Emilie Moisez

Nicolas Spilmont

Laurent Seuront
email: laurent.seuront@cnrs.fr

Microhabitats choice in intertidal gastropods is species-, temperature-and habitat-specific

Keywords:

Understanding how behavioural adaptations can limit thermal stress for intertidal gastropods will be crucial for climate models. Some behavioural adaptations are already known to limit desiccation and thermal stresses as shell-lifting, shell-standing, towering, aggregation of conspecifics or habitat selection. Here we used the IRT (i.e. infrared thermography) to investigate the thermal heterogeneity of a rocky platform at different spatial scales, with four different macrohabitats (i.e. bare rock, rock with barnacles, mussels and mussels incrusted by barnacles) during four seasons. We investigated the body temperature of Littorina littorea and Patella vulgata found on this platform and the temperature of their microhabitat (i.e. the substratum within one body length around of each individual). We also considered the aggregation behaviour of each species and assessed the percentage of thermal microhabitat choice (i.e choice for a microhabitat with a temperature different than the surrounding substrate). We did not find any aggregation of L. littorea on the rocky platform during the four studied months. In contrast, P. vulgata were found in aggregates in all the studied periods and within each habitat, but there was no difference in body temperature between aggregated and solitary individuals. We noted that these two gastropods species were preferentially found on rock covered by barnacles in the four studied months. The presence of a thermal microhabitat choice in L. littorea and P. vulgata is habitat-dependent and also seasondependent. In June, July and November the choice was for a microhabitat with temperatures lower than the temperatures of the surrounding substrate whereas in December, individuals choose microhabitats with higher temperatures than the temperatures of their substratum. All together, these results suggest that gastropods species are able to explore their environment to find sustainable thermal macrohabitats and microhabitats and adapt this behaviour in function of the conditions of temperatures.

INTRODUCTION

For both terrestrial and aquatic animals, habitat selection is a way to attract mates, raise offspring, avoid both predation and extreme climatic conditions, and may be influenced by biotic (competition, predation) and abiotic (temperature, relative humidity) factors. Among marine systems, rocky intertidal shores are considered as one of the most thermally variable and stressful environments [START_REF] Harley | Tidal dynamics, topographic orientation, and temperature-mediated mass mortalities on rocky shores[END_REF]. In these environments, intertidal species have to cope with physical and chemical stresses which intensity depends on the interaction of four major environmental gradients: the vertical gradient (i.e. "tidal" gradient), the horizontal gradient (i.e. exposure to waves), the sediment particle size gradient, and the marinefreshwater gradient of salinity [START_REF] Raffaelli | Intertidal Ecology[END_REF]. These gradients, together with species interactions, lead to a zonation in the distribution of intertidal rocky shores species [START_REF] Little | The biology of rocky shores[END_REF][START_REF] Raffaelli | Intertidal Ecology[END_REF]. Within this meso-scale zonation, mobile species can select different microhabitats, such as boulders, crevices or pools. The thermal stress has been shown to be triggering this microhabitat choice. Indeed, intertidal species can typically face temperatures variations up to ca. 20°C during a tidal cycle [START_REF] Helmuth | How do we measure the environment? Linking intertidal thermal physiology and ecology through biophysics[END_REF] and most intertidal invertebrates live close to the upper limit of their thermal tolerance window [START_REF] Somero | Thermal physiology and vertical zonation of intertidal animals: optima, limits, and costs of living[END_REF] that lead them to choose the most favourable environment for their survival.

The body temperature of intertidal invertebrates can vary by 8-12°C depending on the microhabitat they occupy [START_REF] Seabra | Side matters: microhabitat influence on intertidal heat stress over a large geographical scale[END_REF]; for instance, the gastropod Nerita atramentosa is found on rock platforms and boulder fields with different topographic complexity and thus different substratum temperatures (i.e. up to 4.18 °C of mean temperature difference in autumn; Chapperon and Seuront, 2011a). On the other hand, it has been shown that other littorinid snails, such as Cenchritis muricatus, rarely inhabit open areas and are preferentially found in crevices or in depressions [START_REF] Judge | Physiological consequences of the supralittoral fringe: microhabitat temperature profiles and stress protein levels in the tropical periwinkle Cenchritis muricatus (Linneaus, 1758)[END_REF]. The orientation of the microhabitat towards sunlight can also influence the distribution of intertidal species; for example, the snail Littoraria scabra, is consistently found on the shaded part of its habitat and thus actively chose a microhabitat with lower temperatures than microhabitats directly exposed to solar radiations (Chapperon and Seuront, 2011b).

In a context of global climate change, rare events such as storms, floods, droughts or heat waves are likely to become the norm [START_REF] Meehl | More intense, more frequent, and longer lasting heat waves in the 21st century[END_REF][START_REF] Oswald | A trend analysis of the 1930-2010 extreme heat events in the continental United States[END_REF][START_REF] Rahmstorf | Increase of extreme events in a warming world[END_REF]. In particular, heat waves may have dramatic consequences on intertidal species as massive mortality in juvenile barnacles, limpets and mussels [START_REF] Harley | Tidal dynamics, topographic orientation, and temperature-mediated mass mortalities on rocky shores[END_REF][START_REF] Seuront | Decreased thermal tolerance under recurrent heat stress conditions explains summer mass mortality of the blue mussel Mytilus edulis[END_REF]. Intertidal ectotherms developed some physiological adaptations as heat-shock protein expression, modifications in heart function or in mitochondrial respiration [START_REF] Somero | Thermal physiology and vertical zonation of intertidal animals: optima, limits, and costs of living[END_REF] and behavioural adaptations as habitat selection, aggregation, retraction of the foot into the shell or orientation of the shell (Chapperon andSeuront, 2011a, 2011b;[START_REF] Miller | Importance of behavior and morphological traits for controlling body temperature in littorinid snails[END_REF] to face these heat waves. These adaptations may become the norm under future climatic scenarios and their understanding in thus critical to assess the diversity and functioning of intertidal communities for the next decades.

Temperatures of rocky intertidal substrates are highly variable between shaded and exposed areas [START_REF] Bertness | The role of positive interactions in communities : lessons from intertidal habitats[END_REF], vertical and horizontal surfaces [START_REF] Helmuth | Intertidal mussels microclimates: predicting the body temperature of a sessile invertebrate[END_REF][START_REF] Miller | The role of temperature and desiccation stress in limiting the local-scale distribution of the owl limpet, Lottia gigantea[END_REF], north-and south-facing substrates (Denny et al., 2006;[START_REF] Harley | Tidal dynamics, topographic orientation, and temperature-mediated mass mortalities on rocky shores[END_REF][START_REF] Seabra | Side matters: microhabitat influence on intertidal heat stress over a large geographical scale[END_REF], and between crevices and emergent rock [START_REF] Jackson | Effects of topography on the environment[END_REF]. The presence of ecosystem engineers also contributes to the natural variability in temperatures [START_REF] Bertness | The role of positive interactions in communities : lessons from intertidal habitats[END_REF] and modify the biotic and abiotic properties of the rocky shores [START_REF] Jones | Organisms as ecosystem engineers[END_REF]. Mussels are the dominant ecosystem engineers in the intertidal zone of temperate rocky shores through the formation of multi-layered beds that create microhabitats.

Particularly, mussel beds generate interstitial space where water is trapped and thus reduce the effect of desiccation and heat stress [START_REF] Gutiérrez | Mollusks as ecosystem engineers: the role of shell production in aquatic habitats[END_REF][START_REF] Sousa | Non-indigenous invasive bivalves as ecosystem engineers[END_REF][START_REF] Suchanek | Mussels and their role in structuring rocky shore communities[END_REF]. Their presence also offers protection against wave action during high tides [START_REF] Choat | The influence of sessile organisms on the population biology of three species of Acmaeid limpets[END_REF][START_REF] Lewis | Local habitat-induced variations in the population dynamics of Patella vulgata L[END_REF]. Mussel beds also increase the reflection of solar radiation, which leads to a reduction of substratum temperature [START_REF] Little | The biology of rocky shores[END_REF]. Other ecosystem engineers, such as barnacles, have also been shown to increase the fine-scale topographic complexity of the substratum, reduce the temperature (by up to 8°C compared to bare rock or to rock with low cover of barnacles) of the surrounding substratum by shading and increase interstitial humidity within patches [START_REF] Lathlean | Using infrared imagery to test for quadratlevel temperature variation and effects on the early life history of a rocky-shore barnacle[END_REF]. Barnacles have also been shown to reduce interspecific competition between the limpets Cellana tramoserica and Patelloida latistrigata, reduce heat stress and wave exposure that facilitated settlement of P. latistrigata [START_REF] Lathlean | Not all space is created equal: distribution of free space and its influence on heat-stress and the limpet Patelloida latistrigata[END_REF].

In these environments, the use of thermal imaging has drastically increased over the last 10 years, in particular to investigate the role of thermal stress at scale pertinent for individual organisms [START_REF] Lathlean | On the edge: the use of infrared thermography in monitoring responses of intertidal organisms to heat stress[END_REF][START_REF] Lathlean | Infrared thermography in marine ecology : methods, previous applications and future challenges[END_REF][START_REF] Seuront | A review of the thermal biology and ecology of molluscs, and of the use of infrared thermography in molluscan research[END_REF]. Infrared thermography (IRT) is a non-invasive tool for simultaneously measuring the temperature of multiple organisms and their environment. IRT thus allows assessing the thermal heterogeneity of complex environments such as intertidal rocky shores [START_REF] Lathlean | Infrared thermography in marine ecology : methods, previous applications and future challenges[END_REF][START_REF] Seuront | A review of the thermal biology and ecology of molluscs, and of the use of infrared thermography in molluscan research[END_REF]. This methodology was previously applied on mussel beds, in which observed peak temperatures were 10-15°C lower than on the adjacent bedrock [START_REF] Jurgens | Edge effects reverse facilitation by a widespread foundation species[END_REF], and on barnacle-covered bedrocks, an increasing barnacle density reducing the heat stress for inhabiting gastropods (the limpet Patelloida latistrigata; [START_REF] Lathlean | Not all space is created equal: distribution of free space and its influence on heat-stress and the limpet Patelloida latistrigata[END_REF]. For intertidal mollusks, the body temperature is significantly correlated with the substrate temperature but not with the air temperature [START_REF] Chapperon | Thermally mediated body temperature , water content and aggregation behaviour in the intertidal gastropod Nerita atramentosa[END_REF][START_REF] Seuront | Decreased thermal tolerance under recurrent heat stress conditions explains summer mass mortality of the blue mussel Mytilus edulis[END_REF] and the aggregation behaviour of some gastropod species is suspected to favour a decrease in individual body temperature (e.g. Nerita atramentosa; [START_REF] Chapperon | Thermally mediated body temperature , water content and aggregation behaviour in the intertidal gastropod Nerita atramentosa[END_REF].

Overall, these results stress the fact that climate change models should integrate both abiotic variables, such as substratum temperature (rather than air temperature: [START_REF] Seuront | Decreased thermal tolerance under recurrent heat stress conditions explains summer mass mortality of the blue mussel Mytilus edulis[END_REF], and biotic parameters, such as behavioural adaptations [START_REF] Ng | Linking behaviour and climate change in intertidal ectotherms: insights from littorinid snails[END_REF].

Noticeably, the previous studies that dealt with both habitat and microhabitat choice in intertidal organisms, in particular gastropods [START_REF] Chapperon | Thermally mediated body temperature , water content and aggregation behaviour in the intertidal gastropod Nerita atramentosa[END_REF][START_REF] Chapperon | Mitigating thermal effect of behaviour and microhabitat on the intertidal snail Littorina saxatilis (Olivi) over summer[END_REF]Chapperon andSeuront, 2011a, 2011b;[START_REF] Lathlean | Not all space is created equal: distribution of free space and its influence on heat-stress and the limpet Patelloida latistrigata[END_REF], essentially focused on one season, one species and/or one habitat. In this context, the present study aimed at assessing microhabitat choice in two common temperate gastropod species, Littorina littorea and Patella vulgata, within four distinct habitats (i.e. bare rock, rock covered by barnacles or mussels, and rock covered by mussels encrusted by barnacles) occurring on a topographically homogenous intertidal rock platform extending over the whole intertidal zone during four distinct months. Specifically, we first assessed where these habitats occur on the rocky platform and in which habitat L. littorea and P. vulgata occur on the platform under different meteorological conditions (i.e. different months). We also assessed the presence and intensity of aggregation behaviour of these two species often reported as gregarious [START_REF] Little | The biology of rocky shores[END_REF]. We subsequently inferred the presence of a habitat preference for P. vulgata and L. littorea, and if this choice was species-specific/month-specific. Finally, we described the thermal properties of the four habitats, assessed potential differences between the body temperature of each species and substratum temperature, and inferred the presence of a choice for a thermal microhabitat.

METHODS

Study site

This work was conducted on a temperate intertidal rocky platform, typical of the rocky habitats found along the French coast of the Eastern English Channel, located in Wimereux, France (50°45'09.94"N, 1°35'44.01"E). This platform ranges over the whole intertidal zone and is directly exposed to the dominant South-South West swell direction and submitted to a semi diurnal megatidal regime with a tidal range ranging between 3 and 9 m. We specifically chose this study site for its topographic homogeneity and the presence of four distinct macrohabitats from the supralittoral zone to the low intertidal zone. These habitats were (i) bare rock (Fig. 1A), (ii) rock covered by barnacles Semibalanus balanoides and Austrominus modestus, respectively native and introduced (Fig. 1B), (iii) rock covered by the blue mussels Mytilus edulis (Fig. 1C) and (iv) rock covered by mussels with barnacles (Fig. 1D). The location is submitted to a large range of temperatures over a seasonal cycle (i.e. from -2°C to 32°C in 2019). Thus, four periods of sampling were chosen for the present study, to encompass typical temperature variations of the location: June, November and December 2018 and in July 2019. The common periwinkle Littorina littorea and the common limpet Patella vulgata were observed on this platform and were chosen as target species for the study. These two species can face large variations of temperatures during a year; typically, L. littorea can be exposed to air temperature as low as -15°C [START_REF] Murphy | A comparative study of the freezing tolerances of the marine snails Littorina littorea ( L .) and Nassarius obsoletus ( Say )[END_REF] and its lethal temperature is 46°C [START_REF] Evans | The lethal temperatures of some common British littoral molluscs[END_REF], whereas the lethal temperature of P. vulgata is 42.8°C [START_REF] Evans | The lethal temperatures of some common British littoral molluscs[END_REF].

Thermal imaging

In order to characterize the thermal properties of these four different habitats, thermal imaging was used as a non-invasive temperature measurement [START_REF] Lathlean | Infrared thermography in marine ecology : methods, previous applications and future challenges[END_REF][START_REF] Seuront | A review of the thermal biology and ecology of molluscs, and of the use of infrared thermography in molluscan research[END_REF]. We used a Fluke Ti25 camera (Fluke Corporation, Everett, Washington, USA), which sensitivity is ≤ 0.1°C at 30°C and the temperature measurement accuracy is ±2°C for the temperature range measured in the present study (i.e. temperatures <100°C; Fluke Corporation). The emissivity (i.e. ability of an object to emit thermal radiation; Lathlean and Seuront, 2014) value of substrata was set at 0.95 as previous studies show that emissivity values of rocky shores typically vary between 0.95 and 1 (Chapperon and Seuront, 2011a;[START_REF] Cox | Thermal ecology on an exposed algal reef: infrared imagery a rapid tool to survey temperature at local spatial scales[END_REF]Denny and Harley, 2006;[START_REF] Helmuth | Intertidal mussels microclimates: predicting the body temperature of a sessile invertebrate[END_REF][START_REF] Miller | The role of temperature and desiccation stress in limiting the local-scale distribution of the owl limpet, Lottia gigantea[END_REF]. All thermal imagines were analysed using the software SmartView 4.1 (Fluke Corporation). For each thermal picture, a closed curve marker was drawn around each photographed individual to calculate the mean value of body temperature (BT) and line were drawn to determine the surrounding substrate temperature.

Thermal properties of the four habitats

The experiments were conducted on one day per studied month. Three replicate IR images of 25 cm ´ 25 cm quadrats were haphazardly taken every 10 meters along a transect on the platform from the low intertidal zone to the supralittoral zone. The photo-quadrats were not fixed but varied in their relative position along the shore for each month. Specifically, 26 stations were sampled in June (n = 78), 27 in July (n = 81), 19 in November (n = 57) and 30 in December (n = 90). The difference in the number of stations sampled is due to differences in the tidal range at the different sampling occasions. We recorded the presence of each habitat along each transect and we calculated the percentage of occurrence of each habitat on the rocky platform as Nhab / n, where Nhab is the number of sampling stations where the habitat was observed and n is the total number of sampling stations for each month. This allowed the estimation of the vertical distribution of the different habitats at each sampling date. The abundance of L. littorea and P. vulgata on each habitat were based on the photo-quadrats; specifically, we choose the photo-quadrats where only one habitat was found to calculate the maximal abundance of each species.

To minimise the potential sources of bias in the measurements of intertidal habitat and species temperatures, IR images were consistently taken on days when low tide fell between 11 am and 1 pm (i.e. midday ± 1h) and cloud cover was minimal or absent. Air and water temperature and wind speed were considered as environmental variables that were most likely to affect substrate temperature and the behaviour and distribution of both L. littorea and P. vulgata [START_REF] Seuront | A review of the thermal biology and ecology of molluscs, and of the use of infrared thermography in molluscan research[END_REF]. Hourly air temperature and wind speed were obtained from Meteo France (www.donneespubliques.meteofrance.fr) weather station of Boulogne-Sur-Mer (50°43′54N, 1°35′53E). Water temperature was measured using temperature loggers (DSL1922L iButtons; resolution 0.1 °C) that were installed throughout the rocky platform. iButtons were wrapped in parafilm, epoxied into shallow depressions chiseled into the rock, and covered by a 1-2 mm layer of epoxy, which was flush with the rock surface.

To determine the temperature of each habitat found on each quadrat sampled, 3 horizontal or vertical lines were haphazardly drawn in SmartView 4.1 to measure the range of temperature of the substratum (ST). The temperature of each pixel found on each line was subsequently extracted with SmartView 4.1.

Body temperatures (BT) and microhabitat substrate temperatures (STµhab)

To determine the presence of a thermal microhabitat choice (i.e. choice for a microhabitat with a temperature significantly different from the temperature of the habitat ST) in the intertidal gastropods Littorina littorea and Patella vulgata, we measured the body temperature (BT) of individual organisms and the temperature of their microhabitat (STµhab), i.e. the substratum within one body length of each individual, from infrared images with SmartView 4.1 (Chapperon andSeuront, 2011a, 2012). A thermal microhabitat choice can either be defined as positive (i.e. choice for a microhabitat with a temperature significantly warmer than the temperature of the habitat, i.e. STµhab>ST) or negative (i.e. choice for a microhabitat with temperature significantly cooler than the temperature of the habitat, i.e. STµhab<ST). The percentage of positive choice was defined as n1/N, where N is the total number of observations, n1 the number of observations with STµhab > ST, and the percentage of negative choice was defined as n2/N where n2 is the number of observations with STµhab < ST.

Aggregation status

Patella vulgata and Littorina littorea individuals were considered as being aggregated when there was a direct shell contact with the shell of at least one conspecific (Chapperon andSeuront, 2011a, 2011b;[START_REF] Chapperon | Thermally mediated body temperature , water content and aggregation behaviour in the intertidal gastropod Nerita atramentosa[END_REF].

Statistical analysis

Depending on the normality of the data, body temperature of solitary and aggregated individuals were compared using either a Mann-Whitney test or a t-test, within each habitat for each month. ST were compared between habitats within each month using a Kruskall-Wallis test (hereafter referred to as K-W test) and when necessary a subsequent post-hoc test was performed using a Dunn test [START_REF] Zar | Biostatistical analysis[END_REF]. BT and STµhab were compared using a Mann-Whitney test for all habitats and months for both species. The Spearman's coefficient of correlation ρ between BT and STµhab was calculated for the two species. The mean temperature of the microhabitat (STµhab) of L. littorea and P. vulgata individuals was compared with the habitat temperature (ST) for each habitat at each tidal level using a Mann-Whitney test.

RESULTS

Environmental conditions

Recorded air temperature (Meteo France data) ranged between 15 and 16°C in June in June, November and December, but decreased to 2.1 ± 0.5 m.s -1 in July. Mean sea surface temperature (iButtons recordings) during the periods of experiment was 16.4°C ± 0.4°C in June, 19.1°C ± 0.7°C in July, 11.9°C ± 0.5 °C in November and 7.5°C ± 0.8°C in December.

Habitat distribution

The four habitats were encountered at all tidal levels along the rocky platform, except on the very high shore where only bare rock was found. However, there were variations in the habitats' distribution and overlap during the four months (Fig. 2). Specifically, in June, November and December, all four habitats were present on the platform (Fig. 2A,C,D). In contrast, in July mussels covered by barnacles were absent (Fig. 2B). In June, the habitats the most represented were the rock covered by barnacles found on 80.8% of the platform and the rock covered by mussels with barnacles found on 69.2% of the platform. In contrast, in July, November and December, the habitats the most represented were the rock covered by barnacles, respectively found on 88.9%, 84.2% and 73.3% of the platform, and the bare rock, respectively found on 85.2%, 57.9% and 70% of the platform.

Abundance and occurrence of Patella vulgata and Littorina littorea

Patella vulgata was totally absent from mussel beds during the four studied months and also from mussel beds encrusted by barnacles in November and December. The maximal abundances of Patella vulgata and Littorina littorea were found on the rock incrusted by barnacles for the four studied months excepted in July for P. vulgata which was more abundant on bare rock (Table. 1).

Aggregation was never observed in L. littorea in any habitat, at any sampling date. On the contrary, for P. vulgata, some patches were found in all habitats at each sampling occasion (Table. 1). The size of the patches varied between 2 and 5 individuals, except in June on mussels with barnacles where a patch of 8 individuals was observed.

Substrate temperature (ST)

The substrate temperature of the four habitats exhibited a clear seasonality (Fig. 3), with significant differences consistently observed between months (K-W test, p < 0.001). For each habitat, ST ranked from high to low temperatures as July > June > November > December (Dunn test, p < 0.001). Within each month, there was significant difference in ST between the different habitats (K-W test, p < 0.001). In July, November and December, ST of the four habitats were significantly different (Dunn test, p < 0.001). In June, ST on bare rock was significantly hotter than the three others habitats (Dunn test, p < 0.001). ST was not significantly different between the habitats mussels with barnacles and mussels (Dunn test, p > 0.05). There was no significant difference in ST between the habitats rock covered by barnacles, mussels and mussels with barnacles (Dunn test, p > 0.05).

Body temperature (BT) and microhabitat temperature (STµhab)

For P. vulgata, a significant positive correlation (ρ > 0.805, p < 0.001) was consistently found between BT and STµhab for the three habitats where the species was found, i.e. bare rock, substratum with barnacles and mussels with barnacles (Fig. 4). BT was significantly higher than STµhab in July and November for all the habitats, and only for bare rock and rock with barnacles in June (Mann-Whitney test, p < 0.05). No significant difference was found between BT and STµhab in mussels encrusted with barnacles in June, and in all habitats in December (Mann-Whitney test, p > 0.05). Finally, no significant difference (p > 0.05) was found between the body temperature of solitary and aggregated individuals.

For L. littorea, a significant positive correlation was found between BT and STµhab in the four habitats (ρ > 0.978, p < 0.001; Fig. 5). In July and November on bare rock and on rock with barnacles, BT was significantly higher than STµhab (Mann-Whitney test, p < 0.05). For the others months and habitats there was no significant difference between BT and STµhab (Mann-Whitney test, p > 0.05).

Frequency of thermal microhabitat choice

Thermal microhabitat choice was observed for Patella vulgata in all the habitats and for all the studied months. Specifically, the proportion of individuals performing choice ranged from 16.7% (on rock incrusted by barnacles in November and on mussels incrusted by barnacles in June) to 57.1% (on bare rock in June; Fig. 6A). Both positive and negative choice were observed; on bare rock and on rock with barnacles in November and on mussels with barnacles in June, there was only negative choice, whereas on bare rock in December, only positive choice was detected. For the other habitats and seasons there was a co-occurrence of both negative and positive choice (Fig. 6A) with a predominance of negative choice in June,

July and November and a predominance of positive choice in December.

For Littorina littorea, there was no thermal microhabitat choice in June and November on bare rock, and in November on mussels incrusted by barnacles. The maximum proportion of individuals displaying a choice was observed in June on rock incrusted by barnacles (47.4%; Fig. 6B). As for P. vulgata, both positive and negative were observed: there was only negative choice in July on the three habitats were the species was found, as well as on mussels with barnacles in June whereas in December, we only found positive choice on rock with barnacles, mussels and mussels with barnacles. For the other habitats and months, both negative and positive choice were found (Fig. 6B) with a predominance of negative choice in June, July and November and a predominance of positive choice in December, as for P.

vulgata.

DISCUSSION

Aggregation

The aggregation behaviour of intertidal snails was reported in numerous studies [START_REF] Chapman | Influences of tidal conditions, temperature and desiccation on patterns of aggregation of the high-shore periwinkle, Littorina unifasciata, in New South Wales, Australia[END_REF][START_REF] Chapperon | Thermally mediated body temperature , water content and aggregation behaviour in the intertidal gastropod Nerita atramentosa[END_REF]Chapperon andSeuront, 2009, 2012). These studies suggested that aggregation behaviour was controlled by local climatic conditions [START_REF] Bertness | Intraspecific competition and facilitation in a northern acorn barnacle population[END_REF][START_REF] Chapman | Influences of tidal conditions, temperature and desiccation on patterns of aggregation of the high-shore periwinkle, Littorina unifasciata, in New South Wales, Australia[END_REF][START_REF] Chapperon | Thermally mediated body temperature , water content and aggregation behaviour in the intertidal gastropod Nerita atramentosa[END_REF][START_REF] Chapperon | Keeping warm in the cold : on the thermal benefits of aggregation behaviour in an intertidal ectotherm[END_REF][START_REF] Rojas | Geographic variability in thermal tolerance and water economy of the intertidal gastropod Nodilittorina peruviana[END_REF] or by substrate topography [START_REF] Rickards | Effects of temperature and humidity on activity and microhabitat selection by Littorina subrotundata[END_REF][START_REF] Stafford | Self-organization of intertidal snails facilitates evolution of aggregation behavior[END_REF]. However, in the present study, aggregation of Littorina littorea was not observed on the topographically homogeneous rocky reef. On nearby rocky platform with a highly complex topography, many L. littorea aggregates were previously observed [START_REF] Chapperon | Cue synergy in Littorina littorea navigation following wave dislodgement[END_REF]. These results suggest that, in the study area, the aggregation behaviour of L. littorea was not governed by climatic conditions (i.e. temperature) but mainly by substrate topography. Our observations are also consistent with the hypothesis that aggregation behaviour is selected to cope with thermal and desiccation stresses only on shores with a high crevice density [START_REF] Stafford | Self-organization of intertidal snails facilitates evolution of aggregation behavior[END_REF].

Some aggregates of Patella vulgata were however found. Benefit from living in aggregation for P. vulgata is likely to be a reduction in physical or biological stresses [START_REF] Coleman | Aggregation as a defense: limpet tenacity changes in response to simulated predator attack[END_REF]. Living in group may allow to retain moisture on rock surface and modify air flow over the individuals and thus reduce evaporation. However, there is only relatively weak evidence of the effect of aggregation on desiccation stress in limpets [START_REF] Coleman | Aggregation as a defense: limpet tenacity changes in response to simulated predator attack[END_REF]. Here, we found no difference on body temperature between solitary and aggregated individuals. This result suggests that aggregation behaviour was not used as a response against thermal and desiccation stresses but was driven by others factors. As suggested by [START_REF] Coleman | Limpet aggregation does not alter desiccation in the limpet Cellana tramoserica[END_REF], aggregation in limpets may more likely be a behavioural response against predation risks than a way to reduce desiccation stress [START_REF] Coleman | Limpet aggregation does not alter desiccation in the limpet Cellana tramoserica[END_REF].

The selection of a macrohabitat as refuge

The four habitats described here were encountered at all tidal levels during the study and thus often overlapped (Fig. 2). Both species were consistently observed in these habitats, whatever the tidal level, except for the limpet P. vulgata which was absent from mussel beds whatever the distribution of this habitat along the shore. These two results testify for a meso-habitat choice rather than a distribution of the species related to tidal height of the shore.

The four habitats previously described on the rocky platform (i.e. bare rock, rock covered by barnacles, rock covered by mussels and rock covered by mussels with barnacles) were found in June, November and December. In contrast, in July the habitat "mussels covered by barnacles" was absent and there were only a few mussels present. This absence of mussels is most probably related to the massive mortality of the blue mussel Mytilus edulis observed on the Opal Coast in summer 2018 [START_REF] Seuront | Decreased thermal tolerance under recurrent heat stress conditions explains summer mass mortality of the blue mussel Mytilus edulis[END_REF]. Limpets are usually absent from mussel beds, unless boulders are present, as described for this habitat in the EUNIS classification [START_REF] Connor | The marine habitat classification for Britain and Ireland[END_REF] and previously observed on temperate intertidal mussel beds [START_REF] Saier | Subtidal and intertidal mussel beds (Mytilus edulis L.) in the Wadden Sea: Diversity differences of associated epifauna[END_REF][START_REF] Norling | Structural and functional effects of Mytilus edulis on diversity of associated species and ecosystem functioning[END_REF]. On our study site, Littorina littorea and Patella vulgata were mostly found on substrate covered by barnacles at all sampling months. This result suggests that, at the scale of the platform, the rock covered by barnacles is the most favourable habitat for P. vulgata and L. littorea. This is in accordance with the results of Lathlean, (2014), who showed that the abundance of the limpet Patelloida latistrigata decreased with the diminution of barnacles cover associated to an increase of heat stress which increases with the free space due to the diminution of barnacle cover [START_REF] Lathlean | Not all space is created equal: distribution of free space and its influence on heat-stress and the limpet Patelloida latistrigata[END_REF].

Barnacles create biogenic structures and provide refuges from desiccation and predation, increase the surface for attachment for mobile gastropods species [START_REF] Barnes | The use of intertidal barnacle shells[END_REF] and also create topographically complex microhabitats which greatly reduce flow velocity and allow snails to persist against wave-swept ( [START_REF] Denny | Physical processes that generate patterns in marine communties[END_REF][START_REF] Silva | Habitat provision of barnacle tests for overcrowded periwinkles[END_REF]. Thus, the biogenic structures formed by barnacles can act as thermal refuges for gastropod species.

Regarding littorinids, the body temperature of the temperate snail Littorina saxatilis in summer is on average 1°C cooler than their surrounding substrate when found on substrate with barnacles than on bare rock [START_REF] Chapperon | Mitigating thermal effect of behaviour and microhabitat on the intertidal snail Littorina saxatilis (Olivi) over summer[END_REF]. Furthermore, [START_REF] Cartwright | Seasonal variation in utilization of biogenic microhabitats by littorinid snails on tropical rocky shores[END_REF] showed that Echinolittorina malaccana and E. vidua used the barnacles as refuges during times of the year where the environmental conditions, and in particular temperature, were stressful. They also showed that these two species did not used barnacles as refuges all along the year; when environmental conditions became cooler E. malaccana and E. vidua were found on other areas without the need of thermal refuges.

Microhabitat selection: a way to escape from unfavourable temperatures?

The habitat selection is a consistent behavioural feature in molluscs. This behavioural adaptation allows molluscs to reduce or avoid desiccation stress or predators. For examples Littorina saxatilis hide in crevices to reduce water loss [START_REF] Little | The biology of rocky shores[END_REF] or the high shore snails Cenchritis muricatus climb on terrestrial vegetation to avoid extremes temperature of the rocks surface [START_REF] Emson | Life on a hot dry beach: behavioural, physiological, and ultrastructural adaptations of the littorinid gastropod Cenchritis (Tectarius) muricatus[END_REF]. Limpets usually select their habitat through their homing behaviour, thus reducing desiccation stress and also predation risks [START_REF] Little | The biology of rocky shores[END_REF]. In the present study, food availability was not estimated in the different habitats and we assume that all resting positions in air recorded are based on temperature. Controlled laboratory experiments or controlled field observations would be needed to tease out this potential effect. The rationale behind the thermal microhabitat choice behaviour relies on the fact that gastropods body temperature is primarily determined by the substrate temperature, due to conductance between the foot and the substratum (Chapperon andSeuront, 2011a, 2011b;[START_REF] Garrity | Some adaptations of gastropods to physical stress on a tropical rocky shore[END_REF]; in a thermally heterogeneous environment, gastropods would then be able to actively chose the most favourable thermal habitat, to either favour or limit heat exchange with the substrate. Our results showed that the body temperatures (BT) of Littorina littorea and Patella vulgata were always significantly and positively correlated with the microhabitat temperatures (STµhab) in each habitat for each season which supports the thermal microhabitat choice hypothesis. However, for both species, this choice was both habitatdependent and season-dependent. In December, individuals predominantly choose microhabitats with higher temperatures than the temperatures of their substratum (i.e. positive thermal microhabitat choice). Under cold conditions of temperature, L. littorea individuals drastically reduced their metabolic rate (down to 20% of the regular metabolic rate; [START_REF] Jackson | Common periwinkle, Littorina littorea[END_REF]; the choice for a microhabitat with higher temperature during cold seasons allows individuals to maintain their metabolic rate or at least to limit the reduction of metabolic rate.

On the contrary, in June, July and November the choice was principally for a microhabitat with temperatures lower than the temperatures of the surrounding substrate (i.e. negative thermal microhabitat choice) which suggests a selection of a cool substrate to reduce the increase of body temperature by conduction between gastropod's foot and the substratum.

Individuals, were thus able to navigate in their thermally heterogeneous environment to find the most sustainable temperatures, in particular during the heat wave of July when this choice was predominantly positive for P. vulgata and always positive for L. littorea. P. vulgata and L. littorea cessed their spontaneous movement around 32°C and their lethal temperature are 42°C and 46°C, respectively [START_REF] Evans | The lethal temperatures of some common British littoral molluscs[END_REF]. Thus, the choice of microhabitat with lower temperature, allows individuals to maintain their body temperature under lethal or sub-lethal temperature as previously observed in tropical littorinids (Littoraria scabra; Chapperon and Seuront, 2011b) and limpets (Cellana tramoserica; [START_REF] Sinclair | Phenotypic flexibility in the metabolic response of the limpet Cellana tramoserica to thermally different microhabitats[END_REF]Cellana grata;[START_REF] Williams | Habitat partitioning and thermal tolerance in a tropical limpet , Cellana grata[END_REF]. In addition to microhabitat choice, others behavioural adaptations have previously been described, such as shell-lifting, shell-standing and towering that reduce conduction from the substrate in littorinids [START_REF] Seuront | Standing in the sun: infrared thermography reveals discinct thermal regulatory behaviours in two tropical high-shore littorinid snails[END_REF][START_REF] Ng | Linking behaviour and climate change in intertidal ectotherms: insights from littorinid snails[END_REF] and "mushrooming" in limpets which raise their shell to allow evaporative cooling [START_REF] Garrity | Some adaptations of gastropods to physical stress on a tropical rocky shore[END_REF]. These behaviours were, however, never observed in the present study, not even, to our knowledge, in the study area. This suggests that microhabitat choice is the predominant behaviour and that, under particularly harsh thermal conditions, other behavioural adaptations may be involved. With the expected rise in both mean temperatures and frequency of extreme high temperature events (IPCC, 2018), behavioural adaptations remain highly important for intertidal gastropods to maintain their body temperature under thermal stress and to avoid lethal temperature, and we may observe in the near future in temperate areas (such as the present study site) behaviours that are usually seen in tropical environments (e.g. shell-towering) where the thermal stress level is already high. The critical importance of the survival of herbivores such as limpets and littorinids goes beyond the individual and population interest, since their presence has been shown to dampen the impact of warming on the overall intertidal community composition and functioning [START_REF] Kordas | Herbivory enables marine communities to resist warming[END_REF].

CONCLUSION

This study suggests that the two intertidal gastropods species, Littorina littorea and Patella vulgata showed a clear preference for the thermally favourable biogenic habitat created by barnacles. Moreover, these two gastropods species, at the individual scale, also showed a choice for thermally favourable microhabitats. In winter, both species showed a clear behavioural preference for microhabitats with temperatures higher than the temperature of the substratum, whereas during other seasons, they showed a preference for microhabitats with temperatures lower than the ones measured on the substratum. These differences between temperature of their microhabitats and their substrate may lead to a difference in the way they face thermal stresses such as frost or heat wave. It also means that, temperature variations et very small scales are really important to understand how intertidal gastropods, and intertidal ectotherms in general, can survive in a context of global change. Specially, climate change scenarios do not integrate variations in temperature at pertinent scales for the individuals.

Furthermore, these scenarios do not considerate that the body temperature of gastropods is strongly correlated to the temperature of the substrate and never to the air temperature. These results showed that climate models probably underestimate the effect of climate change in the body temperature of intertidal gastropods. This study also reveals that gastropods species are able to explore their environment to find thermal refuge and thus may find refuges under new environmental conditions and possibly adopt locally new adaptive behaviours.

  , 23 and 27°C in July, 10 and 13°C in November and 2 and 3°C in December. These temperatures were characteristic of the studied months except in July, where the experiment was donneespubliques.meteofrance.fr). Wind speed (Meteo France data) was 5.1 ± 1 m.s -1

Fig. 1 .

 1 Fig. 1. Thermal images and associated digital pictures of the four types of habitats found on the rocky platform (A) bare rock, (B) rock covered by barnacles, (C) rock covered by mussels and (D) rock covered by mussels with barnacles. These images were taken in June.

Fig. 2 .

 2 Fig. 2. Distribution of the four habitats found along the rocky platform (in grey) (BR) bare rock, (B) rock covered by barnacles, (M) mussels, (MB) mussels encrusted by barnacles at the different months (A) June, (B) July, (C) November and (D) December. The black line represents the presence of the habitat.

Fig. 3 .

 3 Fig. 3. Substrate temperature (ST) found on the different habitats in June (light grey), in July (grey), in November (dark grey) and in December (black). The habitat mussels with barnacles was absent in July.

Fig. 4 .Fig. 5 .

 45 Fig. 4. Body temperature (BT) as a function microhabitat substrate temperature (STµhab) for Patella vulgata for the four months sutdied (a) and in particular for June (b), July (c), November (d) and December (e). The open dots represented the bare rock habitat, the diamond ones rock with barnacles, the square markers mussels and the triangle markers represented mussels with barnacles. The black line represents the first bissectrix, i.e. BT = STµhab.

Table 1 .

 1 Maximal abundance (ind.m -2 ) of Patella vulgata and Littorina littorea on the different studied months in the four habitats. The number of patches of P. vulgata are given within brackets. L. littorea was never found on aggregate in this study.

			Bare rock	Rock with	Mussels	Mussels with
				barnacles		barnacles
	Patella vulgata	June July November December	240 (8) 256 (6) 272 (4) 288 (10)	336 (8) 208 (5) 320 (7) 384 (14)	ABS ABS ABS ABS	176 (1) ABS ABS ABS
	Littorina littorea	June July November December	80 272 96 400	700 944 352 416	416 288 160 320	320 ABS 64 144

Acknowledgements

The authors thank Camille Hennion for assistance in the field. This work has been financially supported by a joint PhD fellowship from the Région Hauts-de-France and the Université of Lille to E.M. This work is a contribution to the CPER research project CLIMIBIO. The authors thank the French Ministère de l'Enseignement Supérieur et de la Recherche, the Hauts de France Region and the European Funds for Regional Economical Development for their financial support for this project. Thanks are due to the editor and four anonymous referees, whose comments greatly helped to improve the manuscript.