Abstract
Large‐scale investigations of patterns in heat tolerance often use latitude as a proxy for thermal conditions. We examined how heat tolerance of fishes and its plasticity relate to thermal extremes and seasonal fluctuations in habitat temperatures. Our dataset included over 3000 heat tolerance measurements from 500+ fish species inhabiting freshwater, marine and brackish environments. While heat tolerance varied with latitude, extremes and seasonal fluctuations in temperature better explained this variation. We also found that freshwater fishes had higher heat tolerance with greater plasticity than marine fishes, reflecting exposure to higher and more variable temperatures. Plasticity declined as they neared their thermal limits, highlighting the challenge of physiologically adjusting to extreme heat stress. Finally, we found that the benefits of plasticity in terms of time gained for adaptation varied considerably across the globe (5%–35%), with the lowest values for marine species at lower (warmer) latitudes. The distinct heat tolerance adaptations in freshwater and marine fishes reported here underscore the importance of thermal physiology in predicting responses to climate change.