Abstract
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.
Highlights
Intertidal gastropods are periodically exposed to the terrestrial environment during low tide and, as marine ectotherms, do not physiologically regulate their body temperature; instead their body temperature tracks that of the surrounding environment (Wolcott, 1973; Madeira et al, 2012)
Occurrences of limited heterogeneity were low (15%) for both rock types on the two cooler days sampled at each seashore, where gradients dominated on siltstone (85%) and mosaics on quartzite (≥70%, Table 2, Appendix Fig. A3)
On the hottest day sampled for both rock types, occurrences of limited heterogeneity increased to 40–45% of all boulders sampled (Table 2), with a corresponding decrease in the occurrence of gradients and mosaics, especially mosaics on quartzite (Table 2, Appendix Fig. A3)
Summary
Intertidal gastropods are periodically exposed to the terrestrial environment during low tide and, as marine ectotherms, do not physiologically regulate their body temperature; instead their body temperature tracks that of the surrounding environment (Wolcott, 1973; Madeira et al, 2012). Gastropod body temperature is often positively correlated with substratum temperature at low tide (Soto & Bozinovic, 1998; Caddy-Retalic, Benkendorff & Fairweather, 2011; Chapperon & Seuront, 2011a). Rock surfaces exposed to direct insolation when emersed can warm by 10–20 C (Southward, 1958; Harris, 1990; Bertness, 1999; Helmuth & Hofmann, 2001), with gastropods inhabiting the mid and upper levels of the seashore spending 14–98% of their lives in these thermally-challenging conditions (McMahon, 1988; Harley, 2003; Helmuth et al, 2006b). Rocky seashores represent a thermally-variable and potentially-stressful habitat for the gastropod populations inhabiting them (Helmuth & Hofmann, 2001; Harley, 2008)
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