A Systematic Review of the Effects of Temperature on Anopheles Mosquito Development and Survival: Implications for Malaria Control in a Future Warmer Climate

The rearing temperature of the immature stages can have a significant impact on the life-history traits and the ability of adult mosquitoes to transmit diseases. This review assessed published evidence of the effects of temperature on the immature stages, life-history traits, insecticide susceptibility, and expression of enzymes in the adult Anopheles mosquito. Original articles published through 31 March 2021 were systematically retrieved from Scopus, Google Scholar, Science Direct, PubMed, ProQuest, and Web of Science databases. After applying eligibility criteria, 29 studies were included. The review revealed that immature stages of An. arabiensis were more tolerant (in terms of survival) to a higher temperature than An. funestus and An. quadriannulatus. Higher temperatures resulted in smaller larval sizes and decreased hatching and pupation time. The development rate and survival of An. stephensi was significantly reduced at a higher temperature than a lower temperature. Increasing temperatures decreased the longevity, body size, length of the gonotrophic cycle, and fecundity of Anopheles mosquitoes. Higher rearing temperatures increased pyrethroid resistance in adults of the An. arabiensis SENN DDT strain, and increased pyrethroid tolerance in the An. arabiensis SENN strain. Increasing temperature also significantly increased Nitric Oxide Synthase (NOS) expression and decreased insecticide toxicity. Both extreme low and high temperatures affect Anopheles mosquito development and survival. Climate change could have diverse effects on Anopheles mosquitoes. The sensitivities of Anopeheles mosquitoes to temperature differ from species to species, even among the same complex. Notwithstanding, there seem to be limited studies on the effects of temperature on adult life-history traits of Anopheles mosquitoes, and more studies are needed to clarify this relationship.

Global warming overrides physiological anti-predatory mechanisms in intertidal rock pool fish Gobius paganellus

In nature, a multitude of factors influences the fitness of an organism at a given time, which makes single stressor assessments far from ecologically relevant scenarios. This study focused on the effects of water temperature and predation stress on the metabolism and body mass gain of a common intertidal rock pool fish, Gobius paganellus, addressing the following hypotheses: (1) the energy metabolism of G. paganellus under predation stress is reduced; (2) G. paganellus shows thermal compensation under heat stress; and (3) thermal stress is the dominant stressor that may override predation stress responses. Individuals were exposed to simulated predation stress and temperature increase from 20 °C to 29 °C, and both stressors combined. Physiological effects were addressed using biochemical biomarkers related with energy metabolism (isocitrate dehydrogenase, lactate dehydrogenase, energy available, energy consumption rates), oxidative stress (superoxide dismutase, catalase, DNA damage, lipid peroxidation), and biotransformation (glutathione-S-transferase). The results of this study revealed that predation stress reduced the cellular metabolism of G. paganellus, and enhanced storage of protein reserves. As hypothesized, hyperthermia decreased the aerobic mitochondrial metabolism, indicating thermal compensation mechanisms to resist against unfavourable temperatures. Hyperthermia was the dominant stressor overriding the physiological responses to predation stress. Both stressors combined might further have synergistically activated detoxification pathways, even though not strong enough to counteract lipid peroxidation and DNA damage completely. The synergistic effect of combined thermal and predation stress thus may not only increase the risk of being preyed upon, but also may indicate extra energy trade-off for the basal metabolism, which in turn may have ecologically relevant consequences for general body functions such as somatic growth and reproduction. The present findings clearly underline the ecological importance of multi-stressor assessments to provide a better and holistic picture of physiological responses towards more realistic evaluations of climate change consequences for intertidal populations.

Introduction to the special issue-Beyond CTMAX and CTMIN : Advances in studying the thermal limits of reptiles and amphibians

Two themes emerging from the special issue "Beyond CT_MAX and CT_MIN : Advances in Studying the Thermal Limits of Reptiles and Amphibians" are: (1) the need to identify mechanisms that determine the shape of thermal performance curves and (2) how these curves can be best used predictively.

Summer Is Coming! Tackling Ocean Warming in Atlantic Salmon Cage Farming

Atlantic salmon (Salmo salar) cage farming has traditionally been located at higher latitudes where cold seawater temperatures favor this practice. However, these regions can be impacted by ocean warming and heat waves that push seawater temperature beyond the thermo-tolerance limits of this species. As more mass mortality events are reported every year due to abnormal sea temperatures, the Atlantic salmon cage aquaculture industry acknowledges the need to adapt to a changing ocean. This paper reviews adult Atlantic salmon thermal tolerance limits, as well as the deleterious eco-physiological consequences of heat stress, with emphasis on how it negatively affects sea cage aquaculture production cycles. Biotechnological solutions targeting the phenotypic plasticity of Atlantic salmon and its genetic diversity, particularly that of its southernmost populations at the limit of its natural zoogeographic distribution, are discussed. Some of these solutions include selective breeding programs, which may play a key role in this quest for a more thermo-tolerant strain of Atlantic salmon that may help the cage aquaculture industry to adapt to climate uncertainties more rapidly, without compromising profitability. Omics technologies and precision breeding, along with cryopreservation breakthroughs, are also part of the available toolbox that includes other solutions that can allow cage farmers to continue to produce Atlantic salmon in the warmer waters of the oceans of tomorrow.

Thermal Responses Differ across Levels of Biological Organization

Temperature is one of the most important environmental factors driving the genome-to-phenome relationship. Metabolic rates and related biological processes are predicted to increase with temperature due to the biophysical laws of chemical reactions. However, selection can also act on these processes across scales of biological organization, from individual enzymes to whole organisms. Although some studies have examined thermal responses across multiple scales, there is no general consensus on how these responses vary depending on the level of organization, or whether rates actually follow predicted theoretical patterns such as Arrhenius-like exponential responses or thermal performance curves (TPCs) that show peak responses. Here, we performed a meta-analysis on studies of ectotherms where biological rates were measured across the same set of temperatures, but at multiple levels of biological organization: enzyme activities, mitochondrial respiration, and/or whole-animal metabolic rates. Our final dataset consisted of 235 pairwise comparisons between levels of organization from 13 publications. Thermal responses differed drastically across levels of biological organization, sometimes showing completely opposite patterns. We developed a new effect size metric, "organizational disagreement" (OD) to quantify the difference in responses among levels of biological organization. Overall, rates at higher levels of biological organization (e.g., whole animal metabolic rates) increased more quickly with temperature than rates at lower levels, contrary to our predictions. Responses may differ across levels due to differing consequences of biochemical laws with increasing organization or due to selection for different responses. However, taxa and tissues examined generally did not affect OD. Theoretical TPCs, where rates increase to a peak value and then drop, were only rarely observed (12%), possibly because a broad range of test temperatures was rarely investigated. Exponential increases following Arrhenius predictions were more common (29%). This result suggests a classic assumption about thermal responses in biological rates is rarely observed in empirical datasets, although our results should be interpreted cautiously due to the lack of complete thermal profiles. We advocate for authors to explicitly address OD in their interpretations and to measure thermal responses across a wider, more incremental range of temperatures. These results further emphasize the complexity of connecting the genome to the phenome when environmental plasticity is incorporated: the impact of the environment on the phenotype can depend on the scale of organization considered.

Climate-induced decrease in biomass flow in marine food webs may severely affect predators and ecosystem production

Climate change impacts on marine life in the world ocean are expected to accelerate over the 21st century, affecting the structure and functioning of food webs. We analyzed a key aspect of this issue, focusing on the impact of changes in biomass flow within marine food webs and the resulting effects on ecosystem biomass and production. We used a modeling framework based on a parsimonious quasi-physical representation of biomass flow through the food web, to explore the future of marine consumer biomass and production at the global scale over the 21st century. Biomass flow is determined by three climate-related factors: primary production entering the food web, trophic transfer efficiency describing losses in biomass transfers from one trophic level (TL) to the next, and flow kinetic measuring the speed of biomass transfers within the food web. Using climate projections of three earth system models, we calculated biomass and production at each TL on a 1° latitude ×1° longitude grid of the global ocean under two greenhouse gas emission scenarios. We show that the alterations of the trophic functioning of marine ecosystems, mainly driven by faster and less efficient biomass transfers and decreasing primary production, would lead to a projected decline in total consumer biomass by 18.5% by 2090-2099 relative to 1986-2005 under the "no mitigation policy" scenario. The projected decrease in transfer efficiency is expected to amplify impacts at higher TLs, leading to a 21.3% decrease in abundance of predators and thus to a change in the overall trophic structure of marine ecosystems. Marine animal production is also projected to decline but to a lesser extent than biomass. Our study highlights that the temporal and spatial projected changes in biomass and production would imply direct repercussions on the future of world fisheries and beyond all services provided by Ocean.

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.

Long-term thermal acclimation drives adaptive physiological adjustments of a marine gastropod to reduce sensitivity to climate change

Ocean warming is predicted to challenge the persistence of a variety of marine organisms, especially when combined with ocean acidification. While temperature affects virtually all physiological processes, the extent to which thermal history mediates the adaptive capacity of marine organisms to climate change has been largely overlooked. Using populations of a marine gastropod (Turbo undulatus) with different thermal histories (cool vs. warm), we compared their physiological adjustments following exposure (8-week) to ocean acidification and warming. Compared to cool-acclimated counterparts, we found that warm-acclimated individuals had a higher thermal threshold (i.e. increased CT_max by 2 °C), which was unaffected by the exposure to ocean acidification and warming. Thermal history also strongly mediated physiological effects, where warm-acclimated individuals adjusted to warming by conserving energy, suggested by lower respiration and ingestion rates, energy budget (i.e. scope for growth) and O:N ratio. After exposure to warming, warm-acclimated individuals had higher metabolic rates and greater energy budget due to boosted ingestion rates, but such compensatory feeding disappeared when combined with ocean acidification. Overall, we suggest that thermal history can be a critical mediator of physiological performance under future climatic conditions. Given the relatively gradual rate of global warming, marine organisms may be better able to adaptively adjust their physiology to future climate than what short-term experiments currently convey.

Advances in understanding the impacts of global warming on marine fishes farmed offshore: Sparus aurata as a case study

Monitoring variations in proteins involved in metabolic processes, oxidative stress responses, cell signalling and protein homeostasis is a powerful tool for developing hypotheses of how environmental variations affect marine organisms' physiology and biology. According to the oxygen- and capacity-limited thermal tolerance hypothesis, thermal acclimation mechanisms such as adjusting the activities of enzymes of intermediary metabolism and of antioxidant defence mechanisms, inducing heat shock proteins (Hsps) or activating mitogen-activated protein kinases may all shift tolerance windows. Few studies have, however, investigated the molecular, biochemical and organismal responses by fishes to seasonal temperature variations in the field to link these to laboratory findings. Investigation of the impacts of global warming on fishes farmed offsore, in the open sea, can provide a stepping stone towards understanding effects on wild populations because they experience similar environmental fluctuations. Over the last 30 years, farming of the gilthead sea bream Sparus aurata (Linnaeus 1758) has become widespread along the Mediterranean coastline, rendering this species a useful case study. Based on available information, the prevailing seasonal temperature variations expose the species to the upper and lower limits of its thermal range. Evidence for this includes oxygen restriction, reduced feeding, reduced responsiveness to environmental stimuli, plus a range of molecular and biochemical indicators that change across the thermal range. Additionally, close relationships between biochemical pathways and seasonal patterns of metabolism indicate a connection between energy demand and metabolic processes on the one hand, and cellular stress responses such as oxidative stress, inflammation and autophagy on the other. Understanding physiological responses to temperature fluctuations in fishes farmed offshore can provide crucial background information for the conservation and successful management of aquaculture resources in the face of global change.

Thermally tolerant intertidal triplefin fish (Tripterygiidae) sustain ATP dynamics better than subtidal species under acute heat stress

Temperature is a key factor that affects all levels of organization. Minute shifts away from thermal optima result in detrimental effects that impact growth, reproduction and survival. Metabolic rates of ectotherms are especially sensitive to temperature and for organisms exposed to high acute temperature changes, in particular intertidal species, energetic processes are often negatively impacted. Previous investigations exploring acute heat stress have implicated cardiac mitochondrial function in determining thermal tolerance. The brain, however, is by weight, one of the most metabolically active and arguably the most temperature sensitive organ. It is essentially aerobic and entirely reliant on oxidative phosphorylation to meet energetic demands, and as temperatures rise, mitochondria become less efficient at synthesising the amount of ATP required to meet the increasing demands. This leads to an energetic crisis. Here we used brain homogenate of three closely related triplefin fish species (Bellapiscis medius, Forsterygion lapillum, and Forsterygion varium) and measured respiration and ATP dynamics at three temperatures (15, 25 and 30 °C). We found that the intertidal B. medius and F. lapillum were able to maintain rates of ATP production above rates of ATP hydrolysis at high temperatures, compared to the subtidal F. varium, which showed no difference in rates at 30 °C. These results showed that brain mitochondria became less efficient at temperatures below their respective species thermal limits, and that energetic surplus of ATP synthesis over hydrolysis narrows. In subtidal species synthesis matches hydrolysis, leaving no scope to elevate ATP supply.

Oxygen supply capacity breathes new life into critical oxygen partial pressure (Pcrit)

The critical oxygen partial pressure (Pcrit), typically defined as the PO2 below which an animal's metabolic rate (MR) is unsustainable, is widely interpreted as a measure of hypoxia tolerance. Here, Pcrit is defined as the PO2 at which physiological oxygen supply (α0) reaches its maximum capacity (α; µmol O2 g-1 h-1 kPa-1). α is a species- and temperature-specific constant describing the oxygen dependency of the maximum metabolic rate (MMR=PO2×α) or, equivalently, the MR dependence of Pcrit (Pcrit=MR/α). We describe the α-method, in which the MR is monitored as oxygen declines and, for each measurement period, is divided by the corresponding PO2 to provide the concurrent oxygen supply (α0=MR/PO2). The highest α0 value (or, more conservatively, the mean of the three highest values) is designated as α. The same value of α is reached at Pcrit for any MR regardless of previous or subsequent metabolic activity. The MR need not be constant (regulated), standardized or exhibit a clear breakpoint at Pcrit for accurate determination of α. The α-method has several advantages over Pcrit determination and non-linear analyses, including: (1) less ambiguity and greater accuracy, (2) fewer constraints in respirometry methodology and analysis, and (3) greater predictive power and ecological and physiological insight. Across the species evaluated here, α values are correlated with MR, but not Pcrit. Rather than an index of hypoxia tolerance, Pcrit is a reflection of α, which evolves to support maximum energy demands and aerobic scope at the prevailing temperature and oxygen level.

Effects of temperature, dissolved oxygen, and their interaction on the growth performance and condition of rainbow trout (Oncorhynchus mykiss)

The individual effects of temperature and dissolved oxygen (DO) on rainbow trout (Oncorhynchus mykiss), an important aquaculture species, are clearly established; however, little is known about the interactive effects of these parameters. In this study, the effects of temperature, DO, and their interaction on the growth, antioxidant status, digestive enzyme activity, serum biochemical parameters, and liver IGF-1 expression in rainbow trout were evaluated. Fish (initial weight, 109.98 ± 3.28 g) were reared in a recirculating system for 4 weeks and subjected to 6 treatments at three temperatures (13 °C, 17 °C, and 21 °C) and two DO contents (4.2 mg L^-1 and 9.6 mg L^-1). Physiological parameters were determined at the end of the trial. Specific growth rate and feed consumption were the highest at 17 °C and the lowest at 21 °C. Additionally, lysozyme, trypsin, lipase, and amylase activities, serum glucose and serum triglyceride contents, and IGF-1 expression decreased significantly at 21 °C and total serum protein and albumin contents were significantly higher at 21 °C than at 13 °C and 17 °C, indicating that high temperature impaired the immunity, digestion, and growth of rainbow trout. However, the adverse effects of high temperature can be alleviated by a high DO content, as evidenced by the smaller increments and decrements of these parameters under hyperoxic conditions than under hypoxic conditions. In response to high temperature stress, an increase in antioxidant enzyme activity led to the removal of oxygen free radicals under hyperoxic conditions; however, this increase was inhibited under hypoxia. Our results indicated that high temperatures have adverse effects on rainbow trout, and these harmful effects can be reduced by a high DO content.

Food intake, growth, and expression of neuropeptides regulating appetite in clown anemonefish (Amphiprion ocellaris) exposed to predicted climate changes

The clown anemonefish (Amphiprion ocellaris) is a common model species in studies assessing the impact of climate changes on tropical coral fish physiology, metabolism, growth, and stress. However, the basic endocrine principles for the control of food intake and energy homeostasis, under normal and elevated sea temperatures, in this species remain unknown. In this work, we studied food intake and growth in clown anemonefish reared at different temperatures and with different food availability. We also analyzed expression of genes in the melanocortin system, which is believed to be involved in the control of appetite and feeding behavior. These were two paralogues of pomc: pomca and pomcb; two paralogs of agrp: agrp1 and agrp2; and one mc4r-like. Groups of juvenile clown anemonefish were exposed to four experimental treatments combining (orthogonal design) two rearing temperatures: 28 °C (T28; normal) and 32 °C (T32; high) and two feeding regimes: one (1 M; 08:00) or three (3 M; 08:00, 12:00, 15:00) meals per day, fed to satiety by hand. The results showed that high temperature (T32) did not affect the average growth rate but induced a stronger asymmetrical individual body weight of the fish within the population (tank). Lower feeding frequency (1 M) resulted in lower growth rates at both rearing temperatures. Fish reared at high temperature had higher total daily food intake, which correlated with a lower expression of pomca, supporting an anorexigenic role of this gene. High temperature combined with restricted feeding induced higher agrp1 levels and resulted in a higher food intake in the morning meal compared to the control. This supports an orexigenic role for agrp1. mRNA levels of agrp2 responded differently from agrp1, supporting different roles for the paralogues. Levels of mc4r-like inversely correlated with fish body weight, indicating a possible size/stage dependence of gene expression. In conclusion, our results indicate that the melanocortin system is involved in adjusting appetite and food intake of clown anemonefish in response to elevated temperature and low food availability.

Effects of hypoxia on the thermal physiology of a high-elevation lizard: implications for upslope-shifting species

Montane reptiles are predicted to move to higher elevations in response to climate warming. However, whether upwards-shifting reptiles will be physiologically constrained by hypoxia at higher elevations remains unknown. We investigated the effects of hypoxic conditions on preferred body temperatures (T_pref) and thermal tolerance capacity of a montane lizard (Phrynocephalus vlangalii) from two populations on the Qinghai-Tibet Plateau. Lizards from 2600 m a.s.l. were exposed to O₂ levels mimicking those at 2600 m (control) and 3600 m (hypoxia treatment). Lizards from 3600 m a.s.l. were exposed to O₂ levels mimicking those at 3600 m (control) and 4600 m (hypoxia treatment). The T_pref did not differ between the control and hypoxia treatments in lizards from 2600 m. However, lizards from 3600 m selected lower body temperatures when exposed to the hypoxia treatment mimicking the O₂ level at 4600 m. Additionally, the hypoxia treatment induced lower critical thermal minimum (CT_min) in lizards from both populations, but did not affect the critical thermal maximum (CT_max) in either population. Our results imply that upwards-shifting reptiles may be constrained by hypoxia if a decrease in T_pref reduces thermally dependent fitness traits, despite no observed effect on their heat tolerance.

Research on sablefish (Anoplopoma fimbria) suggests that limited capacity to increase heart function leaves hypoxic fish susceptible to heat waves

Studies of heart function and metabolism have been used to predict the impact of global warming on fish survival and distribution, and their susceptibility to acute and chronic temperature increases. Yet, despite the fact that hypoxia and high temperatures often co-occur, only one study has examined the effects of hypoxia on fish thermal tolerance, and the consequences of hypoxia for fish cardiac responses to acute warming have not been investigated. We report that sablefish (Anoplopoma fimbria) did not increase heart rate or cardiac output when warmed while hypoxic, and that this response was associated with reductions in maximum O2 consumption and thermal tolerance (CTmax) of 66% and approximately 3°C, respectively. Further, acclimation to hypoxia for four to six months did not substantially alter the sablefish's temperature-dependent physiological responses or improve its CTmax. These results provide novel, and compelling, evidence that hypoxia can impair the cardiac and metabolic response to increased temperatures in fish, and suggest that some coastal species may be more vulnerable to climate change-related heat waves than previously thought. Further, they support research showing that cross-tolerance and physiological plasticity in fish following hypoxia acclimation are limited.

Thermal reaction norms of key metabolic enzymes reflect divergent physiological and behavioral adaptations of closely related amphipod species

Lake Baikal is inhabited by more than 300 endemic amphipod species, which are narrowly adapted to certain thermal niches due to the high interspecific competition. In contrast, the surrounding freshwater fauna is commonly represented by species with large-scale distribution and high phenotypic thermal plasticity. Here, we investigated the thermal plasticity of the energy metabolism in two closely-related endemic amphipod species from Lake Baikal (Eulimnogammarus verrucosus; stenothermal and Eulimnogammarus cyaneus; eurythermal) and the ubiquitous Holarctic amphipod Gammarus lacustris (eurythermal) by exposure to a summer warming scenario (6-23.6 °C; 0.8 °C d^-1). In concert with routine metabolic rates, activities of key metabolic enzymes increased strongly with temperature up to 15 °C in E. verrucosus, whereupon they leveled off (except for lactate dehydrogenase). In contrast, exponential increases were seen in E. cyaneus and G. lacustris throughout the thermal trial (Q₁₀-values: 1.6-3.7). Cytochrome-c-oxidase, lactate dehydrogenase, and 3-hydroxyacyl-CoA dehydrogenase activities were found to be higher in G. lacustris than in E. cyaneus, especially at the highest experimental temperature (23.6 °C). Decreasing gene expression levels revealed some thermal compensation in E. cyaneus but not in G. lacustris. In all species, shifts in enzyme activities favored glycolytic energy generation in the warmth. The congruent temperature-dependencies of enzyme activities and routine metabolism in E. verrucosus indicate a strong feedback-regulation of enzymatic activities by whole organism responses. The species-specific thermal reaction norms reflect the different ecological niches, including the spatial distribution, distinct thermal behavior such as temperature-dependent migration, movement activity, and mating season.

Climate impacts on organisms, ecosystems and human societies: integrating OCLTT into a wider context

Physiological studies contribute to a cause and effect understanding of ecological patterns under climate change and identify the scope and limits of adaptation. Across most habitats, this requires analyzing organism responses to warming, which can be modified by other drivers such as acidification and oxygen loss in aquatic environments or excess humidity or drought on land. Experimental findings support the hypothesis that the width and temperature range of thermal performance curves relate to biogeographical range. Current warming causes range shifts, hypothesized to include constraints in aerobic power budget which in turn are elicited by limitations in oxygen supply capacity in relation to demand. Different metabolic scopes involved may set the borders of both the fundamental niche (at standard metabolic rate) and the realized niche (at routine rate). Relative scopes for aerobic performance also set the capacity of species to interact with others at the ecosystem level. Niche limits and widths are shifting and probably interdependent across life stages, with young adults being least thermally vulnerable. The principles of thermal tolerance and performance may also apply to endotherms including humans, their habitat and human society. Overall, phylogenetically based comparisons would need to consider the life cycle of species as well as organism functional properties across climate zones and time scales. This Review concludes with a perspective on how mechanism-based understanding allows scrutinizing often simplified modeling approaches projecting future climate impacts and risks for aquatic and terrestrial ecosystems. It also emphasizes the usefulness of a consensus-building process among experimentalists for better recognition in the climate debate.

How hornbills handle heat: sex-specific thermoregulation in the southern yellow-billed hornbill

At a global scale, thermal physiology is correlated with climatic variables such as temperature and aridity. There is also evidence that thermoregulatory traits vary with fine-scale microclimate, but this has received less attention in endotherms. Here, we test the hypothesis that avian thermoregulation varies with microclimate and behavioural constraints in a non-passerine bird. Male and female southern yellow-billed hornbills (Tockus leucomelas) experience markedly different microclimates while breeding, with the female sealing herself into a tree cavity and moulting all her flight feathers during the breeding attempt, becoming entirely reliant on the male for provisioning. We examined interactions between resting metabolic rate (RMR), evaporative water loss (EWL) and core body temperature (T _b) at air temperatures (T _a) between 30°C and 52°C in male and female hornbills, and quantified evaporative cooling efficiencies and heat tolerance limits. At thermoneutral T _a, neither RMR, EWL nor T _b differed between sexes. At T _a >40°C, however, RMR and EWL of females were significantly lower than those of males, by ∼13% and ∼17%, respectively, despite similar relationships between T _b and T _a, maximum ratio of evaporative heat loss to metabolic heat production and heat tolerance limits (∼50°C). These sex-specific differences in hornbill thermoregulation support the hypothesis that avian thermal physiology can vary within species in response to fine-scale microclimatic factors. In addition, Q ₁₀ for RMR varied substantially, with Q ₁₀ ≤2 in some individuals, supporting recent arguments that active metabolic suppression may be an underappreciated aspect of endotherm thermoregulation in the heat.

Bioenergetics in environmental adaptation and stress tolerance of aquatic ectotherms: linking physiology and ecology in a multi-stressor landscape

Energy metabolism (encompassing energy assimilation, conversion and utilization) plays a central role in all life processes and serves as a link between the organismal physiology, behavior and ecology. Metabolic rates define the physiological and life-history performance of an organism, have direct implications for Darwinian fitness, and affect ecologically relevant traits such as the trophic relationships, productivity and ecosystem engineering functions. Natural environmental variability and anthropogenic changes expose aquatic ectotherms to multiple stressors that can strongly affect their energy metabolism and thereby modify the energy fluxes within an organism and in the ecosystem. This Review focuses on the role of bioenergetic disturbances and metabolic adjustments in responses to multiple stressors (especially the general cellular stress response), provides examples of the effects of multiple stressors on energy intake, assimilation, conversion and expenditure, and discusses the conceptual and quantitative approaches to identify and mechanistically explain the energy trade-offs in multiple stressor scenarios, and link the cellular and organismal bioenergetics with fitness, productivity and/or ecological functions of aquatic ectotherms.

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.

Do aquatic ectotherms perform better under hypoxia after warm acclimation?

Aquatic animals increasingly encounter environmental hypoxia due to climate-related warming and/or eutrophication. Although acute warming typically reduces performance under hypoxia, the ability of organisms to modulate hypoxic performance via thermal acclimation is less understood. Here, we review the literature and ask whether hypoxic performance of aquatic ectotherms improves following warm acclimation. Interpretation of thermal acclimation effects is limited by reliance on data from experiments that are not designed to directly test for beneficial or detrimental effects on hypoxic performance. Most studies have tested hypoxic responses exclusively at test temperatures matching organisms' acclimation temperatures, precluding the possibility of distinguishing between acclimation and acute thermal effects. Only a few studies have applied appropriate methodology to identify beneficial thermal acclimation effects on hypoxic performance, i.e. acclimation to different temperatures prior to determining hypoxic responses at standardised test temperatures. These studies reveal that acute warming predominantly impairs hypoxic performance, whereas warm acclimation tends to be either beneficial or have no effect. If this generalises, we predict that warm-acclimated individuals in some species should outperform non-acclimated individuals under hypoxia. However, acclimation seems to only partially offset acute warming effects; therefore, aquatic ectotherms will probably display overall reduced hypoxic performance in the long term. Drawing on the appropriate methodology, future studies can quantify the ability of organisms to modulate hypoxic performance via (reversible) thermal acclimation and unravel the underlying mechanisms. Testing whether developmental acclimation and multigenerational effects allow for a more complete compensation is essential to allow us to predict species' resilience to chronically warmer, hypoxic environments.

Victims of ancient hyperthermal events herald the fates of marine clades and traits under global warming

Organismic groups vary non-randomly in their vulnerability to extinction. However, it is unclear whether the same groups are consistently vulnerable, regardless of the dominant extinction drivers, or whether certain drivers have their own distinctive and predictable victims. Given the challenges presented by anthropogenic global warming, we focus on changes in extinction selectivity trends during ancient hyperthermal events: geologically rapid episodes of global warming. Focusing on the fossil record of the last 300 million years, we identify clades and traits of marine ectotherms that were more prone to extinction under the onset of six hyperthermal events than during other times. Hyperthermals enhanced the vulnerability of marine fauna that host photosymbionts, particularly zooxanthellate corals, the reef environments they provide, and genera with actively burrowing or swimming adult life-stages. The extinction risk of larger sized fauna also increased relative to non-hyperthermal times, while genera with a poorly buffered internal physiology did not become more vulnerable on average during hyperthermals. Hyperthermal-vulnerable clades include rhynchonelliform brachiopods and bony fish, whereas resistant clades include cartilaginous fish, and ostreid and venerid bivalves. These extinction responses in the geological past mirror modern responses of these groups to warming, including range-shift magnitudes, population losses, and experimental performance under climate-related stressors. Accordingly, extinction mechanisms distinctive to rapid global warming may be indicated, including sensitivity to warming-induced seawater deoxygenation. In anticipation of modern warming-driven marine extinctions, the trends illustrated in the fossil record offer an expedient preview.

Detection of genes positively selected in Cuban Anolis lizards that naturally inhabit hot and open areas and currently thrive in urban areas

Species of Anolis lizards of the West Indies that naturally inhabit hot and open areas also tend to thrive in urban areas. In this study, transcriptome was sequenced for nine species of Cuban Anolis lizards that are closely related to each other, but inhabit different thermal microhabitats. Using PAML and HyPhy software, we attempted to identify genes and amino acid sites under positive selection in the common ancestral branch of A. porcatus and A. allisoni, and the branch of A. sagrei, which inhabit hot and open areas, and thrive in urban areas. Although there were no genes where positive selection was commonly detected on both of the tested branches, positive selection was detected in genes involved in the stress response (e.g., DNA damage and oxidative stress) and cardiac function, which could be related to adaptive evolution of tolerance to heat or ultraviolet radiation, on both branches. These findings suggest that adaptive evolution of the response to stress caused by heat or ultraviolet radiation might have occurred in ancestors of Anolis species inhabiting hot and open areas and might be related to the current thriving in urban areas of them.

Non-invasive MRI Studies of Ventilatory and Cardiovascular Performance in Edible Crabs Cancer pagurus During Warming Under Elevated CO2 Levels

The thermal tolerance of marine decapod crustacea is defined through their capacities for oxygen uptake and distribution. High ambient CO₂ levels were previously shown to reduce hemolymph oxygen levels at enhanced cardiac performance during warming. This study investigated the impacts of warming under two CO₂ levels on ventilation and hemolymph circulation in edible crabs Cancer pagurus. It also highlights changes in the ventilatory and cardiac pauses displayed by Decapoda under routine metabolism. Animals were exposed to step-wise, sub-critical warming (12–20°C over 5 days) under control (470 μatm) and high (1,350 μatm) water PCO₂. Flow-through respirometry was combined with magnetic resonance imaging and infra-red photoplethysmography to allow for simultaneous, non-invasive measurements of metabolic rates (M˙O2), ventilation and cardiovascular performance. Crabs spent significantly more time in a low M˙O2 state (metabolic pause), when experiencing high CO₂ conditions above 16°C, compared to normocapnic warming. Heart rates leveled off beyond 18°C at any CO₂ level. Cardiac output continued to increase with high-CO₂-warming, due to elevated cardiac stroke volumes. Consequently, temperature-dependent branchial hemolymph flow remained unaffected by CO₂. Instead, a suppressing effect of CO₂ on ventilation was found beyond 16°C. These results indicate constrained oxygen uptake at stable cardiovascular performance in a decapod crustacean.

Cancer pagurus: urn:lsid:zoobank.org:act:B750F89A-84B5-448B-8D80-EBD724A1C9D4

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.

Temperature dependence of metabolic rate in tropical and temperate aquatic insects: Support for the Climate Variability Hypothesis in mayflies but not stoneflies

A fundamental gap in climate change vulnerability research is an understanding of the relative thermal sensitivity of ectotherms. Aquatic insects are vital to stream ecosystem function and biodiversity but insufficiently studied with respect to their thermal physiology. With global temperatures rising at an unprecedented rate, it is imperative that we know how aquatic insects respond to increasing temperature and whether these responses vary among taxa, latitudes, and elevations. We evaluated the thermal sensitivity of standard metabolic rate in stream-dwelling baetid mayflies and perlid stoneflies across a ~2,000 m elevation gradient in the temperate Rocky Mountains in Colorado, USA, and the tropical Andes in Napo, Ecuador. We used temperature-controlled water baths and microrespirometry to estimate changes in oxygen consumption. Tropical mayflies generally exhibited greater thermal sensitivity in metabolism compared to temperate mayflies; tropical mayfly metabolic rates increased more rapidly with temperature and the insects more frequently exhibited behavioral signs of thermal stress. By contrast, temperate and tropical stoneflies did not clearly differ. Varied responses to temperature among baetid mayflies and perlid stoneflies may reflect differences in evolutionary history or ecological roles as herbivores and predators, respectively. Our results show that there is physiological variation across elevations and species and that low-elevation tropical mayflies may be especially imperiled by climate warming. Given such variation among species, broad generalizations about the vulnerability of tropical ectotherms should be made more cautiously.

The gill-oxygen limitation theory (GOLT) and its critics

The gill-oxygen limitation theory (GOLT) provides mechanisms for key aspects of the biology (food conversion efficiency, growth and its response to temperature, the timing of maturation, and others) of water-breathing ectotherms (WBEs). The GOLT's basic tenet is that the surface area of the gills or other respiratory surfaces of WBE cannot, as two-dimensional structures, supply them with sufficient oxygen to keep up with the growth of their three-dimensional bodies. Thus, a lower relative oxygen supply induces sexual maturation, and later a slowing and cessation of growth, along with an increase of physiological processes relying on glycolytic enzymes and a declining role of oxidative enzymes. Because the "dimensional tension" underlying this argument is widely misunderstood, emphasis is given to a detailed refutation of objections to the GOLT. This theory still needs to be put on a solid quantitative basis, which will occur after the misconceptions surrounding it are put to rest.

Lower bioenergetic costs but similar immune responsiveness under a heat wave in urban compared to rural damselflies

There is mounting evidence that the widespread phenotypic changes in response to urbanization may reflect adaptations caused by rapid evolutionary processes driven by urban-related stressors. Compared to increased habitat fragmentation and pollution, adaptations towards another typical urban-related stressor, that is higher and longer lasting very high temperatures (heat waves), are much less studied. Notably, the sensitivities to heat waves of life-history traits and important fitness-related physiological traits such as immune responsiveness and bioenergetic variables (energy availability, energy consumption and their balance) have never been contrasted between urban and rural populations. By conducting a laboratory common-garden experiment, we compared effects of a simulated heat wave on life history (survival and growth rate), immune responsiveness and bioenergetic variables between three urban and three rural populations of the damselfly Coenagrion puella. Because energy-mediated trade-off patterns may only be detected under energetically costly manipulations, all larvae were immune-challenged by simulating ectoparasitism by water mites. As expected, the simulated heat wave caused negative effects on nearly all response variables. The immune responsiveness, on the other hand, increased under the heat wave, consistent with a trade-off pattern between immune function and growth, and this similarly between urban and rural populations. A key finding was that urban larvae suffered less from the simulated heat wave compared to the rural larvae in terms of a lower heat wave-induced depletion in energy availability. This suggests an adaptation of urban populations to better cope with the stronger and more frequent heat waves in cities. Notably, this urbanization-driven evolution in the bioenergetic variables was not apparent in the absence of a heat wave. Given that changes in energy budgets have strong fitness consequences, our findings suggest that the evolved higher ability to cope with heat waves is fundamental for the survival of urban damselfly populations.

Urinary and Plasma Antioxidants in Behavioral Symptoms of Individuals With Autism Spectrum Disorder

The balance between antioxidant capacity and oxidative stress-induced free radicals may be crucial in the pathophysiological development factor of autism spectrum disorder (ASD). We measured the following urinary and plasma biomarker levels of oxidative stress and antioxidants. As urinary biomarkers, (1) hexanoyl-lysine (HEL), which is a new biomarker of oxidative stress, (2) the total antioxidant capacity (TAC), and (3) 8-hydroxy-2'-deoxyguanosine (8-OHdG), as a product of oxidative modifications to DNA; and the plasma levels of (4) the antioxidant protein superoxide dismutase (SOD), which is the crucial defense again oxygen reactive species, and (5) transferrin and (6) ceruloplasmin, which are biomarkers of iron and copper neurotransmission and oxidant-antioxidant systems. We examined the relationship between these urinary and plasma biomarkers and behavioral symptoms in 19 individuals with ASD (mean age, 10.8 ± 5.2 years) and 10 age-matched healthy controls (mean age, 14.2 ± 7.0 years). Behavioral symptoms were estimated using the Aberrant Behavior Checklist (ABC). Urinary TAC levels were significantly lower, whereas urinary HEL levels were significantly increased in the ASD group as compared with the control group. The five ABC subscale and total scores were significantly raised in the autism group than in the control group. The results of a linear regression analysis revealed that plasma SOD levels may be a more accurate predictor of differences in ABC scores between individuals with ASD and control individuals. The present study firstly revealed the important findings that the cooperation between the urinary antioxidant TAC and plasma SOD levels may contribute to the ABC subscale scores of stereotypy. Urinary TAC activity and antioxidant protein SOD may be associated with incomplete mineral body store and antioxidant-related transcription factor and browning reactions. Consequently, a critical imbalance between TAC urinary levels and plasma SOD levels may be an important contributor to autistic behavioral symptoms.

Reduced hypoxia tolerance and survival at elevated temperatures may limit the ability of Amazonian fishes to survive in a warming world

The Amazon basin contains more than 20% of the world's freshwater fishes, many of ecological and economical importance. An increase in temperature of 2.2 to 7 °C is predicted to occur within the next century in the worst-case scenario of climate change predictions, which will likely be associated with an increase in the prevalence and duration of reduced water oxygen levels (hypoxia). Furthermore, there is an increasing frequency of heat waves in the Amazon basin, which exacerbates issues related to temperature and hypoxia. Increases in temperature and hypoxia both constrain an organism's ability to supply oxygen to metabolizing tissues, thus the ability to cope with thermal and hypoxic stress may be correlated. Here, we reveal a positive correlation between acute thermal tolerance and acute hypoxia tolerance amongst 37 Amazonian fish species at the current river temperatures of 28-31 °C. The effects of long-term (10 days or 4 weeks) increases in temperature were investigated in a subset of 13 species and demonstrated that 2 species failed to acclimate and survive at 33 °C, 9 species failed at 35 °C, and only 2 species survived up to 35 °C. Of those that survived long-term exposure to 33 or 35 °C, the majority of the species demonstrated only an improvement in acute thermal tolerance. In contrast, hypoxia tolerance was reduced following acute- and long-term exposure to 33, 35 or 37 °C in all species investigated. The results of this study suggest that many of the fish species that inhabit the Amazon may be at risk during both short- and long-term temperature increases and these risks are exacerbated by the associated environmental hypoxia.

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.

The effect of respiratory gases and incubation temperature on early stage embryonic development in sea turtles

Sea turtle embryos at high-density nesting beaches experience relative high rates of early stage embryo death. One hypothesis to explain this high mortality rate is that there is an increased probability that newly constructed nests are located close to maturing clutches whose metabolising embryos cause low oxygen levels, high carbon dioxide levels, and high temperatures. Although these altered environmental conditions are well tolerated by mature embryos, early stage embryos, i.e. embryos in eggs that have only been incubating for less than a week, may not be as tolerant leading to an increase in their mortality. To test this hypothesis, we incubated newly laid sea turtle eggs over a range of temperatures in different combinations of oxygen and carbon dioxide concentrations and assessed embryo development and death rates. We found that gas mixtures of decreased oxygen and increased carbon dioxide, similar to those found in natural sea turtle nests containing mature embryos, slowed embryonic development but did not influence the mortality rate of early stage embryos. We found incubation temperature had no effect on early embryo mortality but growth rate at 27°C and 34°C was slower than at 30°C and 33°C. Our findings indicate that low oxygen and high carbon dioxide partial pressures are not the cause of the high early stage embryo mortality observed at high-density sea turtle nesting beaches, but there is evidence suggesting high incubation temperatures, particularly above 34°C are harmful. Any management strategies that can increase the spacing between nests or other strategies such as shading or irrigation that reduce sand temperature are likely to increase hatching success at high-density nesting beaches.

Chasing away accurate results: exhaustive chase protocols underestimate maximum metabolic rate estimates in European perch Perca fluviatilis

Metabolic rates are one of many measures that are used to explain species' response to environmental change. Static respirometry is used to calculate the standard metabolic rate (SMR) of fish, and when combined with exhaustive chase protocols it can be used to measure maximum metabolic rate (MMR) and aerobic scope (AS) as well. While these methods have been tested in comparison to swim tunnels and chambers with circular currents, they have not been tested in comparison with a no-chase control. We used a repeated-measures design to compare estimates of SMR, MMR and AS in European perch Perca fluviatilis following three protocols: (a) a no-chase control; (b) a 3-min exhaustive chase; and (c) a 3-min exhaustive chase followed by 1-min air exposure. We found that, contrary to expectations, exhaustive chase protocols underestimate MMR and AS at 18°C, compared to the no-chase control. This suggests that metabolic rates of other species with similar locomotorty modes or lifestyles could be similarly underestimated using chase protocols. These underestimates have implications for studies examining metabolic performance and responses to climate change scenarios. To prevent underestimates, future experiments measuring metabolic rates should include a pilot with a no-chase control or, when appropriate, an adjusted methodology in which trials end with the exhaustive chase instead of beginning with it.

Developing in a warming intertidal, negative carry over effects of heatwave conditions in development to the pentameral starfish in Parvulastra exigua

Ocean warming and increasing incidence of marine heat waves (MHW) are having far-reaching impacts on coastal ecosystems. The small intertidal asterinid starfish, Parvulastra exigua, in south-eastern Australia, occurs in a global warming hotspot. Development occurs in the intertidal as this species lays eggs and has benthic larvae. The impact of temperature on development to the juvenile was determined over a broad temperature range (12-28 °C) encompassing temperatures experienced during the breeding season (16-20 °C) and cool (- 4 °C) and warm (+10 °C) extremes with the higher temperatures (24-28 °C) simulating a MHW. As the larva to juvenile transition involves major body reorganisation, we determined the impact of temperature on metamorphosis and formation of the normal five-armed juvenile. Development was faster at the higher temperatures 24-28 °C, but survival decreased from 1 to 5 days post fertilisation (dpf). Mortality was evident from day 15 at 22 °C and no larvae survived to 20 dpf at 28 °C. Thermal tolerance decreased over developmental time and the thermal optimum for 95% survival to the 20 day old juvenile spanned from 12 to 20.0 °C with the lethal temperature for 50% survival being 23.5 °C (5.5 °C above ambient). Juveniles reared in 26 °C were smaller, suggesting application of the temperature size rule. Increased temperature (22-26 °C) perturbed pentamery with three, four, six and no-armed juveniles present, contrasting with the low level of non-pentamerous individuals (<3%) in the cooler cultures and in nature (five populations surveyed). Despite the high thermal tolerance in premetamorphic stages, negative carry over effects were evident in the juveniles. This shows the importance of considering the whole of development in climate warming studies. As sea surface temperatures increase and heatwaves become more prevalent, habitat warming will be detrimental to P. exigua populations.

Different sensitivity to heatwaves across the life cycle of fish reflects phenotypic adaptation to environmental niche

Predicting responses of marine organisms to global change requires eco-physiological assessments across the complex life cycles of species. Here, we experimentally tested the vulnerability of a demersal temperate fish (Sparus aurata) to long-lasting heatwaves, on larval, juvenile and adult life-stages. Fish were exposed to simulated coastal (18 °C), estuarine (24 °C) summer temperatures, and heatwave conditions (30 °C) and their physiological responses were assessed based on cellular stress response biomarkers (heat shock protein 70 kDa, ubiquitin, antioxidant enzymes, lipid peroxidation) and phenotypic measures (histopathology, condition and mortality). Life-stage vulnerability can be ranked as larvae > adults > juveniles, based on mortality, tissue pathology and the capacity to employ cellular stress responses, reflecting the different environmental niches of each life stage. While larvae lacked acclimation capacity, which resulted in damage to tissues and elevated mortality, juveniles coped well with elevated temperature. The rapid induction of cytoprotective proteins maintained the integrity of vital organs in juveniles, suggesting adaptive phenotypic plasticity in coastal and estuarine waters. Adults displayed lower plasticity to heatwaves as they transition to deeper habitats for maturation, showing tissue damage in brain, liver and muscle. Life cycle closure of sea breams in coastal habitats will therefore be determined by larval and adult stages.

Thermal performance curves reveal shifts in optima, limits and breadth in early life

Understanding thermal performance at life stages that limit persistence is necessary to predict responses to climate change, especially for ectotherms whose fitness (survival and reproduction) depends on environmental temperature. Ectotherms often undergo stage-specific changes in size, complexity and duration that are predicted to modify thermal performance. Yet performance is mostly explored for adults, while performance at earlier stages that typically limit persistence remains poorly understood. Here, we experimentally isolate thermal performance curves at fertilization, embryo development and larval development stages in an aquatic ectotherm whose early planktonic stages (gametes, embryos and larvae) govern adult abundances and dynamics. Unlike previous studies based on short-term exposures, responses with unclear links to fitness or proxies in lieu of explicit curve descriptors (thermal optima, limits and breadth), we measured performance as successful completion of each stage after exposure throughout, and at temperatures that explicitly capture curve descriptors at all stages. Formal comparisons of descriptors using a combination of generalized linear mixed modelling and parametric bootstrapping reveal important differences among life stages. Thermal performance differs significantly from fertilization to embryo development (with thermal optimum declining by ∼2°C, thermal limits shifting inwards by ∼8-10°C and thermal breadth narrowing by ∼10°C), while performance declines independently of temperature thereafter. Our comparisons show that thermal performance at one life stage can misrepresent performance at others, and point to gains in complexity during embryogenesis, rather than subsequent gains in size or duration of exposure, as a key driver of thermal sensitivity in early life.

Acidification and hypoxia interactively affect metabolism in embryos, but not larvae, of the coastal forage fish Menidia menidia

Ocean acidification is occurring in conjunction with warming and deoxygenation as a result of anthropogenic greenhouse gas emissions. Multistressor experiments are critically needed to better understand the sensitivity of marine organisms to these concurrent changes. Growth and survival responses to acidification have been documented for many marine species, but studies that explore underlying physiological mechanisms of carbon dioxide (CO2) sensitivity are less common. We investigated oxygen consumption rates as proxies for metabolic responses in embryos and newly hatched larvae of an estuarine forage fish (Atlantic silverside, Menidia menidia) to factorial combinations of CO2×temperature or CO2×oxygen. Metabolic rates of embryos and larvae significantly increased with temperature, but partial pressure of CO2 (PCO2 ) alone did not affect metabolic rates in any experiment. However, there was a significant interaction between PCO2  and partial pressure of oxygen (PO2 ) in embryos, because metabolic rates were unaffected by PO2  level at ambient PCO2 , but decreased with declining PO2  under elevated PCO2 For larvae, however, PCO2  and PO2  had no significant effect on metabolic rates. Our findings suggest high individual variability in metabolic responses to high PCO2 , perhaps owing to parental effects and time of spawning. We conclude that early life metabolism is largely resilient to elevated PCO2  in this species, but that acidification likely influences energetic responses and thus vulnerability to hypoxia.

The power struggle: assessing interacting global change stressors via experimental studies on sharks

Ocean warming and acidification act concurrently on marine ectotherms with the potential for detrimental, synergistic effects; yet, effects of these stressors remain understudied in large predatory fishes, including sharks. We tested for behavioural and physiological responses of blacktip reef shark (Carcharhinus melanopterus) neonates to climate change relevant changes in temperature (28 and 31 °C) and carbon dioxide partial pressures (pCO₂; 650 and 1050 µatm) using a fully factorial design. Behavioural assays (lateralisation, activity level) were conducted upon 7–13 days of acclimation, and physiological assays (hypoxia tolerance, oxygen uptake rates, acid–base and haematological status) were conducted upon 14–17 days of acclimation. Haematocrit was higher in sharks acclimated to 31 °C than to 28 °C. Significant treatment effects were also detected for blood lactate and minimum oxygen uptake rate; although, these observations were not supported by adequate statistical power. Inter-individual variability was considerable for all measured traits, except for haematocrit. Moving forward, studies on similarly ‘hard-to-study’ species may account for large inter-individual variability by increasing replication, testing larger, yet ecologically relevant, differences in temperature and pCO₂, and reducing measurement error. Robust experimental studies on elasmobranchs are critical to meaningfully assess the threat of global change stressors in these data-deficient species.

Environmentally driven changes in Baltic salmon oxidative status during marine migration

The fitness and recruitment of fish stocks can be markedly affected by environmental disturbances including global warming, eutrophication and contamination. Understanding the effects of environmental stressors on salmon physiology during marine residence is of a global concern as marine survival has decreased. We present a unique combination of physiological responses - antioxidant defence and oxidative damage biomarkers, stable isotopes and contaminant exposure biomarkers - measured from adult Atlantic salmon (Salmo salar) collected at the Baltic Sea and studied in relation to environmental variables and fitness estimates. The results demonstrate that feeding populations of salmon display marked temporal and spatial variation in oxidative status. Better oxidative status of salmon was characterized by a higher amount of reduced glutathione (GSH) and decreased lipid peroxidation (LPX), when the weight-at-age of 3-4-year old sprats was higher and contaminant exposure biomarker (EROD) was lower. Summer season conditions, which included cooler sea surface temperature (SST), higher bottom O₂ and less cyanobacteria also indicated conditions for better oxidative status. Summer SST was additionally shown to affected glutathione metabolism enzyme activities. Oxidative status was associated with stable isotopes δ¹³C and δ¹⁵N indicating indirect effect of abiotic conditions and lower levels of the food web. Differences in condition factor and growth were associated with oxidative status in one and two sea winter salmon, respectively. Wild salmon survival was higher in years when they had higher GSH and catalase activity and lower LPX. Enhanced glutathione metabolism and increased protein carbonyls were associated with higher occurrence of yolk-sac fry mortality (M74). Our results show that oxidative status can provide information on exposure to complex combinations of environmental conditions and stressors in the wild and provide a link of physiological function to individual and population level fitness effects.

Heat tolerance and thermal preference of the copepod Tigriopus californicus are insensitive to ecologically relevant dissolved oxygen levels

Shifting climate patterns may impose novel combinations of abiotic conditions on animals, yet understanding of the present-day interactive effects of multiple stressors remains under-developed. We tested the oxygen and capacity limited thermal tolerance (OCLTT) hypothesis and quantified environmental preference of the copepod Tigriopus californicus, which inhabits rocky-shore splashpools where diel fluctuations of temperature and dissolved oxygen (DO) are substantial. Egg-mass bearing females were exposed to a 5 h heat ramp to peak temperatures of 34.1–38.0 °C crossed with each of four oxygen levels: 22, 30, 100 and 250% saturation (4.7–5.3, 5.3–6.4, 21.2–21.3, and 50.7–53.3 kPa). Survival decreased at higher temperatures but was independent of DO. The behavioral preference of females was quantified in seven combinations of gradients of both temperature (11–37 °C) and oxygen saturation (17–206% or 3.6–43.6 kPa). Females avoided high temperatures regardless of DO levels. This pattern was more pronounced when low DO coincided with high temperature. In uniform temperature treatments, the distribution shifted toward high DO levels, especially in uniform high temperature, confirming that Tigriopus can sense environmental pO₂. These results question the ecological relevance of OCLTT for Tigriopus and raise the possibility of microhabitat selection being used within splashpool environments to avoid physiologically stressful combinations of conditions.

Oxygen supply limits the chronic heat tolerance of locusts during the first instar only

Although scientists know that overheating kills many organisms, they do not agree on the mechanism. According to one theory, referred to as oxygen- and capacity-limitation of thermal tolerance, overheating occurs when a warming organism's demand for oxygen exceeds its supply, reducing the organism's supply of ATP. This model predicts that an organism's heat tolerance should decrease under hypoxia, yet most terrestrial organisms tolerate the same amount of warming across a wide range of oxygen concentrations. This point is especially true for adult insects, who deliver oxygen through highly efficient respiratory systems. However, oxygen limitation at high temperatures may be more common during immature life stages, which have less developed respiratory systems. To test this hypothesis, we measured the effects of heat and hypoxia on the survival of South American locusts (Schistocerca cancellata) throughout development and during specific instars. We demonstrate that the heat tolerance of locusts depends on oxygen supply during the first instar but not during later instars. This finding provides further support for the idea that oxygen limitation of thermal tolerance depends on respiratory performance, especially during immature life stages.

Mitochondria and the thermal limits of ectotherms

Temperature is a critical abiotic factor shaping the distribution and abundance of species, but the mechanisms that underpin organismal thermal limits remain poorly understood. One possible mechanism underlying these limits is the failure of mitochondrial processes, as mitochondria play a crucial role in animals as the primary site of ATP production. Conventional measures of mitochondrial performance suggest that these organelles can function at temperatures much higher than those that limit whole-organism function, suggesting that they are unlikely to set organismal thermal limits. However, this conclusion is challenged by recent data connecting sequence variation in mitochondrial genes to whole-organism thermal tolerance. Here, we review the current state of knowledge of mitochondrial responses to thermal extremes and ask whether they are consistent with a role for mitochondrial function in shaping whole-organism thermal limits. The available data are fragmentary, but it is possible to draw some conclusions. There is little evidence that failure of maximal mitochondrial oxidative capacity as assessed in vitro sets thermal limits, but there is some evidence to suggest that temperature effects on ATP synthetic capacity may be important. Several studies suggest that loss of mitochondrial coupling is associated with the thermal limits for organismal growth, although this needs to be rigorously tested. Most studies have utilized isolated mitochondrial preparations to assess the effects of temperature on these organelles, and there remain many untapped opportunities to address these questions using preparations that retain more of their biological context to better connect these subcellular processes with whole-organism thermal limits.

The biophysical basis of thermal tolerance in fish eggs

A warming climate poses a fundamental problem for embryos that develop within eggs because their demand for oxygen (O₂) increases much more rapidly with temperature than their capacity for supply, which is constrained by diffusion across the egg surface. Thus, as temperatures rise, eggs may experience O₂ limitation due to an imbalance between O₂ supply and demand. Here, we formulate a mathematical model of O₂ limitation and experimentally test whether this mechanism underlies the upper thermal tolerance in large aquatic eggs. Using Chinook salmon (Oncorhynchus tshawytscha) as a model system, we show that the thermal tolerance of eggs varies systematically with features of the organism and environment. Importantly, this variation can be precisely predicted by the degree to which these features shift the balance between O₂ supply and demand. Equipped with this mechanistic understanding, we predict and experimentally confirm that the thermal tolerance of these embryos in their natural habitat is substantially lower than expected from laboratory experiments performed under normoxia. More broadly, our biophysical model of O₂ limitation provides a mechanistic explanation for the elevated thermal sensitivity of fish embryos relative to other life stages, global patterns in egg size and the extreme fecundity of large teleosts.

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.

High elevation insect communities face shifting ecological and evolutionary landscapes

Climate change is proceeding rapidly in high mountain regions worldwide. Rising temperatures will impact insect physiology and associated fitness and will shift populations in space and time, thereby altering community interactions and composition. Shifts in space are expected as insects move upslope to escape warming temperatures and shifts in time will occur with changes in phenology of resident high-elevation insects. Clearly, spatiotemporal shifts will not affect all species equally. Terrestrial insects may have more opportunities than aquatic insects to exploit microhabitats, potentially buffering them from warming. Such responses of insects to warming may also fuel evolutionary change, including hitchhiking of maladaptive alleles and genetic rescue. Together, these considerations suggest a striking restructuring of high-elevation insect communities that remains largely unstudied.

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.

Feeling the heat: source–sink mismatch as a mechanism underlying the failure of thermal tolerance

A mechanistic explanation for the tolerance limits of animals at high temperatures is still missing, but one potential target for thermal failure is the electrical signaling off cells and tissues. With this in mind, here I review the effects of high temperature on the electrical excitability of heart, muscle and nerves, and refine a hypothesis regarding high temperature-induced failure of electrical excitation and signal transfer [the temperature-dependent deterioration of electrical excitability (TDEE) hypothesis]. A central tenet of the hypothesis is temperature-dependent mismatch between the depolarizing ion current (i.e. source) of the signaling cell and the repolarizing ion current (i.e. sink) of the receiving cell, which prevents the generation of action potentials (APs) in the latter. A source–sink mismatch can develop in heart, muscles and nerves at high temperatures owing to opposite effects of temperature on source and sink currents. AP propagation is more likely to fail at the sites of structural discontinuities, including electrically coupled cells, synapses and branching points of nerves and muscle, which impose an increased demand of inward current. At these sites, temperature-induced source–sink mismatch can reduce AP frequency, resulting in low-pass filtering or a complete block of signal transmission. In principle, this hypothesis can explain a number of heat-induced effects, including reduced heart rate, reduced synaptic transmission between neurons and reduced impulse transfer from neurons to muscles. The hypothesis is equally valid for ectothermic and endothermic animals, and for both aquatic and terrestrial species. Importantly, the hypothesis is strictly mechanistic and lends itself to experimental falsification.