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The understanding of how climate warming may affect the ecological and geographical distributions of intertidal ectotherms requires insight into how their body temperatures vary in relation to environmental temperature (Angilletta, 2009; Helmuth et al., 2010). This issue is particularly relevant in high-shore environments, in which species experience extreme thermal stress regimes during the periods of aerial exposure (McMahon, 1990; Marshall, Mcquaid & Williams, 2010a; Marshall, Qadirulisyam Bin Mustafa & Williams, 2010b). In fact, highshore intertidal species are considered more vulnerable to climate warming than low-shore or subtidal species, because they are typically living closer to their thermal tolerance physiological limits in the tropics (see Somero, 2012 for a review). While physiological responses may be relatively limited in mitigating heat stress, behavioural responses may, however, be critical to buffer thermal stress as shown in ectotherms that use a wide range of thermal environments (Kearney, Shine & Porter, 2009). Thermal regulatory behaviours are critical on rocky shores due to the latter’s high degree of topographic complexity, hence the related variety of thermal microhabitats (Chapperon & Seuront, 2011a; Lathlean, Ayre & Minchinton, 2012) and considerable temporal and spatial fluctuations in environmental temperature (Denny et al., 2011; Gedan et al., 2011; Lathlean, Ayre & Minchinton, 2014). The behaviours include selection of thermally more benign microhabitats (Chapperon & Seuront, 2011a; Iacarella & Helmuth 2011), formation of aggregations (Chapperon & Seuront, 2012), shell orientation relative to the sun (Muñoz et al., 2005) and shell-posturing behaviour (Marshall et al., 2010a, b). High shore littorinid snails exhibit shell-posturing behaviour during low tide, i.e. individuals retract their foot into their shell, close their operculum and glue themselves to the substratum with a mucus holdfast that allows them to lift their body off the substratum (McMahon, 1990; Lang, Britton & Metz, 1998). This behaviour is considered as an adaptive strategy that minimizes conductive transfer from the substratum to the shell by reducing the amount of the shell in contact with the substratum, while maximizing the shell surface exposed to air currents, hence allowing increased heat transfer from the shell through convective cooling (Miller & Denny, 2011; Marshall & Chua, 2012). More specifically, improved cooling is achieved through shell-standing (Lim, 2008; Miller & Denny, 2011; Marshall & Chua, 2012) and shell-stacking (Marshall et al., 2010b), two related behaviours whereby a snail is respectively attached to the substratum with its aperture facing perpendicularly to the surface (as opposed to downwards) and climbs onto other snails to form a stack or ‘tower’ (Marshall et al., 2010b). Both shell standing and towering behaviours have previously been shown to be effective strategies to mitigate thermal stress in Echinolittorina malaccana (Marshall et al., 2010b; Marshall & Chua, 2012). These studies were, however, based on the use of biomimetic snail models with thermocouples inserted in them. Such models allow continuous long-term measurements, avoid spurious physiological and behavioural consequences of inserting thermocouples into small organisms and show reasonable correlations between body temperature of model snails and live snails, including for littorinids (Iacarella & Helmuth, 2011; Marshall et al., 2010a; Miller & Denny, 2011). Their generality is, however, intrinsically limited by the number of snails that can be simultaneously studied, e.g. a tower of three E. malaccana shells (Marshall et al., 2010b), and four E. malaccana shells glued on a flat rock surface in standing and non-standing position either under direct sunlight or in the shade (Marshall & Chua, 2012). Further, this approach does not allow simultaneous measurement of snail and substratum temperature at spatial scales compatible with the considerable thermal heterogeneity observed on intertidal rocky shores (Denny et al., 2011; Lathlean et al., 2014). This is not the case, however, with infrared thermography (IRT) that allows—in a relatively limited amount of time and with limited logistics compared with traditional thermal methods—non-invasive simultaneous measurements of both the body temperature of multiple snails and the thermal properties of their substratum at scales compatible with the behavioural biology and ecology of individual organisms (see Lathlean & Seuront, 2014, for a review). We used IRT to investigate and compare the potential consequences of standing and towering behaviours on the body temperature of two tropical rocky-shore snails, Echinolittorina malaccana (Philippi, 1847) and E. radiata (Souleyet, 1852) (Caenogastropoda: Littorinidae). These two species were chosen as they co-occur in the high-shore zone of Hong Kong where they experience extreme heating, with rock temperature often exceeding 50 8C in summer (Williams, 1994). Field observations were conducted during both Hong Kong’s cold, dry winter (4 December 2014) and its hot, wet summer (5–6 August 2014;

Standing in the sun: infrared thermography reveals distinct thermal regulatory behaviours in two tropical high-shore littorinid snails