The heritable basis and cost of colour plasticity in coastrange sculpins

Both genetic and plastic traits contribute to adaptation in novel environments. Phenotypic plasticity can facilitate adaptation by allowing for existence in a wider range of conditions and a faster response to environmental change than gene-based selection. Coastrange sculpins (Cottus aleuticus) colonize new and variable streams arising in the wake of receding glaciers in south-east Alaska, and substrate-matching plasticity may enhance colonization success by reducing detection by visual predators. As part of a long-term study of the fitness consequences of colour plasticity and its capacity to respond to both positive and negative selection, we investigated whether it is heritable and costly. We raised full-sib broods of sculpins in the laboratory: one half of each brood was raised in white containers, the other half in black. After 4 months, we digitally analysed their colour and found significant but weak heritability in both baseline colour and colour plasticity. To investigate the cost of colour plasticity, we compared the growth and mortality rates of juvenile sculpins reared under constant substrate colours to those reared on substrates that changed colour frequently, and compared growth rates among sculpin that differed in their colour change ability. We found evidence of small costs of plasticity, consistent with other studies of natural populations. Evidence of heritable genetic variation for plasticity and small costs to its maintenance and expression contributes to explanations of how plasticity is variable and persistent among wild populations and underscores its ability to respond both positively and negatively to selection in variable habitats.