It's about time: Linkages between heat tolerance, thermal acclimation and metabolic rate at different temporal scales in the freshwater amphipod Gammarus fossarum Koch, 1836

Influence of salinity on the thermal tolerance of aquatic organisms

Aquatic organisms are challenged by changes in their external environment, such as temperature and salinity fluctuations. If these variables interacted with each other, the response of organisms to temperature changes would be modified by salinity and vice versa. We tested for potential interaction between temperature and salinity effects on freshwater, brackish, and marine organisms, including algae, macrophytes, heterotrophic protists, parasites, invertebrates, and fish. We performed a meta-analysis that compared the thermal tolerance (characterised by the temperature optimum, lower and upper temperature limits, and thermal breadth) at various salinities. The meta-analysis was based on 90 articles (algae: 15; heterotrophic protists: 1; invertebrates: 43; and fish: 31). Studies on macrophytes and parasites were lacking. We found that decreasing salinity significantly increased and decreased the lower and upper temperature limits, respectively, in all groups. Thus, a lowered salinity increased the thermal sensitivity of organisms. These findings mainly reflect the response of brackish and marine organisms to salinity changes, which dominated our database. The few studies on freshwater species showed that their lower thermal limits increased and the upper thermal limits decreased with increasing salinity, albeit statistically nonsignificant. Although non-significant, the response of thermal tolerance to salinity changes differed between various organism groups. It generally decreased in the order of: algae > invertebrates > fish. Overall, our findings indicate adverse effects of salinity changes on the temperature tolerance of aquatic organisms. For freshwater species, studies are comparatively scarce and further studies on their thermal performance at various salinity gradients are required to obtain more robust evidence for interactions between salinity and temperature tolerance. Considering test conditions such as acclimation temperature and potential infection with parasites in future studies may decrease the variability in the relationship between salinity and thermal tolerance.

The seasonal response of metabolic rate to projected climate change scenarios in aquatic amphipods

The responses of organisms to climate change are mediated primarily by its impact on their metabolic rates, which, in turn, drive various biological and ecological processes. Although there have been numerous seminal studies on the sensitivity of metabolic rate to temperature, little is empirically known about how this rate responds to seasonal temperature ranges and beyond under conservative IPCC climate change scenarios. Here, we measured the SMR of the aquatic amphipod, Gammarus insensibilis, which served as our subject species, with body masses ranging from 0.20 to 7.74 mg ash free weight. We assessed the response of the SMR across nine temperature levels ranging from 12 to 30.2 °C. These temperatures match seasonal temperature norms, with an incremental increase of 0.6-1.2 °C above each seasonal baseline, as projected for the years 2040 and 2100 under the modest climate change scenarios. Overall, our findings showed that the effect of temperature on SMR varies with body mass, as indicated by a negative size-temperature interaction, with larger conspecifics exhibiting less sensitivity to temperature changes than smaller ones. From the cold to warm season, the SMR increased by an average of 14% °C-1, with increases of 18.4% °C-1 in smaller individuals and 11.4% °C-1 in larger ones. The SMR of smaller individuals peaked at a 0.6 °C increase from the current summer baseline (15.08% °C-1, Q10 = 4.2), while in larger ones it peaked with a 1.2 °C increase beyond autumn temperatures (14.9% °C-1, Q10 = 3.9). However, at temperatures reflecting global warming that exceed summer temperatures, the SMR of larger individuals levelled off, while that of smaller ones continued to increase. Overall, our findings suggest that smaller-sized individuals have a broader thermal window for SMR performance, while the SMR of larger-sized ones will become increasingly constrained at summer temperatures as those summer temperatures become hotter.

Predicting organismal response to marine heatwaves using dynamic thermal tolerance landscape models

Marine heatwaves (MHWs) can cause thermal stress in marine organisms, experienced as extreme 'pulses' against the gradual trend of anthropogenic warming. When thermal stress exceeds organismal capacity to maintain homeostasis, organism survival becomes time-limited and can result in mass mortality events. Current methods of detecting and categorizing MHWs rely on statistical analysis of historic climatology and do not consider biological effects as a basis of MHW severity. The re-emergence of ectotherm thermal tolerance landscape models provides a physiological framework for assessing the lethal effects of MHWs by accounting for both the magnitude and duration of extreme heat events. Here, we used a simulation approach to understand the effects of a suite of MHW profiles on organism survival probability across (1) three thermal tolerance adaptive strategies, (2) interannual temperature variation and (3) seasonal timing of MHWs. We identified survival isoclines across MHW magnitude and duration where acute (short duration-high magnitude) and chronic (long duration-low magnitude) events had equivalent lethal effects on marine organisms. While most research attention has focused on chronic MHW events, we show similar lethal effects can be experienced by more common but neglected acute marine heat spikes. Critically, a statistical definition of MHWs does not accurately categorize biological mortality. By letting organism responses define the extremeness of a MHW event, we can build a mechanistic understanding of MHW effects from a physiological basis. Organism responses can then be transferred across scales of ecological organization and better predict marine ecosystem shifts to MHWs.

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.

Cross-species gut microbiota transplantation predictably affects host heat tolerance

The gut microbiome is known to influence and have regulatory effects in diverse physiological functions of host animals, but only recently has the relationship between host thermal biology and gut microbiota been explored. Here, we examined how early-life manipulations of the gut microbiota in larval amphibians influenced their critical thermal maximum (CTmax) at different acclimation temperatures. We stripped the resident microbiome from egg masses of wild-caught wood frogs (Lithobates sylvaticus) via an antibiotic wash, and then inoculated the eggs with pond water (control), no inoculation, or the intestinal microbiota of another species that has a wider thermal tolerance - green frogs (Lithobates clamitans). We predicted that this cross-species transplant would increase the CTmax of the recipient wood frog larvae relative to the other treatments. In line with this prediction, green frog microbiome-recipient larvae had the highest CTmax while those with no inoculum had the lowest CTmax. Both the microbiome treatment and acclimation temperature significantly influenced the larval gut microbiota communities and α-diversity indices. Green frog microbiome-inoculated larvae were enriched in Rikenellaceae relative to the other treatments, which produce short-chain fatty acids and could contribute to greater energy availability and enhanced heat tolerance. Larvae that received no inoculation had a higher relative abundance of potentially pathogenic Aeromonas spp., which negatively affects host health and performance. Our results are the first to show that cross-species gut microbiota transplants alter heat tolerance in a predictable manner. This finding has repercussions for the conservation of species that are threatened by climate change and demonstrates a need to further explore the mechanisms by which the gut microbiota modulate host thermal tolerance.

Toxicokinetic-Toxicodynamic Model to Assess Thermal Stress

Temperature is a crucial environmental factor affecting the distribution and performance of ectothermic organisms. This study introduces a new temperature damage model to interpret their thermal stress. Inspired by the ecotoxicological damage model in the General Unified Threshold model for Survival (GUTS) framework, the temperature damage model assumes that damage depends on the balance between temperature-dependent accumulation and constant repair. Mortality due to temperature stress is driven by the damage level exceeding a threshold. Model calibration showed a good agreement with the measured survival of Gammarus pulex exposed to different constant temperatures. Further, model simulations, including constant temperatures, daily temperature fluctuations, and heatwaves, demonstrated the model's ability to predict temperature effects for various environmental scenarios. With this, the present study contributes to the mechanistic understanding of temperature as a single stressor while facilitating the incorporation of temperature as an additional stressor alongside chemicals in mechanistic multistressor effect models.

Long-term forecast of thermal mortality with climate warming in riverine amphipods

Forecasting long-term consequences of global warming requires knowledge on thermal mortality and how heat stress interacts with other environmental stressors on different timescales. Here, we describe a flexible analytical framework to forecast mortality risks by combining laboratory measurements on tolerance and field temperature records. Our framework incorporates physiological acclimation effects, temporal scale differences and the ecological reality of fluctuations in temperature, and other factors such as oxygen. As a proof of concept, we investigated the heat tolerance of amphipods Dikerogammarus villosus and Echinogammarus trichiatus in the river Waal, the Netherlands. These organisms were acclimated to different temperatures and oxygen levels. By integrating experimental data with high-resolution field data, we derived the daily heat mortality probabilities for each species under different oxygen levels, considering current temperatures as well as 1 and 2°C warming scenarios. By expressing heat stress as a mortality probability rather than a upper critical temperature, these can be used to calculate cumulative annual mortality, allowing the scaling up from individuals to populations. Our findings indicate a substantial increase in annual mortality over the coming decades, driven by projected increases in summer temperatures. Thermal acclimation and adequate oxygenation improved heat tolerance and their effects were magnified on longer timescales. Consequently, acclimation effects appear to be more effective than previously recognized and crucial for persistence under current temperatures. However, even in the best-case scenario, mortality of D. villosus is expected to approach 100% by 2100, while E. trichiatus appears to be less vulnerable with mortality increasing to 60%. Similarly, mortality risks vary spatially: In southern, warmer rivers, riverine animals will need to shift from the main channel toward the cooler head waters to avoid thermal mortality. Overall, this framework generates high-resolution forecasts on how rising temperatures, in combination with other environmental stressors such as hypoxia, impact ecological communities.

Energetic costs increase with faster heating in an aquatic ectotherm

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

Metabolic rate and climate change across latitudes: evidence of mass-dependent responses in aquatic amphipods

Predictions of individual responses to climate change are often based on the assumption that temperature affects the metabolism of individuals independently of their body mass. However, empirical evidence indicates that interactive effects exist. Here, we investigated the response of individual standard metabolic rate (SMR) to annual temperature range and forecasted temperature rises of 0.6-1.2°C above the current maxima, under the conservative climate change scenario IPCC RCP2.6. As a model organism, we used the amphipod Gammarus insensibilis, collected across latitudes along the western coast of the Adriatic Sea down to the southernmost limit of the species' distributional range, with individuals varying in body mass (0.4-13.57 mg). Overall, we found that the effect of temperature on SMR is mass dependent. Within the annual temperature range, the mass-specific SMR of small/young individuals increased with temperature at a greater rate (activation energy: E=0.48 eV) than large/old individuals (E=0.29 eV), with a higher metabolic level for high-latitude than low-latitude populations. However, under the forecasted climate conditions, the mass-specific SMR of large individuals responded differently across latitudes. Unlike the higher-latitude population, whose mass-specific SMR increased in response to the forecasted climate change across all size classes, in the lower-latitude populations, this increase was not seen in large individuals. The larger/older conspecifics at lower latitudes could therefore be the first to experience the negative impacts of warming on metabolism-related processes. Although the ecological collapse of such a basic trophic level (aquatic amphipods) owing to climate change would have profound consequences for population ecology, the risk is significantly mitigated by phenotypic and genotypic adaptation.

Marine heatwaves of differing intensities lead to distinct patterns in seafloor functioning

Marine heatwaves (MHWs) are increasing in frequency and intensity due to climate change. Several well-documented effects of heatwaves on community structure exist, but examples of their effect on functioning of species, communities or ecosystems remain scarce. We tested the effects of short-term, moderate and strong MHWs on macrofauna bioturbation and associated solute fluxes as examples of ecosystem functioning. We also measured macrofaunal excretion rates to assess effects of temperature on macrofauna metabolism. For this experiment, we used unmanipulated sediment cores with natural animal communities collected from a muddy location at 32 m depth in the northern Baltic Sea. Despite the mechanistic effect of bioturbation remaining unchanged between the treatments, there were significant differences in oxygen consumption, solute fluxes and excretion. Biogeochemical and biological processes were boosted by the moderate heatwave, whereas biogeochemical cycling seemed to decrease under a strong heatwave. A prolonged, moderate heatwave could possibly lead to resource depletion if primary production cannot meet the demands of benthic consumption. By contrast, decreased degradation activities under strong heatwaves could lead to a build-up of organic material and potentially hypoxia. The strong variability and the complexity of the response highlight the context dependency of these processes complicating future predictions.

Consequences of thermal plasticity for hypoxic performance in coastal amphipods

Physiological plasticity may confer an ability to deal with the effect of rapid climate change on aquatic ectotherms. However, plasticity induced by one stressor may only be adaptive in situ if it generates cross-tolerance to other stressors. Understanding the consequences of thermal acclimation on hypoxia thresholds is vital to understanding future climate-driven hypoxia. We tested if thermal acclimation benefits hypoxic performance in four closely-related amphipod species. The effects of thermal acclimation (7 days at 10 or 20 °C) on routine metabolic rate (RMR) and critical oxygen tensions (P_crit) were determined at a standardised test temperature (20 °C). Gammarus chevreuxi and Echinogammarus marinus displayed increased P_crit with acute warming but warm acclimation negated this increase. P_crit of Gammarus duebeni was thermally insensitive. Gammarus zaddachi displayed increased P_crit upon acute warming but little change via acclimation. Cross-tolerance between thermal plasticity and hypoxia may improve performance for some, but not all, species under future environmental change.

Interacting Effects of Cell Size and Temperature on Gene Expression, Growth, Development and Swimming Performance in Larval Zebrafish

Cell size may be important in understanding the thermal biology of ectotherms, as the regulation and consequences of cell size appear to be temperature dependent. Using a recently developed model system of triploid zebrafish (which have around 1.5-fold larger cells than their diploid counterparts) we examine the effects of cell size on gene expression, growth, development and swimming performance in zebrafish larvae at different temperatures. Both temperature and ploidy affected the expression of genes related to metabolic processes (citrate synthase and lactate dehydrogenase), growth and swimming performance. Temperature also increased development rate, but there was no effect of ploidy level. We did find interactive effects between ploidy and temperature for gene expression, body size and swimming performance, confirming that the consequences of cell size are temperature dependent. Triploids with larger cells performed best at cool conditions, while diploids performed better at warmer conditions. These results suggest different selection pressures on ectotherms and their cell size in cold and warm habitats.

Sex biased effect of acute heat shock on the antioxidant system of non-native round goby Neogobius melanostomus

Monitoring oxidative stress biomarkers has become a powerful and common tool to estimate organismal condition and response to endogenous and environmental factors. In the present study, we used round goby (Neogobius melanostomus) from non-native European populations, as a model species to test sex differences in oxidative stress biomarkers. Considering sex differences in reproductive investment, we hypothesized that males would display lower resistance to abiotic stress. Fish were exposed to a heat shock (temperature elevated by 10°C) for 1h, 6h, and 12h and catalase activity (CAT), reduced glutathione (GSH), total antioxidant capacity (TAC) and lipid peroxidation (LPO) were measured in liver and muscle tissues. Liver of males was significantly more responsive compared to liver of females in all tested parameters. GSH was found to be the most responsive to heat stress exposure in both sexes. The results supported our hypothesis that male reproductive investment (territoriality, courtship, and brood care) and likelihood of only a single spawning period in their lifetime influenced on higher sensitivity of their antioxidant defence. On the other hand, for females antioxidant defence is considered more important to survive the environmental changes and successfully reproduce in the next season. Our experiments exposed fish to acute thermal stress. Further research should determine the effects of exposure to chronic thermal stress to corroborate our understanding on sex differences in antioxidant defence in the round goby.

Plasticity of thermal performance curves in a narrow range endemic water beetle

Thermal history can plastically alter the response of ectotherms to temperature, and thermal performance curves (TPCs) are powerful tools for exploring how organismal-level performance varies with temperature. Plasticity in TPCs may be favoured in thermally variable habitats, where it can result in fitness benefits. However, thermal physiology remains insufficiently studied for freshwater insects despite freshwater biodiversity being at great risk under global change. Here, we assess how acclimation at either summer or winter average temperatures changes TPCs for locomotion activity and metabolism in Enochrus jesusarribasi (Hydrophilidae), a water beetle endemic to shallow saline streams in SE Spain. This beetle is a bimodal gas exchanger and so we also assessed how aerial and aquatic gas exchange varied across temperatures for both acclimation treatments. Responses of locomotory TPCs to thermal acclimation were relatively weak, but high temperature acclimated beetles tended to exhibit higher maximum locomotor activity and reduced TPC breadth than those acclimated at lower temperature. High temperature acclimation increased the thermal sensitivity of metabolic rates, contrary to the response generally found in aquatic organisms. Higher metabolic rates upon high temperature acclimation were achieved by increasing aerial, rather than aquatic oxygen uptake. Such plastic respiratory behaviour likely contributed to enhanced locomotor performance at temperatures around the optimum and thermal plasticity could thus be an important component in the response of aquatic insects to climate change. However, high temperature acclimation appeared to be detrimental for locomotion in subsequent exposure at upper sublethal temperatures, suggesting that this narrow range endemic may be vulnerable to future climate warming. This study demonstrates that TPCs are context-specific, differing with performance metric as well as thermal history. Such context dependency must be considered when using TPCs to predict organismal responses to climate change.

The critical thermal maximum of diving beetles (Coleoptera: Dytiscidae): a comparison of subterranean and surface-dwelling species

Thermal tolerance limits in animals are often thought to be related to temperature and thermal variation in their environment. Recently, there has been a focus on studying upper thermal limits due to the likelihood for climate change to expose more animals to higher temperatures and potentially extinction. Organisms living in underground environments experience reduced temperatures and thermal variation in comparison to species living in surface habitats, but how these impact their thermal tolerance limits are unclear. In this study, we compare the thermal critical maximum (CTmax) of two subterranean diving beetles (Dytiscidae) to that of three related surface-dwelling species. Our results show that subterranean species have a lower CTmax (38.3-39.0°C) than surface species (42.0-44.5°C). The CTmax of subterranean species is ∼10°C higher than the highest temperature recorded within the aquifer. Groundwater temperature varied between 18.4°C and 28.8°C, and changes with time, depth and distance across the aquifer. Seasonal temperature fluctuations were 0.5°C at a single point, with the maximum heating rate being ∼1000x lower (0.008°C/hour) than that recorded in surface habitats (7.98°C/hour). For surface species, CTmax was 7-10°C higher than the maximum temperature in their habitats, with daily fluctuations from ∼1°C to 16°C and extremes of 6.9°C and 34.9°C. These findings suggest that subterranean dytiscid beetles are unlikely to reach their CTmax with a predicted warming of 1.3-5.1°C in the region by 2090. However, the impacts of long-term elevated temperatures on fitness, different life stages and other species in the beetle's trophic food web are unknown.

Divergence in Thermal Physiology Could Contribute to Vertical Segregation in Intertidal Ecotypes of Littorina saxatilis

AbstractThermal stress is a potentially important selective agent in intertidal marine habitats, but the role that thermal tolerance might play in local adaptation across shore height has been underexplored. Northwest Spain is home to two morphologically distinct ecotypes of the periwinkle Littorina saxatilis, separated by shore height and subject to substantial differences in thermal stress exposure. However, despite other biotic and abiotic drivers of ecotype segregation being well studied, their thermal tolerance has not been previously characterized. We investigated thermal tolerance across multiple life history stages by employing the thermal death time (TDT) approach to determine (i) whether the two ecotypes differ in thermal tolerance and (ii) how any differences vary with life history stage. Adults of the two ecotypes differed in their thermal tolerance in line with their shore position: the upper-shore ecotype, which experiences more extreme temperatures, exhibited greater endurance of thermal stress compared with the lower-shore ecotype. This difference was most pronounced at the highest temperatures tested. The proximate physiological basis for these differences is unknown but likely due to a multifarious interaction of traits affecting different parts of the TDT curve. Differences in tolerance between ecotypes were less pronounced in early life history stages but increased with ontogeny, suggesting partial divergence of this trait during development. Thermal tolerance could potentially play an important role in maintaining population divergence and genetic segregation between the two ecotypes, since the increased thermal sensitivity of the lower-shore ecotype may limit its dispersal onto the upper shore and so restrict gene flow.

Thermal acclimation increases the stability of a predator-prey interaction in warmer environments

Global warming over the next century is likely to alter the energy demands of consumers and thus the strengths of their interactions with their resources. The subsequent cascading effects on population biomasses could have profound effects on food web stability. One key mechanism by which organisms can cope with a changing environment is phenotypic plasticity, such as acclimation to warmer conditions through reversible changes in their physiology. Here, we measured metabolic rates and functional responses in laboratory experiments for a widespread predator-prey pair of freshwater invertebrates, sampled from across a natural stream temperature gradient in Iceland (4-18℃). This enabled us to parameterize a Rosenzweig-MacArthur population dynamical model to study the effect of thermal acclimation on the persistence of the predator-prey pairs in response to warming. Acclimation to higher temperatures either had neutral effects or reduced the thermal sensitivity of both metabolic and feeding rates for the predator, increasing its energetic efficiency. This resulted in greater stability of population dynamics, as acclimation to higher temperatures increased the biomass of both predator and prey populations with warming. These findings indicate that phenotypic plasticity can act as a buffer against the impacts of environmental warming. As a consequence, predator-prey interactions between ectotherms may be less sensitive to future warming than previously expected, but this requires further investigation across a broader range of interacting species.

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

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

Inorganic mercury effects on biomarker gene expressions of a freshwater amphipod at two temperatures

Mercury (Hg) is a global contaminant resulting of both natural processes and human activities. In aquatic environments, studies conducted on vertebrates highlighted changes of gene expression or activity of antitoxic and oxidative enzymes. However, although Hg is a highly toxic compound in aquatic environments, only a few studies have evaluated the lethal and sublethal effects of inorganic Hg on Gammarus sp. Therefore, this study aimed at evaluating the effects of inorganic Hg (HgCl₂) on the expression of 17 genes involved in crucial biological functions or mechanisms for organisms, namely respiration, osmoregulation, apoptosis, immune and endocrine system, and antioxidative and antitoxic defence systems. The study was performed in males of the freshwater amphipod Gammarus pulex exposed to two environmentally relevant concentrations (50 and 500 ng/L) at two temperature regime fluctuations (16 °C and 20 °C +/-2 °C) for 7 and 21 days. Results showed that G. pulex mortality was dependent on Hg concentration and temperature; the higher the concentration and temperature, the higher the mortality rate. In addition, the Integrated Biomarker Response emphasized that HgCl₂ toxicity was dependent on the concentration, time and temperature of exposure. Overall, antioxidant and antitoxic defences, as well as the endocrine and immune systems, were the biological functions most impacted by Hg exposure (based on the concentration, duration, and temperature tested). Conversely, osmoregulation was the least affected biological function. The results also demonstrated a possible adaptation of G. pulex after 21 days at 500 ng/L, regardless of the exposure temperature. This study allowed us to show that Hg deregulates many crucial biological functions after a short exposure, but that during a long exposure, an adaptation phenomenon could occur, regardless of temperature.

Feeding Behavioural Studies with Freshwater Gammarus spp.: The Importance of a Standardised Methodology

Freshwater Gammarids are common leaf-shredding detritivores, and they usually feed on naturally conditioned organic material, in other words leaf litter that is characterised by an increased palatability, due to the action and presence of microorganisms (Chaumot et al. 2015; Cummins 1974: Maltby et al. 2002). Gammarus spp. are biologically omnivorous organisms, so they are involved in shredding leaf litter and are also prone to cannibalism, predation behaviour (Kelly et al. 2002) and coprophagy when juveniles (McCahon and Pascoe 1988). Gammarus spp. is a keystone species (Woodward et al. 2008), and it plays an important role in the decomposition of organic matter (Alonso et al. 2009; Bundschuh et al. 2013) and is also a noteworthy prey for fish and birds (Andrén and Eriksson Wiklund 2013; Blarer and Burkhardt-Holm 2016). Gammarids are considered to be fairly sensitive to different contaminants (Ashauer et al. 2010; Bloor et al. 2005; Felten et al. 2008a; Lahive et al. 2015; Kunz et al. 2010); in fact Amphipods have been reported to be one of the most sensitive orders to metals and organic compounds (Wogram and Liess 2001), which makes them representative test organisms for ecotoxicological studies and valid sentinel species for assessing water quality status (Garcia-Galan et al. 2017).

Effects of developmental plasticity on heat tolerance may be mediated by changes in cell size in Drosophila melanogaster

There is a growing interest in the physiology underpinning heat tolerance of ectotherms and their responses to the ongoing rise in temperature. However, there is no consensus about the underlying physiological mechanisms. According to "the maintain aerobic scope and regulate oxygen supply" hypothesis, responses to warming at different organizational levels contribute to the ability to safeguard energy metabolism via aerobic pathways. At the cellular level, a decrease in cell size increases the capacity for the uptake of resources (e.g., food and oxygen), but the maintenance of electrochemical gradients across cellular membranes implies greater energetic costs in small cells. In this study, we investigated how different rearing temperatures affected cell size and heat tolerance in the fruit fly Drosophila melanogaster. We tested the hypothesis that smaller-celled flies are more tolerant to acute, intense heat stress whereas larger-celled flies are more tolerant to chronic, mild heat stress. We used the thermal tolerance landscape framework, which incorporates the intensity and duration of thermal challenge. Rearing temperatures strongly affected both cell size and survival times. We found different effects of developmental plasticity on tolerance to either chronic or acute heat stress. Warm-reared flies had both smaller cells and exhibited higher survival times under acute, intense heat stress when compared to cold-reared flies. However, under chronic, mild heat stress, the situation was reversed and cold-reared flies, consisting of larger cells, showed better survival. These differences in heat tolerance could have resulted from direct effects of rearing temperature or they may be mediated by the correlated changes in cell size. Notably, our results are consistent with the idea that a smaller cell size may confer tolerance to acute temperatures via enhanced oxygen supply, while a larger cell may confer greater tolerance to chronic and less intense heat stress via more efficient use of resources.

Temperature effects on metabolic scaling of a keystone freshwater crustacean depend on fish-predation regime

According to the metabolic theory of ecology, metabolic rate, an important indicator of the pace of life, varies with body mass and temperature as a result of internal physical constraints. However, various ecological factors may also affect metabolic rate and its scaling with body mass. Although reports of such effects on metabolic scaling usually focus on single factors, the possibility of significant interactive effects between multiple factors requires further study. In this study, we show that the effect of temperature on the ontogenetic scaling of resting metabolic rate of the freshwater amphipod Gammarus minus depends critically on habitat differences in predation regime. Increasing temperature tends to cause decreases in the metabolic scaling exponent (slope) in population samples from springs with fish predators, but increases in population samples from springs without fish. Accordingly, the temperature sensitivity of metabolic rate is not only size-specific, but also its relationship to body size shifts dramatically in response to fish predators. We hypothesize that the dampened effect of temperature on the metabolic rate of large adults in springs with fish, and of small juveniles in springs without fish are adaptive evolutionary responses to differences in the relative mortality risk of adults and juveniles in springs with versus without fish predators. Our results demonstrate a complex interaction among metabolic rate, body mass, temperature and predation regime. The intraspecific scaling of metabolic rate with body mass and temperature is not merely the result of physical constraints related to internal body design and biochemical kinetics, but rather is ecologically sensitive and evolutionarily malleable.

Does global warming intensify cost of antipredator reaction? A case study of freshwater amphipods

Global warming is a worldwide phenomenon affecting the functioning of diverse ecosystems, including fresh waters. Temperature increase affects physiology and behaviour of ectotherms due to growing energetic demands necessary to sustain increased metabolic rate. Anti-predator responses may resemble temperature-induced changes in organisms, suggesting synergism between these factors. To check how temperature shapes physiological and behavioural responses of ectotherms to predation risk, we exposed amphipods: Dikerogammarus villosus and Gammarus jazdzewskii to fish kairomones at 10, 17 or 24 °C. Animals were placed in tanks where temperature was gradually adjusted to the desired test temperature and acclimated under such conditions for 3 subsequent days. Then they were exposed to the predator cue (the Eurasian perch kairomone) for 35 min to test their acute responses. We measured metabolic rate (as respiration), antioxidant defence (CAT: catalase activity, TAS: total antioxidant status), oxidative molecules (TOS: total oxidative status), oxidative damage (TBARS: thiobarbituric acid reactive substances) and behaviour (locomotor activity). Amphipods increased respiration with raising temperature and when exposed to predation risk (all temperatures). Only G. jazdzewskii exhibited increased TOS when exposed to 24 °C or to predation risk at all temperatures. Antioxidant defence increased with raising temperature (CAT, TAS) and decreased under predation risk (CAT). Cellular damage increased in G. jazdzewskii under predation risk at 10 and 24 °C, but raised temperature itself did not generate any damage. Amphipods reduced locomotor activity at 24 °C. Thus, at elevated temperatures, amphipods minimized their cellular damage at the cost of increased antioxidant defence and lower locomotor activity (potentially disadvantageous under higher energetic demands). Under predation risk, the performance of antioxidant systems was reduced, probably due to energy allocation into anti-predatory mechanisms, leading to increased cellular damage at suboptimum temperatures. Thus, negative consequences of elevated temperature for organisms may be amplified by changes in behaviour (compromising food acquisition) and non-consumptive predator effects.

Changes in heat stress tolerance in a freshwater amphipod following starvation: The role of oxygen availability, metabolic rate, heat shock proteins and energy reserves

The ability of organisms to cope with environmental stressors depends on the duration and intensity of the stressor, as well as the type of stress. For aquatic organisms, oxygen limitation has been implicated in limiting heat tolerance. Here we examine how starvation affects heat tolerance in the amphipod Gammarus fossarum (Koch, 1836) and whether observed changes can be explained from alterations in oxidative metabolism, depletion of energy reserves, upregulation of heat shock proteins or susceptibility to oxygen limitation. Starved amphipods showed impaired survival compared to fed amphipods during prolonged exposure to mild heat. In contrast, under acute, high-intensity heat exposure they actually showed improved survival. We observed a lower demand for oxygen in starved amphipods which could make them less susceptible to oxygen limitation. Such a role for oxygen in limiting heat tolerance was verified as hypoxia impaired the heat tolerance of amphipods, especially starved ones. Fed amphipods likely rely more on anaerobic metabolism to maintain energy status during heat stress, whereas for starved amphipods aerobic metabolism appears to be more important. The depletion of their energy reserves constrains their ability to maintain energy status via anaerobic metabolism. We did not find evidence that alterations in heat tolerance following starvation were related to the upregulation of heat shock proteins. In conclusion, starvation can have opposite effects on heat tolerance, acting via pathways that are operating on different time scales.

Experimental evidence for the adaptive response of aquatic invertebrates to chronic predation risk

As acute stress induced by predation risk can generate significant oxidative damage, prey organisms are forced to balance their defence reaction and the cost of activating the cellular defence system. Stress tolerance differs significantly among species; therefore predator pressure indirectly shapes the community structure. To test adaptation abilities of amphipod crustaceans (Dikerogammarus villosus and Gammarus jazdzewskii) we exposed them to acute (35 min.) and chronic (1 or 7 days) predation risk (the Eurasian perch). We measured respiration (related to metabolic rate), cellular defence systems (antioxidant enzyme (catalase) activity and heat shock protein (Hsp70) concentration), and the level of oxidative damage (thiobarbituric acid reactive substances (TBARS) concentration). Both amphipods increased their respiration rate in the presence of predation cues, irrespective of the duration of their pre-exposure to danger. This increase in D. villosus was initiated more quickly (immediately vs. after 10 min. of the test) and lasted for a longer time (20 vs. 10 min.) than in G. jazdzewskii. However, only G. jazdzewskii after a short exposure to predation risk exhibited an increase in its catalase activity, Hsp70 concentration and oxidative damage. No changes in these parameters were exhibited by D. villosus or after a chronic exposure of G. jazdzewskii to predation cues. Our results show that prey organisms are able to reconfigure their physiology to maintain increased metabolic rate under prolonged predator pressure and, at the same time, reduce oxidative damage as well as costs related to anti-oxidant defence.

Different drivers, common mechanism; the distribution of a reef fish is restricted by local-scale oxygen and temperature constraints on aerobic metabolism

The distributions of ectothermic marine organisms are limited to temperature ranges and oxygen conditions that support aerobic respiration, quantified within the metabolic index (ϕ) as the ratio of oxygen supply to metabolic oxygen demand. However, the utility of ϕ at local scales and across heterogenous environments is unknown; yet, these scales are often where actionable management decisions are made. Here, we test if ϕ can delimit the entire distribution of marine organisms when calibrated across an appropriate temperature range and at local scales (~10 km) using the endemic reef fish, Chrysoblephus laticeps, which is found in the highly heterogenous temperature and oxygen environment along the South African coastal zone, as a model species. In laboratory experiments, we find a bidirectional (at 12°C) hypoxia tolerance response across the temperature range tested (8 to 24°C), permitting a piecewise calibration of ϕ. We then project this calibrated ϕ model through temperature and oxygen data from a high spatial resolution (11 to 13 km) ocean model for the periods 2005 to 2009 and 2095 to 2099 to quantify various magnitudes of ϕ across space and time paired with complementary C. laticeps occurrence points. Using random forest species distribution models, we quantify a critical ϕ value of 2.78 below which C. laticeps cannot persist and predict current and future distributions of C. laticeps in line with already observed distribution shifts of other South African marine species. Overall, we find that C. laticeps' distribution is limited by increasing temperatures towards its warm edge but by low oxygen availability towards its cool edge, which is captured within ϕ at fine scales and across heterogenous oxygen and temperature combinations. Our results support the application of ϕ for generating local- and regional-scale predictions of climate change effects on organisms that can inform local conservation management decisions.

Heat Waves, the New Normal: Summertime Temperature Extremes Will Impact Animals, Ecosystems, and Human Communities

A consequence of climate change is the increased frequency and severity of extreme heat waves. This is occurring now as most of the warmest summers and most intense heat waves ever recorded have been during the past decade. In this review, I describe the ways in which animals and human populations are likely to respond to increased extreme heat, suggest how to study those responses, and reflect on the importance of those studies for countering the devastating impacts of climate change.

Sex-dependent effects of sulfamethoxazole exposure on pro-/anti-oxidant status with stimulation on growth, behavior and reproduction in the amphipod Hyalella azteca

Negative effects of environmental antibiotics on non-target organisms were observed in studies at various levels of the biological organization. Yet, studies combining the effects at multiple levels were required to interpret their ecological frequencies in a broader context. Currently, effects of sulfamethoxazole (SMX) was studied on the amphipod Hyalella azteca which is important in ecological stability. At the biochemical level, effects on the antioxidant capacities showed stimulation with an inverse U-shaped change over the concentrations. The stimulation was greater in male than in females. Effects on the oxidative stress showed a U-shaped change which included stimulation and inhibition in males, and solely stimulation in females. The stimulation was less in males than in females. Effects on acetylcholinesterase (AChE) activities in both sexes were well correlated with those on oxidative stress (p < 0.05). At the individual level, effects on the body weight showed an inverse U-shaped change over concentrations, and the stimulation was greater in males than in females. The stimulations were significantly correlated with the male oxidative stress (p < 0.01), and male (p < 0.1) and female AChE activities (p < 0.05). Effects of SMX on the pre-copulation behavior also showed an inverse U-shaped change which correlated with male and female antioxidant capacities (p < 0.05) and the male body weight (p < 0.05). At the population level, effects on the reproduction showed an inverse U-shaped change over concentrations, and they significantly correlated with the male body weight (p < 0.05) and the pre-copulation behavior (p < 0.05). Summing up, SMX provoked simultaneous disturbances on the amphipod at multiple levels with sex-dependent responses.