Evolution of decoration in majoid crabs: a comparative phylogenetic analysis of the role of body size and alternative defensive strategies.

Although experimental studies have demonstrated the antipredatory advantages of camouflage and its associated costs, few studies have examined the evolution of camouflage in a phylogenetic context. We use phylogenetic comparative methods to examine evolutionary trade-offs associated with camouflage in the crab superfamily Majoidea. The majoids, or spider crabs, are known for their decoration behavior in which they attach materials from their environment to hooked setae on their carapace. We found that coverage of hooked setae on a crab (morphology) strongly predicts decoration cover in the field (behavior). Half of the species examined exhibited decreases in the coverage of hooked setae with ontogeny, and we also found a strong negative correlation between the extent of hooked setae and adult body size among species using independent contrasts, suggesting that size may constrain the evolution of camouflage. Finally, using a well-resolved clade of epialtids (kelp crabs)--many of which decorate little but use color change as an alternative camouflage strategy--we found a negative correlation between utilization of decoration versus color camouflage strategies. Our findings suggest that the costs of hook production and decoration maintenance and/or the lowered adaptive value of camouflage for larger species may limit the evolutionary distribution of decoration camouflage among the majoids.