has dramatically declined in the wild. As an ectotherm, it may be further threatened by climate change. Therefore, understanding the thermal physiology of this species should be the priority to formulate related conservation strategies. In this study, the plasticity in metabolic rate and thermal tolerance limits of A. davidianus larvae were studied. Specifically, the larvae were acclimated to three temperature levels (7 °C, cold stress; 15 °C, optimum; and 25 °C, heat stress) and two diet items (red worm or fish fray) for 20 days. Our results indicated that cold-acclimated larvae showed increased metabolic capacity, while warm-acclimated larvae showed a decrease in metabolic capacity. These results suggested the existence of thermal compensation. Moreover, the thermal tolerance windows of cold-acclimated and warm-acclimated larvae shifted to cooler and hotter ranges, respectively. Metabolic capacity is not affected by diet but fish-fed larvae showed superiority in both cold and heat tolerance, potentially due to the input of greater nutrient loads. Overall, our results suggested a plastic thermal tolerance of A. davidianus in response to temperature and diet variations. These results are meaningful in guiding the conservation of this species.

Aerobic scope is not maintained at low temperature and is associated with cardiac aerobic capacity in the three-spined stickleback Gasterosteus aculeatus

Metabolic thermal plasticity is central to the survival of fishes in a changing environment. The eurythermal three-spined stickleback Gasterosteus aculeatus displays thermal plasticity at the cellular level with an increase in the activity of key metabolic enzymes in response to cold acclimation. Nonetheless, it is unknown if these changes are sufficient to completely compensate for the depressive effects of cold temperature on whole organismal metabolic rate (ṀO₂ ). The authors hypothesized that as a cold-tolerant, eurythermal fish, absolute aerobic scope (AAS), the difference between the maximum metabolic rate (MMR) and standard metabolic rate (SMR), would be maintained in G. aculeatus following acclimation to a range of temperatures that span its habitat temperatures. To test this hypothesis, G. aculeatus were acclimated to 5, 12 and 20°C for 20-32 weeks, and SMR, MMR and aerobic scope (AS) were quantified at each acclimation temperature. The maximal activity of citrate synthase (CS), a marker enzyme of aerobic metabolism, was also quantified in heart ventricles to determine if cardiac aerobic capacity is associated with AS at these temperatures. SMR increased with acclimation temperature and was significantly different among all three temperature groups. MMR was similar between animals at 5 and 12°C and between animals at 12 and 20°C but was 2.6-fold lower in fish at 5°C compared with those at 20°C, resulting in a lower AAS in fish at 5°C compared with those at 12 and 20°C. Correlated with a higher AAS in animals acclimated to 12 and 20°C was a larger relative ventricular mass and higher CS activity per 100 g body mass compared with animals at 5°C. Together, the results indicate that despite their eurythermal nature, AS is not maintained at low temperature but is associated with cardiac aerobic metabolic capacity.

Differential expression analyses reveal extensive transcriptional plasticity induced by temperature in New Zealand silver trevally (Pseudocaranx georgianus)

Ectotherm species, such as marine fishes, depend on environmental temperature to regulate their vital functions. In finfish aquaculture production, being able to predict physiological responses in growth and other economic traits to temperature is crucial to address challenges inherent in the selection of grow-out locations. This will become an even more significant issue under the various predicted future climate change scenarios. In this study, we used the marine teleost silver trevally (Pseudocaranx georgianus), a species currently being explored as a candidate for aquaculture in New Zealand, as a model to study plasticity in gene expression patterns and growth in response to different temperatures. Using a captive study population, temperature conditions were experimentally manipulated for 1 month to mimic seasonal extremes. Phenotypic differences in growth were measured in 400 individuals, and gene expression patterns of pituitary gland and liver were determined in a subset of 100 individuals. Results showed that growth increased 50% in the warmer compared with the colder condition, suggesting that temperature has a large impact on metabolic activities associated with growth. A total of 265,116,678 single-end RNA sequence reads were aligned to the trevally genome, and 28,416 transcript models were developed (27,887 of these had GenBank accessions, and 17,980 unique gene symbols). Further filtering reduced this set to 8597 gene models. 39 and 238 differentially expressed genes (DEGs) were found in the pituitary gland and the liver, respectively (|log2FC| > 0.26, p-value < 0.05). Of these, 6 DEGs showed a common expression pattern between both tissues, all involved in housekeeping functions. Temperature-modulated growth responses were linked to major pathways affecting metabolism, cell regulation and signalling, previously shown to be important for temperature tolerance in other fish species. An interesting finding of this study was that genes linked to the reproductive system were up-regulated in both tissues in the high treatment, indicating the onset of sexual maturation. Few studies have investigated the thermal plasticity of the gene expression in the main organs of the somatotropic axis simultaneously. Our findings indicate that trevally exhibit substantial growth differences and predictable plastic regulatory responses to different temperature conditions. We identified a set of genes that provide a list of candidates for further investigations for selective breeding objectives and how populations may adapt to increasing temperatures.

An unusually high upper thermal acclimation potential for rainbow trout

Thermal acclimation, a compensatory physiological response, is central to species survival especially during the current era of global warming. By providing the most comprehensive assessment to date for the cardiorespiratory phenotype of rainbow trout (Oncorhynchus mykiss) at six acclimation temperatures from 15°C to 25°C, we tested the hypothesis that, compared with other strains of rainbow trout, an Australian H-strain of rainbow trout has been selectively inbred to have an unusually high and broad thermal acclimation potential. Using a field setting at the breeding hatchery in Western Australia, thermal performance curves were generated for a warm-adapted H-strain by measuring growth, feed conversion efficiency, specific dynamic action, whole-animal oxygen uptake (ṀO₂) during normoxia and hypoxia, the critical maximum temperature and the electrocardiographic response to acute warming. Appreciable growth and aerobic capacity were possible up to 23°C. However, growth fell off drastically at 25°C in concert with increases in the time required to digest a meal, its total oxygen cost and its peak ṀO₂. The upper thermal tipping points for appetite and food conversion efficiency corresponded with a decrease in the ability to increase heart rate during warming and an increase in the cost to digest a meal. Also, comparison of upper thermal tipping points provides compelling evidence that limitations to increasing heart rate during acute warming occurred well below the critical thermal maximum (CT_max) and that the faltering ability of the heart to deliver oxygen at different acclimation temperatures is not reliably predicted by CT_max for the H-strain of rainbow trout. We, therefore, reasoned the remarkably high thermal acclimation potential revealed here for the Australian H-strain of rainbow trout reflected the existing genetic variation within the founder Californian population, which was then subjected to selective inbreeding in association with severe heat challenges. This is an encouraging discovery for those with conservation concerns for rainbow trout and other fish species. Indeed, those trying to predict the impact of global warming should more fully consider the possibility that the standing intra-specific genetic variation within a fish species could provide a high thermal acclimation potential, similar to that shown here for rainbow trout.

Quantitative mismatch between empirical temperature-size rule slopes and predictions based on oxygen limitation

In ectotherms, adult body size commonly declines with increasing environmental temperature, a pattern known as the temperature-size rule. One influential hypothesis explaining this observation is that the challenge of obtaining sufficient oxygen to support metabolism becomes greater with increasing body size, and more so at high temperatures. Yet, previous models based on this hypothesis do not account for phenotypic plasticity in the physiology of organisms that counteracts oxygen limitation at high temperature. Here, we model the predicted strength of the temperature-size response using estimates of how both the oxygen supply and demand is affected by temperature when allowing for phenotypic plasticity in the aquatic ectotherm Daphnia magna. Our predictions remain highly inconsistent with empirical temperature-size responses, with the prior being close to one order of magnitude stronger than the latter. These results fail to provide quantitative support for the hypothesis that oxygen limitation drives temperature-size clines in aquatic ectotherms. Future studies into the role of oxygen limitation should address how the strength of the temperature-size response may be shaped by evolution under fluctuating temperature regimes. Finally, our results caution against applying deterministic models based on the oxygen limitation hypothesis when predicting future changes in ectotherm size distributions under climate change.

Widespread alterations to hypothalamic-pituitary-gonadal (HPG) axis signaling underlie high temperature reproductive inhibition in the eurythermal sheepshead minnow (Cyprinodon variegatus)

Fish experiencing abnormally high or prolonged elevations in temperature can exhibit impaired reproduction, even for species adapted to warm water environments. Such high temperature inhibition of reproduction has been linked to diminished gonadal steroidogenesis, but the mechanisms whereby hypothalamic-pituitary-gonadal (HPG) axis signaling is impacted by high temperature are not fully understood. Here, we characterized differences in HPG status in adult sheepshead minnow (Cyprinodon variegatus), a eurythermal salt marsh and estuarine species of eastern North America, exposed for 14 d to temperatures of 27 °C or 37 °C. Males and females at 37 °C had lower gonadosomatic index (GSI) values compared to fish at 27 °C, and females at 37 °C had fewer spawning capable eggs and lower circulating 17β-estradiol (E₂). Gene transcripts encoding gonadotropin-inhibitory hormone (gnih) and gonadotropin-releasing hormone-3 (gnrh3) were higher in relative abundance in the hypothalamus of both sexes at 37 °C. While pituitary mRNAs for the β-subunits of follicle-stimulating hormone (fshβ) and luteinizing hormone (lhβ) were lowered only in males at 37 °C, Fsh and Lh receptor mRNA levels in the gonads were at lower relative levels in both the ovary and testis of fish at 37 °C. Females at 37 °C also showed reduced ovarian mRNA levels for steroid acute regulatory protein (star), P450 side-chain cleavage enzyme (cyp11a1), 3β-hydroxysteroid dehydrogenase (3βhsd), 17β-hydroxysteroid dehydrogenase (hsd17β3), and ovarian aromatase (cyp19a1a). Females at the higher 37 °C temperature also had a lower liver expression of mRNAs encoding estrogen receptor α (esr1) and several vitellogenin and choriogenin genes, but elevated mRNA levels for hepatic sex hormone-binding globulin (shbg). Our results substantiate prior findings that exposure of fish to high temperature can inhibit gonadal steroidogenesis and oogenesis, and point to declines in reproductive performance emerging from alterations at several levels of HPG axis signaling including increased hypothalamic Gnih expression, depressed gonadal steroidogenesis, and reduced egg yolk and egg envelope protein production in the liver.

Thermal Acclimation to the Highest Natural Ambient Temperature Compromises Physiological Performance in Tadpoles of a Stream-Breeding Savanna Tree Frog

Amphibians may be more vulnerable to climate-driven habitat modification because of their complex life cycle dependence on land and water. Considering the current rate of global warming, it is critical to identify the vulnerability of a species by assessing its potential to acclimate to warming temperatures. In many species, thermal acclimation provides a reversible physiological adjustment in response to temperature changes, conferring resilience in a changing climate. Here, we investigate the effects of temperature acclimation on the physiological performance of tadpoles of a stream-breeding savanna tree frog (Bokermannohyla ibitiguara) in relation to the thermal conditions naturally experienced in their microhabitat (range: 18.8-24.6°C). We quantified performance measures such as routine and maximum metabolic rate at different test (15, 20, 25, 30, and 34°C) and acclimation temperatures (18 and 25°C). We also measured heart rate before and after autonomic blockade with atropine and sotalol at the respective acclimation temperatures. Further, we determined the critical thermal maximum and warming tolerance (critical thermal maximum minus maximum microhabitat temperature), which were not affected by acclimation. Mass-specific routine and mass-specific maximum metabolic rate, as well as heart rate, increased with increasing test temperatures; however, acclimation elevated mass-specific routine metabolic rate while not affecting mass-specific maximum metabolic rate. Heart rate before and after the pharmacological blockade was also unaffected by acclimation. Aerobic scope in animals acclimated to 25°C was substantially reduced, suggesting that physiological performance at the highest temperatures experienced in their natural habitat is compromised. In conclusion, the data suggest that the tadpoles of B. ibitiguara, living in a thermally stable environment, have a limited capacity to physiologically adjust to the highest temperatures found in their micro-habitat, making the species more vulnerable to future climate change.

Associations between metabolic traits and growth rate in brown trout (Salmo trutta) depend on thermal regime

Metabolism defines the energetic cost of life, yet we still know relatively little about why intraspecific variation in metabolic rate arises and persists. Spatio-temporal variation in selection potentially maintains differences, but relationships between metabolic traits (standard metabolic rate (SMR), maximum metabolic rate (MMR), and aerobic scope) and fitness across contexts are unresolved. We show that associations between SMR, MMR, and growth rate (a key fitness-related trait) vary depending on the thermal regime (a potential selective agent) in offspring of wild-sampled brown trout from two populations reared for approximately 15 months in either a cool or warm (+1.8°C) regime. SMR was positively related to growth in the cool, but negatively related in the warm regime. The opposite patterns were found for MMR and growth associations (positive in warm, negative in the cool regime). Mean SMR, but not MMR, was lower in warm regimes within both populations (i.e. basal metabolic costs were reduced at higher temperatures), consistent with an adaptive acclimation response that optimizes growth. Metabolic phenotypes thus exhibited a thermally sensitive metabolic 'floor' and a less flexible metabolic 'ceiling'. Our findings suggest a role for growth-related fluctuating selection in shaping patterns of metabolic variation that is likely important in adapting to climate change.

The effects of warming on red blood cell carbonic anhydrase activity and respiratory performance in a marine fish

Measures of fitness are valuable tools to predict species' responses to environmental changes, like increased water temperature. Aerobic scope (AS) is a measure of an individual's capacity for aerobic processes, and frequently used as a proxy for fitness. However, AS is complicated by individual variation found not only within a species, but within similar body sizes as well. Maximum metabolic rate (MMR), one of the factors determining AS, is constrained by an individual's ability to deliver and extract oxygen (O₂) at the tissues. Recently, data has shown that red blood cell carbonic anhydrase (RBC CA) is rate-limiting for O₂ delivery in red drum (Sciaenops ocellatus). We hypothesized increased temperature impacts MMR and RBC CA activity in a similar manner, and that an individual's RBC CA activity drives individual variation in AS. Red drum were acutely exposed to increased temperature (+6 °C; 22 °C to 28 °C) for 24 h prior to exhaustive exercise and intermittent-flow respirometry at 28 °C. RBC CA activity was measured before temperature exposure and after aerobic performance. Due to enzymatic thermal sensitivity, acute warming increased individual RBC CA activity by 36%, while there was no significant change in the control (22 °C) treatment. Interestingly, average MMR of the acute warming treatment was 36% greater than that of control drum. However, we found no relationships between individual RBC CA activity and their respective MMR and AS at either temperature. While warming similarly affects RBC CA activity and MMR, RBC CA activity is not a predictor of individual MMR.

Interindividual plasticity in metabolic and thermal tolerance traits from populations subjected to recent anthropogenic heating

To better understand temperature's role in the interaction between local evolutionary adaptation and physiological plasticity, we investigated acclimation effects on metabolic performance and thermal tolerance among natural Fundulus heteroclitus (small estuarine fish) populations from different thermal environments. Fundulus heteroclitus populations experience large daily and seasonal temperature variations, as well as local mean temperature differences across their large geographical cline. In this study, we use three populations: one locally heated (32°C) by thermal effluence (TE) from the Oyster Creek Nuclear Generating Station, NJ, and two nearby reference populations that do not experience local heating (28°C). After acclimation to 12 or 28°C, we quantified whole-animal metabolic (WAM) rate, critical thermal maximum (CTmax) and substrate-specific cardiac metabolic rate (CaM, substrates: glucose, fatty acids, lactate plus ketones plus ethanol, and endogenous (i.e. no added substrates)) in approximately 160 individuals from these three populations. Populations showed few significant differences due to large interindividual variation within populations. In general, for WAM and CTmax, the interindividual variation in acclimation response (log2 ratio 28/12°C) was a function of performance at 12°C and order of acclimation (12-28°C versus 28-12°C). CTmax and WAM were greater at 28°C than 12°C, although WAM had a small change (2.32-fold) compared with the expectation for a 16°C increase in temperature (expect 3- to 4.4-fold). By contrast, for CaM, the rates when acclimatized and assayed at 12 or 28°C were nearly identical. The small differences in CaM between 12 and 28°C temperature were partially explained by cardiac remodeling where individuals acclimatized to 12°C had larger hearts than individuals acclimatized to 28°C. Correlation among physiological traits was dependent on acclimation temperature. For example, WAM was negatively correlated with CTmax at 12°C but positively correlated at 28°C. Additionally, glucose substrate supported higher CaM than fatty acid, and fatty acid supported higher CaM than lactate, ketones and alcohol (LKA) or endogenous. However, these responses were highly variable with some individuals using much more FA than glucose. These findings suggest interindividual variation in physiological responses to temperature acclimation and indicate that additional research investigating interindividual may be relevant for global climate change responses in many species.