Differences in heat tolerance plasticity between supratidal and intertidal snails indicate complex responses to microhabitat temperature variation.

Abstract

Tropical intertidal gastropods that experience extreme and highly variable daily temperatures have evolved significant and complex heat tolerance plasticity, comprising components that respond to different timescales of temperature variation. An earlier study showed different plasticity attributes in snails from differently-heated coastlines, suggesting lifelong irreversible responses that matched habitat thermal regimes. To determine whether heat tolerance plasticity varied at a finer, within-shore spatial scale, we compared the responses of supratidal (predominantly shade-dwelling) and intertidal (frequently solar-exposed) populations of the tropical thermophilic gastropod, Echinolittorina malaccana. Snails modified lethal temperature (LT50) under warm or cool laboratory acclimation, with the overall variation in LT50 being greater in the supratidal (56.0-58.0°C) than in the intertidal population (57.1-58.1°C). Similar maximum LT50s expressed by the populations after warm acclimation suggest a capacity limitation under these temperature conditons. The different minimum LT50s after cool acclimation corresponded with microhabitat temperature and field acclimatization of the snails. Different responses to the same laboratory acclimation treatment imply long-term (and possibly lifelong) thermal acclimatization, which could benefit sedentary organisms that are randomly recruited as larvae from a common thermally-stable aquatic environment to thermally-unpredictable intertidal microhabitats. These findings provide another example of thermal tolerance plasticity operating at microhabitat scales, suggesting the importance of considering microhabitat thermal responses when assessing broad-scale environmental change.