Physiological determinants of biogeography: The importance of metabolic depression to heat tolerance.

A quantitative understanding of physiological thermal responses is vital for forecasting species distributional shifts in response to climate change. Many studies have focused on metabolic rate as a global metric for analyzing the sublethal effects of changing environments on physiology. Thermal performance curves (TPCs) have been suggested as a viable analytical framework, but standard TPCs may not fully capture physiological responses, due in part to failure to consider the process of metabolic depression. We derived a model based on the nonlinear regression of biological temperature-dependent rate processes and built a heart rate data set for 26 species of intertidal molluscs distributed from 33°S to ~40°N. We then calculated physiological thermal performance limits with continuous heating using T 1 / 2 H , the temperature at which heart rate is decreased to 50% of the maximal rate, as a more realistic measure of upper thermal limits. Results indicate that heat-induced metabolic depression of cardiac performance is a common adaptive response that allows tolerance of harsh environments. Furthermore, our model accounted for the high inter-individual variability in the shape of cardiac TPCs. We then used these TPCs to calculate physiological thermal safety margins (pTSM), the difference between the maximal operative temperature (95th percentile of field temperatures) and T 1 / 2 H of each individual. Using pTSMs, we developed a physiological species distribution model (pSDM) to forecast future geographic distributions. pSDM results indicate that climate-induced species range shifts are potentially less severe than predicted by a simple correlative SDM. Species with metabolic depression below the optimum temperature will be more thermal resistant at their warm trailing edges. High intraspecific variability further suggests that models based on species-level vulnerability to environmental change may be problematic. This multi-scale, mechanistic understanding that incorporates metabolic depression and inter-individual variability in thermal response enables better predictions about the relationship between thermal stress and species distributions.

Tracking coastal acidification from erosion of gastropod shells: spatial sensitivity and organism size effect.

The rapidly changing marine environmental chemistry associated with growing industrialisation, urban population expansion, and the unabated rise in atmospheric CO2 necessitates monitoring. Traditional approaches using metres, dataloggers, and buoys to monitor marine acidification have limited application in coastal oceans and intertidal zones subjected to direct wave action. The present study trialled a system to biomonitor coastal acidification (carbonate ion and pH) based on the dissolution of living gastropod shells. We extended on an approach that ranked shell erosion (SER) in Nerita chamaeleon (Nc) in environments where such erosion was found to correlate with exposure to acidified water. We assessed the spatial scale at which the Nc-SER marker could detect change in acidification along rocky shores, and whether snail body size affected this marker. We found that proportional and unique Nc-SERs not only varied between acidified and non-acidified reference shores at a coarse spatial scale (10 km), but also in predictable ways at fine scales (metres), vertically and horizontally within a shore. Differences between acidified and reference shores in the relationship for snail size and Nc-SER were accentuated by less weathered shells at reference localities, highlighting the value of including small, juvenile snails in monitoring protocols. Gastropod shells are shown to be useful for assessing point sources of acidification and the spatial area of affected coastal zones. This cost-effective and easy-to-use approach (potentially even by citizen-scientists) offers an early warning system of acidification of rocky shore ecosystems, where the deployment of instruments is precluded.

Differences in heat tolerance plasticity between supratidal and intertidal snails indicate complex responses to microhabitat temperature variation.

Tropical intertidal gastropods that experience extreme and highly variable daily temperatures have evolved significant and complex heat tolerance plasticity, comprising components that respond to different timescales of temperature variation. An earlier study showed different plasticity attributes in snails from differently-heated coastlines, suggesting lifelong irreversible responses that matched habitat thermal regimes. To determine whether heat tolerance plasticity varied at a finer, within-shore spatial scale, we compared the responses of supratidal (predominantly shade-dwelling) and intertidal (frequently solar-exposed) populations of the tropical thermophilic gastropod, Echinolittorina malaccana. Snails modified lethal temperature (LT50) under warm or cool laboratory acclimation, with the overall variation in LT50 being greater in the supratidal (56.0-58.0 °C) than in the intertidal population (57.1-58.1 °C). Similar maximum LT50s expressed by the populations after warm acclimation suggest a capacity limitation under these temperature conditons. The different minimum LT50s after cool acclimation corresponded with microhabitat temperature and field acclimatization of the snails. Different responses to the same laboratory acclimation treatment imply long-term (and possibly lifelong) thermal acclimatization, which could benefit sedentary organisms that are randomly recruited as larvae from a common thermally-stable aquatic environment to thermally-unpredictable intertidal microhabitats. These findings provide another example of thermal tolerance plasticity operating at microhabitat scales, suggesting the importance of considering microhabitat thermal responses when assessing broad-scale environmental change.

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.

Decoupling of behavioural and physiological thermal performance curves in ectothermic animals: a critical adaptive trait.

Thermal performance curves (TPCs) represent an increasingly popular tool in ecology for anticipating species responses to climate change. TPC theory has been developed using species that experience similar temperatures during activity and at rest and consequently exhibit thermal ranges for activity that closely coincide with their physiological thermal tolerances. Many species, however, experience other stressors, such as desiccation, that limit active behaviour at temperatures below the maximum values experienced. As a result, activity is constrained to a narrow thermal window that is a subset of the range of temperatures that can be tolerated physiologically. This results in a decoupling of behavioural and physiological TPCs that does not conform to the present paradigm. To test the generality of TPC theory, we measured thermal responses for behaviour (crawling speed) and physiological tolerance (heart rate) for six rocky shore gastropods spanning a thermal/desiccation stress gradient. We hypothesized a positive relationship between shore level and the degree of decoupling of behavioural and physiological TPCs. This prediction was confirmed, and was explained by the extension of the physiological TPC beyond the range of the behavioural TPC. Decoupling of behavioural and physiological TPCs is central to predicting accurately the fitness dynamics of ectothermic species subject to multiple stressors. We believe that this decoupling should be explicitly considered as an adaptive trait defining an organism's thermal niche.

Extracellular Antibody Drug Conjugates Exploiting the Proximity of Two Proteins.

The human Na+/K+-ATPase (NKA) is a plasma membrane ion pump that uses ATP to help maintain the resting potential of all human cells. Inhibition of the NKA leads to cell swelling and death. The results of this investigation show that on cancer cells, the NKA either comes in close proximity to, associate with or complexes to important cancer-related proteins, and thus can be targeted with a new type of precision therapy called the extracellular drug conjugate or EDC. The EDCs reported here exhibit EC50 values in the low to mid-picomolar range, and signal to noise ratios > 1,000:1, both of which are dependent on the cell surface expression of the NKA and corresponding cancer-related target. We demonstrate that a potent small molecule inhibitor of the NKA can be covalently attached to antibodies targeting CD20, CD38, CD56, CD147, or dysadherin, to create a series of selective and powerful EDCs that kill cancer cells extracellularly by a mechanism resembling necrosis. This is therefore a framework for the development of a new type of precision therapy wherein exquisite selectivity is achieved for targeting extracellular disease-related proteins.

Can we predict ectotherm responses to climate change using thermal performance curves and body temperatures?

Thermal performance curves (TPCs), which quantify how an ectotherm's body temperature (Tb ) affects its performance or fitness, are often used in an attempt to predict organismal responses to climate change. Here, we examine the key - but often biologically unreasonable - assumptions underlying this approach; for example, that physiology and thermal regimes are invariant over ontogeny, space and time, and also that TPCs are independent of previously experienced Tb. We show how a critical consideration of these assumptions can lead to biologically useful hypotheses and experimental designs. For example, rather than assuming that TPCs are fixed during ontogeny, one can measure TPCs for each major life stage and incorporate these into stage-specific ecological models to reveal the life stage most likely to be vulnerable to climate change. Our overall goal is to explicitly examine the assumptions underlying the integration of TPCs with Tb , to develop a framework within which empiricists can place their work within these limitations, and to facilitate the application of thermal physiology to understanding the biological implications of climate change.

Fluid physico-chemical properties influence capture and diet in Nepenthes pitcher plants.

BACKGROUND AND AIMS: Nepenthes pitcher plants have evolved modified leaves with slippery surfaces and enzymatic fluids that trap and digest prey, faeces and/or plant detritus. Although the fluid's contribution to insect capture is recognized, the physico-chemical properties involved remain underexplored and may vary among species, influencing their diet type. This study investigates the contributions of acidity and viscoelasticity in the fluid's capture efficiency of two ant and two fly species in four Nepenthes species with different nutrition strategies. METHODS: Four Nepenthes species were studied, namely N. rafflesiana, N. gracilis, N. hemsleyana and N. ampullaria. Fluid was collected from pitchers of varying ages from plants growing in the field and immediately transferred to glass vials, and individual ants (tribe Campotini, Fomicinae) and flies (Calliphora vomitoria and Drosophila melanogaster) were dropped in and observed for 5 min. Water-filled vials were used as controls. Survival and lifetime data were analysed using models applied to right-censored observations. Additional laboratory experiments were carried out in which C. vomitoria flies were immersed in pH-controlled aqueous solutions and observed for 5 min. KEY RESULTS: Pitcher fluid differed among Nepenthes species as regards insect retention capacity and time-to-kill, with differences observed between prey types. Only the fluids of the reputedly insectivorous species were very acidic and/or viscoelastic and retained significantly more insects than the water controls. Viscoelastic fluids were fatal to flies and were able to trap the broadest diversity of insects. Younger viscoelastic fluids showed a better retention ability than older fluids, although with less rapid killing ability, suggesting that a chemical action follows a mechanical one. Insect retention increased exponentially with fluid viscoelasticity, and this happened more abruptly and at a lower threshold for flies compared with ants. Flies were more often retained if they fell into the traps on their backs, thus wetting their wings. Insect retention and death rate increased with fluid acidity, with a lower threshold for ants than for flies, and the time-to-kill decreased with increasing acidity. The laboratory experiments showed that fewer flies escaped from acidic solutions compared with water. CONCLUSIONS: In addition to viscoelasticity, the pitcher's fluid acidity and wetting ability influence the fate of insects and hence the diet of Nepenthes. The plants might select the prey that they retain by manipulating the secretion of H(+) ions and polysaccharides in their pitcher fluid. This in turn might participate in possible adaptive radiation of this genus with regard to nutrient sequestration strategy. These plants might even structurally influence insect fall-orientation and capture-probability, inspiring biomimetic designs for pest control.

Gross mismatch between thermal tolerances and environmental temperatures in a tropical freshwater snail: climate warming and evolutionary implications.

The relationship between acute thermal tolerance and habitat temperature in ectotherm animals informs about their thermal adaptation and is used to assess thermal safety margins and sensitivity to climate warming. We studied this relationship in an equatorial freshwater snail (Clea nigricans), belonging to a predominantly marine gastropod lineage (Neogastropoda, Buccinidae). We found that tolerance of heating and cooling exceeded average daily maximum and minimum temperatures, by roughly 20°C in each case. Because habitat temperature is generally assumed to be the main selective factor acting on the fundamental thermal niche, the discordance between thermal tolerance and environmental temperature implies trait conservation following 'in situ' environmental change, or following novel colonisation of a thermally less-variable habitat. Whereas heat tolerance could relate to an historical association with the thermally variable and extreme marine intertidal fringe zone, cold tolerance could associate with either an ancestral life at higher latitudes, or represent adaptation to cooler, higher-altitudinal, tropical lotic systems. The broad upper thermal safety margin (difference between heat tolerance and maximum environmental temperature) observed in this snail is grossly incompatible with the very narrow safety margins typically found in most terrestrial tropical ectotherms (insects and lizards), and hence with the emerging prediction that tropical ectotherms, are especially vulnerable to environmental warming. A more comprehensive understanding of climatic vulnerability of animal ectotherms thus requires greater consideration of taxonomic diversity, ecological transition and evolutionary history.

K-Pg events facilitated lineage transitions between terrestrial and aquatic ecosystems.

We use dated phylogenetic trees for tetrapod vertebrates to identify lineages that shifted between terrestrial and aquatic ecosystems in terms of feeding or development, and to assess the timing of such events. Both stem and crown lineage ages indicate a peak in transition events in correspondence with the K-Pg mass extinction. This meets the prediction that changes in competitive pressure and resource availability following mass extinction events should facilitate such transitions.

Response of an invasive native wetland plant to environmental flows: implications for managing regulated floodplain ecosystems.

The natural flow regimes of rivers underpin the health and function of floodplain ecosystems. However, infrastructure development and the over-extraction of water has led to the alteration of natural flow regimes, resulting in the degradation of river and floodplain habitats globally. In many catchments, including Australia's Murray-Darling Basin, environmental flows are seen as a potentially useful tool to restore natural flow regimes and manage the degradation of rivers and their associated floodplains. In this paper, we investigated whether environmental flows can assist in controlling an invasive native floodplain plant in Barmah Forest, south-eastern Australia. We experimentally quantified the effects of different environmental flow scenarios, including a shallow (20 cm) and deeper (50 cm) flood of different durations (12 and 20 weeks), as well as drought and soil-saturated conditions, on the growth and survival of seedlings of Juncus ingens, a native emergent macrophyte that has become invasive in some areas of Barmah Forest following river regulation and alteration of natural flow regimes. Three height classes of J. ingens (33 cm, 17 cm and 12 cm) were included in the experiment to explicitly test for relationships between treatments, plant survival and growth, and plant height. We found that seedling mortality occurred in the drought treatment and in the 20-week flood treatments of both depths; however, mortality rates in the flood treatments depended on initial plant height, with medium and short plants (initial heights of ≤17 cm) exhibiting the highest mortality rates. Both the 20 cm and 50 cm flood treatments of only 12 weeks duration were insufficient to cause mortality in any of the height classes; indeed, shoots of plants in the 20 cm flood treatment were able to elongate through the water surface at rapid rates. Our findings have important implications for management of Barmah Forest and floodplain ecosystems elsewhere, as it demonstrates the potential for using environmental flows to limit the spread of invasive plants by targeting a life-stage that is particularly sensitive to prolonged submergence. However, there may be narrow thresholds between the conditions that provide effective control of an invasive species, and those that instead facilitate growth and may promote further invasion.

Compensatory shell thickening in corrosive environments varies between related rocky-shore and estuarine gastropods.

Few studies have considered the capabilities of gastropods living in minerally-deficient acidified coastal waters to compensate for outer shell corrosion or compromised growing edge shell production. We compared inner shell thickening between pristine shells (control) and corroded shells (experiment) of two related intertidal neritid gastropod species from reduced salinity and acidified environments. We predicted that the rocky-shore, Nerita chamaeleon, which has greater access to shell building biomineralization substrates, should better control shell thickness than the estuarine, Neripteron violaceum. Accordingly, N. chameleon was found to compensate perfectly for variation in the thickness of the outer calcitic blocky layer (BL). Optimal shell thickness (OST) was maintained by selective reabsorption of the aperture ridge of the distal shell (aragonitic crossed-lamellar layer, CL) and by increased internal deposition of proximal (older) shell (aragonitic protocrossed lamellar, PCL). Despite greater exposure to acidification and hyposalinity, N. violaceum showed no significant compensatory shell thickening. These findings reveal that shell thickening capability may vary greatly among intertidal gastropods and that this may be constrained by environmental biomineralization substrate availability. Such environmentally-related responses carry implications for predicted future reductions in coastal water pH and salinity.

Quantification, characterization and risk assessment of microplastics from five major estuaries along the northern Bay of Bengal coast.

Microplastics (MPs) as hazardous contaminants has drawn the rapid attention of the general public due to their omnipresence and adverse impacts on ecosystems and human health. Despite this, understanding of MPs contamination levels in the estuarine ecosystems along the Bay of Bengal coast remains very limited. This research focused on the presence, spatial distribution, morpho-chemical characteristics and ecological implications of MPs in water and sediment from five key estuaries (Meghna, Karnaphuli, Matamuhuri, Bakkhali, and Naf rivers) within the Bengal delta. Out of the five estuaries, the Meghna exhibited the least amount of MPs in both surface water (150.00 ± 65.62 items/m3) and sediment (30.56 ± 9.34 items/kg). In contrast, the highest occurrence of MPs was recorded in Karnaphuli river water (350.00 ± 69.22 items/m3) and Matamuhuri river sediment (118.33 ± 26.81 items/kg). ANOVA indicated a statistically significant distinction (p < 0.01) among the examined estuaries. Most identified MPs were fibers and < 0.5 mm in size in both water and sediment samples. Transparent MPs were dominant in both water (42.28%) and sediment (45.22%). Besides violet, red, blue, pink and green colored MPs were also observed. Various polymer types, including PE, PP, PET, PS, Nylon, EVA, and ABS, were detected, with PE being the dominant one. Based on the polymer risk index (PHI), the estuaries were classified as hazard level V, signifying a severe level of MP contamination. However, the potential ecological hazardous index (PHI), potential ecological risk index (RI), and pollution load index (PLI) indicated moderate pollution levels. This study offers initial insights into the pollution caused by MPs in major estuaries of Bengal delta, which policymakers can utilize to implement suitable management strategies.

Metabolic energy sensors (AMPK and SIRT1), protein carbonylation and cardiac failure as biomarkers of thermal stress in an intertidal limpet: linking energetic allocation with environmental temperature during aerial emersion.

The effects of heat stress on organisms are manifested at the levels of organ function, metabolic activity, protein stability and gene expression. Here, we examined effects of high temperature on the intertidal limpet Cellana toreuma to determine how the temperatures at which (1) organ failure (cardiac function), (2) irreversible protein damage (carbonylation) and (3) expression of genes encoding proteins involved in molecular chaperoning (hsp70 and hsp90) and metabolic regulation (ampk and sirt1) occur compare with field temperatures, which commonly exceed 30°C and can reach 46°C. Heart failure, indexed by the Arrhenius break temperature, occurred at 34.3°C. Protein carbonylation rose significantly at 38°C. Genes for heat shock proteins HSP70 (hsp70) and HSP90 (hsp90), for two subunits of AMP-activated protein kinase (AMPK) (ampkα and ampkß) and for histone/protein deacetylase SIRT1 (sirt1) all showed increased expression at 30°C. Temperatures of maximal expression differed among genes, as did temperatures at which upregulation ceased. Expression patterns for ampk and sirt1 indicate that heat stress influenced cellular energy homeostasis; above ~30°C, upregulation of ATP-generating pathways is suggested by elevated expression of genes for ampk; an altered balance between reliance on carbohydrate and lipid fuels is indicated by changes in expression of sirt1. These results show that C. toreuma commonly experiences temperatures that induce expression of genes associated with the stress response (hsp70 and hsp90) and regulation of energy metabolism (ampk and sirt1). At high temperatures, there is likely to be a shift away from anabolic processes such as growth to catabolic processes, to provide energy for coping with stress-induced damage, notably to proteins.

Clay-shielded estuarine gastropods are better protected against environmental acidification than unshielded individuals.

The effects of progressive global acidification on the shells of marine organisms is a topic of much current interest. Most studies on molluscan shell resistance to dissolution consider the carbonate mineral component, with less known about the protective role of the outer organic periostracum. Outer-shell resistance would seem especially important to gastropods living in carbonate-undersaturated and calcium-deficient estuarine waters that threaten shell dissolution and constrain CaCO3 production. We tested this prediction using gastropods from an acidified estuarine population (Neripteron violaceum) that form a clay shield outside the periostracum. Specifically, we aimed to show that the carbonate shell component lacks integrity, that the formation of the clay shield is directed by the organism, and that the clay shield functions to protect against shell dissolution. We found no evidence for any specific carbonate dissolution resistance strategy in the thin, predominantly aragonitic shells of these gastropods. Shield formation was directed by an ornamented periostracum which strongly bonded illite elements (e.g., Fe, Al and S), that become available through suspension in the water column. In unshielded individuals, CaCO3 erosion was initiated randomly across the shell (not age-related) and progressed rapidly when the periostracum was breached. A light reflectance technique showed qualitatively that shield consolidation is negatively-related to shell erosion. These findings support a conceptual framework for gastropod outer-shell responses to acidification that considers both environmental and evolutionary constraints on shell construction. We describe a novel strategy for shell protection against dissolution, highlighting the diversity of mechanisms available to gastropods facing extreme coastal acidification.

Organismal Responses to Coastal Acidification Informed by Interrelating Erosion, Roundness and Growth of Gastropod Shells.

urrent understanding of how calcifying organisms respond to externally forced oceanic and coastal acidification (OCA) is largely based on short-term, controlled laboratory or mesocosm experiments. Studies on organismal responses to acidification (reduced carbonate saturation and pH) in the wild, where animals simultaneously interact with a range of biotic and abiotic circumstances, are limited in scope and interpretation. The present study aimed to better understand how gastropod shell attributes and their interrelations can inform about responses to coastal acidification. We investigated shell chemical erosion, shell roundness, and growth rate of Planaxis sulcatus snails, which are locally exposed to acidified and non-acidified rocky intertidal water. We tested a new approach to quantifying shell erosion based on the spiral suture length (EI, erosion index) and found that shell erosion mirrored field acidification conditions. Exposure to acidification caused shells to become rounder (width/length). Field growth rate, determined from apertural margin extension of marked and later recaptured snails, was strongly negatively related to both shell erosion and shell roundness. Since different shell attributes are indicative of different relationships-shell erosion is an extrinsic passive marker of acidification, and shell roundness and growth rate are intrinsic performance responders-analyzing their interrelations can imply causation, enhance predictive power, and bolster interpretation confidence. This study contributes to the methodology and interpretation of findings of trait-based field investigations to understand organismal responses to coastal acidification.

Conservation of thermal physiology in tropical intertidal snails following an evolutionary transition to a cooler ecosystem: climate change implications.

Predictions for animal responses to climate warming usually assume that thermal physiology is adapted to present-day environments, and seldom consider the influence of evolutionary background. Little is known about the conservation of warm-adapted physiology following an evolutionary transition to a cooler environment. We used cardiac thermal performance curves (cTPCs) of six neritid gastropod species to study physiological thermal trait variation associated with a lineage transition from warmer rocky shores to cooler mangroves. We distinguished between functional thermal performance traits, related to energy homeostasis (slope gradient, slope curvature, HRmax, maximum cardiac activity and Topt, the temperature that maximizes cardiac activity) and a trait that limits performance (ULT, the upper lethal temperature). Considering the theory of optimal thermal performance, we predicted that the functional traits should be under greater selective pressure to change directionally and in magnitude than the thermal limit, which is redundant in the cooler environment. We found little variation in all traits across species, habitats and ecosystems, despite a ~20°C reduction in maximum habitat temperature in the mangrove species over 50 million years. While slope gradient was significantly lowered in the mangrove species, the effect difference was negated by greater thermal plasticity in the rocky shore species. ULT showed the least variation and suggested thermal specialization in the warmest habitat studied. The observed muted variation of the functional traits among the species may be explained by their limited role in energy acquisition and rather their association with heat tolerance adaptation, which is redundant in the mangrove species. These findings have implications for the conservation of habitat of intertidal gastropods that transition to cooler environments. Furthermore, they highlight the significance of evolutionary history and physiological conservation when predicting species responses to climate change.

Microplastic accumulation in oysters along a Bornean coastline (Brunei, South China Sea): Insights into local sources and sinks.

The number of studies on microplastic accumulation in marine organisms has increased precipitously recently, though information is geographically-skewed and limited in terms of local effects. We characterized microplastic accumulation in oysters (Saccostrea cucullata) along a Bornean coastline, focusing on spatial variation. Comparisons were made between locally-polluted (Brunei Estuarine System, BES) and relatively pristine, open-shore (South China Sea, SCS) coastlines. Sixteen coloured microplastic types were characterized into three shapes (fragments, fibres, pellets). Fragments (74.9%), especially smaller polypropylene black fragments predominated in the samples (<50 µm, 31.7%). Site-specific levels of microplastic accumulation varied from 0.43 to 7.20 particles/g oyster tissue. BES and SCS sites differed qualitatively, indicating limited interaction. In the BES, accumulation was greatest near the predicted source (Bandar) and declined strongly seawards, implying current flow, environmental sequestration (local sinks) and seawater dilution effects. Such local-scale variation in microplastic loading in estuaries cautions against extrapolating from limited sampling.

Warming reduces metabolic rate in marine snails: adaptation to fluctuating high temperatures challenges the metabolic theory of ecology.

The universal temperature-dependence model (UTD) of the metabolic theory of ecology (MTE) proposes that temperature controls mass-scaled, whole-animal resting metabolic rate according to the first principles of physics (Boltzmann kinetics). Controversy surrounds the model's implication of a mechanistic basis for metabolism that excludes the effects of adaptive regulation, and it is unclear how this would apply to organisms that live in fringe environments and typically show considerable metabolic adaptation. We explored thermal scaling of metabolism in a rocky-shore eulittoral-fringe snail (Echinolittorina malaccana) that experiences constrained energy gain and fluctuating high temperatures (between 25°C and approximately 50°C) during prolonged emersion (weeks). In contrast to the prediction of the UTD model, metabolic rate was often negatively related to temperature over a benign range (30-40°C), the relationship depending on (i) the temperature range, (ii) the degree of metabolic depression (related to the quiescent period), and (iii) whether snails were isolated within their shells. Apparent activation energies (E) varied between 0.05 and -0.43 eV, deviating excessively from the UTD's predicted range of between 0.6 and 0.7 eV. The lowering of metabolism when heated should improve energy conservation in a high-temperature environment and challenges both the theory's generality and its mechanistic basis.

Thermal adaptation in the intertidal snail Echinolittorina malaccana contradicts current theory by revealing the crucial roles of resting metabolism.

Contemporary theory for thermal adaptation of ectothermic metazoans focuses on the maximization of energy gain and performance (locomotion and foraging). Little consideration is given to the selection for mechanisms that minimize resting energy loss in organisms whose energy gain is severely constrained. We tested a hypothetical framework for thermal performance of locomotor activity (a proxy for energy gain) and resting metabolism (a proxy for energy loss) in energetically compromised snails in the littoral fringe zone, comparing this with existing theory. In contrast to theory, the thermal ranges and optima for locomotor performance and metabolic performance of Echinolittorina malaccana are mismatched, and energy gain is only possible at relatively cool temperatures. To overcome thermal and temporal constraints on energy gain while experiencing high body temperatures (23-50°C), these snails depress resting metabolism between 35 and 46°C (thermally insensitive zone). The resulting bimodal relationship for metabolism against temperature contrasts with the unimodal or exponential relationships of most ectotherms. Elevation of metabolism above the breakpoint temperature for thermal insensitivity (46°C) coincides with the induction of a heat shock response, and has implications for energy expenditure and natural selection. Time-dependent mortality is initiated at this breakpoint temperature, suggesting a threshold above which the rate of energy demand exceeds the capacity for cellular energy generation (rate of ATP turnover). Mortality in a thermal range that elevates rather than limits aerobic metabolism contrasts with the hypothesis that cellular oxygen deficiency underlies temperature-related mortality. The findings of this study point to the need to incorporate aspects of resting metabolism and energy conservation into theories of thermal adaptation.