Rapid-warming tolerance correlates with tolerance to slow warming but not growth at non-optimal temperatures in zebrafish

Directional selection, not the direction of selection, affects telomere length and copy number at ribosomal RNA loci

Many fisheries exert directional selection on traits such as body size and growth rate. Whether directional selection impacts regions of the genome associated with traits related to growth is unknown. To address this issue, we characterised copy number variation in three regions of the genome associated with cell division, (1) telomeric DNA, (2) loci transcribed as ribosomal RNA (rDNA), and (3) mitochondrial DNA (mtDNA), in three selection lines of zebrafish reared at three temperatures (22 °C, 28 °C, and 34 °C). Selection lines differed in (1) the direction of selection (two lines experienced directional selection for large or small body size) and (2) whether they experienced any directional selection itself. Lines that had experienced directional selection were smaller, had lower growth rate, shorter telomeres, and lower rDNA copy number than the line that experiencing no directional selection. Neither telomere length nor rDNA copy number were affected by temperature. In contrast, mtDNA content increased at elevated temperature but did not differ among selection lines. Though directional selection impacts rDNA and telomere length, direction of such selection did not matter, whereas mtDNA acts as a stress marker for temperature. Future work should examine the consequences of these genomic changes in natural fish stocks.

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

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

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

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

Energetic costs increase with faster heating in an aquatic ectotherm

The thermal sensitivity of metabolism is widely studied due to its perceived importance for organismal fitness and resilience to future climate change. Almost all such studies estimate metabolism at a variety of constant temperatures, with very little work exploring how metabolism varies during temperature change. However, temperature in nature is rarely static, so our existing understanding from experiments may not reflect how temperature influences metabolism in natural systems. Using closed-chamber respirometry, we estimated the aerobic metabolic rate of an aquatic ectotherm, the Atlantic ditch shrimp Palaemonetes varians, under varying thermal conditions. We continuously measured oxygen consumption of shrimp during heating, cooling and constant temperatures, starting trials at a range of acclimation temperatures and exposing shrimp to a variety of rates of temperature change. In a broad sense, cumulative oxygen consumption estimated from static temperature exposures corresponded to estimates derived from ramping experiments. However, further analyses showed that oxygen consumption increases for both faster heating and faster cooling, with rapid heating driving higher metabolic rates than if shrimp were warmed slowly. These results suggest a systematic influence of heating rate on the thermal sensitivity of metabolism. With influential concepts such as the metabolic theory of ecology founded in data from constant temperature experiments, our results encourage further exploration of how variable temperature impacts organism energetics, and to test the generality of our findings across species. This is especially important given climate forecasts of heat waves that are characterised by both increased temperatures and faster rates of change.

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.

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.

Developmental plasticity in thermal tolerance: Ontogenetic variation, persistence, and future directions

Understanding the factors affecting thermal tolerance is crucial for predicting the impact climate change will have on ectotherms. However, the role developmental plasticity plays in allowing populations to cope with thermal extremes is poorly understood. Here, we meta-analyse how thermal tolerance is initially and persistently impacted by early (embryonic and juvenile) thermal environments by using data from 150 experimental studies on 138 ectothermic species. Thermal tolerance only increased by 0.13°C per 1°C change in developmental temperature and substantial variation in plasticity (~36%) was the result of shared evolutionary history and species ecology. Aquatic ectotherms were more than three times as plastic as terrestrial ectotherms. Notably, embryos expressed weaker but more heterogenous plasticity than older life stages, with numerous responses appearing as non-adaptive. While developmental temperatures did not have persistent effects on thermal tolerance overall, persistent effects were vastly under-studied, and their direction and magnitude varied with ontogeny. Embryonic stages may represent a critical window of vulnerability to changing environments and we urge researchers to consider early life stages when assessing the climate vulnerability of ectotherms. Overall, our synthesis suggests that developmental changes in thermal tolerance rarely reach levels of perfect compensation and may provide limited benefit in changing environments.

Acute thermal stress elicits interactions between gene expression and alternative splicing in a fish of conservation concern

Transcriptomic research provides a mechanistic understanding of an organism's response to environmental challenges such as increasing temperatures, which can provide key insights into the threats posed by thermal challenges associated with urbanization and climate change. Differential gene expression and alternative splicing are two elements of the transcriptomic stress response that may work in tandem, but relatively few studies have investigated these interactions in fishes of conservation concern. We studied the imperilled redside dace (Clinostomus elongatus) as thermal stress is hypothesised to be an important cause of population declines. We tested the hypothesis that gene expression-splicing interactions contribute to the thermal stress response. Wild fish exposed to acute thermal stress were compared with both handling controls and fish sampled directly from a river. Liver tissue was sampled to study the transcriptomic stress response. With a gene set enrichment analysis, we found that thermally stressed fish showed a transcriptional response related to transcription regulation and responses to unfolded proteins, and alternatively spliced genes related to gene expression regulation and metabolism. One splicing factor, prpf38b, was upregulated in the thermally stressed group compared to the other treatments. This splicing factor may have a role in the Jun/AP-1 cellular stress response, a pathway with wide-ranging and context-dependent effects. Given large gene interaction networks and the context-dependent nature of transcriptional responses, our results highlight the importance of understanding interactions between gene expression and splicing for understanding transcriptomic responses to thermal stress. Our results also reveal transcriptional pathways that can inform conservation breeding, translocation, and reintroduction programs for redside dace and other imperilled species by identifying appropriate source populations.

Reduced physiological plasticity in a fish adapted to stable temperatures

Significance Plastic individuals can buffer environmental changes, maintaining a stable performance across gradients. Plasticity is therefore thought to be particularly beneficial for the survival of wild populations that experience large environmental fluctuations, such as diel and seasonal temperature changes. Maintaining plasticity is widely assumed to be costly; however, empirical evidence demonstrating this cost is scarce. Here, we predict that if plasticity is costly, it would be readily lost in a stable environment, such as a laboratory. To test this, we measured a diverse range of phenotypic traits, spanning gene expression, physiology, and behavior, in wild and laboratory zebrafish acclimated to 15 temperatures. We show that laboratory fish have lost plasticity in many traits, demonstrating that maintaining plasticity carries a cost.

The pace-of life explains whether gills improve or exacerbate pesticide sensitivity in a damselfly larva

Trait-based approaches are promising to make generalizations about the sensitivity of species and populations to pesticides. Two traits that may shape the sensitivity to pesticides are the surface area (related to pesticide uptake) and the metabolic rate (related to pesticide elimination). We compared the sensitivity of damselfly larvae to the pesticide chlorpyrifos and how this was modified by loss of external gills (autotomy, reducing the surface area) in both fast pace-of-life (high metabolic rate) and slow pace-of-life (low metabolic rate) populations of Ischnura elegans. The slow-paced populations were more sensitive to the pesticide than the fast-paced populations in terms of survival, growth and energy metabolism. This suggests the higher metabolic rate of fast-paced populations enabled a faster pesticide elimination. Pesticide exposure also reduced heat tolerance, especially in slow-paced larvae under hypoxia. Gill loss had opposite effects on pesticide sensitivity in slow- and fast-paced populations. In slow-paced larvae, gill loss lowered the sensitivity to the pesticide, while in fast-paced larvae, gill loss increased the sensitivity. This difference likely reflects the balance between the roles of the gills in pesticide uptake (more detrimental in slow-paced populations) and oxygen uptake (more important in fast-paced populations). Our results highlight the need to consider trait interactions when applying trait-based approaches to predict the sensitivity to pesticides.

Resilience of cardiac performance in Antarctic notothenioid fishes in a warming climate

Warming in the region of the Western Antarctic Peninsula is occurring at an unprecedented rate, which may threaten the survival of Antarctic notothenioid fishes. Herein, we review studies characterizing thermal tolerance and cardiac performance in notothenioids - a group that includes both red-blooded species and the white-blooded, haemoglobinless icefishes - as well as the relevant biochemistry associated with cardiac failure during an acute temperature ramp. Because icefishes do not feed in captivity, making long-term acclimation studies unfeasible, we focus only on the responses of red-blooded notothenioids to warm acclimation. With acute warming, hearts of the white-blooded icefish Chaenocephalus aceratus display persistent arrhythmia at a lower temperature (8°C) compared with those of the red-blooded Notothenia coriiceps (14°C). When compared with the icefish, the enhanced cardiac performance of N. coriiceps during warming is associated with greater aerobic capacity, higher ATP levels, less oxidative damage and enhanced membrane integrity. Cardiac performance can be improved in N. coriiceps with warm acclimation to 5°C for 6-9 weeks, accompanied by an increase in the temperature at which cardiac failure occurs. Also, both cardiac mitochondrial and microsomal membranes are remodelled in response to warm acclimation in N. coriiceps, displaying homeoviscous adaptation. Overall, cardiac performance in N. coriiceps is malleable and resilient to warming, yet thermal tolerance and plasticity vary among different species of notothenioid fishes; disruptions to the Antarctic ecosystem driven by climate warming and other anthropogenic activities endanger the survival of notothenioids, warranting greater protection afforded by an expansion of marine protected areas.

Low potential for evolutionary rescue from climate change in a tropical fish

Climate change is increasing global temperatures and intensifying the frequency and severity of extreme heat waves. How organisms will cope with these changes depends on their inherent thermal tolerance, acclimation capacity, and ability for evolutionary adaptation. Yet, the potential for adaptation of upper thermal tolerance in vertebrates is largely unknown. We artificially selected offspring from wild-caught zebrafish (Danio rerio) to increase (Up-selected) or decrease (Down-selected) upper thermal tolerance over six generations. Selection to increase upper thermal tolerance was also performed on warm-acclimated fish to test whether plasticity in the form of inducible warm tolerance also evolved. Upper thermal tolerance responded to selection in the predicted directions. However, compared to the control lines, the response was stronger in the Down-selected than in the Up-selected lines in which evolution toward higher upper thermal tolerance was slow (0.04 ± 0.008 °C per generation). Furthermore, the scope for plasticity resulting from warm acclimation decreased in the Up-selected lines. These results suggest the existence of a hard limit in upper thermal tolerance. Considering the rate at which global temperatures are increasing, the observed rates of adaptation and the possible hard limit in upper thermal tolerance suggest a low potential for evolutionary rescue in tropical fish living at the edge of their thermal limits.