Animals from many taxa, from snakes and crabs to caterpillars and lobsters, change appearance with age, but the reasons why this occurs are rarely tested. We show the importance that ontogenetic changes in coloration have on the camouflage of the green shore crabs ( Carcinus maenas ), known for their remarkable phenotypic variation and plasticity in colour and pattern. In controlled conditions, we reared juvenile crabs of two shades, pale or dark, on two background types simulating different habitats for 10 weeks. In contrast to expectations for reversible colour change, crabs did not tune their background match to specific microhabitats, but instead, and regardless of treatment, all developed a uniform dark green phenotype. This parallels changes in shore crab appearance with age observed in the field. Next, we undertook a citizen science experiment at the Natural History Museum London, where human subjects (“predators”) searched for crabs representing natural colour variation from different habitats, simulating predator vision. In concert, crabs were not hardest to find against their original habitat, but instead, the dark green phenotype was hardest to detect against all backgrounds. The evolution of camouflage can be better understood by acknowledging that the optimal phenotype to hide from predators may change over the life history of many animals, including the utilization of a generalist camouflage strategy. A plain language summary is available for this article.

Many animals change appearance with age but the reasons why are rarely tested. Common shore crabs (Carcinus maenas), for example, are known for their ability to change colour over time. Young crabs show remarkable variation in coloration and it has been suggested that their variable appearance may help them to hide from predators in the habitats they use. However, as crabs grow they become more mobile and adult crabs, in contrast, are known to possess a more uniform coloration. This creates a problem: how to remain hidden in habitats that are variable and very different in appearance? To answer this, we first reared young shore crabs of two shades, pale or dark, on two background types resembling different habitats for 10 weeks. We predicted that crabs would adopt a coloration that would improve their match to the background, but instead they all developed a dark green or brown appearance. Next, we undertook an experiment at the Natural History Museum London during the Colour and Vision exhibition, where visitors searched for crabs representing natural colour variation from different habitats. Remarkably, crabs were not hardest to find against their original habitat, but instead the dark green or 'mudflat crabs' were hardest to detect against all backgrounds. These findings suggest that shore crabs change into similar colour with age, which appears to be a good general solution to match several habitat types and therefore to hide from predators. We conclude that the evolution of camouflage can be better understood by recognizing that the optimal appearance to hide may change over the lifespan of many animals.