Where three snail species attach while emersed in relation to heterogenous substrate temperatures underneath intertidal boulders

Mobile intertidal gastropods can employ behavioural thermoregulation to mitigate thermal stress, which may include retreating under boulders when emersed. However, little is known about how gastropod occupancy of under-boulder habitats is associated with any variations in substrate temperature that exist under boulders. Thermal imagery was used to measure the temperature of boulder lower surfaces and investigate how three snail species were associated at low tide with the maximum and average temperatures underneath grey siltstone and quartzite. Lower boulder surfaces had heterogeneous temperatures, with grey siltstone having temperature gradients and quartzite temperature showing mosaics. Temperature differences between the hottest and coolest gradient or mosaic locations were >5 °C; thus there was a range of temperatures that snails could interact with. All three snail species occupied cooler parts of temperature mosaics or gradients, avoiding the hottest areas. Stronger associations were detected on the hotter grey siltstone and for the more-thermally sensitive Nerita atramentosa and Diloma concameratum. Even though snails were associated with cooler areas, some individuals were still exposed to extreme substratum heat (>50 °C). These results suggest that gastropod thermoregulatory behaviour is far more complex than simply retreating underneath boulders at low tide, as there is also a range of under-boulder temperatures that they interact with. Untangling interactions between intertidal gastropods and heterogenous substrate temperatures is important given rocky seashores already represent a thermally-variable and potentially-stressful habitat, which may be exacerbated further given predictions of warming temperatures associated with climate change.