Patterns of epibiont colonisation on the spider crab Inachus communissimus (Decapoda, Inachidae) from the Northern Adriatic Sea (Mediterranean Sea)

Active masking behaviours have been widely reported for Majoid spider crabs, although it is known that species living on sandy and muddy bottoms are frequently less masked than those living on rocky substrates. On the other hand, crabs living on soft bottoms may represent rare hard substrates available for larval settlement of other organisms. Inachus communissimus Rizza, 1839 is a spider crab belonging to the Inachidae family, common in the Mediterranean Sea. Individuals collected in two different locations on the sandy–muddy bottoms of the Northern Adriatic Sea (Senigallia and Pedaso), showed exoskeletons mainly covered by the colonial ascidian Diplosoma listerianum (Milne-Edwards, 1841). The large size of the ascidians, encompassing all the portions of the crabs’ exoskeleton, and the lack of differences in the abundance of the covering, between adult males and females, support the idea that D. listerianum is an epibiont on the carapace of the crabs. The low level of carapace covering in the juveniles may be attributed to the regular moulting that the crabs undergo until puberty. On the other hand, juveniles actively mask themselves with small fragments of filamentous green algae, affixing them to the hooked setae. This active behaviour is lost when the crabs reach their final size after the pubertal moult.