Decorator crabs

Decorator crabs are crabs of several different species, belonging to the superfamily Majoidea (not all of which are decorators), that use materials from their environment to hide from, or ward off, predators. They stick mostly sedentary animals and plants to their bodies as camouflage or, if the attached organisms are noxious, to ward off predators through aposematism.

In 1889, William Bateson observed in detail the way that decorator crabs fix materials on their backs. He noted that "[t]he whole proceeding is most human and purposeful", and that if a Stenorhynchus crab is cleaned, it will "immediately begin to clothe itself again with the same care and precision as before".

In his The Colours of Animals (1890), Edward Bagnall Poulton classified protective animal coloration into types such as warning colours and protective mimicry. He included self decoration under the heading "Adventitious Protection", quoting Bateson's account of decorator crabs.

In his Adaptive Coloration in Animals (1940), Hugh Bamford Cott describes self decoration under the heading "adventitious concealing coloration", also naming it "adventitious resemblance". He describes it as a device "perhaps unrivalled" for effective concealment, and points out that it is brought about and depends on "highly specialized behaviour". Further, it grades into other means of protection including "the borrowing of protection from aposematic partners" and the use of "fortified hiding-places" and burrows. Cott compares the way Australian aborigines once used water lily leaves over their faces to swim up to waterfowl until they could catch them by the legs.

Cott described the decorator crabs as using "concealment afforded by masks of adventitious material", giving as example the great spider crab Hyas araneus of Britain "which disguises itself very perfectly". When H. araneus specimens were moved from an environment where all the crabs were camouflaged with short pieces of seaweed into different environments, all of them had redecorated themselves with local materials after one night. One was among corallines, and was covered with a dense bush of the hydrozoan Sertularia abietina. Another was on small shells and gravel, and was decorated with those. A third was among the crinoid Antedon rosaceus, and had broken off pieces of crinoid arms as decoration.

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