Active background selection facilitates camouflage in shore crabs, Carcinus maenas

Camouflage plays a significant role in preventing and facilitating predation. A common method used by many species to avoid detection is through matching aspects of the visual background. Behaviour can comprise a valuable element of camouflage through enabling animals to choose appropriate substrates, yet how widespread this is remains relatively underexplored. Through a series of substrate choice experiments we tested whether the highly phenotypically diverse common shore crab (Carcinus maenas) shows substrate preferences, and whether preferences reflected choices that actively improve individual camouflage. Using image analysis, we compared brightness and colour metrics of crabs to their chosen versus alternative substrates. Crabs tended to choose substrates with a brightness that better matched their own appearance. However, choices depended on the exact backgrounds offered, for example with crabs preferring backgrounds resembling native rock pool colour patterns over those resembling mudflats, but showing little difference in choice between red and green substrates. The results help explain observations that shore crabs and other animals show phenotype-environment associations at a micro scale, and demonstrate how individuals can maintain camouflage in highly variable visual environments. Our study shows that substrate preferences can be a key route to enabling camouflage in a broad spectrum of species.