Untangling the roles of microclimate, behaviour and physiological polymorphism in governing vulnerability of intertidal snails to heat stress

Abstract

Biogeographic distributions are driven by cumulative effects of smaller scale processes. Thus, vulnerability of animals to thermal stress is the result of physiological sensitivities to body temperature (T b ), microclimatic conditions, and behavioural thermoregulation. To understand interactions among these variables, we analysed the thermal tolerances of three species of intertidal snails from different latitudes along the Chinese coast, and estimated potential T b in different microhabitats at each site. We then empirically determined the temperatures at which heart rate decreased sharply with rising temperature (Arrhenius breakpoint temperature, ABT) and at which it fell to zero (flat line temperature, FLT) to calculate thermal safety margins (TSM). Regular exceedance of FLT in sun-exposed microhabitats, a lethal effect, was predicted for only one mid-latitude site. However, ABTs of some individuals were exceeded at sun-exposed microhabitats in most sites, suggesting physiological impairment for snails with poor behavioural thermoregulation and revealing inter-individual variations (physiological polymorphism) of thermal limits. An autocorrelation analysis of T b showed that predictability of extreme temperatures was lowest at the hottest sites, indicating that the effectiveness of behavioural thermoregulation is potentially lowest at these sites. These results illustrate the critical roles of mechanistic studies at small spatial scales when predicting effects of climate change.

The complex drivers of thermal acclimation and breadth in ectotherms

“…genetically based differences) and intraindividual (e.g. life-stage differences) variability in thermal responses that are unaccounted for in the current analyses (Dong et al 2017). Although our empirical analyses were consistent with most of the hypotheses derived from our mathematical model, there were also important areas where the model and empirical analyses deviated.…”

“…life‐stage differences) variability in thermal responses that are unaccounted for in the current analyses (Dong et al . ).…”

“…One possible explanation for this deviation is that terrestrial species might be better able to regulate body temperature through behavioural means than aquatic species (Dong et al . ). Alternatively, given that oxygen availability is more circumscribed in aquatic than terrestrial environments, this deviation from theory might also be a product of limited oxygen availability restricting whole‐animal tolerances to thermal extremes (Pörtner et al .…”

“…A final noteworthy deviation between theory and empirical data was the lack of greater thermal acclimation abilities of terrestrial than aquatic organisms, despite terrestrial species experiencing greater temperature variability. One possible explanation for this deviation is that terrestrial species might be better able to regulate body temperature through behavioural means than aquatic species (Dong et al 2017). Alternatively, given that oxygen availability is more circumscribed in aquatic than terrestrial environments, this deviation from theory might also be a product of limited oxygen availability restricting whole-animal tolerances to thermal extremes (P€ ortner et al 2017).…”

Survival in spatially variable thermal environments: Consequences of induced thermal defense

“…In particular, even when the mean maximum temperature of an event exceeds the mean tolerance (which at first glance might lead one to suppose that all individuals die), some individuals with above average tolerance will, by chance, experience below average temperatures and thereby survive (Denny et al 2011). In short, latitudinal patterns and population-wide consequences of rising average temperatures are likely to be strongly affected by individual variation in body temperature and thermal physiology (Dong et al 2017).…”