Intraspecific variation in thermal tolerance differs between tropical and temperate fishes

How ectothermic animals will cope with global warming, especially more frequent and intense heatwaves, is a critical determinant of the ecological impacts of climate change. There has been extensive study of upper thermal tolerance limits among fish species but how intraspecific variation in tolerance may be affected by habitat characteristics and evolutionary history has not been considered. Intraspecific variation is a primary determinant of species vulnerability to climate change, with implications for global patterns of impacts of ongoing warming. Using published critical thermal maximum (CTmax) data on 203 marine and freshwater fish species, we found that intraspecific variation in upper thermal tolerance varies according to a species latitude and evolutionary history. Notably, freshwater tropical species have lower variation in tolerance than temperate species in the northern hemisphere, which implies increased vulnerability to impacts of thermal stress. The extent of variation in CTmax among fish species has a strong phylogenetic signal, which may indicate a constraint on evolvability to rising temperatures in tropical fishes. That is, in addition to living closer to their upper thermal limits, tropical species may have higher sensitivity and lower adaptability to global warming compared to temperate counterparts. This is evidence that tropical fish communities, worldwide, are especially vulnerable to ongoing climate change.