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Elaborate Accessories Keep Predators Away From Decorator Crabs

542 Elaborate Accessories Keep Predators Away From Decorator Crabs
Cigdem Sean Cooper on shutterstock

Many animal species, from bugs to birds to boars, decorate themselves by accumulating various materials from their surroundings and attaching them to their bodies. A new review of the evolution of decorating behavior published in Biology Letters this month, found that the phenomenon is predominantly an aquatic one: The buoyancy helps with all the heavy lifting. 

Graeme Ruxton from the University of St Andrews and University of Exeter’s Martin Stevens reviewed the literature to explore what we know about non-human decorators, who use these objects very differently than we do. "The most common reason to decorate is to avoid predators," told Stevens to National Geographic. Decorations also help with parasite reduction and UV protection, though some animals do adorn themselves to appear more attractive to mates. 


The most widely studied group of decorators are crabs from the superfamily Majoidea. About 75 percent of the 900 species in this group boast decorations on some or all of their body. They even have specialized, hooked setae (hair-like bristles) for attaching materials. These crabs appear to have two kinds of decorations: Repellent or camouflage. 

As accessories, chemically defended plants or sessile animals (like sponges or stinging anemone) ward off predators, thanks to their repugnant smell or taste. The long-legged spider crab (Macropodia rostrata) and the longnose spider crab (Libinia dubia), for example, cover their shells with toxic seaweed, New Scientist reports. Predators might see them, but they just move along. Other kinds of decorations help crabs blend into their backgrounds. The yellowline arrow crab (Stenorhynchus seticornis) would chew a piece of seaweed to make it look rougher and more likely to catch on its shell, New Scientist explains. It also helps to stay still around lurking predators.

Sometimes, the threat of predators leads to aggression among juvenile crabs, who may compete with each other for decorating materials. Accordingly, the extent of decorations on any individual crab predicts dominance in these encounters. Crabs decorate themselves less as they get bigger: Carrying all these decorations around can be energetically costly, even in water. When they’re bigger, the crabs can defend themselves with their claws and thicker carapaces. Plus, they don’t fit into fish mouths as easily. 

On land, many insects are capable of generating enough power to haul around material weighing more than the insects themselves. Even still, decorating is more common in the larval stages of insects. Flying adult instinct need to be lightweight to evade potential predators.


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