Arctic charr phenotypic responses to abrupt temperature change: an insight into how cold water fish could respond to extreme climatic events.

Phenotypic plasticity, the ability of an organism to express multiple phenotypes in response to the prevailing environmental conditions without genetic change, may occur as a response to anthropogenic environmental change. Arguably, the most significant future anthropogenic environment change is contemporary climate change. Given that increasing climate variability is predicted to pose a greater risk than directional climate change, we tested the effect of a water temperature differential of 4 {o}C on the Arctic charr phenotypic response within a generation. We demonstrate that Arctic charr phenotype can respond rapidly and markedly to an environmental cue. The plastic response to different temperature regimes comprised a shift in the mean phenotype coupled with a reduction in the between-individual phenotypic variation in the expressed head shape. The magnitude of shape difference was cumulative over time but the rate of divergence diminished as fish became larger. Individuals raised in the elevated temperature treatment expressed a phenotype analogous to a benthivorous ecotype of this species rather than that of the parental pelagic feeding form. The response of cold-water freshwater species to temperature change is likely to be an interaction between the capacity of the organism for phenotypic plasticity, the speed of mean change in the environment (e.g., temperature), and the degree of short interval variation in the environment.