Does Variable Coloration in Juvenile Marine Crabs Reduce Risk of Visual Predation?

We discovered that newly settled marine rock crabs, Cancer irroratus, exhibit a variety of non-adult colors early in life. This color polymorphism predominates in populations of minute juvenile crabs living in polychromatic habitats where it apparently renders them inconspicuous to visual predators such as fish. Experiments revealed lower frequencies of non-adult color morphs in monochromatic three-dimensional habitats with predators and polychromatic habitats from which predators were excluded. These patterns result from selective predation on visually contrasting color morphs. Adult crabs are monochromatic, conspicuous, and not associated with shelters. Both polymorphic newly settled and monochromatic large individuals occur in environments dominated by small predatory fish where larger adult crabs are at low risk of predation. Behavioral and visual crypsis may only be important early in life when post-settlement mortality is high, and survival at that stage determines recruitment and ultimately population densities. The well-known examples of camouflage among insects usually apply to adults who, unlike these marine counterparts, are small relative to their predators and thus remain vulnerable throughout their lives. Many other large marine crustaceans are cryptic only early in life, suggesting that this early developmental color polymorphism might be an important difference between marine and terrestrial arthropods.