Metabolic responses to thermal ramping in two endemic eurybathic amphipods of the genus Ommatogammarus from ancient Lake Baikal

Lake Baikal harbors freshwater profundal amphipods, including the eurybathic species Ommatogammarus flavus and O. albinus. O. flavus occupies shallower habitats, including the littoral zone, which is subject to greater temperature variability. Consequently, O. flavus may demonstrate a higher degree of tolerance to temperature fluctuations in comparison with O. albinus. We compared the metabolic responses of these two amphipod species to thermal ramping. Specimens were collected from a range of depths and acclimated to a temperature of 4 °C. They were then subjected to controlled temperature up- and downshifts to assess the parameters of tissue energy status, antioxidant enzyme activity, and survival. O. flavus showed a higher degree of tolerance to warming than the deeper-dwelling O. albinus, while both species were resilient to temperature decreases. The depth of sampling had a minor effect on glycogen levels in O. flavus as well as catalase and glutathione S-transferase activity in O. albinus but had no effect on survival during experimental warming. Glucose level was shown to be the most sensitive biochemical marker to temperature variations, indicating that it could be used as a stress indicator for Baikal deep-water amphipods. This finding might provide insight into their adaptability to ambient temperature fluctuations.

Climate-Driven Warming Disrupts the Symbiosis of Bobtail Squid Euprymna scolopes and the Luminous Bacterium Vibrio fischeri

Under the current climate crisis, marine heatwaves (MHW) are expected to intensify and become more frequent in the future, leading to adverse effects on marine life. Here, we aimed to investigate the impact of environmental warming on the symbiotic relationship between the Hawaiian bobtail squid (Euprymna scolopes) and the bioluminescent bacterium Vibrio fischeri. We exposed eggs of E. scolopes to three different temperatures during embryogenesis, namely: (i) 25°C (yearly average), (ii) 27°C (summer maximum) or (iii) 30°C (category IV MHW), followed by a colonisation assay under the same conditions. Decreased hatching success and reduced developmental time were observed across warmer conditions compared to 25°C. Moreover, exposure to the category IV MHW led to a significant decrease in survival after 48 h. With increasing temperature, bobtail squids required more bacteria in the surrounding seawater for successful colonisation. When colonised, the regression of the light organ's appendages was not dependent on temperature, but the opposite was found in non-colonised bobtail squids. Furthermore, the capacity for crypt 3 formation in the squid's light organ, which is crucial for enhancing resilience under stress, also declined with warming conditions. This study emphasises the critical need to study the dynamics of microbial symbiosis under the projected conditions for the ocean of tomorrow.

Corticotropin-releasing hormone receptor 1 mediates the enhanced locomotor activity and metabolic demands to an acute thermal stress in adult zebrafish

We recently showed that Crh-Crhr1 signalling is essential for acute stress-related locomotor activity in zebrafish larvae. However, the possibility that Crhr1 activation may also initiate the acute metabolic demands for stress coping was unexplored. Here, we tested the hypothesis that Crhr1 signalling is essential for the thermal stressor-induced increases in the acute metabolic rate, a key response for coping with the enhanced energy demands during stress. We tested this by using a wildtype (WT) and a ubiquitous Crhr1 knockout (crhr1-/-) zebrafish and subjecting them to an acute thermal stressor (TS: +5°C above ambient for 60 min). The TS induced the heat shock proteins response in both genotypes, but the elevated cortisol response observed in the WT was absent in the crhr1-/- mutant. The TS also increased the locomotor activity and the metabolic rate in the WT fish, but this response was inhibited in the crhr1-/- mutants. To test if this was due to a lack of TS-induced cortisol elevation in the crhr1-/- mutant, we mimicked the response in the WT fish by treating them with metyrapone, an 11β-hydroxylase inhibitor. While metyrapone inhibited the TS-induced cortisol elevation in the WT, it did not affect the metabolic rate. The lack of Crhr1 also reduced the swimming performance, and the lower Ucrit in the mutants corresponded with alterations in muscle energy metabolism. Together, our results indicate that Crh-Crhr1 signalling, independent of downstream cortisol action, is essential for the TS-induced acute hyperlocomotor activity and the associated increases in the metabolic demand for stress coping.

Multiple Models of European Marine Fish Stocks: Regional Winners and Losers in a Future Climate

Climate change continues to alter the productivity of commercially and culturally important fisheries with major consequences for food security and coastal economies. We provide the first, multi-model projections of changes in the distribution and productivity of 18 key fish stocks across seven European regional seas spanning the Mediterranean to the Arctic, using 11 state-of-the-art bio-ecological models. Our projections indicate species- and region-specific changes in abundance and distributions of these stocks by the mid- to late 21st century. The varied responses are caused by differences in species' physiology, regional food web dynamics, and physical habitat characteristics. Important drivers include not only warming of Europe's seas (from 1°C to 3°C in RCP 4.5, and 2°C to 4°C in RCP 8.5 by 2100) and changes in primary productivity but also oxygen-limited fish growth, changes in pH, and benthic dissolved organic carbon. Warming and altered levels of secondary production are projected to lead to declines in some stocks (Norwegian and Barents Sea herring) and increases in others (Bay of Biscay anchovy). While some temperate and cold-water stocks are projected to decline markedly in some regions (e.g., North Sea, Western Mediterranean), the immigration of species from the south and/or increase in productivity of warm-water species may offer new opportunities for fisheries. Species-level changes will likely have ecosystem-level consequences that have yet to be fully assessed, and responses in some sub-areas may be more pronounced due to local processes not captured in projections. Projections are consistent despite differences in model structures, and the results of our multi-model analysis align with other modelling exercises while delving into details often overlooked at the species or spatial level. This represents a novel approach to projecting the impacts of climate change on fisheries, which should be considered in future efforts to support climate-ready management strategies for marine fish stocks.

It's a good thing that severely hypoxic salmon (Salmo salar) have a limited capacity to increase heart rate when warmed

With climate change, fish are facing rising temperatures, an increase in the frequency and severity of heat waves and hypoxia, sometimes concurrently. However, only limited studies have examined the combined effects of increases in temperature and hypoxia on fish physiology and survival. We measured the cardiorespiratory physiology of 12°C-acclimated Atlantic salmon when exposed acutely to normoxia [100% air saturation (sat.)] versus 75 and 50% air sat., and then warmed to their critical thermal maximum (CTmax) at 2°C h-1. Fish exposed to 50% air sat. became bradycardic, were unable to increase heart rate (fH) when warmed, and had lower values for metabolic scope and CTmax (21.3 vs 26.1°C in normoxic fish). The effects of 75% air sat. on cardiorespiratory parameters and CTmax were intermediate. We then used atropine (1.2 mg kg-1) and 8-cyclopentyltheophylline (CPT; 50 nmol kg-1) to investigate what role(s) cholinergic tone on the heart and cardiac adenosinergic effects, respectively, play in preventing severely hypoxic salmon (40% air sat.) from increasing fH when warmed. CPT had no/limited effects on salmon cardiorespiratory parameters and thermal tolerance. However, atropine increased fH in hypoxic fish and allowed it to rise with temperature, and this resulted in salmon that were much less tolerant to warming. Collectively, these results: (1) show that fish in severely hypoxic environments will be very susceptible to climate change-associated heat waves; and (2) suggest that cholinergic tone on the heart is not removed when severely hypoxic fish are exposed to rising temperatures to protect the heart's pumping capacity.

Narrow Margins: Aerobic Performance and Temperature Tolerance of Coral Reef Fishes Facing Extreme Thermal Variability

Climate change is driving rising average sea temperatures and the intensification of thermal variability. Tropical coral reef fishes have evolved under thermally stable conditions to function optimally within a narrow temperature range, with many currently living close to their upper thermal limits. However, recent work has demonstrated that some species possess additional capacity, such as reductions in basal metabolic rates (i.e., 'plastic floors'), to compensate for the acute effects of thermal challenges when assessed over multigenerational timeframes. In this study, we use the 'plastic floors and concrete ceilings' hypothesis to generate and then test predictions regarding the thermal physiology of reef fishes in the world's hottest and most thermally variable coral reef ecosystem (southern Arabian/Persian Gulf). By comparing three species of reef fishes (Scolopsis ghanam, Ecsenius pulcher and Cheilodipterus novemstriatus) from the southern Arabian/Persian Gulf, with an annual temperature range of 18.0°C-36.5°C, to conspecifics from nearby but more thermally benign (~21.0°C-32.0°C) reefs in the Gulf of Oman, we find enhanced upper thermal limits and a broadening of the temperature performance curves for aerobic scope in the Arabian/Persian Gulf, but no evidence for changes in basal metabolic rates ('plastic floors'). Despite these conserved increases in temperature tolerance, the summer thermal safety margins of Arabian/Persian Gulf fishes were 1.47°C lower than those of conspecifics from the Gulf of Oman, demonstrating that while the temperature tolerance of tropical coral reef fishes is somewhat plastic over multigenerational timeframes, its rate of change is likely insufficient to keep pace with the rising average temperatures and growing thermal variability expected under climate change.

Seasonal effects of extreme climate events and sea surface temperature indicators on the vulnerability of marine pelagic fisheries in the Bay of Bengal region

The study highlights the vulnerability of the eastern coast to tropical cyclones and the unique characteristics of the Bay of Bengal region. Seasonality, driven by global climatic events and geography, significantly affects the marine ecosystem. Furthermore, it underscores the effects of extreme climate events on marine pelagic fisheries and advocates for a state-based approach to raise awareness among government institutions and fishing communities. Findings reveal a significant negative relationship between Sea Surface Temperature (SST) and fish-catch (coefficient = -0.09, p < 0.01), indicating that rising SST adversely affects pelagic fish populations. Moreover, increased frequency of extreme events (-0.12 to -2.06, p < 0.05 to p < 0.01) and disturbances (-0.149 to -0.679, p < 0.05) exhibits detrimental impacts across various models. Notably, seasonal variations play a crucial role, with quarters 1, 3, and 4 demonstrating positive associations (0.186-0.604, p < 0.1 to p < 0.05) with fish-catch, signifying potentially favourable conditions during specific seasons. State-specific analysis highlights diverse impacts, wherein West Bengal experiences substantial negative effects from extreme events (-2.056, p < 0.01), emphasizing regional disparities. These findings underscore the need for regionalized mitigation strategies and sustainable fishing practices to ensure the future of the Bay of Bengal's marine ecosystem.

What makes a competent aquatic invader? Considering saline niches of invertebrates and ray-finned fishes

Aquatic invasive species are of growing concern globally, especially in fresh water. The problem is intensified by climate change, which often causes salinization of coastal fresh waters. Animals deal with salinity through the function of osmoregulation, and osmoregulatory ability can be informative when considering invasive potential. A species is said to be 'euryhaline' if it can tolerate a wide range of salinities, either through osmoregulation (tightly controlling its extracellular fluid osmolality) or osmoconformation (matching the osmotic concentration of its internal fluids with that of the environment). Euryhaline animals display a large fundamental saline niche (FSN); i.e. a wide physiological tolerance of salinity change. However, the range of salinities of the habitats where a species actually occurs define its realized saline niche (RSN). Importantly, aquatic species living in stable habitats (i.e. those with little variation in salinity) will have a small RSN, but may have large FSNs, depending on their evolutionary history. Species with large FSNs are more likely to be successful invaders of new habitats with different salinities. Here, I propose the term 'osmotic comfort' as a concept that is associated with the FSN. The core of the FSN corresponds to ∼100% osmotic comfort, or 'optimum salinity', putatively meaning minimum stress. Physiological markers of osmotic comfort can provide raw data for mechanistic niche modelling in aquatic habitats. A species with a larger FSN is more likely to remain 'osmotically comfortable' in a different saline habitat, and is less likely to suffer local extinction in fresh waters, for example, that undergo salinization.

Linking Inferred Laboratory-Derived Temperature Stress to the Immunocompetence of Wild Octopus maya (Mayan Octopus) G.L. Voss & Solís, 1966

The "oxygen capacity-dependent thermal tolerance" (OCLTT) hypothesis suggests that the ability of ectotherms to tolerate heat is limited by their ability to supply oxygen to their tissues at various temperatures set by the capacity of the cardiovascular and respiratory systems. Optimal temperatures and oxygen can supply enough energy through adenosine triphosphate (ATP) via the electron transport chain to support fitness-related processes. Conversely, stressful temperatures indicate an energetic limitation that could describe physiological parameters and biogeographical patterns. Our study aimed to determine if stressful temperatures could be related to immunological performance under a macroecological approach. To prove this hypothesis, we recapitulated key immune parameters, including total hemocyte count, hemagglutination, phenoloxidase system, and lysozyme activity, of wild mayan octopus (Octopus maya), an endemic species in Mexico's Yucatan Peninsula, with physiological data via thermal metabolic scope (a proxy of the aerobic scope) from its fishing regions. Our results indicate that stressful temperatures (> 27°C) are associated with depression in the immunocompetence of the mayan octopus. Specifically, we found that favorable temperatures (< 27°C) are positively correlated with a better immunocompetence of wild octopus. This study provides evidence that temperature stress inferred from laboratory studies presents a potential tool to determine wild populations' health. However, predictions and modeling should consider additional factors such as demographic distribution, seasonality, biotic/abiotic interactions, and ontogenetic development.

Transcriptome analyses reveal differences in the response to temperature in Florida and Northern largemouth bass (Micropterus spp.) during early life stages

Temperature is one of the most relevant factors influencing the development of aquatic species, making it a key parameter to consider for aquaculture. Largemouth bass (LMB; Micropterus spp.) are highly relevant for human consumption and sport fishing, representing one of North America's most important freshwater fisheries. Yet, questions remain on how LMB raised in recirculating aquaculture systems (RAS) respond to different temperatures. The main objective of this study was to determine the impact of thermal rearing conditions (21°C, 24°C, and 27°C) on gene expression of Florida and Northern LMB larvae at 8- and 28-days post hatch (DPH). Using de novo transcriptomes as a reference, our results suggest that gene expression differences for Florida LMB were mostly associated with temperature, while differences for Northern LMB were controlled by temperature and developmental stage. In general, both lineages showed activation of molecular pathways associated growth, such as development of muscle, nervous system, and vascular system. There were molecular signatures of stress with warming as well, including immune function, apoptosis, regulation of inflammation, and heat shock proteins. Florida LMB showed large differences between temperatures at both stages, while differences were much larger for Northern LMB at 28 DPH, specifically for individuals reared at 27°C. The results from this study are in line with previous phenotypic studies that indicated faster growth at warmer temperatures and better performance of Northern LMB raised in RAS. Overall, this study exemplifies how controlling developmental temperatures during the critical early life stages can be essential to guarantee the success of commercial hatchery production techniques.

Marine species and assemblage change foreshadowed by their thermal bias over Early Jurassic warming

A mismatch of species' thermal preferences to their environment may indicate how they will respond to future climate change. Averaging this mismatch across species may forewarn that some assemblages will undergo greater reorganization, extirpation, and possibly extinction, than others. Here, we examine how regional warming determines species occupancy and assemblage composition of marine bivalves, brachiopods, and gastropods over one-million-year time steps during the Early Jurassic. Thermal bias, the difference between modelled regional temperatures and species' long-term thermal optima, predicts a gradient of species occupancy response to warming. Species that become extirpated or extinct tend to have cooler temperature preferences than immigrating species, while regionally persisting species fell midway. Larger regional changes in summer seawater temperatures (up to +10 °C) strengthen the relationship between species thermal bias and the response gradient, which is also stronger for brachiopods than for bivalves, while the relationship collapses during severe seawater deoxygenation. At +3 °C regional seawater warming, around 5 % of pre-existing benthic species in a regional assemblage are extirpated, and immigrating species comprise around one-fourth of the new assemblage. Our results validate thermal bias as an indicator of immigration, persistence, extirpation, and extinction of marine benthic species and assemblages under modern-like magnitudes of climate change.

Effects of oxygen level on thermal tolerance in Amazonian catfishes with bimodal respiration: physiological and behavioural changes

The degree of tolerance to adverse conditions ultimately shapes a species' vulnerability to environmental changes. Some studies have reported limited thermal tolerance due to hypoxia in fish employing aquatic respiration. However, there is a lack of information regarding the effects of hypoxia on thermal tolerance in fish exhibiting bimodal respiration. A set of Amazonian fish species has adaptations to breathe air when oxygen in water is not enough to fulfil demand. Additionally, loricariid species within this group possess stomach adaptations for air breathing. The Loricariidae family exhibits varying stomach types and observed morphological differences could influence their ability to obtain oxygen from the air. This ability may, in turn, have consequences for the thermal tolerance of these species. Our objective was to assess the effects of hypoxia on thermal tolerance, along with the physiological (whole-animal metabolic rates and mitochondrial respiration) and behavioural mechanisms involved, in two facultative air-breathing species: Pterygoplichthys pardalis and Ancistrus dolichopterus. These species showcase morphological distinctions in their stomachs, with the former having a higher capacity to obtain oxygen from the air. Thermal tolerance in P. pardalis remained unaffected by dissolved oxygen in the water when air access was available but decreased when access to the water surface was restricted, specifically in hypoxic conditions. Conversely, the thermal tolerance of A. dolichopterus decreased below the critical oxygen partial pressure (Pcrit), even with access to air, highlighting their limited ability to obtain oxygen through their adapted stomach. Our results underscore that air breathing enhances thermal tolerance, but this effect is prominent only in species with a higher capacity for air breathing.

Temperature-Related Effects on Disease Susceptibility and Immune Response in Redband Trout (Oncorhynchus mykiss gairdneri) Following Challenge With Flavobacterium columnare

Heat stress can increase disease risk in fishes by reducing immune function. Interactions between redband trout (Oncorhynchus mykiss gairdneri) and Flavobacterium columnare, a causative agent of columnaris disease, provide an opportunity to investigate the effects of temperature on immune function and disease resistance during periods of thermal stress. We conducted three trials to characterise differences in immune function and mortality between redband trout held at 18°C and 21°C following challenge with F. columnare. In trial 1, cumulative per cent mortality (CPM) was low and not statistically different between 18°C and 21°C. In trials 2 and 2, we administered higher challenge doses and observed increased CPM overall and significantly greater CPM at 21°C than 18°C. Redband trout upregulated il-8, tnf-α, igm and igt following infection by F. columnare, suggesting that all of these genes may be involved in immune responses to F. columnare infection. We found no differences in the strength of the immune responses between fish held at 21°C versus 18°C. This indicated that 21°C did not elicit sufficient thermal stress to impair immune function and that increased CPM at 21°C versus 18°C was due to enhanced F. columnare virulence.

Cardiac performance mirrors the passive thermal tolerance range in the oyster Ostrea edulis

Increasing frequencies of heatwaves threaten marine ectotherm species but not all alike. In exposed habitats, some species rely on a higher capacity for passive tolerance at higher temperatures, thereby extending time-dependent survival limits. Here, we assessed how the involvement of the cardiovascular system in extended tolerance at the margins of the thermal performance curve is dependent on warming rate. We studied organismal and heart tissue cellular responses of the European oyster, Ostrea edulis, challenged by rapid warming (+2°C per hour) and gradual warming (+2°C per 24 h). Starting at 22°C, cardiac activity was monitored as temperature was increased, tracking cardiac performance curves. Hearts were collected at discrete temperatures to determine cardiomyocyte metabolic profiles. Heart rate peaked at a lower Arrhenius breakpoint temperatures (ABT) of 30.5°C under rapid warming versus 33.9°C under gradual warming. However, oysters survived to higher temperatures under rapid than under gradual warming, with half of oysters dying (LT50) by 36.9°C versus 34.8°C, respectively. As rapid warming passed 30°C, heart rate fell and cardiomyocyte metabolic profiles suddenly changed as oysters switched to anaerobic metabolism for survival. By 36°C, severe fluctuations in Krebs cycle-related metabolites accompanied cardiac failure. In contrast, oysters exposed to gradual warming made gradual, extensive adjustments to intracellular metabolic pathways, prolonging aerobic cardiomyocyte metabolism to higher temperatures. This extended survival duration and ABT, beyond which cardiac activity decreased sharply and ceased. Our results emphasize how the rate of warming forces a trade-off between temperature maxima and survival duration, via tissue- and cellular-level impacts. European oysters possess adaptations that enable extended tolerance and survival of intertidal populations.

Reproductive effort affects cellular response in the mantle of Nodipecten subnodosus scallops exposed to acute hyperthermia

In marine ecosystems, temperature regulates the energy metabolism of animals. In the last decades, the temperature increase was related to mass mortality events of marine ectotherms, particularly during high-energy investment for reproduction. In scallops, the mantle has been poorly investigated while this tissue covers more than 40 % of the body mass, contributing to the perception of surrounding environmental stimuli. Our aim was to assess the cellular and molecular responses linked to energy metabolism in the mantle of adult N. subnodosus facing acute hyperthermia during reproductive effort. Scallops collected in spring (late gametogenesis) and summer (ripe gonads) were exposed to a control temperature (22 °C) or acute hyperthermia (30 °C) for 24 h. In spring, increased arginine kinase (AK) activity together with increased pyruvate kinase/citrate synthase ratio (PK/CS) suggested an enhanced carbohydrate, pyruvate, and arginine metabolism to maintain the adenylate energy charge (AEC) in the mantle of scallops coping with acute thermal increase. In summer, animals decreased their AEC (5 %) and arginine phosphate pool (40 %) and increased their anaerobic metabolism as shown by enhanced activities of lactate dehydrogenase (LDH) and octopine dehydrogenase (ODH), respectively. The abundance of twenty proteins involved in energy metabolism (isocitrate dehydrogenase, ATP synthase subunit β), protein protection (cognate heat shock protein 70), and cytoskeleton (actins and tubulins) were affected only by season. These results underlie the role of the mantle of N. subnodosus in the seasonal responses of this tissue to thermal fluctuations during reproductive effort with possible implications for the physiological performance of scallops under heat waves in wild or harvest conditions.

Constraints of digestion on swimming performance and stress tolerance vary with habitat in freshwater fish species

Limited aerobic scope (AS) during digestion might be the main constraint on the performance of bodily functions in water-breathing animals. Thus, investigating the postprandial changes in various physiological functions and determining the existence of a shared common pattern because of possible dependence on residual AS during digestion in freshwater fish species are very important in conservation physiology. All species from slow-flow habitats showed impaired swimming speed while digesting, whereas all species from fast-flow habitats showed strong swimming performance, which was unchanged while digesting. Only two species from slow-flow habitats showed impaired heat tolerance during digestion, suggesting that whether oxygen limitation is involved in the heat tolerance process is species-specific. Three species from slow- or intermediate-flow habitats showed impaired hypoxia tolerance during digestion because feeding metabolism cannot cease completely under hypoxia. Overall, there was no common pattern in postprandial changes in different physiological functions because: (1) the digestion process was suppressed under oxygen-limiting conditions, (2) the residual AS decreased during digestion, and (3) performance was related to residual AS, while digestion was context-dependent and species-specific. However, digestion generally showed a stronger effect on bodily functions in species from slow-flow habitats, whereas it showed no impairment in fishes from fast-flow habitats. Nevertheless, the postprandial change in physiological functions varies with habitat, possibly due to divergent selective pressure on such functions. More importantly, the present study suggests that a precise prediction of how freshwater fish populations will respond to global climate change needs to incorporate data from postprandial fishes.

The weak association between hypoxia tolerance and thermal tolerance increases the susceptibility of abalone to climate change

The simultaneous occurrence of high temperatures and hypoxia events caused mass die-offs of aquatic animals. It is crucial to investigate the relationship between hypoxia tolerance and thermal tolerance of aquatic animals to predict the biological and ecological outcomes under global climate change scenarios. In this study, the hypoxia tolerance and thermal tolerance of Pacific abalone, Haliotis discus hannai, were measured by methods based on adhesion capacity (hypoxia adhesion duration and heat adhesion duration) and heart rate fluctuations (breakpoint of dissolved oxygen and Arrhenius breakpoint temperature). Weak correlations were found between hypoxia tolerance and thermal tolerance (Spearman correlation, r = -0.09, P = 0.2069; Pearson correlation, r = -0.04, P = 0.3313). Furthermore, a total of 21 significant SNPs and 19 candidate genes (such as cubn, lrp6, gria2, rft2, and casp8) were identified to be associated with hypoxia tolerance of Pacific abalone by conducting whole genome resequencing and genome-wide association study (GWAS). But there was no overlap between candidate genes associated with hypoxia tolerance and candidate genes associated with thermal tolerance, validating the weak correlation between hypoxia tolerance and thermal tolerance. This study highlights that individuals with greater hypoxia tolerance do not necessarily have greater thermal tolerance. Global warming and hypoxia may pose a greater threat to population size and genetic diversity of some aquatic animals than previously believed.

Plasticity cannot fully compensate evolutionary differences in heat tolerance across fish species

Understanding how evolution and phenotypic plasticity contribute to variation in heat tolerance is crucial to predicting responses to warming. Here, we analyze 272 thermal death time curves of 53 fish species acclimated to different temperatures and quantify their relative contributions. Analyses show that evolution and plasticity account, respectively, for 80.5% and 12.4% of the variation in elevation across curves, whereas their slope remained invariant. Evolutionary and plastic adaptive responses differ in magnitude, with heat tolerance increasing to 0.54 °C between species and 0.32 °C within species for every 1 °C increase in environmental temperatures. After successfully predicting critical temperatures under ramping conditions to validate these estimates, we show that fish populations can only partly ameliorate the impact of warming waters via thermal acclimation, and this deficit in plasticity could increase as the warming accelerates.

Temperature alters antioxidant status and induces cell damage in the Amazonian fish tambaqui

Since Amazonian fish live close to their maximum thermal limits, this makes them vulnerable to the effects of global warming. The aim of this study was to evaluate the oxidative stress and antioxidant enzymatic and biochemical responses of the plasma, liver and muscle of tambaqui (Colossoma macropomum) exposed to a rising gradient of water temperature. One hundred and twenty (N = 120) juvenile tambaqui were exposed to four temperature levels, these being: the environmental temperature of the season (Tenv - 25.7-30 °C), 31 °C, 34 °C and 37 °C, following a completely randomized design with three replicates for a period of 60 days. Liver and muscle samples were used to determine the levels of the enzymes superoxide dismutase (SOD), catalase (CAT) glutathione peroxidase (GPx) and lipid peroxidation (LPO). Plasma levels of aspartate aminotransferase (AST) and alanine aminotransferase (ALT) were measured. A histopathological damage assessment (HAI) was performed using liver samples and the results showed an increase in lipid peroxidation in the muscle and liver of animals kept at 37 °C in relation to other temperatures. Enzyme responses were tissue-specific in the liver and muscle. In the liver, the reduction of CAT, SOD and GPx levels of the animals was observed at 37 °C compared to those maintained at Tenv and SOD and GPx in relation to animals maintained at 31 and 34 °C. The GPx enzyme showed higher activity at 34 and 37 °C compared to the other evaluated temperatures. At 37 °C, plasma levels of ALT and AST were higher than the other temperatures evaluated, as well as an increase in histopathological damage. In this way, in a scenario of warming of the waters of the Amazon or even of the systems used for rearing of the species, the tambaqui will be able to cope with temperatures of up to 34 °C, without affecting its antioxidant capacity. However, at 37 °C, oxidative stress levels and increased liver damage suggest a reduction in antioxidant capacity due to tissue impairment of the organ and general loss of animal performance as it approaches the upper thermal limit of the species.

Cumulative Heat Stress in Fluctuating Temperatures and Implications for the Distribution of Freshwater Fish

Predicting how rising temperatures will impact different species and communities is imperative and increasingly urgent with ongoing global warming. Here, we describe how thermal-death time curves obtained in the laboratory can be combined with an envelope model to predict the mortality of freshwater fish under field conditions and their distribution limits. We analyze the heat tolerance and distribution of 22 fish species distributed across North America and demonstrate that high temperatures imposed a distribution boundary for 11 of them, employing a null model. Importantly, predicted thermal boundaries closely match the warmest suitable locality of the envelope model. Simulated warming suggests that the distribution of fish species with lower heat tolerances will be disproportionately affected by rising temperatures, and the rate of local extinctions will be higher across fish communities in warmer localities. Ultimately, our analyses illustrate how physiological information can be combined with distribution models to forecast how warming temperatures are expected to impact different species and ecological communities.

SNP Polymorphisms Are Associated with Environmental Factors in Sockeye Salmon Populations Across the Northwest Pacific: Insights from Redundancy Analysis

The SNP variation in sockeye salmon across the Asian part of its range was studied in 23 samples from 16 lake-river systems of the West Pacific Coast to improve understanding of genetic adaptation in response to spawning watersheds conditions. Identification of candidate SNPs and environmental factors that can contribute to local adaptations in sockeye salmon populations was carried out using redundancy analysis (RDA), a powerful tool for landscape genetics proven to be effective in genotype-environment association studies. Climatic and hydrographic indices (7 indices in total), reflecting abiotic conditions in freshwater habitats of sockeye salmon and characterizing the temperature regime in the river basin, its variability during the year, the amount of precipitation, as well as the height of the maximum tide in the estuary, were used as predictor factors. Among the 45 analyzed SNPs, several loci (ALDOB-135, HGFA, and RAG3-93) correlated with predictors gradients along the northwest Pacific coast were identified. The putative candidate loci localized in genes involved in the immune and inflammatory responses, as well as genes encoding temperature-sensitive enzymes and some hormones regulating ion homeostasis in fish during the anadromous migration and smoltification, were potentially associated with environmental conditions in natal rivers. The findings could have implications for aquaculture, conservation, and resource management in the context of global climate change.

The Impact of Acclimation on Standard and Maximum Metabolic Rate in a Small Freshwater Fish

AbstractThe ability of freshwater fish to acclimate quickly to water temperature variation is imperative when living in shallow changeable environments. However, while it has often been assumed that maximum metabolic rate is constant and therefore that metabolic scope (the difference between maximum and standard metabolic rates) decreases with ambient temperature, this assumption is weakly supported and remains controversial. We investigated acclimation in a temperate, shallow-dwelling Australian freshwater fish, the Pacific blue-eye (Pseudomugil signifer), to rising water temperatures. We placed wild-caught fish into three acclimation treatments (24°C, 28°C, and 30°C) and measured metabolic rate at three test temperatures (24°C, 28°C, and 30°C). We found that fish acclimated (recovered standard metabolic rate) to housing temperatures before the first measurement at 10 d. Moreover, we found that regardless of acclimation temperature, standard metabolic rate, maximum metabolic rate, and aerobic scope all increased with test temperature. Our findings suggest that maximum metabolic rate and metabolic scope can adjust rapidly to ambient temperature. More research is needed to understand the generality of these effects, as well as their consequences for fitness.

Hyperoxia does not improve the acute upper thermal tolerance of a tropical marine fish (Lutjanus apodus)

Fish can experience hyperoxia in shallow environments due to photosynthetic activity and this has been suggested to provide them with a metabolic refuge during acute warming. However, this hypothesis has never been tested on a tropical marine species. Thus, we fitted 29°C-acclimated wild schoolmaster snapper (Lutjanus apodus; a species known to experience diel hyperoxia in mangrove creeks and coastal waters) with Transonic® flow probes and exposed them to an acute increase in temperature (at 1°C h-1) in respirometers under normoxia and hyperoxia (150% air saturation), until their critical thermal maximum (CTmax). The CTmax of both groups was ∼39°C, and no differences in maximum cardiac function were recorded as the fish were warmed. However, temperature-induced factorial aerobic scope was significantly greater in fish tested under hyperoxia. These data suggest that hyperoxia will not protect coastal tropical fish species during marine heat waves, despite its effects on metabolic scope/capacity.

Bivalves under extreme weather events: A comparative study of five economically important species in the South China sea during marine heatwaves

Marine heatwaves (MHWs) are increasing in frequency and intensity, threatening marine organisms and ecosystems they support. Yet, little is known about impacts of intensifying MHWs on ecologically and economically important bivalves cultured in the South China Sea. Here, we compared survival and physiological responses of five bivalve species, Pinctada fucata, Crassostrea angulata, Perna viridis, Argopecten irradians and Paphia undulata, to two consecutive MHWs events (3 days of thermal exposure to + 4 °C or + 8 °C, following 3 days of recovery under ambient conditions). While P. fucata, P. viridis, and P. undulata are native to the South China Sea region, C. angulata and A. irradians are not. Individuals of P. fucata, C. angulata and P. viridis had higher stress tolerance to MHWs than A. irradians and P. undulata, the latter already experiencing 100% mortality under +8 °C conditions during the first event. With increasing intensity of MHWs, standard metabolic rates of all five species increased significantly, in line with significant depressions of function-related energy-metabolizing enzymes (CMA, NKA, and T-ATP). Likewise, activities of antioxidant enzymes (SOD, CAT, and MDA) and shell mineralization-related enzymes (AKP and ACP) responded significantly to MHWs, despite species-specific performances observed. These findings demonstrate that some bivalve species can likely fail to accommodate intensifying MHWs events in the South China Sea, but some may persist. If this is the case, then one would expect substantial loss of fitness in bivalve aquaculture in the South China Sea under intensifying MHWs conditions.

Realised Thermal Niches in Marine Ectotherms Are Shaped by Ontogeny and Trophic Interactions

Understanding the response of marine organisms to temperature is crucial for predicting climate change impacts. Fundamental physiological thermal performance curves (TPCs), determined under controlled conditions, are commonly used to project future species spatial distributions or physiological performances. Yet, real-world performances may deviate due to extrinsic factors covarying with temperature (food, oxygen, etc.). Using a bioenergetic marine ecosystem model, we evaluate the differences between fundamental and realised TPCs for fish species with contrasted ecology and thermal preferences. Food limitation is the primary cause of differences, decreasing throughout ontogeny and across trophic levels due to spatio-temporal variability of low-trophic level prey availability with temperature. Deoxygenation has moderate impact, despite increasing during ontogeny. This highlights the lower sensitivity of early life stages to hypoxia, which is mechanistically explained by lower mass-specific ingestion at older stages. Understanding the emergence of realised thermal niches offers crucial insights to better determine population's persistence under climate warming.

The heat is on: sensitivity of goldsinny wrasse to global climate change

Unsustainable harvesting practices have drastically reduced fish populations globally and developments in aquaculture have increased. Unexpectedly, Atlantic salmon farming caused the opening of a new fishery in northern European countries, where previously unharvested mesopredatory species, like the goldsinny wrasse (Ctenolabrus rupestris), are captured for use as cleaner fish in pens along the coast and fjords. The goldsinny wrasse is widespread in coastal areas where it plays an ecologically important role as a predator of small invertebrates. Since climate change effects are particularly pronounced in coastal waters, it becomes urgent to understand how fish like the goldsinny will respond to global climate change, including the increasing frequency and intensity of marine heatwaves (MHWs), ocean freshening (OF) and ocean acidification (OA). To address this, we conducted a multi-stressor experiment exposing adult goldsinny to each stressor individually, as well as to all three combined. The results indicated that the goldsinny is highly affected by MHWs and extremely sensitive to a multi-stressor environment, with 34% and 53% mortality, respectively. Additionally, exposure to a MHW event, OF and multi-stressor conditions affected fish metabolism, with the highest standard metabolic- and maximum metabolic-oxygen consumption rates observed for the MHW treatment. Increases in oxidized glutathione (GSSG) and percent oxidized glutathione (% GSSG) in the livers, indicative of oxidative stress, were also seen in the MHW, OF and multi-stressor treatments. As a single stressor, OA showed no significant impacts on the measured parameters. This information is important for conservation of coastal marine environments, given the species' important role in shallow-water habitats and for management of goldsinny or other mesopredatory fish harvested in coastal ecosystems. The sensitivity of the goldsinny wrasse to future stressors is of concern, and any potential reductions in abundance as a result of climate change may lead to cascade effects with ecosystem-wide consequences.

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.

Effects of temperature on fish aggression and the combined impact of temperature and turbidity on thermal tolerance

Deforestation can increase light penetration and runoff entering adjacent freshwaters leading to increased average water temperature, stronger diel temperature fluctuations, and increased water turbidity. Changes in temperature extremes (particularly upper peaks) are important for fishes as their body temperature and rate of oxygen consumption varies with environmental temperature. Here, we compare effects of diel-fluctuating versus stable water temperature regimes on the behaviour and upper thermal tolerance (measured as Critical Thermal Maximum, CTmax) of the Bluntnose Minnow, Pimephales notatus. Fish were acclimated to either a static 18°C, static 24°C or a diel-fluctuating treatment of low to high (18-24°C) for a total of 10 weeks. Activity level and aggression were measured for 6 consecutive weeks during the acclimation period. Activity level remained high across treatments and over time. However, fish from the diel-fluctuating treatment exhibited a significant increase in aggression over the day as temperatures increased from 18°C to 24°C. Following acclimation, upper thermal limits of fish from each treatment were measured under two conditions: clear water (<2 NTU) and turbid water (25 NTU). This was to evaluate effects of acute turbidity exposure that might arise with heavy rain on deforested streams. CTmax was lowest in fish acclimated to static 18°C and highest in fish acclimated to static 24°C; fish acclimated to diel 18-24°C showed an intermediate CTmax. Exposure to acute turbidity during CTmax trials significantly lowered CTmax across all treatments, highlighting the importance of multiple-stressor studies in evaluating upper thermal tolerance of fishes.

Extreme Temperatures Reduce Copepod Performance and Change the Relative Abundance of Internal Microbiota

Copepods are one of the most abundant invertebrate groups in the seas and oceans and are a significant food source for marine animals. Copepods are also particularly sensitive to elevated temperatures. However, it is relatively unknown how the internal microbiome influences copepod susceptibility to warming. We addressed this fundamental knowledge gap by assessing key life history traits (survival, development, and reproduction) and changes in the internal microbiome in the tropical calanoid copepod Acartia sp. in response to warming (26°C, 30°C, and 34°C). Copepod microbiomes were analyzed using high throughput DNA sequencing of V1-V9 of 16S rRNA hypervariable regions. Copepod performance was better at 30°C than at 26°C, as indicated by faster development, a higher growth rate, and fecundity. However, these parameters strongly decreased at 34°C. We recorded 1,262,987 amplicon sequence reads, corresponding to 392 total operational taxonomic units (OTUs) at 97% similarity. Warming did not affect OTU numbers and the biodiversity indices, but it substantially changed the relative abundance of three major phyla: Proteobacteria, Actinobacteria, and Bacteroidota. The thermophilic and opportunistic Proteobacteria and Bacteroidota increased under extreme temperatures (34°C) while Actinobacteria abundance was strongly reduced. Changes in the relative abundance of these bacteria might be related to reduced copepod growth, survival, and reproduction under extreme temperatures. Profiling the functional role of all internal bacterial groups in response to the temperature change will fundamentally advance our mechanistic understanding of the performance of tropical copepods and, more generally, marine invertebrates to a warming climate.

Within the optimal thermal range, temperature fluctuations with similar means have little effect on offspring phenotypes: A comparison of two approaches that simulate natural nest conditions

Temperature influences nearly every aspect of organismal function. Because aspects of global change such as urbanization and climate change influence temperature, researchers must consider how altering thermal regimes will impact biodiversity across the planet. To do so, they often measure temperature in natural and/or human-modified habitats, replicate those temperatures in laboratory experiments to understand organismal responses, and make predictions under models of future change. Consequently, accurately representing temperature in the laboratory is an important concern, yet few studies have assessed the consequences of simulating thermal conditions in different ways. We used nest temperatures for two urban-dwelling, invasive lizards (Anolis sagrei and A. cristatellus) to create two egg incubation treatments in the laboratory. Like most studies of thermal developmental plasticity, we created daily repeating thermal fluctuations; however, we used different methods to create temperature treatments that differed in the magnitude and breadth of thermal cycles, and then evaluated the effects of these different approaches on embryo development and hatchling phenotypes. Additionally, we measured embryo heart rate, a proxy for metabolism, across temperature to understand the immediate effects of treatments. We found that treatments had minimal effect on phenotypes likely because temperatures were within the optimal thermal range for each species and were similar in mean temperature. We conclude that slight differences in thermal treatments may be unimportant so long as temperatures are within a range appropriate for development, and we make several recommendations for future studies of developmental plasticity.

Hyperoxia disproportionally benefits the aerobic performance of large fish at elevated temperature

Increasing evidence shows that larger fish are more vulnerable to acute warming than smaller individuals of the same species. This size-dependency of thermal tolerance has been ascribed to differences in aerobic performance, largely owing to a decline in oxygen supply relative to demand. To shed light on these ideas, we examined metabolic allometry in 130 rainbow trout ranging from 12 to 358 g under control conditions (17°C) and in response to acute heating (to 25°C), with and without supplemental oxygen (100% versus 150% air saturation). Under normoxia, high temperature caused an average 17% reduction in aerobic scope compared with 17°C. Aerobic performance disproportionally deteriorated in bigger fish as the scaling exponent (b) for aerobic scope declined from b=0.87 at 17°C to b=0.74 at 25°C. Hyperoxia increased maximum metabolic rate and aerobic scope at both temperatures and disproportionally benefited larger fish at 25°C as the scaling exponent for aerobic scope was reestablished to the same level as at 17°C (b=0.86). This suggests that hyperoxia may provide metabolic refuge for larger individuals, allowing them to sustain aerobic activities when facing acute warming. Notably, the elevated aerobic capacity afforded by hyperoxia did not appear to improve thermal resilience, as mortality in 25°C hyperoxia (13.8%, n=4) was similar to that in normoxia (12.1%, n=4), although we caution that this topic warrants more targeted research. We highlight the need for mechanistic investigations of the oxygen transport system to determine the consequences of differential metabolic scaling across temperature in a climate warming context.

Temperature-dependent responses and trophic interaction strengths of a predatory marine gastropod and rock oyster under ocean warming

Predator-prey interactions are important in shaping ecosystem structure. Consequently, impacts of accelerating global warming on predators will have notable implications. Effects are likely to be particularly marked for tropical organisms which are anticipated to be sensitive to further thermal stress. Here, we investigated effects of future ocean warming on the predatory dogwhelk Reishia clavigera and its predation of Saccostrea cucullata. Mortality of the predators rapidly increased under the extreme elevated temperature, while those exposed to moderate elevated temperature displayed similar mortality as the ambient. Predators that survived moderate temperature increases altered their oxygen consumption patterns, increased average feeding rates, and functional responses, although condition index and energy reserves were unchanged. Overall, we show extreme ocean warming scenarios can remove predators and their consumption of prey from an ecosystem, whereas moderate warming can intensify predator-prey interactions. Such temperature-dependent alterations to predator-prey interactions would lead to fundamental changes of ecosystem structure as the ocean warms.

In Hot Water: Current Thermal Threshold Methods Unlikely to Predict Invasive Species Shifts in NW Atlantic

As global temperatures continue to rise, accurate predicted species distribution models will be important for forecasting the movement of range-shifting species. These predictions rely on measurements of organismal thermal tolerance, which can be measured using classical threshold concepts such as Arrhenius break temperatures and critical thermal temperatures, or through ecologically relevant measurements such as the temperature at which reproduction and growth occur. Many species, including invasive species, exhibit thermal plasticity, so these thresholds may change based on ambient temperature, life stage, and measurement techniques. Here, we review thermal thresholds for 15 invertebrate species invasive to the Gulf of Maine. The high degree of variability within a species and between applied conceptual frameworks suggests that modeling the future distribution of these species in all ecosystems, but especially in the rapidly warming northwest Atlantic and Gulf of Maine, will be challenging. While each of these measurement techniques is valid, we suggest contextualization and integration of threshold measurements for accurate modeling.

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

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

Evolution and development of Drosophila melanogaster under different thermal conditions affected cell sizes and sensitivity to paralyzing hypoxia

Environmental gradients cause evolutionary and developmental changes in the cellular composition of organisms, but the physiological consequences of these effects are not well understood. Here, we studied experimental populations of Drosophila melanogaster that had evolved in one of three selective regimes: constant 16 °C, constant 25 °C, or intergenerational shifts between 16 °C and 25 °C. Genotypes from each population were reared at three developmental temperatures (16 °C, 20.5 °C, and 25 °C). As adults, we measured thorax length and cell sizes in the Malpighian tubules and wing epithelia of flies from each combination of evolutionary and developmental temperatures. We also exposed flies from these treatments to a short period of nearly complete oxygen deprivation to measure hypoxia tolerance. For genotypes from any selective regime, development at a higher temperature resulted in smaller flies with smaller cells, regardless of the tissue. At every developmental temperature, genotypes from the warm selective regime had smaller bodies and smaller wing cells but had larger tubule cells than did genotypes from the cold selective regime. Genotypes from the fluctuating selective regime were similar in size to those from the cold selective regime, but their cells of either tissue were the smallest among the three regimes. Evolutionary and developmental treatments interactively affected a fly's sensitivity to short-term paralyzing hypoxia. Genotypes from the cold selective regime were less sensitive to hypoxia after developing at a higher temperature. Genotypes from the other selective regimes were more sensitive to hypoxia after developing at a higher temperature. Our results show that thermal conditions can trigger evolutionary and developmental shifts in cell size, coupled with changes in body size and hypoxia tolerance. These patterns suggest links between the cellular composition of the body, levels of hypoxia within cells, and the energetic cost of tissue maintenance. However, the patterns can be only partially explained by existing theories about the role of cell size in tissue oxygenation and metabolic performance.

The microbiome at the interface between environmental stress and animal health: an example from the most threatened vertebrate group

Nitrate pollution and global warming are ubiquitous stressors likely to interact and affect the health and survival of wildlife, particularly aquatic ectotherms. Animal health is largely influenced by its microbiome (commensal/symbiotic microorganisms), which responds to such stressors. We used a crossed experimental design including three nitrate levels and five temperature regimes to investigate their interactive and individual effects on an aquatic ectotherm, the European common frog. We associated health biomarkers in larvae with changes in gut bacteria diversity and composition. Larvae experienced higher stress levels and lower body condition under high temperatures and nitrate exposure. Developmental rate increased with temperature but decreased with nitrate pollution. Alterations in bacteria composition but not diversity are likely to correlate with the observed outcomes in larvae health. Leucine degradation decreased at higher temperatures corroborating accelerated development, nitrate degradation increased with nitrate level corroborating reduced body condition and an increase in lysine biosynthesis may have helped larvae deal with the combined effects of both stressors. These results reinforce the importance of associating traditional health biomarkers with underlying microbiome changes. Therefore, we urge studies to investigate the effects of environmental stressors on microbiome composition and consequences for host health in a world threatened by biodiversity loss.

The effects of marine heatwaves on a coral reef snapper: insights into aerobic and anaerobic physiology and recovery

Marine heatwaves (MHWs) are increasing in frequency and intensity. Coral reefs are particularly susceptible to MHWs, which cause mass coral bleaching and mortality. However, little is known about how MHWs affect coral reef fishes. Here, we investigated how MHWs affect the physiology of a coral reef mesopredator, Lutjanus carponotatus. Specifically, we exposed mature adults to two different MHW intensities, +1°C (29.5°C) and + 2°C (30.5°C) and measured physiological performance at 2 and 4 weeks of exposure and at 2 weeks post-exposure. At these time points, we measured oxygen consumption at rest and after a simulated fishing capture event, recovery time, excess post-exercise oxygen consumption (EPOC) and associated biochemical markers in the blood (baseline lactate, post-capture lactate, glucose, haemoglobin levels and haematocrit proportion). We found that 2 weeks of exposure to MHW conditions increased resting oxygen consumption (+1°C = 23%, +2°C = 37%), recovery time (+1°C = 62%, +2°C = 77%), EPOC (+1°C = 50%, +2°C = 68%), baseline lactate (+1°C = 27%, +2°C = 28%), post-capture lactate (+1°C = 62%, +2°C = 109%) and haemoglobin levels (+1°C = 13%, +2°C = 28%). This pattern was maintained at 4 weeks of exposure except for post-capture lactate which was reduced (+1°C = -37%, +2°C = 27%). In combination, these results suggest a greater reliance on anaerobic glycolysis to maintain homeostasis in MHW conditions. At 2 weeks post-exposure, when compared to control fish, we found that capture oxygen consumption was increased (+1°C = 25%, +2°C = 26%), recovery rate was increased (+2°C = 38%) and haemoglobin was still higher (+1°C = 15%, +2°C = 21%). These results show that MHW conditions have direct physiological demands on adult coral reef snapper and ecologically relevant residual effects can last for at least 2 weeks post-MHW; however, individuals appear to recover from the negative effects experienced during the MHW. This provides new insight into the effects of MHWs on the physiological performance of coral reef fishes.

Regional thermal variation in a coral reef fish

How species respond to climate change will depend on the collective response of populations. Intraspecific variation in traits, evolved through genetic adaptation and phenotypic plasticity, can cause thermal performance curves to vary over species' distributions. Intraspecific variation within marine species has received relatively little attention due to the belief that marine systems lack dispersal barriers strong enough to promote locally adapted traits. Here we show that intraspecific variation is present between low- and high-latitude populations of a coral reef damselfish (Acanthochromis polyacanthus). Co-gradient variation was observed when examining aerobic physiology across a thermal gradient that reflected mean summer temperatures of high- and low-latitude regions, as well as projected future ocean temperatures (i.e. 27, 28.5, 30, 31.5°C). Whilst thermally sensitive, no significant differences were observed between high- and low-latitude regions when measuring immunocompetence, haematocrit and anaerobic enzyme activity. The presence of co-gradient variation suggests that dispersal limitations in marine systems can promote local adaptive responses; however, intraspecific variation may not be ubiquitous amongst traits. Identifying locally adapted traits amongst populations remains necessary to accurately project species responses to climate change and identify differences in adaptive potential.

Warm and thermally variable incubation conditions reduce embryonic performance and carry over to influence hatchling tradeoffs

Animals' thermal sensitivities have long been characterized by thermal performance curves (TPCs) or reaction norms, and TPCs may predict animals' responses to climate change. Typically, TPCs are parameterized by measuring performance at a range of constant temperatures. Yet, animals encounter a range of thermal environments, and temperature variability is an aspect of climate change that may affect animals more than gradual warming. Daily temperature variability is particularly important for eggs in most taxa because they are highly sensitive to temperature and cannot behaviorally avoid stressful temperatures. Thus, the legacy of thermal conditions experienced during incubation may carryover to subsequent life stages. Here, I factorially manipulated mean temperature (20, 25, or 30 °C) and daily temperature range (DTR; ±0, 5, or 10 °C) during incubation for eggs of the variable field cricket (Gryllus lineaticeps) to integrate the role of DTR into the established paradigm of TPCs. Low DTR (±5 °C) was not generally costly, and it even improved hatchling starvation resistance (sensu hormesis). However, high DTR (±10 °C) reduced and delayed hatching at a warm mean temperature (30 °C). The effects of high DTR carried over to accelerate hatchling development at an expense to hatchling starvation resistance-therefore, thermal conditions during incubation can shape tradeoffs among important traits related to life history and stress tolerance later in life. In sum, animals may exhibit complex responses to their increasingly warmer, more thermally variable environments.

Relatedness of hypoxia and hyperthermia tolerances in the Nile tilapia (Oreochromis niloticus) and their relationships with cardiac and gill traits

In fish, thermal and hypoxia tolerances may be functionally related, as suggested by the oxygen- and capacity-limited thermal tolerance (OCLTT) concept, which explains performance failure at high temperatures due to limitations in oxygen delivery. In this study the interrelatedness of hyperthermia and hypoxia tolerances in the Nile tilapia (Oreochromis niloticus), and their links to cardiorespiratory traits were examined. Different groups of O. niloticus (n = 51) were subjected to hypoxia and hyperthermia challenges and the O2 tension for aquatic surface respiration (ASR pO2) and critical thermal maximum (CTmax) were assessed as measurement endpoints. Gill filament length, total filament number, ventricle mass, length and width were also measured. Tolerance to hypoxia, as evidenced by ASR pO2 thresholds of the individual fish, was highly variable and varied between 0.26 and 3.39 kPa. ASR events increased more profoundly as O2 tensions decreased below 2 kPa. The CTmax values recorded for the O. niloticus individuals ranged from 43.1 to 44.8 °C (Mean: 44.2 ± 0.4 °C). Remarkably, there was a highly significant correlation between ASR pO2 and CTmax in O. niloticus (r = -0.76, p < 0.0001) with ASR pO2 increasing linearly with decreasing CTmax. There were, however, no discernible relationships between the measured cardiorespiratory properties and hypoxia or hyperthermia tolerances. The strong relationship between hypoxia and hyperthermia tolerances in this study may be related to the ability of the cardiorespiratory system to provide oxygen to respiring tissues under thermal stress, and thus provides some support for the OCLTT concept in this species, at least at the level of the entire organism.

The hard life of an octopus embryo is seen through gene expression, energy metabolism, and its ability to neutralize radical oxygen species

The reproductive process in Octopus maya was analyzed to establish the amount of reactive oxygen species that the embryos inherit from females, during yolk synthesis. At the same time, respiratory metabolism, ROS production, and the expression of some genes of the antioxidant system were monitored to understand the ability of embryos to neutralize maternal ROS and those produced during development. The results indicate that carbonylated proteins and peroxidized lipids (LPO) were transferred from females to the embryos, presumably derived from the metabolic processes carried out during yolk synthesis in the ovary. Along with ROS, females also transferred to embryos glutathione (GSH), a key element of the antioxidant defense system, thus facilitating the neutralization of inherited ROS and those produced during development. Embryos are capable of neutralizing ROS thanks to the early expression of genes such as catalase (CAT) and superoxide dismutase (SOD), which give rise to the synthesis of enzymes when the circulatory system is activated. Also, it was observed that the levels of the routine metabolic rate of embryos are almost as high as those of the maximum activity metabolism, which leads, on the one hand, to the elevated production of ROS and suggests that, at this stage of the life cycle in octopuses, energy production is maximum and is physically limited by the biological properties inherent to the structure of embryonic life (oxygen transfer through the chorion, gill surface, pumping capacity, etc.). Due to its role in regulating vascularization, a high expression of HIf-1A during organogenesis suggests that circulatory system development has begun in this phase of embryo development. The results indicate that the routine metabolic rate and the ability of O. maya embryos to neutralize the ROS are probably the maximum possible. Under such circumstances, embryos cannot generate more energy to combat the free radicals produced by their metabolism, even when environmental factors such as high temperatures or contaminants could demand excess energy.

Large potential impacts of marine heatwaves on ecosystem functioning

Ocean warming is driving significant changes in the structure and functioning of marine ecosystems, shifting species' biogeography and phenology, changing body size and biomass and altering the trophodynamics of the system. Particularly, extreme temperature events such as marine heatwaves (MHWs) have been increasing in intensity, duration and frequency. MHWs are causing large-scale impacts on marine ecosystems, such as coral bleaching, mass mortality of seagrass meadows and declines in fish stocks and other marine organisms in recent decades. In this study, we developed and applied a dynamic version of the EcoTroph trophodynamic modelling approach to study the cascading effects of individual MHW on marine ecosystem functioning. We simulated theoretical user-controlled ecosystems and explored the consequences of various assumptions of marine species mortality along the food web, associated with different MHW intensities. We show that an MHW can lead to a significant biomass reduction of all consumers, with the severity of the declines being dependent on species trophic levels (TLs) and biomes, in addition to the characteristics of MHWs. Biomass of higher TLs declines more than lower TLs under an MHW, leading to changes in ecosystem structure. While tropical ecosystems are projected to be sensitive to low-intensity MHWs, polar and temperate ecosystems are expected to be impacted by more intense MHWs. The estimated time to recover from MHW impacts is twice as long for polar ecosystems and one-third longer for temperate biomes compared with tropical biomes. This study highlights the importance of considering extreme weather events in assessing the effects of climate change on the structures and functions of marine ecosystems.

Accelerometer-based swimming metabolism of smallmouth bass (Micropterus dolomieu)

Bioenergetics is informative for a range of fundamental and applied resource management questions, but findings are often constrained by a lack of ecological realism due to the challenges of remotely estimating key parameters such as metabolic rate. To enable field applications, we conducted a calibration study with smallmouth bass (Micropterus dolomieu, 0.7-2 kg) surgically implanted with accelerometer transmitters and exposed to a ramp-Ucrit swimming protocol in a swim tunnel respirometer across a range of water temperatures (6, 12, 18, and 24°C). There was an exponential increase in fish acceleration with swimming speed, and acceleration per speed was higher in smaller fish and female fish, and at colder temperatures. Mass-specific fish metabolic rate (MO2; mg O2 kg-1 h-1) increased with swimming speed, acceleration, and temperature, and decreased with fish mass, which when combined were strong predictors of MO2. Maximum metabolic rate (MMR) was estimated to peak at 22°C, but maximum sustained swimming speed (Ucrit) remained high at c. 90-100 m s-1 above 20°C, based on second-order polynomial functions. Aerobic scope (AS) estimates peaked at 20°C (>90% AS at 17-24°C; >50% AS at 11-28°C). Males exhibited marginally higher MMR, AS, and Ucrit than females at higher temperatures. Larger fish generally exhibited higher Ucrit, but smaller fish had a marginally broader performance range (AS, Ucrit) among temperatures, benefiting from higher MMR despite a steeper increase in resting metabolic rate with temperature. These findings enable field studies to estimate metabolic metrics of smallmouth bass in situ to characterize their ecological energetics and inform bioenergetics models.

Can upwelling regions be potential thermal refugia for marine fishes during climate warming?

Species are expected to migrate to higher latitudes as warming intensifies due to anthropogenic climate change since physiological mechanisms have been adapted to maximize fitness under specific temperatures. However, literature suggests that upwellings could act as thermal refugia under climate warming protecting marine ecosystem diversity. This research aimed to predict the effects of climate warming on commercial and non-commercial fish species reported in official Mexican documents (>200 species) based on their thermal niche to observe if upwellings can act as potential thermal refugia. Present (2000-2014) and Representative Concentration Pathway (6.0 and 8.5) scenarios (2040-2050 and 2090-2100) have been considered for this work. Current and future suitability patterns, species distribution, richness, and turnover were calculated using the minimum volume ellipsoids as algorithm. The results in this study highlight that beyond migration to higher latitudes, upwelling regions could protect marine fishes, although the mechanism differed between the innate characteristics of upwellings. Most modeled species (primarily tropical fishes) found refuge in the tropical upwelling in Northern Yucatan. However, the highest warming scenario overwhelmed this region. In contrast, the Baja California region lies within the Eastern Boundary Upwelling Systems. While the area experiences an increase in suitability, the northern regions have a higher upwelling intensity acting as environmental barriers for many tropical species. Conversely, in the southern regions where upwelling is weaker, species tend to congregate and persist even during elevated warming, according to the turnover analysis. These findings suggest that tropicalization in higher latitudes may not be as straightforward as previously assumed. Nevertheless, climate change affects numerous ecosystem features, such as trophic relationships, phenology, and other environmental variables not considered here. In addition, uncertainty still exists about the assumption of increasing intensity of upwelling systems.

Oxygen consumption rate during recovery from loss of equilibrium induced by warming, hypoxia, or exhaustive exercise in rainbow darter (Etheostoma caeruleum)

Animals routinely encounter environmental (e.g., high temperatures and hypoxia) as well as physiological perturbations (e.g., exercise and digestion) that may threaten homeostasis. However, comparing the relative threat or "disruptiveness" imposed by different stressors is difficult, as stressors vary in their mechanisms, effects, and timescales. We exploited the fact that several acute stressors can induce the loss of equilibrium (LOE) in fish to (i) compare the metabolic recovery profiles of three environmentally relevant stressors and (ii) test the concept that LOE could be used as a physiological calibration for the intensity of different stressors. We focused on Etheostoma caeruleum, a species that routinely copes with environmental fluctuations in temperature and oxygen and that relies on burst swimming to relocate and avoid predators, as our model. Using stop-flow (intermittent) respirometry, we tracked the oxygen consumption rate (MO2) as E. caeruleum recovered from LOE induced by hypoxia (PO2 at LOE), warming (critical thermal maximum, CTmax), or exhaustive exercise. Regardless of the stressor used, E. caeruleum recovered rapidly, returning to routine MO2 within ~3 h. Fish recovering from hypoxia and warming had similar maximum MO2, aerobic scopes, recovery time, and total excess post-hypoxia or post-warming oxygen consumption. Though exhaustive exercise induced a greater maximum MO2 and corresponding higher aerobic scope than warming or hypoxia, its recovery profile was otherwise similar to the other stressors, suggesting that "calibration" to a physiological state such as LOE may be a viable conceptual approach for investigators interested in questions related to multiple stressors, cross tolerance, and how animals cope with challenges to homeostasis.

Seasonal energy investment and metabolic patterns in a farmed fish

The present research focuses on the seasonal changes in the energy content and metabolic patterns of red porgy (Pagrus pagrus) sampled in a fish farm in North Evoikos Gulf (Greece). The study was designed in an effort to evaluate the influence of seasonality in several physiological feauteres of high commercial importance that may affect feed intake and growth. We determined glycogen, lipids and proteins levels, and cellular energy allocation (CEA) as a valuable marker of exposure to stress, which integrates available energy (Ea) and energy consumption (Ec). Metabolic patterns and aerobic oxidation potential were based on the determination of glucose transporter (GLU), carnitine transporter (CTP), L-lactate dehydrogenase (L-LDH), citrate synthase (CS), cytochrome C oxidase subunit IV isoform 1 (COX1) and 3-hydroxyacyl CoA dehydrogenase (HOAD) relative gene expression. To integrate metabolic patterns and gene expression, L-LDH, CS, COX and HOAD activities were also determined. For further estimation of biological stores oxidized during seasonal acclimatization, we determined the blood levels of glucose, lipids and lactate. The results indicated seasonal changes in energy content, different patterns in gene expression and reorganization of metabolic patterns during cool acclimatization with increased lipid oxidation. During warm acclimatization, however, energy consumption was mostly based on carbohydrates oxidation. The decrease of Ec and COX1 activity in the warm exposed heart seem to be consistent with the OCLTT hypothesis, suggesting that the heart may be one of the first organs to be limited during seasonal warming. Overall, this study has profiled changes in energetics and metabolic patterns occurring at annual temperatures at which P. pagrus is currently farmed, suggesting that this species is living at the upper edge of their thermal window, at least during summer.

A decade of submersible observations revealed temporal trends in elasmobranchs in a remote island of the Eastern Tropical Pacific Ocean

No-take marine protected areas (MPAs) can mitigate the effects of overfishing, climate change and habitat degradation, which are leading causes of an unprecedented global biodiversity crisis. However, assessing the effectiveness of MPAs, especially in remote oceanic islands, can be logistically challenging and often restricted to relatively shallow and accessible environments. Here, we used a long-term dataset (2010-2019) collected by the DeepSee submersible of the Undersea Hunter Group that operates in Isla del Coco National Park, Costa Rica, to (1) determine the frequency of occurrence of elasmobranch species at two depth intervals (50-100 m; 300-400 m), and (2) investigate temporal trends in the occurrence of common elasmobranch species between 2010 and 2019, as well as potential drivers of the observed changes. Overall, we observed 17 elasmobranch species, 15 of which were recorded on shallow dives (50-100 m) and 11 on deep dives (300-400 m). We found a decreasing trend in the probability of occurrence of Carcharhinus falciformis over time (2010-2019), while other species (e.g. Taeniurops meyeni, Sphyrna lewini, Carcharhinus galapagensis, Triaenodon obesus, and Galeocerdo cuvier) showed an increasing trend. Our study suggests that some species like S. lewini may be shifting their distributions towards deeper waters in response to ocean warming but may also be sensitive to low oxygen levels at greater depths. These findings highlight the need for regional 3D environmental information and long-term deepwater surveys to understand the extent of shark and ray population declines in the ETP and other regions, as most fishery-independent surveys from data-poor countries have been limited to relatively shallow waters.

Mitochondrial Protein-Coding Gene Expression in the Lizard Sphenomorphus incognitus (Squamata:Scincidae) Responding to Different Temperature Stresses

In the context of global warming, the frequency of severe weather occurrences, such as unexpected cold spells and heat waves, will grow, as well as the intensity of these natural disasters. Lizards, as a large group of reptiles, are ectothermic. Their body temperatures are predominantly regulated by their environment and temperature variations directly impact their behavior and physiological activities. Frequent cold periods and heat waves can affect their biochemistry and physiology, and often their ability to maintain their body temperature. Mitochondria, as the center of energy metabolism, are crucial for maintaining body temperature, regulating metabolic rate, and preventing cellular oxidative damage. Here, we used RT-qPCR technology to investigate the expression patterns and their differences for the 13 mitochondrial PCGs in Sphenomorphus incognitus (Squamata:Scincidae), also known as the brown forest skink, under extreme temperature stress at 4 °C, 8 °C, 34 °C, and 38 °C for 24 h, compared to the control group at 25 °C. In southern China, for lizards, 4 °C is close to lethal, and 8 °C induces hibernation, while 34/38 °C is considered hot and environmentally realistic. Results showed that at a low temperature of 4 °C for 24 h, transcript levels of ATP8, ND1, ND4, COI, and ND4L significantly decreased, to values of 0.52 ± 0.08, 0.65 ± 0.04, 0.68 ± 0.10, 0.28 ± 0.02, and 0.35 ± 0.02, respectively, compared with controls. By contrast, transcript levels of COIII exhibited a significant increase, with a mean value of 1.86 ± 0.21. However, exposure to 8 °C for 24 h did not lead to an increase in transcript levels. Indeed, transcript levels of ATP6, ATP8, ND1, ND3, and ND4 were significantly downregulated, to 0.48 ± 0.11, 0.68 ± 0.07, 0.41 ± 0.08, 0.54 ± 0.10, and 0.52 ± 0.07, respectively, as compared with controls. Exposure to a hot environment of 34 °C for 24 h led to an increase in transcript levels of COI, COII, COIII, ND3, ND5, CYTB, and ATP6, with values that were 3.3 ± 0.24, 2.0 ± 0.2, 2.70 ± 1.06, 1.57 ± 0,08, 1.47 ± 0.13, 1.39 ± 0.56, and 1.86 ± 0.12, respectively, over controls. By contrast, ND4L exhibited a significant decrease (to 0.31 ± 0.01) compared with controls. When exposed to 38 °C, the transcript levels of the 13 PCGs significantly increased, ranging from a 2.04 ± 0.23 increase in ND1 to a 6.30 ± 0.96 rise in ND6. Under two different levels of cold and heat stress, the expression patterns of mitochondrial genes in S. incognitus vary, possibly associated with different strategies employed by this species in response to low and high temperatures, allowing for rapid compensatory adjustments in mitochondrial electron transport chain proteins in response to temperature changes. Furthermore, this underscores once again the significant role of mitochondrial function in determining thermal plasticity in reptiles.

Acclimation to warming but not hypoxia alters thermal tolerance and metabolic sensitivity in an estuarine crustacean

Coastal species are challenged by multiple anthropogenic stressors. Plasticity may buffer the effects of environmental change, but investigation has largely been restricted to single-stressor performance. Multistressor studies have often been short-term and relatively less is known about the consequences of plasticity under one stressor for performance under another. Here, we aimed to test for the effects of thermal or hypoxic acclimation on thermal tolerance in the amphipod Gammarus chevreuxi. Animals were chronically exposed to raised temperature or hypoxia prior to determination of upper thermal limits and routine metabolic rate (RMR). Warm acclimation increased all metrics of thermal tolerance, but hypoxic acclimation had no effect. Different responses to the two stressors was also observed for the thermal sensitivity of RMR. Consequently, this species possesses the ability to increase thermal tolerance via plasticity in response to chronic warming but increasing duration of hypoxic episodes will not confer cross-tolerance to a warming environment.

Climate change consequences on the systemic heart of female Octopus maya: oxidative phosphorylation assessment and the antioxidant system

There is evidence that indicates that temperature modulates the reproduction of the tropical species Octopus maya, through the over- or under-expression of many genes in the brain. If the oxygen supply to the brain depends on the circulatory system, how temperature affects different tissues will begin in the heart, responsible for pumping the oxygen to tissues. The present study examines the impact of heat stress on the mitochondrial function of the systemic heart of adult O. maya. The mitochondrial metabolism and antioxidant defense system were measured in the systemic heart tissue of female organisms acclimated to different temperatures (24, 26, and 30°C). The results show that acclimation temperature affects respiratory State 3 and State 4o (oligomycin-induced) with higher values observed in females acclimated at 26°C. The antioxidant defense system is also affected by acclimation temperature with significant differences observed in superoxide dismutase, glutathione S-transferase activities, and glutathione levels. The results suggest that high temperatures (30°C) could exert physical limitations on the circulatory system through the heart pumping, affecting nutrient and oxygen transport to other tissues, including the brain, which exerts control over the reproductive system. The role of the cardiovascular system in supporting aerobic metabolism in octopus females is discussed.