Feeding and masking selection in Inachus phalangium (Decapoda, Majidae): dressing up has never been so complicated

We investigated the choice preferences exhibited by the spider crab Inachus phalangium when using algae for feeding and masking, together with the pattern of decoration present on the body surface and the morphology of the setae, which are used to attach masking material. We showed that feeding and masking preferences are decoupled, and that I. phalangium exploited the algal nutritional and chemical properties differently. When feeding, the crab revealed a differential choice among the offered algae, preferring the protein-rich cyanobacteria. When decorating, both algal components that match the background and the Phaeophyta Dictyota dichotorna, which acts as a chemical deterrent against predators, were actively chosen. At the same time, decoration in I. phalangium was used as a short-term food store. We also provided evidence that masking patterns varied in different body regions, depending on the functional morphology of the hooked setae. This work discusses the origin and evolution of the decorative habit, showing for the first time that decoration is a multifaceted mechanism, tightly linked with morpho-functional and behavioural adaptations, and that masking material can also be used as a food store.