Heat tolerance of marine ectotherms in a warming Antarctica

Warm acclimation reduces the sensitivity of Drosophila species to heat stress at ecologically relevant scales

Thermal acclimation is presumed to affect heat tolerance, though it is unclear how this should impact populations under realistic natural conditions. In this study, we quantified how thermal acclimation affects heat tolerance landscapes in Drosophila and, as a consequence, their predicted mortality in the field based on modelling with a dynamic thermal tolerance algorithm. We measured the thermal tolerance of four Drosophila species (D. repleta, D. hydei, D. simulans and D. virilis) acclimated to five constant temperatures covering a range from 18 to 30°C. We then combined this information with field temperatures to construct dynamic tolerance landscapes for these species and examine how survival varies over the course of a year. Our analyses reveal the effect of acclimation on an ecologically relevant scale, specifically through the study of cumulative mortality under natural thermal regimes. We explore how different species respond to thermal challenges during acclimation, generally showing an increase in critical temperature (CTmax) while either reducing or maintaining constant thermal sensitivity (z). Furthermore, we show that while acclimation presents a relatively modest improvement in thermal tolerance during short ramping laboratory trials, this response becomes stronger when tolerance estimates are translated into ecologically relevant timescales, such as annual survival. Our results indicate that acclimation to warm conditions can substantially increase Drosophila thermal tolerance, contradicting the idea that thermal acclimation in ectotherms has only a minor effect. Our work applies novel approaches to studying thermal tolerance and aims to highlight the role of acclimation in ameliorating the impact of global warming.

Exploring thermal tolerance across time and space in a tropical bivalve, Pinctada margaritifera

Ectotherm vulnerability to climate change is predicted to increase with temperature variation. Still, translating laboratory observations of organisms' heat-stress responses to the natural fluctuating environment remains challenging. In this study, we used an integrative framework combining insights from thermal death time (TDT) curves and physiological reaction norms to precisely capture Pinctada margaritifera's thermal performance and tolerance landscape. We then applied this integrative model to predict individuals' cumulative heat injury as a function of actual temperature conditions documented at five contrasting islands across French Polynesia. Substantial injury was predicted for spats (ranging from 30.24% to 29.62%) when exposed to eight consecutive extreme low tide events in Nuku Hiva. Overall, this study highlights the potential of this framework to effectively quantify the impact of extreme events, such as marine heatwaves, and to guide resource management initiatives.

Species-specific seasonal variations in thermal performance curves shape the direct and transgenerational vulnerability to marine heatwaves

Research Highlights: Sasaki, M., Finiguerra, M. & Dam, H.G. (2024). Seasonally variable thermal performance curves prevent adverse effects of heatwaves. Journal of Animal Ecology. https://doi.org/10.1111/1365-2656.14221. Marine heatwaves (MHWs) emerge as a devastating stressor that can have direct and transgenerational effects on marine organisms. However, we know very little about how seasonal variations in thermal performance curves (TPCs) may help marine zooplankton cope with these direct and transgenerational effects of MHWs. In a recent study, Sasaki et al. (2024) combined field observations and simulated laboratory heatwave experiments, uncovering seasonal variations in TPCs for key fitness-related traits, including egg and offspring production, hatching success and survivorship in two ecologically important copepod species Acartia tonsa and A. hudsonica. They discovered that the TPC of A. tonsa was highly seasonally variable, allowing them to maintain their thermal optimum of at least 5°C above the field temperature. The transgenerational effects of parental exposure to MHWs on the offspring were minor. In contrast, the TPC of A. hudsonica was relatively unchanged across seasons, suggesting that this species may be highly vulnerable to MHWs, especially during summer. These findings agree with distinct seasonal abundances of the two species in nature: A. hudsonica is primarily abundant during winter and spring while A. tonsa dominates the summer and fall. These findings enhance our understanding of how seasonal variations in TPCs can determine the vulnerability of marine species to heatwaves through direct and transgenerational effects, which are important for ecological risk assessments of marine ecosystems under a rapidly changing climate.

The importance of upwelling conditions as drivers of feeding behavior and thermal tolerance in a prominent intertidal fish

Upwelling, as a large oceanographic phenomenon, increases coastal productivity and influences all levels of biological complexity. Despite decades of research on it, much remains to be understood about the impact of upwelling on the feeding behavior and thermal tolerance of important groups such as fish. Hence, our aim was to investigate how upwelling conditions modify the feeding behavior and thermal tolerance of a prominent intertidal fish, Girella laevifrons. We collected purple mussels (Perumytilus purpuratus) from upwelling (U) and downwelling sites (DU) in central Chile, and used them as prey in feeding trials and measuring the concentration of organic matter and proteins in their tissues. We assessed fish consumption rates and growth in fish collected from the same U and DU sites, feeding on either U or DU mussels. Lastly, we assessed the thermal tolerance of U and DU fish fed with the aforementioned U vs DU mussels. We found that U mussels held higher concentrations of organic matter and proteins compared to their DU counterparts. U mussels were also selected and consumed in larger amounts than DU mussels, although the origin of the fish also influenced consumption rates. Thermal tolerance assays revealed that U fish exhibited higher maximum performance (Max.pf) and critical thermal maxima (Ctmax) and lower sensitivity to temperature changes (as measured by Q10), compared to DU fish. Altogether, these results point to a strong influence of upwelling on the quality of organisms' tissues, indirectly altering key aspects of fish feeding behavior and thermal tolerance. These findings also contribute to understanding the physiological adjustments organisms make in productive upwelling systems, and how they may change in the future with ongoing climate events.

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.

Climate change and polar marine invertebrates: life-history responses in a warmer, high CO2 world

Polar marine invertebrates serve as bellwethers for species vulnerabilities in the face of changing climate at high latitudes of the Earth. Ocean acidification, warming/heatwaves, freshening, sea ice retreat and productivity change are challenges for polar species. Adaptations to life in cold water with intensely seasonal productivity has shaped species traits at both poles. Polar species have life histories often characterised as K-strategist or K-selected (e.g. slow growth and development, larval hypometabolism) that make them sensitive to climate stress and altered seasonal productivity. Moderate warming results in faster development and can have positive effects on development, up to a limit. However, ocean acidification can retard development, impair skeletogenesis and result in smaller larvae. Given the fast pace of warming, data on the thermal tolerance of larvae from diverse species is urgently needed, as well as knowledge of adaptive responses to ocean acidification and changes to sea ice and productivity. Predicted productivity increase would benefit energy-limited reproduction and development, while sea ice loss negatively impacts species with reproduction that directly or indirectly depend on this habitat. It is critical to understand the interactive effects between warming, acidification and other stressors. Polar specialists cannot migrate, making them susceptible to competition and extinction from range-extending subpolar species. The borealisation and australisation of Arctic and Antarctic ecosystems, respectively, is underway as these regions become more hospitable for the larval and adult life-history stages of lower-latitude species. Differences in biogeography and pace of change point to different prospects for Arctic and Antarctic communities. In this Commentary, we hypothesise outcomes for polar species based on life history traits and sensitivity to climate change and suggest research avenues to test our predictions.

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.

Heat hardening improves thermal tolerance in abalone, without the trade-offs associated with chronic heat exposure

Marine animals are challenged by chronically raised temperatures alongside an increased frequency of discrete, severe warming events. Exposure to repeated heat shocks could result in heat hardening, where sub-lethal exposure to thermal stress temporarily enhances thermotolerance, and may be an important mechanism by which marine species will cope with future thermal challenges. However, we have relatively little understanding of the effects of heat hardening in comparison to chronic exposure to elevated temperatures. Therefore, we compared the effects of heat hardening from repeated exposure to acute heat shocks and chronic exposure to elevated temperatures on thermal tolerance in the European abalone, Haliotis tuberculata. Adult abalones were exposed to either control temperature (15 °C), chronic warming (20 °C) or a regime of two events of repeated acute heat shock cycles (23-25 °C) during six months, and their thermal tolerance and performance, based upon cardiac activity, compared using a dynamic ramping assay. The cost associated with each treatment was also estimated via measurements of condition index (CI). Abalone exposed to both temperature treatments had higher upper thermal limits than the control, but heat-hardened individuals had significantly higher CI values, indicating an enhancement in condition status. Differences in the shape of the thermal performance curve suggest different mechanisms may be at play under different temperature exposure treatments. We conclude that heat hardening can boost thermal tolerance in this species, without performance trade-offs associated with chronic warming.

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 capacity to global warming of amphibians and freshwater fishes: Drivers, patterns, and data limitations

Amphibians and fishes play a central role in shaping the structure and function of freshwater environments. These organisms have a limited capacity to disperse across different habitats and the thermal buffer offered by freshwater systems is small. Understanding determinants and patterns of their physiological sensitivity across life history is, therefore, imperative to predicting the impacts of climate change in freshwater systems. Based on a systematic literature review including 345 experiments with 998 estimates on 96 amphibian (Anura/Caudata) and 93 freshwater fish species (Teleostei), we conducted a quantitative synthesis to explore phylogenetic, ontogenetic, and biogeographic (thermal adaptation) patterns in upper thermal tolerance (CTmax) and thermal acclimation capacity (acclimation response ratio, ARR) as well as the influence of the methodology used to assess these thermal traits using a conditional inference tree analysis. We found globally consistent patterns in CTmax and ARR, with phylogeny (taxa/order), experimental methodology, climatic origin, and life stage as significant determinants of thermal traits. The analysis demonstrated that CTmax does not primarily depend on the climatic origin but on experimental acclimation temperature and duration, and life stage. Higher acclimation temperatures and longer acclimation times led to higher CTmax values, whereby Anuran larvae revealed a higher CTmax than older life stages. The ARR of freshwater fishes was more than twice that of amphibians. Differences in ARR between life stages were not significant. In addition to phylogenetic differences, we found that ARR also depended on acclimation duration, ramping rate, and adaptation to local temperature variability. However, the amount of data on early life stages is too small, methodologically inconsistent, and phylogenetically unbalanced to identify potential life cycle bottlenecks in thermal traits. We, therefore, propose methods to improve the robustness and comparability of CTmax/ARR data across species and life stages, which is crucial for the conservation of freshwater biodiversity under climate change.

Testing the Resilience, Physiological Plasticity and Mechanisms Underlying Upper Temperature Limits of Antarctic Marine Ectotherms

Antarctic marine ectotherms live in the constant cold and are characterised by limited resilience to elevated temperature. Here we tested three of the central paradigms underlying this resilience. Firstly, we assessed the ability of eight species, from seven classes representing a range of functional groups, to survive, for 100 to 303 days, at temperatures 0 to 4 °C above previously calculated long-term temperature limits. Survivors were then tested for acclimation responses to acute warming and acclimatisation, in the field, was tested in the seastar Odontaster validus collected in different years, seasons and locations within Antarctica. Finally, we tested the importance of oxygen limitation in controlling upper thermal limits. We found that four of 11 species studied were able to survive for more than 245 days (245-303 days) at higher than previously recorded temperatures, between 6 and 10 °C. Only survivors of the anemone Urticinopsis antarctica did not acclimate CTmax and there was no evidence of acclimatisation in O. validus. We found species-specific effects of mild hyperoxia (30% oxygen) on survival duration, which was extended (two species), not changed (four species) or reduced (one species), re-enforcing that oxygen limitation is not universal in dictating thermal survival thresholds. Thermal sensitivity is clearly the product of multiple ecological and physiological capacities, and this diversity of response needs further investigation and interpretation to improve our ability to predict future patterns of biodiversity.

Beyond a single temperature threshold: Applying a cumulative thermal stress framework to plant heat tolerance

Most plant thermal tolerance studies focus on single critical thresholds, which limit the capacity to generalise across studies and predict heat stress under natural conditions. In animals and microbes, thermal tolerance landscapes describe the more realistic, cumulative effects of temperature. We tested this in plants by measuring the decline in leaf photosynthetic efficiency (FV/FM) following a combination of temperatures and exposure times and then modelled these physiological indices alongside recorded environmental temperatures. We demonstrate that a general relationship between stressful temperatures and exposure durations can be effectively employed to quantify and compare heat tolerance within and across plant species and over time. Importantly, we show how FV/FM curves translate to plants under natural conditions, suggesting that environmental temperatures often impair photosynthetic function. Our findings provide more robust descriptors of heat tolerance in plants and suggest that heat tolerance in disparate groups of organisms can be studied with a single predictive framework.

Climate, Oxygen, and the Future of Marine Biodiversity

The ocean enabled the diversification of life on Earth by adding O2 to the atmosphere, yet marine species remain most subject to O2 limitation. Human industrialization is intensifying the aerobic challenges to marine ecosystems by depleting the ocean's O2 inventory through the global addition of heat and local addition of nutrients. Historical observations reveal an ∼2% decline in upper-ocean O2 and accelerating reports of coastal mass mortality events. The dynamic balance of O2 supply and demand provides a unifying framework for understanding these phenomena across scales from the global ocean to individual organisms. Using this framework, we synthesize recent advances in forecasting O2 loss and its impacts on marine biogeography, biodiversity, and biogeochemistry. We also highlight three outstanding uncertainties: how long-term global climate change intensifies ocean weather events in which simultaneous heat and hypoxia create metabolic storms, how differential species O2 sensitivities alter the structure of ecological communities, and how global O2 loss intersects with coastal eutrophication. Projecting these interacting impacts on future marine ecosystems requires integration of climate dynamics, biogeochemistry, physiology, and ecology, evaluated with an eye on Earth history. Reducing global and local impacts of warming and O2 loss will be essential if humankind is to preserve the health and biodiversity of the future ocean.

Coral Reef Population Genomics in an Age of Global Change

Coral reefs are both exceptionally biodiverse and threatened by climate change and other human activities. Here, we review population genomic processes in coral reef taxa and their importance for understanding responses to global change. Many taxa on coral reefs are characterized by weak genetic drift, extensive gene flow, and strong selection from complex biotic and abiotic environments, which together present a fascinating test of microevolutionary theory. Selection, gene flow, and hybridization have played and will continue to play an important role in the adaptation or extinction of coral reef taxa in the face of rapid environmental change, but research remains exceptionally limited compared to the urgent needs. Critical areas for future investigation include understanding evolutionary potential and the mechanisms of local adaptation, developing historical baselines, and building greater research capacity in the countries where most reef diversity is concentrated.

Temperature adaptation and its impact on the shape of performance curves in Drosophila populations

Understanding how species adapt to different temperatures is crucial to predict their response to global warming, and thermal performance curves (TPCs) have been employed recurrently to study this topic. Nevertheless, fundamental questions regarding how thermodynamic constraints and evolution interact to shape TPCs in lineages inhabiting different environments remain unanswered. Here, we study Drosophila simulans along a latitudinal gradient spanning 3000 km to test opposing hypotheses based on thermodynamic constrains (hotter-is-better) versus biochemical adaptation (jack-of-all-temperatures) as primary determinants of TPCs variation across populations. We compare thermal responses in metabolic rate and the egg-to-adult survival as descriptors of organismal performance and fitness, respectively, and show that different descriptors of TPCs vary in tandem with mean environmental temperatures, providing strong support to hotter-is-better. Thermodynamic constraints also resulted in a strong negative association between maximum performance and thermal breadth. Lastly, we show that descriptors of TPCs for metabolism and egg-to-adult survival are highly correlated, providing evidence of co-adaptation, and that curves for egg-to-adult survival are systematically narrower and displaced toward lower temperatures. Taken together, our results support the pervasive role of thermodynamics constraining thermal responses in Drosophila populations along a latitudinal gradient, that are only partly compensated by evolutionary adaptation.

A pioneer morphological and genetic study of the intertidal fauna of the Gerlache Strait (Antarctic Peninsula)

The underexplored intertidal ecosystems of Antarctica are facing rapid changes in important environmental factors. Associated with temperature increase, reduction in coastal ice will soon expose new ice-free areas that will be colonized by local or distant biota. To enable detection of future changes in faunal composition, a biodiversity baseline is urgently required. Here, we evaluated intertidal faunal diversity at 13 locations around the Gerlache Strait (western Antarctic Peninsula), using a combination of a quadrat approach, morphological identification and genetic characterization. Our data highlight a community structure comprising four generally distributed and highly abundant species (the flatworm Obrimoposthia wandeli, the bivalve Kidderia subquadrata, and the gastropods Laevilitorina umbilicata and Laevilitorina caliginosa) as well as 79 rarer and less widely encountered species. The most abundant species thrive in the intertidal zone due to their ability to either survive overwinter in situ or to rapidly colonize this zone when conditions allow. In addition, we confirmed the presence of multiple trophic levels at nearly all locations, suggesting that complex inter-specific interactions occur within these communities. Diversity indices contrasted between sampling locations (from 3 to 32 species) and multivariate approaches identified three main groups. This confirms the importance of environmental heterogeneity in shaping diversity patterns within the investigated area. Finally, we provide the first genetic and photographic baseline of the Antarctic intertidal fauna (106 sequences, 137 macrophotographs), as well as preliminary insights on the biogeography of several species. Taken together, these results provide a timely catalyst to assess the diversity and to inform studies of the potential resilience of these intertidal communities.

Responding to warming in polar oceans: A commentary on Molina et al. (2022)

Antarctic marine species live in one of the most thermally stable environments on Earth. They have evolved in these cold stable conditions for many millions of years. The long period for evolution, the isolation and mixing of populations produced by glacial cycles and the environmental heterogeneity in terms of light, productivity and physical disturbance, has produced a diverse fauna with an estimated 20 000 species, or more, living on the seabed. It has also produced a fauna that is possibly the most sensitive to warming on Earth in an environment that is changing faster than most, if not all, others. There is a great need to understand this threatened biodiversity and to find ways to mitigate the future prospects of species loss in this special environment that supports unique biology including the only vertebrate species on Earth that live without haemoglobin.