Variation in the carapace pattern of juvenile Carcinus maenas

Juvenile shore crabs (Carcinus meanas L.) are highly variable with respect to carapace pattern; young crabs often have conspicuous patches of white or other pigment. Adult crabs, in contrast, are drab and usually very similar in pattern. It is probable that the patterns serve to make young crabs cryptic on certain backgrounds. Juvenile crabs of carapace widths lying between 5 and 15 mm were collected from a range of British shores (totalling 48). Pattern incidence was found to relate to two factors: the percentage of weed cover (mainly of fucoids) on the shore, and the amount of mud present. Shores of high weed cover, or with much mud present, had low frequencies of conspicuously patterned crabs. The presence of patterned crabs among the juvenile population, and their absence among adults, could result from differential visual predation removing patterned crabs progressively so that only unpatterned crabs would survive to adulthood. Alternatively, individual crabs could lose their conspicuous patterns with age. Evidence is presented in support of the latter hypothesis.