Phenotype-environment matching and colour change for camouflage in the shore crab Carcinus maenas

Camouflage is one of the most common antipredator defences found in nature (Ruxton et al., 2004). The shore crab Carcinus maenas is vulnerable to attack by shore birds, fish species and even other crabs.. C. maenas exhibits a wide range of colours and patterns that appear to be associated with the habitat in which a crab lives. This is commonly referred to as phenotype-environment matching. This thesis investigates whether the colours and patterns provide a camouflage benefit to encompass the wavelengths that can be seen by its putative avian predators. I found that crabs from homogenous, plain mudflat environments have a significantly better camouflage to areas of their own habitat than to backgrounds from other habitat types, in terms of luminance (perceived lightness), colour, and pattern, indicating phenotype specialisation across all appearance metrics. Individual crabs from heterogeneous rockpool and mussel bed environments show mixed levels of phenotype-environment matching. One possible explanation for how shore crabs tune their phenotype to that of the environment is through morphological colour change. I reared crabs on black, white, red, or green backgrounds over short (48 h) and long term (5 weeks) time periods. I found no significant changes in appearance metrics in the short term. However, over a longer time period individuals reared on white backgrounds significantly increased their luminance. Crabs raised on red backgrounds significantly increased their red hue; the changes in luminance and hue significantly improved camouflage as birds were increasingly unable to distinguish them from the background. Phenotype environment matches in C. maenas improves camouflage for crabs in homogenous environments, and this appears to be driven by morphological colour change in juveniles. This study provides the first evidence of how species in complex homogenous environments may stay camouflaged to avoid avian predators.