Rapid and reversible plasticity of upper thermal limit, but no effects of multigenerational warming in medaka

The global rise of atmospheric temperature and the escalating occurrence of extreme climatic events urge for a better understanding of the factors influencing thermal tolerance to extreme temperatures. In particular, the critical thermal maximum (CTmax) is a proxy of the upper thermal tolerance that is key in assessing species vulnerability to warming. While several mechanisms, such as short-term acclimation and intergenerational effects, can influence the CTmax of ectotherm species, their relative contributions and interactions under natural conditions remain unclear. Using the medaka fish (Oryzias latipes), this study investigates whether multigenerational warming has shaped CTmax after multiple generations of laboratory rearing and explores the interplay between potential intergenerational effects and plastic responses during an artificial heatwave in mesocosms. Our findings reveal that seven years of multigenerational exposure to different temperatures (30 °C vs. 20 °C) did not cause any detectable durable change in CTmax. This suggests that evolution of CTmax is slow or requires strong selective pressures to evolve fast enough. In contrast, medaka fish exhibited rapid acclimation with a 1.5 °C increase in their CTmax following a two-week heatwave. Interestingly, two weeks after the heatwaves, we could not detect a significant effect of heatwave exposure on CTmax showing that this plastic response can reverse rapidly. These results, along with other studies on ectotherms thermal tolerance, emphasize the potential for short-term response of species to changing climate through acclimation. Our findings highlight the importance of integrating plasticity into vulnerability assessments to improve predictions of climate change impacts.