Thermal reaction norms and the scale of temperature variation: latitudinal vulnerability of intertidal nacellid limpets to climate change

Evaluation of Sub-Antarctic and Antarctic sea urchins' thermal reaction norm through righting behavior and comparison with in situ measurements

Sea urchin's survival may depend on their capacity to recover proper orientation rapidly and effectively after inversion, enabling escape from predator and preventing desiccation. This righting behavior has been used as a repeatable and reliable indicator to assess echinoderms performance across environmental conditions, including thermal sensitivity and thermal stress. The current study aims at evaluating and comparing the thermal reaction norm for righting behavior (time for righting (TFR) and capacity to self-right) of three common sea urchins from high latitude, the Patagonian sea urchins Loxechinus albus and Pseudechinus magellanicus, and the Antarctic sea urchin Sterechinus neumayeri. In addition, to infer the ecological implications of our experiments, we compared laboratory-based and in situ TFR of these three species. We observed that populations of the Patagonian sea urchins L. albus and P. magellanicus presented similar trend of righting behavior, overly accelerating with increasing temperature (from 0 to 22°C). Little variations and high inter-individual variability were observed below 6°C in the Antarctic sea urchin TFR, and righting success strongly decreased between 7 and 11°C. For the three species, TFR was lower in in situ experiments compared to the laboratory. Overall, our results suggest that the populations of Patagonian sea urchin exhibit a wide thermal tolerance and, based on S. neumayeri's TFR, aligning with the narrow thermal tolerance of Antarctic benthos. Finally, the differences between laboratory and in situ experiments highlights the importance of considering the complexity of marine environments for future predictions.

Evolutionary constraints on physiology confound range shift predictions of two nacellid limpets

Physiological comparisons are fundamental to quantitative assessments of the capacity of species to persist within their current distribution and to predict their rates of redistribution in response to climate change. Yet, the degree to which physiological traits are conserved through evolutionary history may fundamentally constrain the capacity for species to adapt and shift their geographic range. Taxa that straddle major climate transitions provide the opportunity to test the mechanisms underlying evolutionary constraints and how such constraints may influence range shift predictions. Here we focus on two abundant and shallow water nacellid limpets which have representative species on either side of the Polar front. We test the thermal thresholds of the Southern Patagonian limpet, Nacella deaurata and show that its optimal temperatures for growth (4 °C), activity (-1.2 to -0.2 °C) and survival (1 to 8 °C) are mismatched to its currently experienced annual sea surface temperature range (5.9 to 10 °C). Comparisons with the congeneric Antarctic limpet, N. concinna, reveal an evolutionary constraint on N. deaurata physiology, with overlapping thermal capacities, suggesting that a cold climate legacy has been maintained through the evolution of these species. These physiological assessments predict that the South American range of N. deaurata will likely decline with continued warming. It is, however, one of the first species with demonstrated physiological capacity to successfully colonize the cold Southern Ocean. With the expected increase in opportunities for transport within high southern latitudes, N. deaurata has the potential to establish and drive ecological change within the shallow Southern Ocean.

An integrated, multi-level analysis of thermal effects on intertidal molluscs for understanding species distribution patterns

Elucidating the physiological mechanisms that underlie thermal stress and discovering how species differ in capacities for phenotypic acclimatization and evolutionary adaptation to this stress is critical for understanding current latitudinal and vertical distribution patterns of species and for predicting their future state in a warming world. Such mechanistic analyses require careful choice of study systems (species and temperature-sensitive traits) and design of laboratory experiments that reflect the complexities of in situ conditions. Here, we critically review a wide range of studies of intertidal molluscs that provide mechanistic accounts of thermal effects across all levels of biological organization - behavioural, organismal, organ level, cellular, molecular, and genomic - and show how temperature-sensitive traits govern distribution patterns and capacities for coping with thermal stress. Comparisons of congeners from different thermal habitats are especially effective means for identifying adaptive variation. We employ these mechanistic analyses to illustrate how species differ in the severity of threats posed by rising temperature. Counterintuitively, we show that some of the most heat-tolerant species may be most threatened by increases in temperatures because of their small thermal safety margins and minimal abilities to acclimatize to higher temperatures. We discuss recent molecular biological and genomic studies that provide critical foundations for understanding the types of evolutionary changes in protein structure, RNA secondary structure, genome content, and gene expression capacities that underlie adaptation to temperature. Duplication of stress-related genes, as found in heat-tolerant molluscs, may provide enhanced capacity for coping with higher temperatures. We propose that the anatomical, behavioural, physiological, and genomic diversity found among intertidal molluscs, which commonly are of critical importance and high abundance in these ecosystems, makes this group of animals a highly appropriate study system for addressing questions about the mechanistic determinants of current and future distribution patterns of intertidal organisms.

Evaluating the effects of ocean warming and freshening on the physiological energetics and transcriptomic response of the Antarctic limpet Nacella concinna

In the Southern Ocean, warming and freshening are expected to be prominent signals of climate change and the reduced ability of Antarctic marine organisms to cope with changing environmental conditions could challenge their future survival. The Antarctic limpet Nacella concinna is a macroinvertebrate of rocky ecosystems, which occurs in high densities in the shallow subtidal zone. Subtidal individuals were exposed to a combination of temperatures (1, 4, 8, 11, 14 °C) and salinities (20 and 30 psu) for a 60-day period. A drastic increment in mortality was observed with seawater warming, showing that N. concinna is highly stenothermal, with limited ability to survive at temperatures warmer than 4 °C, although there was some degree of acclimation at 4 °C and ambient salinity (30 psu). This study confirmed the stenohaline characteristic of this species, with mortality reaching 50% and lower scope for growth at low salinity (20 psu) even at the control temperature (1 °C). At the sub-cellular level, limpets' low tolerance to out-of range salinity is illustrated by the activation of cell remodelling processes whereas the down-regulation of chaperones proteins and plasma membrane ATPase suggest that under the combination of warming and freshening N. concinna experiences a severe level of stress and devote much of its energy to somatic maintenance and survival. The drastic effect observed can be explained by its subtidal origin, an environment with more stable conditions. The surviving individuals at 1 °C and lowered salinity (20 psu) were either more tolerant or showing signs of acclimation after 60 days, but the combination of warming and freshening have a greater combined stress. Projections of climate change for end of the century for this part of the Antarctic can, therefore, result in a significant diminution of the subtidal population of N. concinna, affecting ecological interactions and diversity of the food web.

Interpreting empirical estimates of experimentally derived physiological and biological thermal limits in ectotherms

Whole-organism function is underpinned by physiological and biological processes, which respond to temperature over a range of time scales. Given that environmental temperature controls biological rates within ectotherms, different experimental protocols are needed to assess the ability of organisms to withstand extreme weather events versus gradual temperature change. Here we emphasize the importance of time in shaping ecological and evolutionary processes, and as an experimental parameter that is key when interpreting physiology studies reporting thermal limits. We discuss how acute and chronic thermal performance is underpinned by mechanisms operating at different time scales — resistance, acclimation, and adaptation. We offer definitions of common physiological and biological temperature metrics and identify challenges inherent to compiling the wealth of historical temperature limit data now available into meta-analytic frameworks. We use a case study, data across temperate fishes, to highlight that false positives may occur when differences in the thermal tolerances of species are in fact due to experimental protocols. We further illustrate that false negatives can arise if researchers fail to recognize differences in thermal limits of species emerging from macrophysiological approaches that are due to biological mechanisms. We strongly advocate for the careful design, interpretation, and reporting of experimental results to ensure that conclusions arising from data synthesis efforts are grounded in theory.

Effect of a gradually increasing temperature on the behavioural and physiological response of juvenile Hippocampus erectus: Thermal preference, tolerance, energy balance and growth

The physiological and behavioural responses of ectotherms to temperature is strongly dependent on the individuals' previous thermal history. Laboratory based studies investigating the mechanisms of thermoregulation in marine ectotherms, however, rarely consider key temporal elements of thermal exposure, such as the rate at which temperature changes. We tested the hypothesis that juvenile seahorses, Hippocampus erectus, from a tropical coastal lagoon in Yucatan, Mexico, would exhibit variations in physiological and behavioural descriptors of thermoregulation when submitted to contrasting regimes during 30 days: temperature constant at 25 °C (C 25); gradually increasing 1 °C every 5 days from 25 to 30 °C (GI 25-30); and constant at 30 °C (C 30). Immediately after exposure, critical maximum temperature, thermal preference, oxygen consumption, partial energy balance, growth rate and survival of seahorses were measured. Seahorses exposed to GI 25-30 showed a significantly higher critical thermal maxima (37.8 ± 0.9 °C), preference (28.7 ± 0.4 °C), growth (1.10 ± 0.49%) and survival (97.6%) than those exposed to C 30 (36.5 ± 1, 29.4 ± 0.3 °C, 0.48 ± 0.32%, 73.8%, respectively). Both high temperature regimes induced metabolic depression, but ramping resulted in a greater amount of energy assimilated (278.9 ± 175.4 J g^-1 day^-1) and higher energy efficiency for growth (89.8%) than constant exposure to 30 °C (115.4 ± 63.4 J g^-1 day^-1, 65.3%, respectively). Gradually increasing temperature allowed physiological mechanisms of thermal adjustment to take place, reflecting the capacity of juvenile H. erectus to respond to environmental change. Despite its advantage, this capacity is limited in time, since the cumulative effect of thermal exposure affected metabolic performance, eventually compromising survival. The study of seahorse response to thermal variations in the context of ocean warming needs to consider the temporal elements of thermal exposure to foresee its vulnerability under future scenarios.

The shape of abundance distributions across temperature gradients in reef fishes

Improving predictions of ecological responses to climate change requires understanding how local abundance relates to temperature gradients, yet many factors influence local abundance in wild populations. We evaluated the shape of thermal‐abundance distributions using 98 422 abundance estimates of 702 reef fish species worldwide. We found that curved ceilings in local abundance related to sea temperatures for most species, where local abundance declined from realised thermal ‘optima’ towards warmer and cooler environments. Although generally supporting the abundant‐centre hypothesis, many species also displayed asymmetrical thermal‐abundance distributions. For many tropical species, abundances did not decline at warm distribution edges due to an unavailability of warmer environments at the equator. Habitat transitions from coral to macroalgal dominance in subtropical zones also influenced abundance distribution shapes. By quantifying the factors constraining species’ abundance, we provide an important empirical basis for improving predictions of community re‐structuring in a warmer world.

Variability among individuals is generated at the gene expression level

Selection acts on individuals, specifically on their differences. To understand adaptation and responses to change therefore requires knowledge of how variation is generated and distributed across traits. Variation occurs on different biological scales, from genetic through physiological to morphological, yet it is unclear which of these carries the most variability. For example, if individual variation is mainly generated by differences in gene expression, variability should decrease progressively from coding genes to morphological traits, whereas if post-translational and epigenetic effects increase variation, the opposite should occur. To test these predictions, we compared levels of variation among individuals in various measures of gene expression, physiology (including activity), and morphology in two abundant and geographically widespread Antarctic molluscs, the clam Laternula elliptica and the limpet Nacella concinna. Direct comparisons among traits as diverse as heat shock protein QPCR assays, whole transcription profiles, respiration rates, burying rate, shell length, and ash-free dry mass were made possible through the novel application of an established metric, the Wentworth Scale. In principle, this approach could be extended to analyses of populations, communities, or even entire ecosystems. We found consistently greater variation in gene expression than morphology, with physiological measures falling in between. This suggests that variability is generated at the gene expression level. These findings have important implications for refining current biological models and predictions of how biodiversity may respond to climate change.

Thermal variation, thermal extremes and the physiological performance of individuals

In this review we consider how small-scale temporal and spatial variation in body temperature, and biochemical/physiological variation among individuals, affect the prediction of organisms' performance in nature. For ‘normal’ body temperatures – benign temperatures near the species' mean – thermal biology traditionally uses performance curves to describe how physiological capabilities vary with temperature. However, these curves, which are typically measured under static laboratory conditions, can yield incomplete or inaccurate predictions of how organisms respond to natural patterns of temperature variation. For example, scale transition theory predicts that, in a variable environment, peak average performance is lower and occurs at a lower mean temperature than the peak of statically measured performance. We also demonstrate that temporal variation in performance is minimized near this new ‘optimal’ temperature. These factors add complexity to predictions of the consequences of climate change. We then move beyond the performance curve approach to consider the effects of rare, extreme temperatures. A statistical procedure (the environmental bootstrap) allows for long-term simulations that capture the temporal pattern of extremes (a Poisson interval distribution), which is characterized by clusters of events interspersed with long intervals of benign conditions. The bootstrap can be combined with biophysical models to incorporate temporal, spatial and physiological variation into evolutionary models of thermal tolerance. We conclude with several challenges that must be overcome to more fully develop our understanding of thermal performance in the context of a changing climate by explicitly considering different forms of small-scale variation. These challenges highlight the need to empirically and rigorously test existing theories.