A comparative analysis of the upper thermal tolerance limits of eastern Pacific porcelain crabs, genus Petrolisthes: influences of latitude, vertical zonation, acclimation, and phylogeny

Marine intertidal organisms are subjected to a variety of abiotic stresses, including aerial exposure and wide ranges of temperature. Intertidal species generally have higher thermal tolerance limits than do subtidal species, and tropical species have higher thermal tolerance limits than do temperate species. The adaptive significance of upper thermal tolerance limits of intertidal organisms, however, has not been examined within a comparative context. Here, we present a comparative analysis of the adaptive significance of upper thermal tolerance limits in 20 congeneric species of porcelain crabs, genus Petrolisthes, from intertidal and subtidal habitats throughout the eastern Pacific. Upper thermal tolerance limits are positively correlated with surface water temperatures and with maximal microhabitat temperatures. Analysis of phylogenetically independent contrasts (from a phylogenetic tree on the basis of the 16s rDNA gene sequence) suggests that upper thermal tolerance limits have evolved in response to maximal microhabitat temperatures. Upper thermal tolerance limits increased during thermal acclimation at elevated temperatures, the amount of increase being greater for subtidal than for intertidal species. This result suggests that the upper thermal tolerance limits of some intertidal species may be near current habitat temperature maxima, and global warming thus may affect the distribution limits of intertidal species to a greater extent than for subtidal species.     

Upper temperature tolerances of three molluscs from South African sandy beaches

The upper temperature tolerances of two South African bivalves, Donax serra Röding and D. sordidus Hanley, and a gastropod, Bullia rhodostoma (Reeve), from sandy beaches in Algoa Bay. were compared by means of median lethal temperatures (LT₅₀) and median burial temperature (BT₅₀) determinations for periods of exposure up to 96 h. Donax serra and D. sordidus adults showed a similar temperature tolerance of 29°C. D. serra juveniles showed a lower tolerance of 27°C. Bullia rhodostoma had a slightly higher thermal tolerance (≈ 31°C) than the bivalves, with small individuals having a greater thermal tolerance than large individuals for the longer exposure periods. These thermal tolerances are discussed in relation to distribution, and compared with those of related species from European waters.     

Differential thermal tolerance between algae and corals may trigger the proliferation of algae in coral reefs

Marine heatwaves can lead to rapid changes in entire communities, including in the case of shallow coral reefs the potential overgrowth of algae. Here we tested experimentally the differential thermal tolerance between algae and coral species from the Red Sea through the measurement of thermal performance curves and the assessment of thermal limits. Differences across functional groups (algae vs. corals) were apparent for two key thermal performance metrics. First, two reef‐associated algae species (Halimeda tuna and Turbinaria ornata) had higher lethal thermal limits than two coral species (Pocillopora verrucosa and Stylophora pistillata) conferring those species of algae with a clear advantage during heatwaves by surpassing the thermal threshold of coral survival. Second, the coral species had generally greater deactivation energies for net and gross primary production rates compared to the algae species, indicating greater thermal sensitivity in corals once the optimum temperature is exceeded. Our field surveys in the Red Sea reefs before and after the marine heatwave of 2015 show a change in benthic cover mainly in the southern reefs, where there was a decrease in coral cover and a concomitant increase in algae abundance, mainly turf algae. Our laboratory and field observations indicate that a proliferation of algae might be expected on Red Sea coral reefs with future ocean warming.     

THE EVOLUTION OF ENVIRONMENTAL TOLERANCE AND RANGE SIZE: A COMPARISON OF GEOGRAPHICALLY RESTRICTED AND WIDESPREAD MIMULUS

The geographic ranges of closely related species can vary dramatically, yet we do not fully grasp the mechanisms underlying such variation. The niche breadth hypothesis posits that species that have evolved broad environmental tolerances can achieve larger geographic ranges than species with narrow environmental tolerances. In turn, plasticity and genetic variation in ecologically important traits and adaptation to environmentally variable areas can facilitate the evolution of broad environmental tolerance. We used five pairs of western North American monkeyflowers to experimentally test these ideas by quantifying performance across eight temperature regimes. In four species pairs, species with broader thermal tolerances had larger geographic ranges, supporting the niche breadth hypothesis. As predicted, species with broader thermal tolerances also had more within‐population genetic variation in thermal reaction norms and experienced greater thermal variation across their geographic ranges than species with narrow thermal tolerances. Species with narrow thermal tolerance may be particularly vulnerable to changing climatic conditions due to lack of plasticity and insufficient genetic variation to respond to novel selection pressures. Conversely, species experiencing high variation in temperature across their ranges may be buffered against extinction due to climatic changes because they have evolved tolerance to a broad range of temperatures.     

Aerial exposure and body temperature of the intertidal sea anemone Anthopleura elegantissima

The sea anemone Anthopleura elegantissima is a common member of intertidal communities along the west coast of North America, and can experience extended periods of increased temperature during summertime low tides. Internal body temperatures of emersed individuals of A. elegantissima were monitored in a laboratory wind tunnel and in the field, and factors influencing the anemones’ thermal experience were examined. Larger body size and aggregation with conspecifics slowed body temperature increases in controlled wind tunnel conditions. In the field, anemones in the interior of an aggregation stayed cooler than those on the edges, and microhabitat features related to light exposure and surface orientation overshadowed any direct effects of body size. In the warmest month only (July), aggregations of A. elegantissima were significantly larger at the upper limit of their distribution than they were at the mid and lower limits, suggesting aggregation in high intertidal zones may be a behavioral response to desiccation and temperature stress. As this sea anemone can host multiple species of symbiotic algae with different thermal tolerances, the ability to slow body heating may affect the type of algae hosted and thus the potential contribution of this abundant anemone to primary production in the intertidal zone.     

Integrating metabolic performance,thermal tolerance,and plasticity enables for more accurate predictions on species vulnerability to acute and chronic effects of global warming

Predicting species vulnerability to global warming requires a comprehensive, mechanistic understanding of sublethal and lethal thermal tolerances. To date, however, most studies investigating species physiological responses to increasing temperature have focused on the underlying physiological traits of either acute or chronic tolerance in isolation. Here we propose an integrative, synthetic approach including the investigation of multiple physiological traits (metabolic performance and thermal tolerance), and their plasticity, to provide more accurate and balanced predictions on species and assemblage vulnerability to both acute and chronic effects of global warming. We applied this approach to more accurately elucidate relative species vulnerability to warming within an assemblage of six caridean prawns occurring in the same geographic, hence macroclimatic, region, but living in different thermal habitats. Prawns were exposed to four incubation temperatures (10, 15, 20 and 25 °C) for 7 days, their metabolic rates and upper thermal limits were measured, and plasticity was calculated according to the concept of Reaction Norms, as well as Q₁₀ for metabolism. Compared to species occupying narrower/more stable thermal niches, species inhabiting broader/more variable thermal environments (including the invasive Palaemon macrodactylus) are likely to be less vulnerable to extreme acute thermal events as a result of their higher upper thermal limits. Nevertheless, they may be at greater risk from chronic exposure to warming due to the greater metabolic costs they incur. Indeed, a trade‐off between acute and chronic tolerance was apparent in the assemblage investigated. However, the invasive species P. macrodactylus represents an exception to this pattern, showing elevated thermal limits and plasticity of these limits, as well as a high metabolic control. In general, integrating multiple proxies for species physiological acute and chronic responses to increasing temperature helps providing more accurate predictions on species vulnerability to warming.     

Thermal tolerance of sympatric hymenopteran parasitoid species: does it match seasonal activity?

Climatic changes result in an increased in mean temperature and in a higher incidence of extreme weather events such as heat and cold waves. For ectotherms, such as insect parasitoids, the ability to remain active under extreme climatic conditions is a significant key to fitness. The body size of individuals, and in particular their surface to volume ratio, may play a role in their resistance to thermal conditions. The thermal tolerances are investigated of two closely‐related sympatric parasitoid species [Aphidius avenae Haliday and Aphidius rhopalosiphi De Stefani‐Perez (Hymenoptera: Aphidiinae)] that have a similar ecology but differ in body size and phenologies. The critical thermal limits of individuals are assessed in both sexes of each parasitoid species and the influence of surface–volume ratios on their thermal tolerances. Aphidius avenae is less resistant to low temperatures and more resistant to high temperatures than A. rhopalosiphi. The lower surface to volume ratio of A. avenae individuals may help them to remain active in summer when experiencing heat waves. However, body size is not the sole factor that plays a role in differences of thermal tolerance between species and body size may not be an adaptation to extreme temperatures but rather a by‐product of developmental regulation. Closely‐related sympatric species from the same ecological guild can have different thermal tolerances that may allow them to occur within the same habitat. The present study also highlights the importance of clearly defining how to measure critical thermal limits to determine the thermal tolerance of a species.     

Species interactions and thermal constraints on ant community structure

Patterns of species occurrence and abundance are influenced by abiotic factors and biotic interactions, but these factors are difficult to disentangle without experimental manipulations. In this study, we used observational and experimental approaches to investigate the role of temperature and interspecific competition in controlling the structure of ground‐foraging ant communities in forests of the Siskiyou Mountains of southwestern Oregon. To assess the potential role of competition, we first used null model analyses to ask whether species partition temporal and/or spatial environments. To understand how thermal tolerances influence the structure of communities, we conducted a laboratory experiment to estimate the maximum thermal tolerance of workers and a field experiment in which we added shaded microhabitats and monitored the response of foragers. Finally, to evaluate the roles of temperature and interspecific competition in the field, we simultaneously manipulated shading and the presence of a dominant competitor (Formica moki). The foraging activity of species broadly overlapped during the diurnal range of temperatures. Species co‐occurrence patterns varied across the diurnal temperature range: species were spatially segregated at bait stations at low temperatures, but co‐occurred randomly at high temperatures. The decreased abundance of the co‐occurring thermophilic Temnothorax nevadensis in shaded plots was a direct effect of shading and not an indirect effect of competitive interactions. Thermal tolerance predicted the response of ant species to the shading experiment: species with the lowest tolerances to high temperatures showed the greatest increase in abundance in the shaded plots. Moreover, species with more similar thermal tolerance values segregated more frequently on baits than did species that differed in their thermal tolerances. Collectively, our results suggest that thermal tolerances of ants may mediate competitive effects in habitats that experience strong diurnal temperature fluctuations.     

Differential thermal tolerance and energetic trajectories during ontogeny in porcelain crabs,genus Petrolisthes

Thermal tolerance limits of marine intertidal zone organisms are elevated compared to subtidal species, but are typically just slightly higher than maximal habitat temperatures. The small thermal safety margins maintained by intertidal zone organisms suggest that high thermal tolerance is associated with a physiological cost. If true, we hypothesize that species that transition between intertidal zone and planktonic habitats during ontogeny, will adjust their thermal tolerance accordingly to capitalize upon potential energy savings while in a thermally benign habitat. We tested this hypothesis in porcelain crabs that transition between the thermally stressful, intertidal zone as embryos, to the thermally benign pelagic zone as larvae, and back at settlement. We found the more thermally tolerant, mid-intertidal zone species, Petrolisthes cinctipes, and the less thermally tolerant, subtidal zone species, Petrolisthes manimacilis, exhibited reduced thermal tolerance (LT₅₀) in the transition from embryos to larvae. This was associated with an increased oxygen consumption rate in both species, though P. cinctipes exhibited a significantly greater increase in oxygen consumption. P. cinctipes also showed an increase in thermal tolerance in settled juveniles compared to pelagic zoea I larvae, resulting in an overall V-shaped thermal tolerance relationship during ontogeny, while in P. manimaculis thermal tolerance was significantly lower in juveniles compared to zoea I. In neither species were these changes (zoea I to juvenile) associated with a significant change in metabolism. While embryos and juveniles of P. cinctipes have thermal tolerance limits near intertidal habitat thermal maxima (∼32.5 °C), all three life-history stages in P. manimaculis (especially embryos and larvae) exhibit considerable thermal safety margins. The mechanisms underlying this “excess” thermal tolerance in P. manimacilis embryos are unknown, but suggest that patterns of thermal tolerance in early life history stages are species-specific.     

Temperature Tolerances of North American Freshwater Fishes Exposed to Dynamic Changes in Temperature

Traditionally lower and upper temperature tolerances of fishes have been quantified in the laboratory via three different experimental approaches: the Fry or incipient lethal temperature (ILT), critical thermal (CTM) and chronic lethal (CLM) methodologies. Although these three experimental laboratory approaches generate endpoints which are quantitatively expressed as a temperature, are determined experimentally with random samples of fish acclimated to specific temperatures, and involve both time and temperature as major test variables, they do not quantify the same response. All three approaches generate valuable, albeit different, information concerning the temperature tolerance of a species. In this review we have summarized published research concerning the tolerance of North American freshwater fishes to dynamic changes in temperature, i.e., tolerance is tested by methods that gradually change temperatures until biological stress is observed. We found more than 450 individual temperature tolerances listed in 80 publications which present original dynamic temperature tolerance data for 116 species, 7 subspecies and 7 hybrids from 19 families of North American freshwater fishes. This total represents about 1/3 of the families and 1/6 of the known North American freshwater species. Temperature tolerance data were partitioned by experimental approach, i.e., critical thermal method (CTM) and chronic lethal method (CLM), and direction of temperature change. Although both CTM and CLM expose fish to dynamic changes in water temperature, these two methods differ in temperature change rates and test endpoints, and hence measure different aspects of thermal stress. A majority of the 80 studies employed CTM to assess temperature tolerance, in particular determination of CTmaxima. One or more CTmaxima has been reported for 108 fishes. Twenty-two fishes have reported highest CTmaxima of 40°C or higher. Several species in the family Cyprinodontidae have generated some of the highest CTmaxima reported for any ectothermic vertebrate. For a variety of reasons, data concerning tolerance of low temperatures are less plentiful. Low temperature tolerance quantified as either CTminima or CLminima were found for a total of 37 fishes. Acclimation temperature exerts a major effect on the temperature tolerance of most North American fish species and it is usually strongly linearly related to both CTmaxima and CTminima. Although we uncovered dynamic temperature tolerance data for 130 fishes, only a single dynamic, temperature tolerance polygon has been published, that for the sheepshead minnow, Cyprinodon variegatus.     

Heat tolerance in Drosophila subobscura along a latitudinal gradient: Contrasting patterns between plastic and genetic responses

Susceptibility to global warming relies on how thermal tolerances respond to increasing temperatures through plasticity or evolution. Climatic adaptation can be assessed by examining the geographic variation in thermal‐related traits. We studied latitudinal patterns in heat tolerance in Drosophila subobscura reared at two temperatures. We used four static stressful temperatures to estimate the thermal death time (TDT) curves, and two ramping assays with fast and slow heating rates. Thermal death time curves allow estimation of the critical thermal maximum (CT_max), by extrapolating to the temperature that would knock down the flies almost “instantaneously,” and the thermal sensitivity to increasing stressful temperatures. We found a positive latitudinal cline for CT_max, but no clinal pattern for knockdown temperatures estimated from the ramping assays. Although high‐latitude populations were more tolerant to an acute heat stress, they were also more sensitive to prolonged exposure to less stressful temperatures, supporting a trade‐off between acute and chronic heat tolerances. Conversely, developmental plasticity did not affect CT_max but increased the tolerance to chronic heat exposition. The patterns observed from the TDT curves help to understand why the relationship between heat tolerance and latitude depends on the methodology used and, therefore, these curves provide a more complete and reliable measurement of heat tolerance.     

Thermal Buffering of Microhabitats is a Critical Factor Mediating Warming Vulnerability of Frogs in the Philippine Biodiversity Hotspot

Species may circumvent the impacts of climate warming if the habitats they use reduce ambient temperature. In this study, we identified which frog species from a tropical montane rain forest in the Philippines may be vulnerable to climate warming. To do so, we selected five anuran species that utilize four breeding habitats and identified the sensitivity and exposure of tadpoles and direct‐developer eggs to heat by measuring their critical thermal maximums (CT_max) and the habitat‐specific temperatures they experience. Our study species included two direct‐developer frogs—one species that lays its eggs on exposed leaves, and another that lays its eggs in ferns—and three species that produce aquatic free‐swimming tadpoles—two stream breeders, and one phytotelm (tree hole) breeder. We compared thermal tolerances derived from microclimates of breeding habitats with tolerances derived from macroclimate (i.e., non‐buffered air temperature taken from the rain forest canopy). We also examined whether differences in CT_max existed across life‐history stages (egg, metamorph/young‐of‐year, and adult) for the two direct‐developer frog species. Habitats buffered ambient temperature and expanded thermal tolerances of all frog species. We found that direct‐developers, however, are more vulnerable to increased temperatures than aquatic breeders—indicated by their high sensitivity to temperature, and exposure to high temperatures. Direct‐developer eggs were more sensitive to warming than both metamorph and adult life‐history stages. Thermally buffered microhabitats may represent the only protection against current and impending climate warming. Our data highlight the importance of considering sensitivity and exposure in unison when deciphering warming vulnerability of frogs.     

Scale‐dependent thermal tolerance variation in Australian mountain grasshoppers

Physiological variation among and within species is thought to play a key role in determining distribution patterns across environmental gradients. We tested inter‐ and intraspecific variation in cold and heat tolerances for three grasshopper species (genus Kosciuscola) with overlapping elevation distributions, across their respective ranges in the Australian mountains. Of the three cold tolerance traits measured, the critical thermal minimum was the only trait to vary among species, with greater cold tolerance associated with a distribution extending to a higher elevation. Cold tolerance limits were regularly exceeded in exposed microhabitats, suggesting a role for cold adaptation in structuring species distribution patterns. In contrast to cold tolerance, heat tolerance variation was primarily partitioned within species. For two species, populations from treeless alpine habitat were more heat tolerant than their lower‐elevation counterparts, supporting recent models that suggest greater exposure to temperature extremes at higher elevations. These contrasting patterns of physiological variation among and within species emphasise the importance of considering variation within species when attempting to understand how species distributions are affected by thermal extremes.     

Thermal Reaction Norms and the Scale of Temperature Variation: Latitudinal Vulnerability of Intertidal Nacellid Limpets to Climate Change

The thermal reaction norms of 4 closely related intertidal Nacellid limpets, Antarctic (Nacella concinna), New Zealand (Cellana ornata), Australia (C. tramoserica) and Singapore (C. radiata), were compared across environments with different temperature magnitude, variability and predictability, to test their relative vulnerability to different scales of climate warming. Lethal limits were measured alongside a newly developed metric of “duration tenacity”, which was tested at different temperatures to calculate the thermal reaction norm of limpet adductor muscle fatigue. Except in C. tramoserica which had a wide optimum range with two break points, duration tenacity did not follow a typical aerobic capacity curve but was best described by a single break point at an optimum temperature. Thermal reaction norms were shifted to warmer temperatures in warmer environments; the optimum temperature for tenacity (T_opt) increased from 1.0°C (N. concinna) to 14.3°C (C. ornata) to 18.0°C (an average for the optimum range of C. tramoserica) to 27.6°C (C. radiata). The temperature limits for duration tenacity of the 4 species were most consistently correlated with both maximum sea surface temperature and summer maximum in situ habitat logger temperature. Tropical C. radiata, which lives in the least variable and most predictable environment, generally had the lowest warming tolerance and thermal safety margin (WT and TSM; respectively the thermal buffer of CT_max and T_opt over habitat temperature). However, the two temperate species, C. ornata and C. tramoserica, which live in a variable and seasonally unpredictable microhabitat, had the lowest TSM relative to in situ logger temperature. N. concinna which lives in the most variable, but seasonally predictable microhabitat, generally had the highest TSMs. Intertidal animals live at the highly variable interface between terrestrial and marine biomes and even small changes in the magnitude and predictability of their environment could markedly influence their future distributions.     

Thermal specialization across large geographical scales predicts the resilience of mangrove crab populations to global warming

The broad prediction that ectotherms will be more vulnerable to climate change in the tropics than in temperate regions includes assumptions about centre/edge population effects that can only be tested by within‐species comparisons across wide latitudinal gradients. Here, we investigated the thermal vulnerability of two mangrove crab species, comparing populations at the centre (Kenya) and edge (South Africa) of their distributions. At the same time, we investigated the role of respiratory mode (water‐ versus air‐breathing) in determining the thermal tolerance in amphibious organisms. To do this, we compared the vulnerability to acute temperature fluctuations of two sympatric species with two different lifestyle adaptations: the free living Perisesarma guttatum and the burrowing Uca urvillei, both pivotal to the ecosystem functioning of mangroves. The results revealed the air‐breathing U. urvillei to be a thermal generalist with much higher thermal tolerances than P. guttatum. Importantly, however, we found that, while U. urvillei showed little difference between edge and centre populations, P. guttatum showed adaptation to local conditions. Equatorial populations had elevated tolerances to acute heat stress and mechanisms of partial thermoregulation, which make them less vulnerable to global warming than temperate conspecifics. The results reveal both the importance of respiratory mode to thermal tolerance and the unexpected potential for low latitude populations/species to endure a warming climate. The results also contribute to a conceptual model on the latitudinal thermal tolerance of these key species. This highlights the need for an integrated population‐level approach to predict the consequences of climate change.     

Heat freezes niche evolution

Climate change is altering phenology and distributions of many species and further changes are projected. Can species physiologically adapt to climate warming? We analyse thermal tolerances of a large number of terrestrial ectotherm (n = 697), endotherm (n = 227) and plant (n = 1816) species worldwide, and show that tolerance to heat is largely conserved across lineages, while tolerance to cold varies between and within species. This pattern, previously documented for ectotherms, is apparent for this group and for endotherms and plants, challenging the longstanding view that physiological tolerances of species change continuously across climatic gradients. An alternative view is proposed in which the thermal component of climatic niches would overlap across species more than expected. We argue that hard physiological boundaries exist that constrain evolution of tolerances of terrestrial organisms to high temperatures. In contrast, evolution of tolerances to cold should be more frequent. One consequence of conservatism of upper thermal tolerances is that estimated niches for cold‐adapted species will tend to underestimate their upper thermal limits, thereby potentially inflating assessments of risk from climate change. In contrast, species whose climatic preferences are close to their upper thermal limits will unlikely evolve physiological tolerances to increased heat, thereby being predictably more affected by warming.     

Exposure to solar radiation drives organismal vulnerability to climate: Evidence from an intertidal limpet

Understanding the physiological abilities of organisms to cope with heat stress is critical for predictions of species’ distributions in response to climate change. We investigated physiological responses (respiration and heart beat rate) of the ectotherm limpet Patella vulgata to heat stress events during emersion and the role of seasonal and microclimatic acclimatization for individual thermal tolerance limits. Individuals were collected from 5 microhabitats characterized by different exposure to solar radiation in the high intertidal zone of a semi-exposed rocky shore in winter and summer of 2014. Upper thermal tolerance limits (heat coma temperatures – HCTs, and heart rate Arrhenius break temperatures - ABTs) were determined for individuals from each microhabitat in both seasons under laboratory conditions. While we found a clear seasonal acclimatization, i.e., higher HCTs and ABTs in summer than in winter, we did not find evidence for microhabitat-specific responses that would suggest microclimatic acclimatization. However, operative limpet temperatures derived from in-situ temperature measurements suggest that individuals from sun exposed microhabitats have a much narrower thermal safety margins than those from less exposed surfaces or within crevices. Microhabitat specific thermal safety margins caused by high thermal heterogeneity at small spatial scales and the lack of short term acclimatization will likely shape small scale distribution patterns of intertidal species in response to the predicted increase in the frequency and intensity of heat waves.     

Thermal tolerance and geographical range size in the Agabus brunneus group of European diving beetles (Coleoptera: Dytiscidae)

Aim Within clades, most taxa are rare, whilst few are common, a general pattern for which the causes remain poorly understood. Here we investigate the relationship between thermal performance (tolerance and acclimation ability) and the size of a species’ geographical range for an assemblage of four ecologically similar European diving beetles (the Agabus brunneus group) to examine whether thermal physiology relates to latitudinal range extent, and whether Brown’s hypothesis and the environmental variability hypothesis apply to these taxa. Location Europe. Methods In order to determine the species tolerances to either low or high temperatures we measured the lethal thermal limits of adults, previously acclimated at one of two temperatures, by means of thermal ramping experiments (± 1°C min^?1). These measures of upper and lower thermal tolerances (UTT and LTT respectively) were then used to estimate each species’ thermal tolerance range, as total thermal tolerance polygons and marginal UTT and LTT thermal polygons. Results Overall, widespread species have higher UTTs and lower LTTs than restricted ones. Mean upper lethal limits of the Agabus brunneus group (43 to 46°C), are similar to those of insects living at similar latitudes, whilst mean lower lethal limits (?6 to ?9°C) are relatively high, suggesting that this group is not particularly cold‐hardy compared with other mid‐temperate‐latitude insects. Widespread species possess the largest thermal tolerance ranges and have a relatively symmetrical tolerance to both high and low temperatures, when compared with range‐restricted relatives. Over the temperature range employed, adults did not acclimate to either high or low temperatures, contrasting with many insect groups, and suggesting that physiological plasticity has a limited role in shaping distribution. Main conclusions Absolute thermal niche appears to be a good predictor of latitudinal range, supporting both Brown’s hypothesis and the environmental variability hypothesis. Restricted‐range species may be more susceptible to the direct effect of climate change than widespread species, notwithstanding the possibility that even ‘thermally‐hardy’, widespread species may be influenced by the indirect effects of climate change such as reduction in habitat availability in Mediterranean areas.     

Interactive effects of food availability and aerial body temperature on the survival of two intertidal Mytilus species

Intertidal organisms must episodically contend with the rigors of both the terrestrial and the marine environments. While body temperatures during high tide are driven primarily by water temperature, aerial body temperatures are driven by multiple environmental factors such that temperature of an organism during low tide is usually quite different from air temperature. Thus, whereas decades of research have investigated the effects of water temperature on intertidal species, considerably less is known about the physiological impacts of temperature during aerial exposure at low tide, especially with regard to the interaction of aerial body temperature with other stressors. We examined the interactive effects of aerial body temperature and food supply on the survival of two intertidal blue mussels, Mytilus galloprovincialis and Mytilus trossulus. Survival was monitored for nine weeks using a simulated tidal cycle, with two levels of food and three levels of aerial body temperature (30, 25, and 20 °C). Decreased food supply significantly reduced the survival of mussels, but only under the 30 °C treatment. In the other two thermal regimes there were no significant effect of food on survival. When aerial body temperatures are high, food availability may have a greater effect on intertidal organisms. Decreases in ocean productivity have been linked to increased in ocean temperatures, thus intertidal organisms may become more susceptible to thermal stress as climates shift.