Critical Thermal Limits Research Articles

Shifts in the Thermal Dependence of Locomotor Performance across an Altitudinal Gradient in Native Populations of Xenopus laevis.

AbstractEctothermic species are dependent on temperature, which drives many aspects of their physiology, including locomotion. The distribution of the native populations of Xenopus laevis is characterized by an exceptional range in latitude and altitude. Along altitudinal gradients, thermal environments change, and populations experience different temperatures. In this study, we compared critical thermal limits and thermal performance curves of populations from the native range across an altitudinal gradient to test whether optimal temperatures for exertion differ depending on altitude. Data on exertion capacity were collected at six different temperatures (8°C, 12°C, 16°C, 19°C, 23°C, and 27°C) for four populations spanning an altitudinal gradient (60, 1,016, 1,948, and 3,197 m asl). Results show that the thermal performance optimum differs among populations. Populations from cold environments at high altitudes exhibit a lower optimal performance temperature than populations from warmer environments at lower altitudes. The ability of this species to change its optimal temperature for locomotor exertion across extremely different climatic environments within the native range may help explain its exceptional invasive potential. These results suggest that ectothermic species capable of adapting to broad altitudinal ranges may be particularly good at invading novel climatic areas, given their ability to cope with a wide range of variation in environmental temperatures.

Multi‐scale relationships in thermal limits within and between two cold‐water frog species uncover different trends in physiological vulnerability

Abstract1. Critical thermal limits represent an important component of an organism's capacity to cope with future temperature changes. Understanding the drivers of variation in these traits may uncover patterns in physiological vulnerability to climate change. Local temperature extremes have emerged as a major driver of thermal limits, although their effects can be mediated by the exploitation of fine‐scale spatial variation in temperature through behavioural thermoregulation.2. Here, we investigated thermal limits along elevation gradients within and between two cold‐water frog species (Ascaphus spp.), one with a coastal distribution (A. truei) and the other with a continental range (A. montanus). We quantified thermal limits for over 700 tadpoles, representing multiple populations from each species. We combined local temporal and fine‐scale spatial temperature data to quantify local thermal landscapes (i.e., thermalscapes), including the opportunity for behavioural thermoregulation.3. Lower thermal limits for either species could not be reached experimentally without the water freezing, suggesting that cold tolerance is <0.3°C. By contrast, upper thermal limits varied among populations, but this variation only reflected local temperature extremes in A. montanus, perhaps as a consequence of the greater variation in stream temperatures across its range. Lastly, we found minimal fine‐scale spatial variability in temperature, suggesting limited opportunity for behavioural thermoregulation and thus increased vulnerability to warming for all populations.4. By quantifying local thermalscapes, we uncovered different trends in the relative vulnerability of populations across elevation for each species. In A. truei, physiological vulnerability decreased with elevation, whereas in A. montanus, all populations were equally physiologically vulnerable. These results highlight how similar environments can differentially shape physiological tolerance and patterns of vulnerability of species, and in turn impact their vulnerability to future warming.

A Theoretical Thermal Tolerance Function for Ectothermic Animals and Its Implications for Identifying Thermal Vulnerability across Large Geographic Scales

The time-to-thermal-death curve, or thermal death curve, seeks to represent all the combinations of exposure time and temperature that kill individuals of a species. We present a new theoretical function to describe that time in lizards based on traditional measures of thermal tolerance (i.e., preferred body temperatures, voluntary thermal maximum, and the critical thermal maximum). We evaluated the utility of this function in two ways. Firstly, we compared thermal death curves among four species of lizards for which enough data are available. Secondly, we compared the geography of predicted thermal vulnerability based on the thermal death curve. We found that the time to loss of function or death may evolve independently from the critical thermal limits. Moreover, the traditional parameters predicted fewer deleterious sites, systematically situated at lower latitudes and closer to large water bodies (lakes or the coast). Our results highlight the urgency of accurately characterizing thermal tolerance across species to reach a less biased perception of the geography of climatic vulnerability.

Larger pollen loads increase risk of heat stress in foraging bumblebees.

Global declines in bumblebee populations are linked to climate change, but specific mechanisms imposing thermal stress on these species are poorly known. Here we examine the potential for heat stress in workers foraging for pollen, an essential resource for colony development. Laboratory studies have shown that pollen foraging causes increased thoracic temperatures (Tth) in bees, but this effect has not been examined in bumblebees nor in real-world foraging situations. We examine the effects of increasing pollen load size on Tth of Bombus impatiens workers in the field while accounting for body size and microclimate. We found that Tth increased by 0.07°C for every milligram of pollen carried (p = 0.007), resulting in a 2°C increase across the observed range of pollen load sizes. Bees carrying pollen were predicted to have a Tth 1.7-2.2°C hotter than those without pollen, suggesting that under certain conditions, pollen loads could cause B. impatiens workers to heat from a safe Tth to one within the range of their critical thermal limits that we measured (41.3°C to 48.4°C). Bumblebees likely adopt behavioural or physiological strategies to counteract the thermal stress induced by pollen transport, and these may limit their foraging opportunities as environmental temperatures continue to increase.

Interspecific differences in thermal tolerance landscape explain aphid community abundance under climate change

A single critical thermal limit is often used to explain and infer the impact of climate change on geographic range and population abundance. However, it has limited application in describing the temporal dynamic and cumulative impacts of extreme temperatures. Here, we used a thermal tolerance landscape approach to address the impacts of extreme thermal events on the survival of co-existing aphid species (Metopolophium dirhodum, Sitobion avenae and Rhopalosiphum padi). Specifically, we built the thermal death time (TDT) models based on detailed survival datasets of three aphid species with three ages across a broad range of stressful high (34–40 °C) and low (−3∼−11 °C) temperatures to compare the interspecific and developmental stage variations in thermal tolerance. Using these TDT parameters, we performed a thermal risk assessment by calculating the potential daily thermal injury accumulation associated with the regional temperature variations in three wheat-growing sites along a latitude gradient. Results showed that M. dirhodum was the most vulnerable to heat but more tolerant to low temperatures than R. padi and S. avenae. R. padi survived better at high temperatures than Sitobion avenae and M. dirhodum but was sensitive to cold. R. padi was estimated to accumulate higher cold injury than the other two species during winter, while M. dirhodum accrued more heat injury during summer. The warmer site had higher risks of heat injury and the cooler site had higher risks of cold injury along a latitude gradient. These results support recent field observations that the proportion of R. padi increases with the increased frequency of heat waves. We also found that young nymphs generally had a lower thermal tolerance than old nymphs or adults. Our results provide a useful dataset and method for modelling and predicting the consequence of climate change on the population dynamics and community structure of small insects.

Do thermal requirements of Dichrorampha odorata, a shoot-boring moth for the biological control of Chromolaena odorata, explain its failure to establish in South Africa?

Chromolaena odorata (L.) RM King and H Rob. (Asteraceae) has been subject to a biological control programme in South Africa for over three decades. A shoot-tip boring moth, Dichrorampha odorata Brown and Zachariades (Lepidoptera: Tortricidae), originating from Jamaica, was released as a biological control agent in 2013 but despite the release of substantial numbers of the insect, it has not established a permanent field population. Because climate incompatibility is a major constraint for classical biological control of invasive plants, and based on the differences in climate between Jamaica and South Africa and field observations at release sites, aspects of the thermal physiology of D. odorata were investigated to elucidate reasons for its failure to establish. Developmental time decreased with increasing temperatures ranging from 20 °C to 30 °C, with incomplete development for immature stages at 18 °C and 32 °C. The developmental threshold, t, was calculated as 8.45 °C with 872.4 degree-days required to complete development (K). A maximum of 6.5 generations per year was projected for D. odorata in South Africa, with the heavily infested eastern region of the country being the most eco-climatically suitable for establishment. The lower lethal temperature (LLT50) of larvae and adults was –4.5 and 1.8 °C, respectively. The upper lethal temperature (ULT50) for larvae was 39.6 °C whilst that of adults was 41.0 °C. Larvae thus had better cold tolerance compared to adults whereas adults had better heat tolerance compared to larvae. The critical thermal (CT) limits for adults were 3.4 ± 0.07 to 43.7 ± 0.12 °C. Acclimation at 20 °C for 7 days resulted in increased cold and heat tolerance with a CTmin and CTmax of 1.9 ± 0.06 and 44.4 ± 0.07 °C respectively, compared to the relative control, acclimated at 25 °C. Acclimation at 30 °C improved neither cold (CTmin: 5.9 ± 0.08 °C) nor heat tolerance (CTmax: 42.9 ± 0.10 °C). These results suggest that thermal requirements fall within field temperatures and are thus not the main constraining factor leading to poor establishment of D. odorata in South Africa.

Carbohydrate‐rich diet increases critical thermal maximum in ants

AbstractTo understand species’ responses to climate change, we must better comprehend the factors shaping physiological critical thermal limits. One factor of potential importance is nutrient availability. Carbohydrates are an energy source that can directly affect an organism's physiological state. Ants are among the most omnipresent and ecologically relevant animal groups on Earth, and many ant species consume carbohydrate‐based diets. Additionally, as ectotherms, ants are highly vulnerable to the effects of climate change. Here, we examined the relationship between foraging temperature, carbohydrate availability, and critical thermal maximum (CTmax) in ants (Hymenoptera: Formicidae). First, we conducted a laboratory experiment using 3–4 colonies of two species that forage at high temperatures (Camponotus blandus Smith and Dorymyrmex thoracicus Gallardo) and two species that forage at lower temperatures (Nylanderia fulva Mayr and Dolichoderus quadridenticulatus Roger). Each colony was divided into two experimental subcolonies, which were given diets containing different carbohydrate concentrations (5 vs. 20% sucrose solutions). We then measured CTmax. We also collected ants belonging to these species in the field and measured their CTmax. We found that CTmax was highest for the two species that forage at higher temperatures. For C. blandus and D. thoracicus, workers given 20% sucrose had higher CTmax than workers given 5% sucrose. No diet‐mediated differences in CTmax were seen for N. fulva and D. quadridenticulatus workers. Additionally, the experimental ants in both treatment groups had higher CTmax than their field‐collected conspecifics, except in the case of C. blandus. If carbohydrate‐rich diets can boost heat tolerance in some species, it is possible that changes in resource availability could determine how climate change affects ants, especially species with carbohydrate‐based diets. Furthermore, these impacts could ripple across the entire trophic network.

High thermal tolerance in high-elevation species and laboratory-reared colonies of tropical bumble bees.

Bumble bees are key pollinators with some species reared in captivity at a commercial scale, but with significant evidence of population declines and with alarming predictions of substantial impacts under climate change scenarios. While studies on the thermal biology of temperate bumble bees are still limited, they are entirely absent from the tropics where the effects of climate change are expected to be greater. Herein, we test whether bees' thermal tolerance decreases with elevation and whether the stable optimal conditions used in laboratory-reared colonies reduces their thermal tolerance. We assessed changes in the lower (CTMin) and upper (CTMax) critical thermal limits of four species at two elevations (2600 and 3600 m) in the Colombian Andes, examined the effect of body size, and evaluated the thermal tolerance of wild-caught and laboratory-reared individuals of Bombus pauloensis. We also compiled information on bumble bees' thermal limits and assessed potential predictors for broadscale patterns of variation. We found that CTMin decreased with increasing elevation, while CTMax was similar between elevations. CTMax was slightly higher (0.84°C) in laboratory-reared than in wild-caught bees while CTMin was similar, and CTMin decreased with increasing body size while CTMax did not. Latitude is a good predictor for CTMin while annual mean temperature, maximum and minimum temperatures of the warmest and coldest months are good predictors for both CTMin and CTMax. The stronger response in CTMin with increasing elevation, and similar CTMax, supports Brett's heat-invariant hypothesis, which has been documented in other taxa. Andean bumble bees appear to be about as heat tolerant as those from temperate areas, suggesting that other aspects besides temperature (e.g., water balance) might be more determinant environmental factors for these species. Laboratory-reared colonies are adequate surrogates for addressing questions on thermal tolerance and global warming impacts.

Critical thermal maxima of three small-bodied fish species (Cypriniformes) of different origin and protection status

Environmental changes related to global warming are both a threat to cold-water fishes and simultaneously create favourable conditions for the spread of eurythermic or warm-water species. In both cases, it is necessary to develop conservation strategies supported by precise ecological data, including thermal requirements. In this study, we determined the upper thermal tolerance thresholds and the critical maxima for three small, non-commercial Eurasian freshwater fish species; i.e. lake minnow Eupallasella (=Rhynchocypris) percnurus, sunbleak Leucaspius delineatus and topmouth gudgeon Pseudorasbora parva using the Critical Thermal Method at different acclimation temperatures, i.e. 18, 23, 28, and 33°C (highest treatment for topmouth gudgeon only). We hypothesized that lake minnow would have the lowest thermal tolerance and the topmouth gudgeon the highest. The response to temperature rise and the upper thermal limits were species-specific and correlated with the acclimation temperature, but not with fish length. Sunbleak showed the lowest thermal tolerance, though at 28°C both E. percnurus and L. delineatus reached a similar critical thermal limit. Topmouth gudgeon showed distinctly higher upper thresholds of thermal tolerance and at the highest acclimation temperature the critical upper limit for the species was close to 42°C. The results obtained for L. delineatus were surprising as we predicted that this leuciscid fish would be more tolerant of high temperatures than E. percnurus. We discuss the results in relation to the threat of extinction and the risk of species spreading beyond their natural range in the context of a warming environment.

Critical thermal limits and temperature-dependent walking speed may mediate coexistence between the native winter ant (Prenolepis imparis) and the invasive Argentine ant (Linepithemahumile)

Comparing the thermal tolerance and performance of native and invasive species from varying climatic origins may explain why some native and invasive species can coexist. We compared the thermal niches of an invasive and native ant species. The Argentine ant (Linepithema humile) is an invasive species that has spread to Mediterranean climates worldwide, where it is associated with losses in native arthropod biodiversity. In northern California, long-term surveys of ant biodiversity have shown that the winter ant (Prenolepis imparis) is the native species best able to coexist with Argentine ants. Both species tend hemipteran scales for food, and previous research suggests that these species’ coexistence may depend on seasonal partitioning: winter ants are active primarily in the colder winter months, while Argentine ants are active primarily in the warmer months in northern California.We investigated the physiological basis of seasonal partitioning in Argentine and winter ants by a) measuring critical thermal limits, and b) comparing how ant walking speed varies with temperature. While both species had similar CTmax values, we found differences between the two species' critical thermal minima that may allow winter ants to remain functional at ecologically relevant temperatures between 0 and 2.5 °C. We also found that winter ants' walking speeds are significantly less temperature-dependent than those of Argentine ants. Winter ants walk faster than Argentine ants at low temperatures, which may allow the winter ants to remain active and forage at lower winter temperatures. These results suggest that partitioning based on differences in temperature tolerance promotes the winter ant's continued occupation of areas invaded by the Argentine ant.

Jack, master or both? The invasive ladybird Harmonia axyridis performs better than a native coccinellid despite divergent trait plasticity

The plasticity of performance traits can promote the success of biological invasions and therefore, precisely estimating trait reaction norms can help to predict the establishment and persistence of introduced species in novel habitats. Most studies focus only on a reduced set of traits and rarely include trait variability that may be vital to predicting establishment success. Here, using a split-brood full-sib design, we acclimated the globally invasive ladybird Harmonia axyridis and a native co-occurring and competing species Cheilomenes lunata to cold, medium and warm temperature regimes, and measured critical thermal limits, life-history traits, and starvation resistance. We used the conceptual framework of “Jack, Master or both” to test predictions regarding performance differences of these two species. The native C. lunata had a higher thermal plasticity of starvation resistance and a higher upper thermal tolerance than H. axyridis. By contrast, H. axyridis had a higher performance than C. lunata for preoviposition period, fecundity and adult emergence from pupae. We combined trait responses, transport duration and propagule pressure to predict the size of the populations established in a novel site following cold, medium and warm scenarios. Although C. lunata initially had a higher performance than the invasive species during transport, more individuals of H. axyridis survived in all simulated environments due to the combined life-history responses, and in particular, higher fecundity. Despite an increased starvation mortality in the warm scenario, given a sufficient propagule size, H. axyridis successfully established. This study underscores how the combination and plasticity of multiple performance traits can strongly influence establishment potential of species introduced into novel environments.

Thermal limits of Africanized honey bees are influenced by temperature ramping rate but not by other experimental conditions

Interest in assessing the critical thermal limits of bees is rapidly increasing, as these physiological traits are good predictors of bees' potential responses to extreme temperature changes, which is relevant in the context of global climate change. However, estimates of thermal limits may be influenced by several factors and published studies differ in experimental methods and conditions, such as the rate of temperature change (ramping rate) and feeding status, which might yield inaccurate predictions and limit comparisons across taxa and regions. Using Africanized honey bees as a model organism, we assessed the effect of ramping rate (0.25, 0.5, 0.75, 1.0 and 1.5 °C min−1) and length of starvation (recently fed vs. fasted for 6, 12, and 18 h) on foragers’ lower (CTMin) and upper (CTMax) thermal limits, as well as the effect of cold stress on CTMax. In addition, we evaluated the two approaches currently used to assess CTMax with a water bath: floating or submerging the testing vials in the bath. We found that critical thermal limits were influenced by ramping rates but not by the other assessed experimental conditions. On average, at ramping rates faster than 0.5 °C min−1, bees displayed a CTMin 1.1–2.6 °C lower and a CTMax 5.3–6.9 °C higher than those of the slowest ramping rate. We discuss the implications of these results and provide suggestions for future thermal studies on bees.

Phenology and plasticity can prevent adaptive clines in thermal tolerance across temperate mountains: The importance of the elevation-time axis.

Critical thermal limits (CTmax and CTmin) decrease with elevation, with greater change in CTmin, and the risk to suffer heat and cold stress increasing at the gradient ends. A central prediction is that populations will adapt to the prevailing climatic conditions. Yet, reliable support for such expectation is scant because of the complexity of integrating phenotypic, molecular divergence and organism exposure. We examined intraspecific variation of CTmax and CTmin, neutral variation for 11 microsatellite loci, and micro‐ and macro‐temperatures in larvae from 11 populations of the Galician common frog (Rana parvipalmata) across an elevational gradient, to assess (1) the existence of local adaptation through a PST‐FST comparison, (2) the acclimation scope in both thermal limits, and (3) the vulnerability to suffer acute heat and cold thermal stress, measured at both macro‐ and microclimatic scales. Our study revealed significant microgeographic variation in CTmax and CTmin, and unexpected elevation gradients in pond temperatures. However, variation in CTmax and CTmin could not be attributed to selection because critical thermal limits were not correlated to elevation or temperatures. Differences in breeding phenology among populations resulted in exposure to higher and more variable temperatures at mid and high elevations. Accordingly, mid‐ and high‐elevation populations had higher CTmax and CTmin plasticities than lowland populations, but not more extreme CTmax and CTmin. Thus, our results support the prediction that plasticity and phenological shifts may hinder local adaptation, promoting thermal niche conservatism. This may simply be a consequence of a coupled variation of reproductive timing with elevation (the “elevation‐time axis” for temperature variation). Mid and high mountain populations of R. parvipalmata are more vulnerable to heat and cool impacts than lowland populations during the aquatic phase. All of this contradicts some of the existing predictions on adaptive thermal clines and vulnerability to climate change in elevational gradients.

More time for aliens? Performance shifts lead to increased activity time budgets propelling invasion success

In the Grinnellian niche concept, the realized niche and potential distribution is characterized as an interplay among the fundamental niche, biotic interactions and geographic accessibility. Climate is one of the main drivers for this concept and is essential to predict a taxon’s distribution. Mechanistic approaches can be useful tools, which use fitness-related aspects like locomotor performance and critical thermal limits to predict the potential distribution of an organism. These mechanistic approaches allow the inclusion key ecological processes like local adaptation and can account for thermal performance traits of different life-history stages. The African Clawed Frog, Xenopus laevis, is a highly invasive species occurring on five continents. The French population is of special interest due to an ongoing expansion for 40 years and a broad base of knowledge. We hypothesize that (1) the French population exhibits increased activity time in the invasive European range that could be devoted to fitness-relevant activity and (2) tadpoles may have less activity time available than adult frogs from the same range. We investigate how thermal performance traits translate into activity time budgets and how local adaptation and differences in the thermal responses of life-history stages may boost the European Xenopus invasion. We use a mechanistic approach based on generalized additive mixed models, where thermal performance curves were used to predict the hours of activity and to compare the potential activity time budgets for two life-history stages of native and invasive populations. Our results show that adult French frogs have more activity time available in Europe compared to South African frogs, which might be an advantage in searching for prey or escaping from predators. However, French tadpoles do not have more activity time in Europe compared to the native South African populations suggesting that tadpoles do not suffer the same strong selective pressure as adult frogs.

Thermal tolerance of western corn rootworm: Critical thermal limits, knock-down resistance, and chill coma recovery

Western corn rootworm, Diabrotica virgifera virgifera, is one of the most economically important crop pests in the world with estimates of damage and control approximating over $1 billion USD annually. Despite an abundance of research devoted to studying rootworm biology in the central Corn Belt of the United States, key aspects on their thermal ecology are still lacking. Here we address this knowledge gap by measuring critical thermal limits, knock-down resistance, and chill coma recovery. In doing so, we also address methodological questions surrounding measurements of thermal tolerance using a variety of dynamic and static assays. The average critical thermal maxima across all trials was 43.0 °C, while the average critical thermal minima was 2.5 °C. Critical thermal limits were relatively invariant across all treatments except at faster ramping rates. Knock-down resistance decreased with increasing temperature as survival dropped from 100% at 39 °C to 0% within 10 min at 44 °C. Recovery from chill coma increased by 1.62 min for each hour of exposure at 0 °C, while survival decreased by 50% after only 24 h. Combined, our results present the first composite picture of different thermal traits for western corn rootworm, which will be vital for predicting their survival and potential spread under future climate change scenarios.

Meta-analysis reveals weak but pervasive plasticity in insect thermal limits

Extreme temperature events are increasing in frequency and intensity due to climate change. Such events threaten insects, including pollinators, pests and disease vectors. Insect critical thermal limits can be enhanced through acclimation, yet evidence that plasticity aids survival at extreme temperatures is limited. Here, using meta-analyses across 1374 effect sizes, 74 studies and 102 species, we show that thermal limit plasticity is pervasive but generally weak: per 1 °C rise in acclimation temperature, critical thermal maximum increases by 0.09 °C; and per 1 °C decline, critical thermal minimum decreases by 0.15 °C. Moreover, small but significant publication bias suggests that the magnitude of plasticity is marginally overestimated. We find juvenile insects are more plastic than adults, highlighting that physiological responses of insects vary through ontogeny. Overall, we show critical thermal limit plasticity is likely of limited benefit to insects during extreme climatic events, yet we need more studies in under-represented taxa and geographic regions.

Prevalence and mechanisms of environmental hyperoxia-induced thermal tolerance in fishes.

Recent evidence has suggested environmental hyperoxia (O2 supersaturation) can boost cardiorespiratory performance in aquatic ectotherms, thereby increasing resilience to extreme heat waves associated with climate change. Here, using rainbow trout (Oncorhynchus mykiss) as a model species, we analysed whether improved cardiorespiratory performance can explain the increased thermal tolerance of fish in hyperoxia (200% air saturation). Moreover, we collated available literature data to assess the prevalence and magnitude of hyperoxia-induced thermal tolerance across fish species. During acute warming, O2 consumption rate was substantially elevated under hyperoxia relative to normoxia beyond 23°C. This was partly driven by higher cardiac output resulting from improved cardiac contractility. Notably, hyperoxia mitigated the rise in plasma lactate at temperatures approaching upper limits and elevated the critical thermal maximum (+0.87°C). Together, these findings show, at least in rainbow trout, that hyperoxia-induced thermal tolerance results from expanded tissue O2 supply capacity driven by enhanced cardiac performance. We show 50% of the fishes so far examined have increased critical thermal limits in hyperoxia (range: 0.4–1.8°C). This finding indicates environmental hyperoxia could improve the ability of a large number of fishes to cope with extreme acute warming, thereby increasing resilience to extreme heat wave events resulting from climate change.

Thermal physiology of Tropical House Geckos (Hemidactylus mabouia) in a cool temperate region of South Africa.

ABSTRACT Temperature profoundly influences the functioning of ectotherms and understanding the thermal biology of invasive species affords valuable insights regarding the suitability of different habitats. Yet, this remains an understudied aspect in the field of invasion biology. Here, we investigated the thermal physiology of the highly invasive Tropical House Gecko (Hemidactylus mabouia) in a coastal region of South Africa. The study site is near to the southernmost extent of the species’ distribution and we expected the physiology of individuals at the study site to differ compared to that of individuals inhabiting tropical regions. Relative to tropical geckos, we observed a decrease of 3.4°C in preferred body temperature (Tpref) to 24.0 °C, which was likely driven by selection for improved functioning within the cooler temperate climate. However, the lower (CTmin) and upper (CTmax) critical thermal limits were higher in geckos at the study site. CTmin was similar between sexes at 11.9 °C, whereas males had a higher CTmax by ≈ 1.9 °C reaching 39.7 °C. A broader tolerance range is typical of a highly variable climate, and we presume that the increase in CTmax allows geckos to better cope with occasional high temperatures. Further, sex-specific differences are uncommon in reptiles, but our study supports claims of improved heat tolerance in male geckos, argued to be related to their aggressive and territorial behaviour. The results also support claims that tropical geckos are more adaptable than previously thought, allowing them to invade new areas. Thereafter, their aggressive behaviour, dietary flexibility and year-round activity likely provides house geckos with a competitive advantage over some native reptiles.

Physiological characterization of Typhlatya cave shrimps: linking habitat with aerobic metabolism

The anchialine environment is characterized by a vertical stratification of water masses with different salinities. Cave shrimps of the genus Typhlatya are widespread inhabitants of the aquifer in fresh, brackish, and marine groundwater. Here we describe physiological aspects of three of the most abundant and widespread Typhlatya species that thrive in the fresh and marine groundwater habitats of the anchialine ecosystem of the Yucatan Peninsula. The aerobic scope (AS) of Typhlatya mitchelli, Typhlatya pearsei and Typhlatya dzilamensis was estimated through induced physical activity, whilst monitoring protein carbonylation and lipid peroxidation (as indicators of cellular damage), lactate accumulation (as an indicator of anaerobic metabolism) and the antioxidant system response. The critical thermal limits (CTL) of all three species as an additional measure of physiological plasticity were also determined. Our results showed that metabolic rates, AS and CTL were similar amongst the two species that inhabit fresh groundwater habitats, and differed markedly from T. dzilamensis, a species typically found in marine groundwater. The antioxidant system response in all three Typhlatya species accompanied the levels of aerobic metabolism following physical activity. However, the large amount of GSH observed in T. dzilamensis may be indicative of an adaptive trait to a more heterogeneous environment. The differences observed among Typhlatya species reflect different physiological adaptations that correspond to the environmental heterogeneity of their natural habitats. Our results suggest that the marine groundwater species, T. dzilamensis, could be better prepared to respond to a naturally more heterogeneous environment, in contrast to Typhlatya mitchelli and Typhlatya pearsei which rarely face environmental clines in the fresh groundwater habitat. Our findings contribute to a better understanding of the consequences of environmental change on ecologically important species that are restricted to live in the aquifer.

Thermal tolerance, safety margins and acclimation capacity assessments reveal the climate vulnerability of large yellow croaker aquaculture

With the ongoing climate change, marine aquaculture and other food production sectors will face new significant challenges in the next century. Despite the importance of these new challenges, understanding the dynamic thermal tolerance and thermal safety status of some major culture species remains scarce, including large yellow croaker (Larimichthys crocea), the most productive mariculture fish in China. Here, we conducted a relatively complete thermal biology assessment of cultured large yellow croaker using long-term acclimation experiments, critical thermal testing and the collected habitat sea surface temperature (SST) data. Changes in body size, oxidative stress and metabolic biomarkers during acclimation were also measured to evaluate the effects of prolonged thermal stress on performance. Results showed that the critical thermal limits of large yellow croaker acclimated at 11–30 °C ranged from 5.27 °C to 36.64 °C, and the total thermal tolerance zone was 444.74 °C2. Thermal limits increased as acclimation temperature rise, and the estimated acclimation response ratios ranged from 0.32 to 0.63, showing obvious thermal plasticity. The estimated post-acclimation thermal sensitivity values (0.80–1.28) for large yellow croaker acclimated at 11–30 °C were close to 1, and the heat- (5.78 °C) and cold-thermal safety margins (6.33 °C) were both high, showing sufficient thermal tolerance to cope with current observed highest SST and the predicted SST caused by climate change. However, performance results showed that long-term exposure to 30 °C would induce energy redistribution and tradeoffs between health/growth and heat tolerance, leading to significantly decreased body weight and increased superoxide dismutase activity in the gill. This result indicates a potential disadvantage of growing large yellow croaker in floating sea-cages, because the shallow culture depth, high density and limited moving spaces in this farming mode make the fish susceptible to fluctuations in SSTs, constrain their ability in behavioral thermoregulation, and increase the risks of hypoxia and disease infections. Thus, these tradeoffs may make the current floating sea-cage farming more vulnerable to elevated temperatures on production. Finally, since our recorded highest in situ SST and warming rate (in summer) have reached up to 30.1 °C and 1.2 °C/d, we propose that the large yellow croaker farms currently operating in the main producing area of China are already at the risk of summer heatwaves. In the future, more efforts should be made to the selection of heat-resilient strains or the exploration of alternative farming systems in large yellow croaker aquaculture industry to avoid or mitigate potential production decline.