A central role for phenotypic plasticity in adaptive evolution is often posited yet lacks empirical support. Selection for the stable production of an induced phenotype is hypothesized to modify the regulation of preexisting developmental pathways, producing rapid adaptive change. We examined the role of plasticity in rapid adaptation of the zooplankton Daphnia melanica to novel fish predators. Here we show that plastic up-regulation of the arthropod melanin gene dopa decarboxylase ( Ddc ) in the absence of UV radiation is associated with reduced pigmentation in D. melanica . Daphnia populations coexisting with recently introduced fish exhibit environmentally invariant up-regulation of Ddc , accompanied by constitutive up-regulation of the interacting arthropod melanin gene ebony . Both changes in regulation are associated with adaptive reduction in the plasticity and mean expression of melanin. Our results provide evidence that the developmental mechanism underlying ancestral plasticity in response to an environmental factor has been repeatedly co-opted to facilitate rapid adaptation to an introduced predator.