BERKE, S.K.*; WOODIN, S.A.; Univ. of South Carolina; Univ. of South Carolina: Do energy-mortality tradeoffs drive sexual dimorphism in a crab�s decoration? (Oregonia gracilis, Brachyura: Majidae)

Decorator crabs camouflage themselves by attaching sessile organisms to their carapace and legs. This behavior reduces predation risk, but incurs significant energetic costs. We asked whether this energy-mortality tradeoff might drive sexual dimorphism in the decorator crab Oregonia gracilis. Sexually mature O. gracilis females decorate extensively, as do juveniles of both sexes. Mature males, however, decorate very little. This variance is largely driven by differences in setal morphology. All decorating Majids use velcro-like hooked setae to attach decoration. Many species also have straight, bristled setae, which can work in concert with hooked setae to facilitate decoration. The density of bristled setae�and consequently the amount of decoration carried�varies sexually and over ontogeny in O. gracilis. Mature females have numerous bristled setae, as do juveniles of both sexes. In males, however, setosity decreases with each molt as they approach maturity. At the same time, males� claws grow larger relative to body size. We asked whether this inverse relationship results from energetic tradeoffs between decorating and having large claws. We measured degrowth during starvation for males having artificial decoration, artificially enlarged chelae, or both. Artificial decoration alone causes degrowth similar to artificial chelae alone. A combination of decoration and chelae causes increased degrowth relative to either treatment alone and controls. These data suggest that male sexual maturity accompanies energy re-allocation from decorating into chelar growth, perhaps because large chelae increase mating success. Since females have small claws and brood their eggs for long time-periods, females might experience greater selective pressure to avoid predation, making it advantageous to decorate throughout their lives.