Behavioral and morphological aspects of decorating in Oregonia gracilis (Brachyura: Majoidea)

. Decorator crabs (Brachyura: Majoidea) are known for attaching sessile organisms to specialized velcro‐like hooked setae. Here we describe behavioral and morphological aspects of decorating for a common northern Pacific majoid, Oregonia gracilis . Members of O. gracilis decorate with a diverse array of sessile organisms. These are attached to hooked setae, and also physically interact with long, pappose setae that occur in the same body regions as hooked setae. We describe these pappose setae and document the occurrence of similar setae in 36 other decorating majoids across seven families. In O. gracilis , the density of pappose and hooked setae independently covary with decoration amount, which is sexually dimorphic—juveniles and adult females decorate heavily, whereas adult males decorate sparsely. Adult males have reduced numbers of hooked and pappose setae, but the ontogenetic patterns for the two setal types are different, suggesting that they are quasi‐independent characters. We experimentally ablated pappose setae to ask if they functionally contribute to decorating in O. gracilis . Surprisingly, we found that pappose setae are not necessary for decorating under laboratory conditions. Pappose setae could play an auxiliary mechanical role or a sensory role in decorating, or may have another as‐yet‐unidentified function distinct from decorating.