Intraspecific variation in thermal tolerance differs between tropical and temperate fishes

How ectothermic animals will cope with global warming 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 fish species, we found that intraspecific variation in upper thermal tolerance varies according to a species’ latitude and evolutionary history. Overall, tropical species show a lower intraspecific variation in thermal tolerance than temperate species. Notably, freshwater tropical species have a lower variation in tolerance than freshwater temperate species, 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 freshwater tropical fish communities, worldwide, are especially vulnerable to ongoing climate change.

Low potential for evolutionary rescue from climate change in a tropical fish

Climate change is increasing global temperatures and intensifying the frequency and severity of extreme heat waves. How organisms will cope with these changes depends on their inherent thermal tolerance, acclimation capacity, and ability for evolutionary adaptation. Yet, the potential for adaptation of upper thermal tolerance in vertebrates is largely unknown. We artificially selected offspring from wild-caught zebrafish (Danio rerio) to increase (Up-selected) or decrease (Down-selected) upper thermal tolerance over six generations. Selection to increase upper thermal tolerance was also performed on warm-acclimated fish to test whether plasticity in the form of inducible warm tolerance also evolved. Upper thermal tolerance responded to selection in the predicted directions. However, compared to the control lines, the response was stronger in the Down-selected than in the Up-selected lines in which evolution toward higher upper thermal tolerance was slow (0.04 ± 0.008 °C per generation). Furthermore, the scope for plasticity resulting from warm acclimation decreased in the Up-selected lines. These results suggest the existence of a hard limit in upper thermal tolerance. Considering the rate at which global temperatures are increasing, the observed rates of adaptation and the possible hard limit in upper thermal tolerance suggest a low potential for evolutionary rescue in tropical fish living at the edge of their thermal limits.

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 vsariation 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.