Brain dysfunction during warming is linked to oxygen limitation in larval zebrafish

Effects of acute hypoxia exposure and acclimation on the thermal tolerance of an imperiled Canadian minnow

Elevated water temperatures and low dissolved oxygen (hypoxia) are pervasive stressors in aquatic systems that can be exacerbated by climate change and anthropogenic activities, and there is growing interest in their interactive effects. To explore this interaction, we quantified the effects of acute and long-term hypoxia exposure on the critical thermal maximum (CTmax) of Redside Dace (Clinostomus elongatus), a small-bodied freshwater minnow with sparse populations in the Great Lakes Basin of Canada and designated as Endangered under Canada's Species at Risk Act. Fish were held at 18°C and acclimated to four levels of dissolved oxygen (>90%, 60%, 40%, and 20% air saturation). CTmax was measured after 2 and 10 weeks of acclimation and after 3.5 weeks of reoxygenation, and agitation behavior was quantified during CTmax trials. Aquatic surface respiration behavior was also quantified at 14 weeks of acclimation to oxygen treatments. Acute hypoxia exposure decreased CTmax in fish acclimated to normoxia (>90% air saturation), but acclimation to hypoxia reduced this effect. There was no effect of acclimation oxygen level on CTmax when measured in normoxia, and there was no effect of exposure time to hypoxia on CTmax. Residual effects of hypoxia acclimation on CTmax were not seen after reoxygenation. Agitation behavior varied greatly among individuals and was not affected by oxygen conditions. Fish performed aquatic surface respiration with low frequency, but performed it earlier when acclimated to higher levels of oxygen. Overall, this work sheds light on the vulnerability of fish experiencing acute hypoxia and heat waves concurrently.

Resilience of circuits to environmental challenge

Animals of all kinds evolved to deal with anticipated and unanticipated changes in a variety of features in their environments. Consequently, all environmental perturbations, adaptations, and acclimation involve a myriad of factors that, together, contribute to environmental resilience. New work highlights the importance of neuromodulation in the control of environmental resilience, and illustrates that different components of the nervous system may be differentially resilient to environmental perturbations. Climate change is today pushing animals to deal with previously unanticipated environmental challenges, and therefore understanding the complex biology of adaptation and acclimation to various environmental conditions takes on new urgency.

Impacts of climate-related stressors on social group cohesion and individual sociability in fish

Group-living in animals comes with a number of benefits associated with predator avoidance, foraging, and reproduction. A large proportion of fish species display grouping behaviour. Fish may also be particularly vulnerable to climate-related stressors including thermal variation, hypoxia, and acidification. As climate-related stressors are expected to increase in magnitude and frequency, any effects on fish behaviour may be increased and affect the ability of fish species to cope with changing conditions. Here we conduct a systematic review of the effects of temperature, hypoxia, and acidification on individual sociability and group cohesion in shoaling and schooling fishes. Searches of the published and grey literature were carried out, and studies were included or excluded based on selection criteria. Data from studies were then included in a meta-analysis to examine broad patterns of effects of climate-related stressors in the literature. Evidence was found for a reduction in group cohesion at low oxygen levels, which was stronger in smaller groups. While several studies reported effects of temperature and acidification, there was no consistent effect of either stressor on sociability or cohesion. There was some evidence that marine fishes are more strongly negatively affected by acidification compared with freshwater species, but results are similarly inconsistent and more studies are required. Additional studies of two or more stressors in combination are also needed, although one study found reduced sociability following exposure to acidification and high temperatures. Overall, there is some evidence that hypoxia, and potentially other climate-related environmental changes, impact sociability and group cohesion in fishes. This may reduce survival and adaptability in shoaling and schooling species and have further ecological implications for aquatic systems. However, this synthesis mainly highlights the need for more empirical studies examining the effects of climate-related factors on social behaviour in fishes.

Acute warming tolerance (CTmax) in zebrafish (Danio rerio) appears unaffected by changes in water salinity

Tolerance against acute warming is an essential trait that can determine how organisms cope during heat waves, yet the mechanisms underlying it remain elusive. Water salinity has previously been suggested to modulate warming tolerance in fish and may therefore provide clues towards these limiting mechanisms. Here, using the critical thermal maximum (CTmax) test, we investigated whether short (2 hours) and long (10 days) term exposure to different water salinities (2 hours: 0-5 ppt, 10 days: 0-3 ppt) affected acute warming tolerance in zebrafish (N = 263). We found that water salinity did not affect the warming tolerance of zebrafish at either time point, indicating that salinity does not affect the mechanism limiting acute warming tolerance in zebrafish at these salinity ranges, and that natural fluctuations in salinity levels might not have a large impact on acute warming tolerance in wild zebrafish.

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.

Respiratory acidosis and O2 supply capacity do not affect the acute temperature tolerance of rainbow trout (Oncorhynchus mykiss)

The mechanisms that determine the temperature tolerances of fish are poorly understood, creating barriers to disentangle how additional environmental challenges-such as CO2-induced aquatic acidification and fluctuating oxygen availability-may exacerbate vulnerability to a warming climate and extreme heat events. Here, we explored whether two acute exposures (~0.5 hours or ~72 hours) to increased CO2 impact acute temperature tolerance limits in a freshwater fish, rainbow trout (Oncorhynchus mykiss). We separated the potential effects of acute high CO2 exposure on critical thermal maximum (CTmax), caused via either respiratory acidosis (reduced internal pH) or O2 supply capacity (aerobic scope), by exposing rainbow trout to ~1 kPa CO2 (~1% or 10 000 μatm) in combination with normoxia or hyperoxia (~21 or 42 kPa O2, respectively). In normoxia, acute exposure to high CO2 caused a large acidosis in trout (blood pH decreased by 0.43 units), while a combination of hyperoxia and ~1 kPa CO2 increased the aerobic scope of trout by 28%. Despite large changes in blood pH and aerobic scope between treatments, we observed no impacts on the CTmax of trout. Our results suggest that the mechanisms that determine the maximum temperature tolerance of trout are independent of blood acid-base balance or the capacity to deliver O2 to tissues.

Thermal tolerance and survival are modulated by a natural gradient of infection in differentially acclimated hosts

Wild ectotherms are exposed to multiple stressors, including parasites, that can affect their responses to environmental change. Simultaneously, unprecedented warm temperatures are being recorded worldwide, increasing both the average and maximum temperatures experienced in nature. Understanding how ectotherms, such as fishes, will react to the combined stress of parasites and higher average temperatures can help predict the impact of extreme events such as heat waves on populations. The critical thermal method (CTM), which assesses upper (CTmax) and lower (CTmin) thermal tolerance, is often used in acclimated ectotherms to help predict their tolerance to various temperature scenarios. Despite the widespread use of the CTM across taxa, few studies have characterized the response of naturally infected fish to extreme temperature events or how acute thermal stress affects subsequent survival. We acclimated naturally infected pumpkinseed sunfish (Lepomis gibbosus) to four ecologically relevant temperatures (10, 15, 20 and 25°C) and one future warming scenario (30°C) for 3 weeks before measuring CTmax and CTmin. We also assessed individual survival the week following CTmax. Parasites were counted and identified following trials to relate infection intensity to thermal tolerance and survival. Interestingly, trematode parasites causing black spot disease were negatively related to CTmax, suggesting that heavily infected fish are less tolerant to acute warming. Moreover, fish infected with yellow grub parasites showed decreased survival in the days following CTmax implying that the infection load has negative survival consequences on sunfish during extreme warming events. Our findings indicate that, when combined, parasite infection and high prolonged average temperatures can affect fish thermal tolerance and survival, emphasizing the need to better understand the concomitant effects of stressors on health outcomes in wild populations. This is especially true given that some parasite species are expected to thrive in warming waters making host fish species especially at risk.

Thermal tolerance of cultured and wild Icelandic arctic charr (Salvelinus alpinus) at self-selected flow rates

Climate change is predicted to change not only the temperature of many freshwater systems but also flow dynamics. Understanding how fishes will fare in the future requires knowing how they will respond to both extended variations of temperature and flow. Arctic charr have had their thermal tolerance measured, but never with respect to flow. Additionally, this circumpolar species has multiple populations exhibiting dramatic phenotypic plasticity which may mean that regional differences in thermal tolerance are unaccounted for. In Iceland, Arctic charr populations have experienced highly variable flow and temperature conditions over the past 10,000 years. The Icelandic climate, topography and geothermal activity have created a mosaic of freshwater habitats inhabited by charr that vary substantially in both temperature and flow. Our purpose was to test whether populations from these varied environments had altered thermal tolerance and whether phenotypic plasticity of thermal tolerance in charr depends on flow. We raised cultured Icelandic charr from hatch under a 2 X 2 matrix of flow and temperature and compared them to wild charr captured from matching flow and temperature environments. Wild fish were more thermally tolerant than cultured fish at both acclimation temperatures and were more thermally plastic. Icelandic Arctic charr were more thermally tolerant than comparison charr populations across Europe and North America, but only when acclimated to 13 °C; fish acclimated to 5 °C compared equably with comparison charr populations. Icelandic Arctic charr were also more thermally plastic than all but one other salmonine species. Neither flow of rearing or the flow selected during a thermal tolerance (CTmax) test factored into thermal tolerance. Thermal tolerance was also independent of body size, condition factor, heart and gill size. In summary, wild Icelandic Arctic charr have greater thermal tolerance and plasticity than predicted from the literature and their latitude, but artificial selection for properties like growth rate or fecundity may be breeding that increased tolerance out of cultured fish. As the world moves toward a warmer climate and increased dependence on cultured fish, this is a noteworthy result and merits further study.

Thermal tolerance in an extremophile fish from Mexico is not affected by environmental hypoxia

The thermal ecology of ectotherm animals has gained considerable attention in the face of human-induced climate change. Particularly in aquatic species, the experimental assessment of critical thermal limits (CTmin and CTmax) may help to predict possible effects of global warming on habitat suitability and ultimately species survival. Here we present data on the thermal limits of two endemic and endangered extremophile fish species, inhabiting a geothermally heated and sulfur-rich spring system in southern Mexico: The sulfur molly (Poecilia sulphuraria) and the widemouth gambusia (Gambusia eurystoma). Besides physiological challenges induced by toxic hydrogen sulfide and related severe hypoxia during the day, water temperatures have been previously reported to exceed those of nearby clearwater streams. We now present temperature data for various locations and years in the sulfur spring complex and conducted laboratory thermal tolerance tests (CTmin and CTmax) both under normoxic and severe hypoxic conditions in both species. Average CTmax limits did not differ between species when dissolved oxygen was present. However, critical temperature (CTmax=43.2°C) in P. sulphuraria did not change when tested under hypoxic conditions, while G. eurystoma on average had a lower CTmax when oxygen was absent. Based on this data we calculated both species' thermal safety margins and used a TDT (thermal death time) model framework to relate our experimental data to observed temperatures in the natural habitat. Our findings suggest that both species live near their thermal limits during the annual dry season and are locally already exposed to temperatures above their critical thermal limits. We discuss these findings in the light of possible physiological adaptions of the sulfur-adapted fish species and the anthropogenic threats for this unique system.

Neurobiology and Changing Ecosystems: Mechanisms Underlying Responses to Human-Generated Environmental Impacts

Human generated environmental change profoundly affects organisms that reside across diverse ecosystems. Although nervous systems evolved to flexibly sense, respond, and adapt to environmental change, it is unclear whether the rapid rate of environmental change outpaces the adaptive capacity of complex nervous systems. Here, we explore neural systems mediating responses to, or impacted by, changing environments, such as those induced by global heating, sensory pollution, and changing habitation zones. We focus on rising temperature and accelerated changes in environments that impact sensory experience as examples of perturbations that directly or indirectly impact neural function, respectively. We also explore a mechanism involved in cross-species interactions that arises from changing habitation zones. We demonstrate that anthropogenic influences on neurons, circuits, and behaviors are widespread across taxa and require further scientific investigation to understand principles underlying neural resilience to accelerating environmental change.SIGNIFICANCE STATEMENT Neural systems evolved over hundreds of millions of years to allow organisms to sense and respond to their environments - to be receptive and responsive, yet flexible. Recent rapid, human-generated environmental changes are testing the limits of the adaptive capacity of neural systems. This presents an opportunity and an urgency to understand how neurobiological processes, including molecular, cellular, and circuit-level mechanisms, are vulnerable or resilient to changing environmental conditions. We showcase examples that range from molecular to circuit to behavioral levels of analysis across several model species, framing a broad neuroscientific approach to explore topics of neural adaptation, plasticity, and resilience. We believe this emerging scientific area is of great societal and scientific importance and will provide a unique opportunity to reexamine our understanding of neural adaptation and the mechanisms underlying neural resilience.

Ontogenetic Plasticity in Shoaling Behavior in a Forage Fish under Warming

Shoaling behavior is known to increase survival rates during attacks from predators, minimize foraging time, favor mating, and potentially increase locomotor efficiency. The onset of shoaling typically occurs during the larval phase, but it is unclear how it may improve across ontogenetic stages in forage fishes. Warming is known to increase metabolic rates during locomotion in solitary fish, and shoaling species may adjust their collective behavior to offset the elevated costs of swimming at higher temperatures. In this study, we quantified the effects of warming on shoaling performance across the ontogeny of a small forage fish, zebrafish (Danio rerio) at different speeds. Shoals of larval, juvenile, and adult zebrafish were acclimated at two temperatures (28°C and 32°C), and metabolic rates were quantified prior to and following nonexhaustive exercise at high speed. Shoals of five individuals were filmed in a flow tank to analyze the kinematics of collective movement. We found that zebrafish improve shoaling swimming performance from larvae to juveniles to adults. In particular, shoals become more cohesive, and both tail beat frequency (TBF) and head-to-tail amplitude decrease with ontogeny. Early life stages have higher thermal sensitivity in metabolic rates and TBF especially at high speeds, when compared to adults. Our study shows that shoaling behavior and thermal sensitivity improve as zebrafish shift from larval to juvenile to adult stages.

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.

Intergenerational plasticity to cycling high temperature and hypoxia affects offspring stress responsiveness and tolerance in zebrafish

Predicted climate change-induced increases in heat waves and hypoxic events will have profound effects on fishes, yet the capacity of parents to alter offspring phenotype via non-genetic inheritance and buffer against these combined stressors is not clear. This study tested how prolonged adult zebrafish exposure to combined diel cycles of thermal stress and hypoxia affect offspring early survival and development, parental investment of cortisol and heat shock proteins (HSPs), larval offspring stress responses, and both parental and offspring heat and hypoxia tolerance. Parental exposure to the combined stressor did not affect fecundity, but increased mortality, produced smaller embryos and delayed hatching. The combined treatment also reduced maternal deposition of cortisol and increased embryo hsf1, hsp70a, HSP70, hsp90aa and HSP90 levels. In larvae, basal cortisol levels did not differ between treatments, but acute exposure to combined heat stress and hypoxia increased cortisol levels in control larvae with no effect on larvae from exposed parents. In contrast, whereas larval basal hsf1, hsp70a and hsp90aa levels differed between parental treatments, the combined acute stressor elicited similar transcriptional responses across treatments. Moreover, the combined acute stressor only induced a marked increase in HSP47 levels in the larvae derived from exposed parents. Finally, combined hypoxia and elevated temperatures increased both thermal and hypoxia tolerance in adults and conferred an increase in offspring thermal but not hypoxia tolerance. These results demonstrate that intergenerational acclimation to combined thermal stress and hypoxia elicit complex carryover effects on stress responsiveness and offspring tolerance with potential consequences for resilience.

A novel method for measuring acute thermal tolerance in fish embryos

Aquatic ectotherms are vulnerable to thermal stress, with embryos predicted to be more sensitive than juveniles and adults. When examining the vulnerability of species and life stages to warming, comparable methodology must be used to obtain robust conclusions. Critical thermal methodology is commonly used to characterize acute thermal tolerances in fishes, with critical thermal maximum (CTmax) referring to the acute upper thermal tolerance limit. At this temperature, fish exhibit loss of controlled locomotion due to a temperature-induced collapse of vital physiological functions. While it is relatively easy to monitor behavioural responses and measure CTmax in larval and adult fish, this is more challenging in embryos, leading to a lack of data on this life stage, or that studies rely on potentially incomparable metrics. Here, we present a novel method for measuring CTmax in fish embryos, defined by the temperature at which embryos stop moving. Additionally, we compare this measurement with the temperature of the embryos' last heartbeat, which has previously been proposed as a method for measuring embryonic CTmax. We found that, like other life stages, late-stage embryos exhibited a period of increased activity, peaking approximately 2-3°C before CTmax. Measurements of CTmax based on last movement are more conservative and easier to record in later developmental stages than measurements based on last heartbeat, and they also work well with large and small embryos. Importantly, CTmax measurements based on last movement in embryos are similar to measurements from larvae and adults based on loss of locomotory control. Using last heartbeat as CTmax in embryos likely overestimates acute thermal tolerance, as the heart is still beating when loss of response/equilibrium is reached in larvae/adults. The last movement technique described here allows for comparisons of acute thermal tolerance of embryos between species and across life stages, and as a response variable to treatments.

Impact of ocean warming on a coral reef fish learning and memory

Tropical ectotherms are highly sensitive to environmental warming, especially coral reef fishes, which are negatively impacted by an increase of a few degrees in ocean temperature. However, much of our understanding on the thermal sensitivity of reef fish is focused on a few traits (e.g., metabolism, reproduction) and we currently lack knowledge on warming effects on cognition, which may endanger decision-making and survival. Here, we investigated the effects of warming on learning and memory in a damselfish species, Acanthochromis polyacanthus. Fish were held at 28-28.5 °C (control group), 30-30.5 °C (moderate warming group) or 31.5-32 °C (high warming group) for 2 weeks, and then trained to associate a blue tag (cue) to the presence of a conspecific (reward). Following 20 training trials (5 days), fish were tested for associative learning (on the following day) and memory storage (after a 5-days interval). The control group A. polyacanthus showed learning of the task and memory retention after five days, but increasing water temperature impaired learning and memory. A thorough understanding of the effects of heat stress, cognition, and fitness is urgently required because cognition may be a key factor determining animals' performance in the predicted scenario of climate changes. Knowing how different species respond to warming can lead to better predictions of future community dynamics, and because it is species specific, it could pinpoint vulnerable/resilience species.

Slow heating rates increase thermal tolerance and alter mRNA HSP expression in juvenile white sturgeon (Acipenser transmontanus)

Freshwater fish such as white sturgeon (Acipenser transmontanus) are particularly vulnerable to the effects of anthropogenically induced global warming. Critical thermal maximum tests (CTmax) are often conducted to provide insight into the impacts of changing temperatures; however, little is known about how the rate of temperature increase in these assays affects thermal tolerance. To assess the effect of heating rate (0.3 °C/min, 0.03 °C/min, 0.003 °C/min) we measured thermal tolerance, somatic indices, and gill Hsp mRNA expression. Contrary to what has been observed in most other fish species, white sturgeon thermal tolerance was highest at the slowest heating rate of 0.003 °C/min (34.2 °C, and CTmax of 31.3 and 29.2 °C, for rates 0.03 and 0.3 °C/min, respectively) suggesting an ability to rapidly acclimate to slowly increasing temperatures. Hepatosomatic index decreased in all heating rates relative to control fish, indicative of the metabolic costs of thermal stress. At the transcriptional level, slower heating rates resulted in higher gill mRNA expression of Hsp90a, Hsp90b, and Hsp70. Hsp70 mRNA expression was increased in all heating rates relative to controls, whereas expression of Hsp90a and Hsp90b mRNA only increased in the two slower trials. Together these data indicate that white sturgeon have a very plastic thermal response, which is likely energetically costly to induce. Acute temperature changes may be more detrimental to sturgeon as they struggle to acclimate to rapid changes in their environment, however under slower warming rates they demonstrate strong thermal plasticity to warming.

The ecological relevance of critical thermal maxima methodology for fishes

Critical thermal maxima methodology (CTM) has been used to infer acute upper thermal tolerance in fishes since the 1950s, yet its ecological relevance remains debated. In this study, the authors synthesize evidence to identify methodological concerns and common misconceptions that have limited the interpretation of critical thermal maximum (CT_max ; value for an individual fish during one trial) in ecological and evolutionary studies of fishes. They identified limitations of, and opportunities for, using CT_max as a metric in experiments, focusing on rates of thermal ramping, acclimation regimes, thermal safety margins, methodological endpoints, links to performance traits and repeatability. Care must be taken when interpreting CTM in ecological contexts, because the protocol was originally designed for ecotoxicological research with standardized methods to facilitate comparisons within study individuals, across species and contexts. CTM can, however, be used in ecological contexts to predict impacts of environmental warming, but only if parameters influencing thermal limits, such as acclimation temperature or rate of thermal ramping, are taken into account. Applications can include mitigating the effects of climate change, informing infrastructure planning or modelling species distribution, adaptation and/or performance in response to climate-related temperature change. The authors' synthesis points to several key directions for future research that will further aid the application and interpretation of CTM data in ecological contexts.

Invited review - the effects of anthropogenic abiotic stressors on the sensory systems of fishes

Climate change is a growing global issue with many countries and institutions declaring a climate state of emergency. Excess CO₂ from anthropogenic sources and changes in land use practices are contributing to many detrimental changes, including increased global temperatures, ocean acidification and hypoxic zones along coastal habitats. All senses are important for aquatic animals, as it is how they can perceive and respond to their environment. Some of these environmental challenges have been shown to impair their sensory systems, including the olfactory, visual, and auditory systems. While most of the research is focused on how ocean acidification affects olfaction, there is also evidence that it negatively affects vision and hearing. The effects that temperature and hypoxia have on the senses have also been investigated, but to a much lesser extent in comparison to ocean acidification. This review assembles the known information on how these anthropogenic challenges affect the sensory systems of fishes, but also highlights what gaps in knowledge remain with suggestions for immediate action. Olfaction, vision, otolith, pH, freshwater, seawater, marine, central nervous system, electrophysiology, mechanism.

Taurine depletion impairs cardiac function and affects tolerance to hypoxia and high temperatures in brook char (Salvelinus fontinalis)

Physiological and environmental stressors can cause osmotic stress in fish hearts, leading to a reduction in intracellular taurine concentration. Taurine is a β-amino acid known to regulate cardiac function in other animal models but its role in fish has not been well characterized. We generated a model of cardiac taurine deficiency (TD) by feeding brook char (Salvelinus fontinalis) a diet enriched in β-alanine, which inhibits cardiomyocyte taurine uptake. Cardiac taurine levels were reduced by 21% and stress-induced changes in normal taurine handling were observed in TD brook char. Responses to exhaustive exercise and acute thermal and hypoxia tolerance were then assessed using a combination of in vivo, in vitro and biochemical approaches. Critical thermal maximum was higher in TD brook char despite significant reductions in maximum heart rate. In vivo, TD brook char exhibited a lower resting heart rate, blunted hypoxic bradycardia and a severe reduction in time to loss of equilibrium under hypoxia. In vitro function was similar between control and TD hearts under oxygenated conditions, but stroke volume and cardiac output were severely compromised in TD hearts under severe hypoxia. Aspects of mitochondrial structure and function were also impacted in TD permeabilized cardiomyocytes, but overall effects were modest. High levels of intracellular taurine are required to achieve maximum cardiac function in brook char and cardiac taurine efflux may be necessary to support heart function under stress. Taurine appears to play a vital, previously unrecognized role in supporting cardiovascular function and stress tolerance in fish.

Cardiovascular oxygen transport and peripheral oxygen extraction capacity contribute to acute heat tolerance in European seabass

This study evaluated whether different parameters describing cardiovascular function, energy metabolism, oxygen transport and oxidative stress were related to the critical thermal maximum (CT_MAX) of European seabass (Dicentrarchus labrax) and if there were differential changes in these parameters during and after heat shock in animals with different CT_MAX in order to characterize which physiological features make seabass vulnerable to heat waves. Seabass (n = 621) were tested for CT_MAX and the physiological parameters were measured in individuals with good or poor temperature tolerance before and after a heat shock (change in temperature from 15 °C to 28 °C in 1.5 h). Fish with good thermal tolerance had larger ventricles with higher maximal heart rate during the heat shock than individuals with poor tolerance. Furthermore, they initially had a high ventricular Ca²⁺-ATPase activity, which was reduced to a similar level as in fish with poor tolerance following heat shock. The activity of heart lactate dehydrogenase increased in fish with high tolerance, when they were exposed to heat shock, while the aerobic enzyme activity did not differ between groups. The tolerant individuals had smaller red muscle fibers with higher myoglobin content than the poorly tolerant ones. The poorly tolerant individuals had higher hematocrit, which increased with heat shock in both groups. The poorly tolerant individuals had also higher activity of enzymes related to oxidative stress especially after heat shock. In general, CT_MAX was not depending on merely one physiological factor but several organ and cellular parameters were related to the CT_MAX of seabass and when working in combination they might protect the highly tolerant seabass from future heat waves.

The upper thermal limit of epaulette sharks (Hemiscyllium ocellatum) is conserved across three life history stages, sex and body size

Owing to climate change, most notably the increasing frequency of marine heatwaves and long-term ocean warming, better elucidating the upper thermal limits of marine fishes is important for predicting the future of species and populations. The critical thermal maximum (CT_max), or the highest temperature a species can tolerate, is a physiological metric that is used to establish upper thermal limits. Among marine organisms, this metric is commonly assessed in bony fishes but less so in other taxonomic groups, such as elasmobranchs (subclass of sharks, rays and skates), where only thermal acclimation effects on CT_max have been assessed. Herein, we tested whether three life history stages, sex and body size affected CT_max in a tropical elasmobranch, the epaulette shark (Hemiscyllium ocellatum), collected from the reef flats surrounding Heron Island, Australia. Overall, we found no difference in CT_max between life history stages, sexes or across a range of body sizes. Findings from this research suggest that the energetically costly processes (i.e. growth, maturation and reproduction) associated with the life history stages occupying these tropical reef flats do not change overall acute thermal tolerance. However, it is important to note that neither embryos developing in ovo, neonates, nor females actively encapsulating egg cases were observed in or collected from the reef flats. Overall, our findings provide the first evidence in an elasmobranch that upper thermal tolerance is not impacted by life history stage or size. This information will help to improve our understanding of how anthropogenic climate change may (or may not) disproportionally affect particular life stages and, as such, where additional conservation and management actions may be required.

Finding the right thermal limit: a framework to reconcile ecological, physiological and methodological aspects of CTmax in ectotherms

Upper thermal limits (CTmax) are frequently used to parameterize the fundamental niche of ectothermic animals and to infer biogeographical distribution limits under current and future climate scenarios. However, there is considerable debate associated with the methodological, ecological and physiological definitions of CTmax. The recent (re)introduction of the thermal death time (TDT) model has reconciled some of these issues and now offers a solid mathematical foundation to model CTmax by considering both intensity and duration of thermal stress. Nevertheless, the physiological origin and boundaries of this temperature–duration model remain unexplored. Supported by empirical data, we here outline a reconciling framework that integrates the TDT model, which operates at stressful temperatures, with the classic thermal performance curve (TPC) that typically describes biological functions at permissive temperatures. Further, we discuss how the TDT model is founded on a balance between disruptive and regenerative biological processes that ultimately defines a critical boundary temperature (Tc) separating the TDT and TPC models. Collectively, this framework allows inclusion of both repair and accumulation of heat stress, and therefore also offers a consistent conceptual approach to understand the impact of high temperature under fluctuating thermal conditions. Further, this reconciling framework allows improved experimental designs to understand the physiological underpinnings and ecological consequences of ectotherm heat tolerance.