High thermal stress responses of Echinolittorina snails at their range edge predict population vulnerability to future warming

Coping with harsh heat environments: molecular adaptation of metabolic depression in the intertidal snail Echinolittorina radiata

Harsh thermal environments in the rocky intertidal zone pose serious physiological and molecular challenges to the inhabitants. Metabolic depression is regarded as an energy-conserving feature of intertidal species. To understand the molecular mechanism of metabolic depression, we investigated physiological and transcriptomic responses in the intertidal snail Echinolittorina radiata. The metabolic rate and expression of most genes were insensitive to temperatures ranging from 33 to 45 °C and then increased with further heating to 52 °C. Different from other genes, the genes involved in heat shock response (HSR) and oxidative stress response (OSR) (e.g., genes encoding heat shock protein 70 (HSP70) and cytochrome P450 protein (CYP450)) kept upregulating during metabolic depression. These high levels of HSR and OSR genes should be important for surviving the harsh thermal environments on the rocky shore. In the population experiencing more frequent moderate heat events, the depression breadth was larger, and the change in magnitude of upregulation was insensitive for HSR genes (e.g., HSP70s) but heat-sensitive for OSR genes (e.g., CYP450s) at the temperature of 37 to 45 °C. These findings indicate that both the thermal sensitivity of HSR and OSR genes and the insensitivity of metabolic genes are crucial for surviving extreme intertidal environments, and different populations of the same species rely on various physiological mechanisms to differing extents to deal with heat stress. The cellular stress response is not a "one size fits all" response across populations largely depending on local thermal regimes.

Thermodynamic effects drive countergradient responses in the thermal performance of Littorina saxatilis across latitude

Thermal performance curves (TPCs) provide a powerful framework to assess the evolution of thermal sensitivity in populations exposed to divergent selection regimes across latitude. However, there is a lack of consensus regarding the extent to which physiological adjustments that compensate for latitudinal temperature variation (metabolic cold adaptation; MCA) may alter the shape of TPCs, including potential repercussion on upper thermal limits. To address this, we compared TPCs for cardiac activity in latitudinally-separated populations of the intertidal periwinkle Littorina saxatilis. We applied a non-linear TPC modelling approach to explore how different metrics governing the shape of TPCs varied systematically in response to local adaptation and thermal acclimation. Both critical upper limits, and the temperatures at which cardiac performance was maximised, were higher in the northernmost (cold-adapted) population and displayed a countergradient latitudinal trend which was most pronounced following acclimation to low temperatures. We interpret this response as a knock-on consequence of increased standard metabolic rate in high latitude populations, indicating that physiological compensation associated with MCA may indirectly influence variation in upper thermal limits across latitude. Our study highlights the danger of assuming that variation in any one aspect of the TPC is adaptive without appropriate mechanistic and ecological context.

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.

City limits: Heat tolerance is influenced by body size and hydration state in an urban ant community

Cities are rapidly expanding, and global warming is intensified in urban environments due to the urban heat island effect. Therefore, urban animals may be particularly susceptible to warming associated with ongoing climate change. We used a comparative and manipulative approach to test three related hypotheses about the determinants of heat tolerance or critical thermal maximum (CT max) in urban ants-specifically, that (a) body size, (b) hydration status, and (c) chosen microenvironments influence CT max. We further tested a fourth hypothesis that native species are particularly physiologically vulnerable in urban environments. We manipulated water access and determined CT max for 11 species common to cities in California's Central Valley that exhibit nearly 300-fold variation in body size. There was a moderate phylogenetic signal influencing CT max, and inter (but not intra) specific variation in body size influenced CT max where larger species had higher CT max. The sensitivity of ants' CT max to water availability exhibited species-specific thresholds where short-term water limitation (8 hr) reduced CT max and body water content in some species while longer-term water limitation (32 hr) was required to reduce these traits in other species. However, CT max was not related to the temperatures chosen by ants during activity. Further, we found support for our fourth hypothesis because CT max and estimates of thermal safety margin in native species were more sensitive to water availability relative to non-native species. In sum, we provide evidence of links between heat tolerance and water availability, which will become critically important in an increasingly warm, dry, and urbanized world that others have shown may be selecting for smaller (not larger) body size.

Non-reversible and Reversible Heat Tolerance Plasticity in Tropical Intertidal Animals: Responding to Habitat Temperature Heterogeneity

The theory for thermal plasticity of tropical ectotherms has centered on terrestrial and open-water marine animals which experience reduced variation in diurnal and seasonal temperatures, conditions constraining plasticity selection. Tropical marine intertidal animals, however, experience complex habitat thermal heterogeneity, circumstances encouraging thermal plasticity selection. Using the tropical rocky-intertidal gastropod, Echinolittorina malaccana, we investigated heat tolerance plasticity in terms of laboratory acclimation and natural acclimatization of populations from thermally-dissimilar nearby shorelines. Laboratory treatments yielded similar capacities of snails from either population to acclimate their lethal thermal limit (LT50 variation was ∼2°C). However, the populations differed in the temperature range over which acclimatory adjustments could be made; LT50 plasticity occurred over a higher temperature range in the warm-shore snails compared to the cool-shore snails, giving an overall acclimation capacity for the populations combined of 2.9°C. In addition to confirming significant heat tolerance plasticity in tropical intertidal animals, these findings reveal two plasticity forms, reversible (laboratory acclimation) and non-reversible (population or shoreline specific) plasticity. The plasticity forms should account for different spatiotemporal scales of the environmental temperature variation; reversible plasticity for daily and tidal variations in microhabitat temperature and non-reversible plasticity for lifelong, shoreline temperature conditions. Non-reversible heat tolerance plasticity, likely established after larvae settle on the shore, should be energetically beneficial in preventing heat shock protein overexpression, but also should facilitate widespread colonization of coasts that support thermally-diverse shorelines. This first demonstration of different plasticity forms in benthic intertidal animals supports the hypothesis that habitat heterogeneity (irrespective of latitude) drives thermal plasticity selection. It further suggests that studies not making reference to different spatial scales of thermal heterogeneity, nor seeking how these may drive different thermal plasticity forms, risk misinterpreting ectothermic responses to environmental warming.

Substantial heat tolerance acclimation capacity in tropical thermophilic snails, but to what benefit?

The theory for thermal acclimation of ectotherms suggests that (1) heat tolerance is traded off for thermal acclimation in thermophilic species and that (2) plasticity is constrained in tropically distributed ectotherms, which commonly experience relatively thermally stable environments. We observed substantial heat tolerance plasticity in a test of this theory using tropical, thermophilic marine intertidal snails that inhabit extremely hot and highly variable thermal environments. The implication of these results is that plasticity selection is largely driven by habitat temperature conditions irrespective of basal heat tolerance or latitude. However, heat tolerance of field-fresh snails was comparable with that of laboratory warm-acclimated snails, suggesting that snails in the field may often be unable to improve heat hardening with further environmental warming. These findings suggest that field referencing is crucial to using laboratory-measured acclimation capacity when inferring climate warming vulnerability in ectotherms, and overall they question how well current thermal biology theory predicts the outcomes of global change in intertidal environments.