Metabolic depression as a key mechanism to survive high on the shore

Abstract

Intertidal zones are highly dynamic environments with strongly contrasting thermal regimes between tidal emersion and submergence. During tidal emersion, environmental temperatures fluctuate rapidly depending on local weather conditions. Temperatures on tropical high shores often exceed the thermal tolerance of organisms during emersion, and species living in such environments need a variety of physiological adaptations to survive in these extreme habitats. One key physiological adaptation is metabolic depression, where cellular metabolism is arrested and energy expenditure minimized, effectively decoupling physiological processes of individuals from their external thermal environments. The high shore oyster, Isognomon nucleus, demonstrates an extreme and distinctive metabolic depression. Initially, heart rate increases with body temperature until 38 °C, where a 55% depression of metabolism occurs until recovery between 42 – 46 °C, after which heart rate crashes irreversibly. Depression of metabolism at 38 °C coincides with a rapid increase in the risk for future environmental temperatures (until next submergence time or dusk) to reach the cardiac breakpoint, suggesting that the depression temperature might act as an early warning signal for the oysters to initiate physiological modifications to prepare for future thermal extremes. Whilst the early warning function might represent an ultimal cause driving the onset of metabolic depression, the oyster could initiate depression by responding to variations in body temperature, oxygen concentrations and/or water content as proximal mechanisms. The alignment between physiological mechanisms and early warning temperatures may, therefore, represent a key mechanism for species persistence in extreme and highly dynamic thermal regimes such as on tropical high shores.