The aptly-nicknamed decorator crab—a designation that includes three-quarters of over 900 species of crab in the family Majoidea—stands out from its fellow crustaceans because of the accessories it carries. But these crabs aren't looking to make a statement: they collect items such as seaweed, corals, and sponges and stick them to the hooked hairs on their shells (called setae) in order to better blend in.
"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," says John J. Stachowicz, an ecologist at the University of California, Davis. The crabs' various "decorations" allow them to do just that, changing out their appearance to suit their surroundings.
The crabs are careful and deliberate in their dressing. Some decorator crabs intentionally choose noxious materials to repel predators, collecting toxic seaweed and even stinging sea anemones to adorn their shells. The yellowline arrow crab (Stenorhynchus seticornis) doesn't just plop a pile of fresh seaweed on its shell and call it a day; it first tears it up and even chews it to make it rough enough to stick to the shell's setae. When decorator crabs, like all crustaceans, have to molt, they delicately remove each accessory from their old shells and reattach them once the new one has hardened.
[h/t New Scientist]