International audienceTropical species are vulnerable to global warming because they live at, or near to, their upper thermal threshold limits. Therefore, the predicted increase in the frequency of warming events in the tropics is expected to be critical for the survival of local species. This study explored the major environmental variables which were thought to be correlated with body temperatures (BTs) of the tropical snail Littoraria scabra at the niche level. A correlation between BT and substrate temperature (ST) was detected from field observations which suggests a possible causal relationship between both substrate and BTs. In contrast, there was no correlation between BT and air temperature. Field observations suggest that 33.4 1C may be L. scabra upper limit of substrate surface temperature, although further experiments are needed to assess if the upper limit of physiological tolerance is actually different. As L. scabra individuals were free to choose their substrata, the observed distribution pattern at the niche level is related to L. scabra's behavior. Additionally, substrate surface temperatures were very heterogeneous at centimeter scale (i.e. from 22.5 to 53.1 1C) and L. scabra was shown to select specific STs (i.e. between 22.5 and 33.4 1C) rather than microhabitat type. Therefore, L. scabra did not seem to behaviorally thermoregulate through microhabitat selection nor aggregation. In contrast, behavioral experiments showed that L. scabra has the ability to actively select a thermally favorable site over short temporal scale (i.e. individual average speed of 1.26 cm min 1 ) following exposure to high temperatures above 33.4 1C. Hence, this study supports the crucial need to integrate intertidal invertebrate behavioral responses to thermal constraints in climate change studies

Summary1. Behavioural adaptations of ectotherms to thermally heterogeneous environments are still overlooked in the literature despite the fact that organismal behaviour could enhance survival in the warming world. This is particularly critical in the intertidal where most ectotherms live at, or near to the upper limit of thermal tolerance. 2. This study investigated (i) the environmental factors determining the body temperatures of the intertidal gastropod Nerita atramentosa, (ii) the space-time variability in environmental and individual body temperatures and (iii) the potential variability in N. atramentosa thermoregulatory behaviours, i.e. microhabitat selection and aggregation. 3. Thermal imaging was used to assess the body temperatures of N. atramentosa and surrounding substrata over two seasons (autumn and summer), at two shore levels (low-vs. high-shore levels) within two habitats of different topographic complexity (rock platform and boulders) on the same rocky shore. 4. Snail body and substratum temperatures were significantly and positively correlated within each habitat at both seasons. Substratum temperature may thus be considered as a primary driver of body temperatures of organisms that attach to a substratum. Substratum temperature and other variables such as solar irradiance critically need to be integrated in climate-change models that use single climatic variables (e.g. air temperature) that are not necessarily correlated with individual body temperatures in nature. 5. The high space-time variability in both substratum and body temperatures reinforces the growing evidence that small spatial scale variations may surpass those observed at larger spatial scales. 6. Nerita atramentosa thermoregulatory behaviour under high thermal stress appeared to be habitat specific. 7. The small spatial scale heterogeneity in environmental and individual temperatures and in thermoregulatory behaviours has stressed the need to focus on body temperature patterns at the niche level and to integrate the organismal behaviour in climate-change models.

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Intertidal organisms are vulnerable to global warming as they already live at, or near to, the upper limit of their thermal tolerance window. The behaviour of ectotherms could, however, dampen their limited physiological abilities to respond to climate change (e.g. drier and warmer environmental conditions) which could substantially increase their survival rates. The behaviour of ectotherms is still mostly overlooked in climate change studies. Here, we investigate the potential of aggregation behaviour to compensate for climate change in an intertidal gastropod species (Nerita atramentosa) in South Australia. We used thermal imaging to investigate (1) the heterogeneity in individual snail water content and body temperature and surrounding substratum temperature on two topographically different habitats (i.e. rock platform and boulders) separated by 250 m at both day‐ and night‐times, (2) the potential relationship between environment temperature (air and substratum) and snail water content and body temperature, and (3) the potential buffering effect of aggregation behaviour on snail water content and body temperature. Both substratum and snail temperature were more heterogeneous at small spatial scales (a few centimetres to a few metres) than between habitats. This reinforces the evidence that mobile intertidal ectotherms could survive locally under warmer conditions if they can locate and move behaviourally in local thermal refuges. N. atramentosa behaviour, water content and body temperature during emersion seem to be related to the thermal stability and local conditions of the habitat occupied. Aggregation behaviour reduces both desiccation and heat stresses but only on the boulder field. Further investigations are required to identify the different behavioural strategies used by ectothermic species to adapt to heat and dehydrating conditions at the habitat level. Ultimately, this information constitutes a fundamental prerequisite to implement conservation management plans for ectothermic species identified as vulnerable in the warming climate.

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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.

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