Zoologger is our weekly column highlighting extraordinary animals – and occasionally other organisms – from around the world
Species: Decorator crabs (superfamily Majoidea)
Habitat: Shallow waters worldwide
If there's one thing we have in common with these crabs, it's our keen sense of fashion. Many of us love to don new clothes, wear jewellery or get tattoos and piercings.
So it is with the decorator crabs. About three-quarters of over 900 species of crab in the family Majoidea decorate themselves, making them perhaps nature's most fashion-conscious animal.
Although not the only animals known to decorate themselves, the crabs are the most well-researched group, according to a study reviewing such behaviours.
They improvise accessories using whatever is around, grabbing items such as seaweed, corals and sponges, and sticking them on their shells. Everything stays in place thanks to the hooked hairs, called setae, which line their shells and act like Velcro.
Blending in
But while we adorn ourselves to be noticed, crabs do it for the opposite reason: the decorations often provide camouflage against predators like fish and octopuses. Against the proper background, a decorator crab can blend in perfectly.
"The nice thing about being a decorator is that wherever you go, you can pull off the old decoration and stick on something new and quickly adapt yourself to whatever environment surrounds you," says John J. Stachowicz, an ecologist at the University of California, Davis. "If you're a slow-moving, roving animal, being able to quickly adopt the coloration or background of wherever you are is likely very adaptive."
For instance, the yellowline arrow crab (Stenorhynchus seticornis) tears a piece of seaweed in its claws and then chews it to make it rougher and more likely to catch on its shell. It backs up the camouflage by remaining still during the day and freezing when predators approach.
Repellent dress sense
Other decorator crabs are picky: any old outfit won't do. They go for materials that are chemically noxious or otherwise repugnant to predators.
"They are selecting decorations that make them toxic or bad to eat," says Stachowicz.
The long-legged spider crab (Macropodia rostrata) and the longnose spider crab (Libinia dubia), for example, cover their shells with toxic seaweed. Stachowicz says you can trick these crabs into donning just about anything if you extract a chemical that fish find repellent from their preferred seaweed and paint it on to materials that they normally don't use.
Yet others adorn themselves with stinging sea anemones. Predators might be able to detect the crabs, but will avoid attacking them.
Like all crustaceans, decorator crabs must shed their shells in order to grow, but they will often recycle their decorations after they moult. Carefully removing all the seaweed, anemones, sponges and other accessories from their old shell, they attach them to the new one when it hardens.
But some species outgrow the urge to accessorise altogether, possibly because they have fewer predators once they reach a certain size.
Journal reference: Royal Society Biology Letters, DOI: 10.1098/rsbl.2015.0325
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