Heat tolerance and its plasticity in freshwater and marine fishes are linked to their thermal regimes.

Responses to climate change are rooted in thermal physiology, and many studies have focussed on heat tolerance and plasticity of heat tolerance. Latitudinal patterns in heat tolerance are commonly considered to reflect latitudinal differences in thermal regimes, but direct tests are few. Here we show that the extremes and fluctuations in habitat temperature explain variation in heat tolerance of freshwater and marine fishes. Furthermore, we found that freshwater fish exhibit greater plasticity in heat tolerance than their marine counterparts. This reflects that, compared to marine fishes, freshwater fishes are exposed to greater thermal fluctuations. Our findings underscore the importance of thermal physiology for predicting responses to climate change and highlight that plasticity in heat tolerance is an important mechanism to cope with thermal extremes, especially for organisms living in thermally variable habitats such as freshwater fish.