Why Is This Crab Wearing Red and Green Pom-Poms?

It’s that time of the year again. Snow is falling and festive ornaments are going up. Humans take their decorations pretty seriously, but animals do too, especially majoid crabs. About 75 percent of majoid crabs are known for their habit of decorating their carapaces with bits of algae, sponges, and other marine debris. These species are colloquially known as decorator crabs.

Scientists are still trying to understand exactly why these crustaceans engage in such behavior. One hypothesis suggests that the decorations help them scare off or confuse predators. A few weeks ago, Danielle Dixson, a marine scientist at the University of Delaware, designed an experiment to investigate this question.

Dixson and her team placed samples of decorator crabs, namely the species Camposcia retusa, in individual containers and provided them with decorative materials, specifically red and green pom-poms. Half of the crabs were provided with shelter while the other half of the crabs were left in open water.

During the course of one day, the team photographed the marine creatures every hour for the first 12 hours, and then again during the final hour. This time-lapse record helped scientists understand two important features of their behavior.

First, crabs are fast decorators. Most of them were adorned with pom-poms within the first six hours and all of them were “fully decorated” after 24 hours. Dixon’s analysis of their speed suggests that decoration is indeed an important defensive strategy.

The photos also revealed a second and more surprising behavior. Those crabs that had access to shelter adorned their arms and legs first. “We know from the literature that crabs usually decorate their body first as that’s where their vital organs are, but in this case, they didn’t,” Dixson says. “Maybe they didn’t because their body was under a shelter and it was legs and arms that were hanging out, suggesting they choose what to protect depending on context.”

So what’s next? The team will conduct more experiments to understand if majoid crabs use decorations to visually hide from predators or if they engage in what’s known as “chemical camouflage”—taking on a different scent to confuse predators. “Humans tend to project their bias when thinking of the animal world, so when most people think about hiding or camouflage they think about vision,” Dixson says. “But we know that in the ocean, chemistry is super important as well.”