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ABSTRACT Natural and anthropogenic stressors are dramatically altering environments, impacting key animal physiological traits, including cardiac performance. Animals require energy and nutrients from their diet to support cardiac performance and plasticity; however, the nutritional landscape is changing in response to environmental perturbations. Diet quantity, quality and options vary in space and time across heterogeneous environments, over the lifetime of an organism and in response to environmental stressors. Variation in dietary energy and nutrients (e.g. lipids, amino acids, vitamins, minerals) impact the heart's structure and performance, and thus whole-animal resilience to environmental change. Notably, many animals can alter their diet in response to environmental cues, depending on the context. Yet, most studies feed animals ad libitum using a fixed diet, thus underestimating the role of food in impacting cardiac performance and resilience. By applying an ecological lens to the study of cardiac plasticity, this Commentary aims to further our understanding of cardiac function in the context of environmental change. 

ABSTRACT The thermal sensitivity of heart rate (fH) in fishes has fascinated comparative physiologists for well over a century. We now know that elevating fH is the primary mechanism through which fishes increase convective oxygen delivery during warming to meet the concomitant rise in tissue oxygen consumption. Thus, limits on fH can constrain whole-animal aerobic metabolism. In this Review, we discuss an increasingly popular methodology to study these limits, the measurement of pharmacologically induced maximum fH (fH,max) during acute warming of an anaesthetized fish. During acute warming, fH,max increases exponentially over moderate temperatures (Q10∼2–3), but this response is blunted with further warming (Q10∼1–2), with fH,max ultimately reaching a peak (Q10≤1) and the heartbeat becoming arrhythmic. Because the temperatures at which these transitions occur commonly align with whole-animal optimum and critical temperatures (e.g. aerobic scope and the critical thermal maximum), they can be valuable indicators of thermal performance. The method can be performed simultaneously on multiple individuals over a few hours and across a broad size range (<1 to >6000 g) with compact equipment. This simplicity and high throughput make it tractable in lab and field settings and enable large experimental designs that would otherwise be impractical. As with all reductionist approaches, the method does have limitations. Namely, it requires anaesthesia and pharmacological removal of extrinsic cardiac regulation. Nonetheless, the method has proven particularly effective in the study of patterns and limits of thermal plasticity and holds promise for helping to predict and mitigate outcomes of environmental change. 

ABSTRACT The environment is changing rapidly, and considerable research is aimed at understanding the capacity of organisms to respond. Changes in environmental temperature are particularly concerning as most animals are ectothermic, with temperature considered a key factor governing their ecology, biogeography, behaviour and physiology. The ability of ectotherms to persist in an increasingly warm, variable, and unpredictable future will depend on their nutritional status. Nutritional resources (e.g. food availability, quality, options) vary across space and time and in response to environmental change, but animals also have the capacity to alter how much they eat and what they eat, which may help them improve their performance under climate change. In this review, we discuss the state of knowledge in the intersection between animal nutrition and temperature. We take a mechanistic approach to describe nutrients (i.e. broad macronutrients, specific lipids, and micronutrients) that may impact thermal performance and discuss what is currently known about their role in ectotherm thermal plasticity, thermoregulatory behaviour, diet preference, and thermal tolerance. We finish by describing how this topic can inform ectotherm biogeography, behaviour, and aquaculture research. 

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

IntroductionChanges in temperature can fundamentally transform how species interact, causing wholesale shifts in ecosystem dynamics and stability. Yet we still have a limited understanding of how temperature-dependence in physiology drives temperature-dependence in species-interactions. For predator-prey interactions, theory predicts that increases in temperature drive increases in metabolism and that animals respond to this increased energy expenditure by ramping up their food consumption to meet their metabolic demand. However, if consumption does not increase as rapidly with temperature as metabolism, increases in temperature can ultimately cause a reduction in consumer fitness and biomass via starvation. MethodsHere we test the hypothesis that increases in temperature cause more rapid increases in metabolism than increases in consumption using the California spiny lobster (Panulirus interruptus) as a model system. We acclimated individual lobsters to temperatures they experience sacross their biogeographic range (11, 16, 21, or 26°C), then measured whether lobster consumption rates are able to meet the increased metabolic demands of rising temperatures. Results and discussionWe show positive effects of temperature on metabolism and predation, but in contrast to our hypothesis, rising temperature caused lobster consumption rates to increase at a faster rate than increases in metabolic demand, suggesting that for the mid-range of temperatures, lobsters are capable of ramping up consumption rates to increase their caloric demand. However, at the extreme ends of the simulated temperatures, lobster biology broke down. At the coldest temperature, lobsters had almost no metabolic activity and at the highest temperature, 33% of lobsters died. Our results suggest that temperature plays a key role in driving the geographic range of spiny lobsters and that spatial and temporal shifts in temperature can play a critical role in driving the strength of species interactions for a key predator in temperate reef ecosystems. 

ABSTRACT Thermal acclimation is a key process enabling ectotherms to cope with temperature change. To undergo a successful acclimation response, ectotherms require energy and nutritional building blocks obtained from their diet. However, diet is often overlooked as a factor that can alter acclimation responses. Using a temperate omnivorous fish, opaleye (Girella nigricans), as a model system, we tested the hypotheses that (1) diet can impact the magnitude of thermal acclimation responses and (2) traits vary in their sensitivity to both temperature acclimation and diet. We fed opaleye a simple omnivorous diet (ad libitum Artemia sp. and Ulva sp.) or a carnivorous diet (ad libitum Artemia sp.) at two ecologically relevant temperatures (12 and 20°C) and measured a suite of whole-animal (growth, sprint speed, metabolism), organ (cardiac thermal tolerance) and cellular-level traits (oxidative stress, glycolytic capacity). When opaleye were offered two diet options compared with one, they had reduced cardiovascular thermal performance and higher standard metabolic rate under conditions representative of the maximal seasonal temperature the population experiences (20°C). Further, sprint speed and absolute aerobic scope were insensitive to diet and temperature, while growth was highly sensitive to temperature but not diet, and standard metabolic rate and maximum heart rate were sensitive to both diet and temperature. Our results reveal that diet influences thermal performance in trait-specific ways, which could create diet trade-offs for generalist ectotherms living in thermally variable environments. Ectotherms that alter their diet may be able to regulate their performance at different environmental temperatures. 

ABSTRACT In the 12 years since Dudgeonet al. (2006) reviewed major pressures on freshwater ecosystems, the biodiversity crisis in the world's lakes, reservoirs, rivers, streams and wetlands has deepened. While lakes, reservoirs and rivers cover only 2.3% of the Earth's surface, these ecosystems host at least 9.5% of the Earth's described animal species. Furthermore, using the World Wide Fund for Nature's Living Planet Index, freshwater population declines (83% between 1970 and 2014) continue to outpace contemporaneous declines in marine or terrestrial systems. The Anthropocene has brought multiple new and varied threats that disproportionately impact freshwater systems. We document 12 emerging threats to freshwater biodiversity that are either entirely new since 2006 or have since intensified: (i) changing climates; (ii) e‐commerce and invasions; (iii) infectious diseases; (iv) harmful algal blooms; (v) expanding hydropower; (vi) emerging contaminants; (vii) engineered nanomaterials; (viii) microplastic pollution; (ix) light and noise; (x) freshwater salinisation; (xi) declining calcium; and (xii) cumulative stressors. Effects are evidenced for amphibians, fishes, invertebrates, microbes, plants, turtles and waterbirds, with potential for ecosystem‐level changes through bottom‐up and top‐down processes. In our highly uncertain future, the net effects of these threats raise serious concerns for freshwater ecosystems. However, we also highlight opportunities for conservation gains as a result of novel management tools (e.g. environmental flows, environmental DNA) and specific conservation‐oriented actions (e.g. dam removal, habitat protection policies, managed relocation of species) that have been met with varying levels of success. Moving forward, we advocate hybrid approaches that manage fresh waters as crucial ecosystems for human life support as well as essential hotspots of biodiversity and ecological function. Efforts to reverse global trends in freshwater degradation now depend on bridging an immense gap between the aspirations of conservation biologists and the accelerating rate of species endangerment.