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 decorate themselves by sticking mostly sedentary animals and plants to their bodies as camouflage, or if the attached organisms are noxious, to ward off predators through aposematism. [1]
In 1889, William Bateson observed in detail the way that decorator crabs fix materials on their backs. He noted that "the 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". [2]
In his The Colours of Animals (1890), Edward Bagnall Poulton [3] 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. [4]
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. [5]
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 Sertularia abietina (a hydrozoan). Another was on small shells and gravel, and was decorated with those. A third was among the Antedon rosaceus (a crinoid), and had broken off pieces of crinoid arms as decoration. [5]
Bateson, in a much quoted passage, [6] describes crabs of the genera Stenorhynchus and Inachus : the crab tears a piece of seaweed in its claws, chews it, and then rubs it firmly on its body until it catches on the "Velcro-like hooked setae", [1] curved hairs which permit camouflage materials to be attached. [2] The carefully chosen decoration is supplemented by cryptic behaviour, such as remaining still by day, and freezing when predators approach. [1]
The surface of Tiarinia cornigera is covered with clusters of setae which have a rough "flowery" surface forming a shape like a haystack. This appears to encourage small organisms to attach themselves mechanically and chemically to the surface. [7]
Kristin Hultgren and Jay Stachowicz showed in 2011 that the species of Majoidea whose juveniles camouflage themselves are scattered about the phylogenetic tree – some species do, some do not, and some do so only a little. About 75% of the Majoidea decorate themselves to some extent during at least one phase of their lifecycle, and this number includes all 8 families in that superfamily. [1]
Reviewing studies conducted by different authors, Hultgren and Stachowicz showed that some species are highly specialised in their choices of camouflage. For example, Pelia tumida decorates itself only with sponges; Macropodia rostrata and Libinia dubia choose the noxious alga Dictyota menstrualis in some places, while Inachus phalangium chooses the same alga for the most exposed parts of its body; and Stenocionops furcata selects the stinging sea anemone Calliactis tricolor for its carapace. The choice of noxious or stinging organisms as decorations implies that attaching them provides protection from predators by aposematism rather than by crypsis, and there is direct observational evidence for this, e.g. that octopuses in tanks avoided decorated crabs. [1] The relationship of crab and organisms such as sea anemones used as decoration may be mutualistic, offering protection to the crab and food to the anemone. [8]
Decorator crabs come from many different genera, and are distributed worldwide. Some examples are described in the table.
| Species | Common names | Distribution | Decoration behaviour |
|---|---|---|---|
| Acanthonyx dentatus | Toothed crab | South Africa | |
| Naxia tumida | Little seaweed crab | Australia and surrounding waters [9] | |
| Macropodia rostrata | Long-legged spider crab | Coasts of Europe, Mediterranean, Black Sea | Chooses the chemically noxious alga Dictyota linearis [1] |
| Stenocionops furcata | Furcate spider crab | Atlantic coast of North America [10] | Chooses stinging anemone Calliactis tricolor [1] |
| Oregonia gracilis | Graceful decorator crab | Northern Pacific Ocean [11] | |
| Hyastenus elatus | Sponge decorator crab | Australia | Decorates with aposematic sponges |
| Achaeus spinosus | Spider crab | Indonesia, Philippines, Japan | Can use stinging hydrozoa |
| Maja crispata | Small spider crab | Eastern Atlantic, Mediterranean Sea [1] | |
| Herbstia parvifrons | Crevice spider crab | Pacific coast of North America [1] | Always uses chemically noxious sponges |
| Libinia emarginata | Portly or nine-spined spider crab | Atlantic coast of North America [1] | |
| Libinia dubia | Longnose spider crab | Atlantic coast of North America | Uses noxious alga Dictyota menstrualis [1] |
| Loxorhynchus crispatus | Masking or moss crab | Eastern Pacific Ocean [1] | Prefers to decorate with bryozoan Bugula neritina |
| Hyas araneus | Great spider crab | North Atlantic, North Sea [5] | |
| Camposcia retusa | Harlequin crab or spider decorator crab | Indonesia [12] | |
| Stenorhynchus seticornis | Yellowline arrow crab | Caribbean [13] | Chooses materials for best camouflage against local background |
Camouflage is the use of any combination of materials, coloration, or illumination for concealment, either by making animals or objects hard to see, or by disguising them as something else. Examples include the leopard's spotted coat, the battledress of a modern soldier, and the leaf-mimic katydid's wings. A third approach, motion dazzle, confuses the observer with a conspicuous pattern, making the object visible but momentarily harder to locate, as well as making general aiming easier. The majority of camouflage methods aim for crypsis, often through a general resemblance to the background, high contrast disruptive coloration, eliminating shadow, and countershading. In the open ocean, where there is no background, the principal methods of camouflage are transparency, silvering, and countershading, while the ability to produce light is among other things used for counter-illumination on the undersides of cephalopods such as squid. Some animals, such as chameleons and octopuses, are capable of actively changing their skin pattern and colours, whether for camouflage or for signalling. It is possible that some plants use camouflage to evade being eaten by herbivores.
Stenorhynchus seticornis, the yellowline arrow crab or simply arrow crab, is a species of marine crab.
In ecology, crypsis is the ability of an animal or a plant to avoid observation or detection by other animals. It may be a predation strategy or an antipredator adaptation. Methods include camouflage, nocturnality, subterranean lifestyle and mimicry. Crypsis can involve visual, olfactory or auditory concealment. When it is visual, the term cryptic coloration, effectively a synonym for animal camouflage, is sometimes used, but many different methods of camouflage are employed by animals or plants.
Countershading, or Thayer's law, is a method of camouflage in which an animal's coloration is darker on the top or upper side and lighter on the underside of the body. This pattern is found in many species of mammals, reptiles, birds, fish, and insects, both in predators and in prey.
Animal colouration is the general appearance of an animal resulting from the reflection or emission of light from its surfaces. Some animals are brightly coloured, while others are hard to see. In some species, such as the peafowl, the male has strong patterns, conspicuous colours and is iridescent, while the female is far less visible.
Underwater camouflage is the set of methods of achieving crypsis—avoidance of observation—that allows otherwise visible aquatic organisms to remain unnoticed by other organisms such as predators or prey.
Acanthonyx dentatus, the toothed decorator crab, is a species of crab in the family Inachidae.
Inachus phalangium, Leach's spider crab, is a species of crabs from the north-eastern Atlantic Ocean and Mediterranean Sea. It is up to 20.5 mm (0.81 in) wide, and is very similar to other species in the genus Inachus.
Oregonia gracilis, commonly known as the graceful decorator crab, is a species of crab belonging to the family Oregoniidae. Like other decorator crabs it habitually attaches other organisms to its back. The sessile organisms are attached to hooked setae that act as a sort of velcro attachment. This decoration provides visual and chemical camouflage thus reducing predation risk. Pacific halibut are a major predator of O. gracilis. Other predators include octopus and sea otters. The main food source of O. gracilis is floating kelp and algae that they capture utilizing a waiting strategy in order to maintain cryptosis.
Libinia dubia, the longnose spider crab, is a species of crab in the family Epialtidae. It is found in shallow waters on the eastern coast of North America.
Disruptive coloration is a form of camouflage that works by breaking up the outlines of an animal, soldier or military hardware with a strongly contrasting pattern. It is often combined with other methods of crypsis including background colour matching and countershading; special cases are coincident disruptive coloration and the disruptive eye mask seen in some fishes, amphibians, and reptiles. It appears paradoxical as a way of not being seen, since disruption of outlines depends on high contrast, so the patches of colour are themselves conspicuous.
Adaptive Coloration in Animals is a 500-page textbook about camouflage, warning coloration and mimicry by the Cambridge zoologist Hugh Cott, first published during the Second World War in 1940; the book sold widely and made him famous.
The Colours of Animals is a zoology book written in 1890 by Sir Edward Bagnall Poulton (1856–1943). It was the first substantial textbook to argue the case for Darwinian selection applying to all aspects of animal coloration. The book also pioneered the concept of frequency-dependent selection and introduced the term "aposematism".
Dorippe frascone, the urchin crab or carrier crab, is a small species of crab in the family Dorippidae that was first described scientifically by J.F.W. Herbst, in 1785. It is found in the Red Sea and parts of the western and eastern Indian Ocean. It often has a symbiotic relationship with a long-spined sea urchin and carries one around on its carapace.
Animal Coloration, or in full Animal Coloration: An Account of the Principal Facts and Theories Relating to the Colours and Markings of Animals, is a book by the English zoologist Frank Evers Beddard, published by Swan Sonnenschein in 1892. It formed part of the ongoing debate amongst zoologists about the relevance of Charles Darwin's theory of natural selection to the observed appearance, structure, and behaviour of animals, and vice versa.
Moreiradromia antillensis is a species of decapods in the family sponge crab. This species lives in the Western and central Atlantic Ocean.
Animal coloration provided important early evidence for evolution by natural selection, at a time when little direct evidence was available. Three major functions of coloration were discovered in the second half of the 19th century, and subsequently used as evidence of selection: camouflage ; mimicry, both Batesian and Müllerian; and aposematism.
Self-decoration camouflage is a method of camouflage in which animals or soldiers select materials, sometimes living, from the environment and attach these to themselves for concealment.
Libinia spinosa is a majoid crab found in mud and sand bottoms of the Southwestern Atlantic and Pacific oceans. It is a generalist feeder on organisms such as algae, sponges, cnidarians, mollusks, polychaetes, crustaceans, and small fish. It commonly engages in a symbiotic relationship with the medusa Lychnorhiza lucerna.
