Rose E O'Dea, Malgorzata Lagisz, Andrew P. Hendry, Shinichi Nakagawa 

Fishes are sensitive to their thermal environment, and face an uncertain future in a warming world. Theoretically, populations in novel environments might express greater levels of phenotypic variability to increase the chance of surviving – and eventually thriving – in the new conditions. Most research on the effect of the early thermal environment in fish species focuses on average phenotypic effects rather than phenotypic variability, but to understand how fishes will respond to rising temperatures we need to consider both the average response of the population, as well as the breadth of individual responses. Here we present the first meta-analysis on the effects of developmental temperature in fishes. Using data from 43 species and over 6,000 individual fish we show that a change in developmental temperature induces a significant change in phenotypic means and variability, but differently depending on whether the temperature is increased or decreased. Decreases in temperature (cool environments) showed a significant decrease in phenotypic means and no change in phenotypic variability. Increases in temperature (warm environments) showed a non-significant increase in phenotypic means, and a significant increase in phenotypic variability. Larger increases in temperature saw grater increases in phenotypic variability, but no increase in the mean phenotypic response. Together, our results suggest that fishes exhibit both directed and stochastic developmental plasticity in response to warming temperatures, which could facilitate or accelerate adaptation to a changing environment.