Measures of the upper thermal limit of the medaka fish (Oryzias latipes) with the critical thermal maximum (CTmax) protocol, for several temperature conditions.

Context: 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. Study design: The experiment was conducted at the INRAE laboratory in Aix-en-Provence, France. Using the medaka fish (Oryzias latipes), we measured their CTmax on different temperature conditions in a full factorial design with two fish treatments (lineage raised in cold or warm water for multiple generations) and two temperature treatments (heatwave and control without heatwave).  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.