Much more than hooked: Setal adaptations for camouflage in Macrocoeloma trispinosum (Latreille, 1825) (Crustacea: Decapoda: Brachyura).

Several majoid crabs are known to adhere exogenous materials to their bodies, a behaviour called decoration. Until now, the adhesion of exogenous materials to the body is most attributed to the well-known hooked setae. Here, we analysed the carapace of Macrocoeloma trispinosum (Latreille, 1825) under light and electron microscopy to study the different mechanisms allowing majoid crabs to decorate themselves. Five setal types are described here, of which four for the first time: velvet type I, velvet type II, depressa and cattail seta. These setae are morphologically and histologically detailed, and new hypotheses about the fixation of exogenous material on the carapace are explored. M. trispinosum has a complex setal apparatus for the adhesion of the decoration, with tegumental ducts along the shaft of most setae. These tegumental ducts are connected to glands formed by large cells arranged radially (rosette or acini) at the base of the setae, in the connective tissue, just below the epithelium. We could observe these glands in different stages of maturation, and no valve-like structure was observed, which may indicate a continuous flow of protein secretion that could serve as an adhesive substance found in the apex of most setae. This is the first record indicating a potential chemical adhesion mechanism aiding the masking process in decorator crabs.

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