Camouflage by the masking crab,Notomithrax ursus(Herbst, 1788) (Decapoda: Brachyura: Majidae): is it a decorator or a dressmaker?

Abstract

AbstractMany spider crabs attach various kinds of living material to camouflage themselves and thereby escape detection by predators. This strategy uses a suite of co-evolved characters including a pyriform carapace shape, long agile chelipeds, hooked setae and a stereotyped behaviour pattern which is used to gather and plant pieces of other organisms amongst hooked setae on their body. Notomithrax ursus (Herbst, 1788) is a New Zealand majid crab that lives amongst seaweeds and mainly masks itself with the same. In doing so, it uses all the skills that we expect of a dressmaker. Denuded of its mask, the crab chooses algae it can manipulate, measures and cuts each piece, passes them to its mouth, and then plants them on its body in a roughly regular order, such that the severed ends are inserted into the hooked setae with only intact end visible. Longer pieces are planted on the body and smaller pieces on its walking legs to make a profile resembling a clump of seaweed. Distribution of the seaweed follows the distribution of the hooked setae on the crab. The number of pieces attached is independent of crab size and, as carapace width increases, the crabs compensate by cutting out longer pieces of seaweed. Cheliped allometry explains scaling of the mask. Total length of seaweed used can range 0.6–2.6 m (mean 1.36 m). Increasing mask load is accompanied by increases in the number and size of hooked setae thereby strengthening attachment. The behaviour of N. ursus should be called “dressmaking” not “decorating.” It uses a stereotyped behaviour pattern to follow a design, dictated by the distribution of hooked setae, and produce a customized garment concealing it from predators. A hypothesis on the evolution of masking, from passive to active camouflage is proposed, based on the original ideas of C.W.S. Aurivillius.