Energy availability influences the dynamics of thermal phenotypic plasticity

We tested whether food availability limits phenotypic plasticity in thermal tolerance in the amphipod Echinogammarus marinus. We shifted specimens from 10°C to an acclimation temperature of 20°C, and kept them there for different durations with and without food before measuring the time to immobilization at 30°C. Our results show that thermal tolerance increases with acclimation duration, but this response was about two times more pronounced in fed than in unfed individuals. We also decomposed the plastic response into a rate component (how fast the trait changes) and a capacity component (by how much it changes). This showed that the overall effect of food treatment on the temporal dynamics of thermal tolerance was primarily driven by the effect on capacity. We conclude that laboratory derived thermal tolerance data from experiments where ecological conditions are otherwise optimal may provide overly optimistic estimates of how well organisms deal with extreme events through phenotypic plasticity.