There are more than 700 species of decorator crabs that belong to the Majoidea family, ranging from spider and harlequin crabs to moss and seaweed crabs, as well as toothed and arrow crabs. These crabs have a unique characteristic of covering their bodies with unexpected ornaments, but their purpose is not to impress or attract attention. Rather, they use these decorations for camouflage or luring prey.
So how do they adorn themselves? Marine Madness explains that most decorator crabs use other marine creatures as their “building material.” There are many different species of decorators, and each one has its own set of preferred decor, including corals, seaweeds, sponges, sea fans, anemones, seagrasses, sea arches, bryozoans, hydrozoans, and even other small crustaceans.
Although these organisms are taken over by the decorators, it does not necessarily mean that they will die. On the contrary, they can benefit from this relationship. In fact, the connection between crabs and their “accessories” can be highly mutually beneficial, helping plants and animals to move over large distances and reduce their chance of being preyed upon.
The article discusses the decorator crab, which is believed to be Achaeus spinosus, and how it covers itself in stinging hydrozoa. This unique behavior benefits both the crab and the hydrozoa as the crab gets protection from the stings, while the hydrozoa can feed on a larger area with the crab’s mobility. Decorator crabs can attach almost any marine organism smaller or bigger than themselves to their shells using special hooked bristles called setae. The process is similar to Velcro as the crab selects what it wants to attach, picks it up, and places it on the bristles. However, the process is not always easy as decorator crabs can be quite picky about what they wear. Some crabs have only a small covering of bristles, while others like the moss crab are covered in them from head to claw. The downside is that when the crabs molt their carapace to grow further, they lose all their carefully selected ornaments and have to start over from the beginning.
The decorator crab is an expert in camouflage, using materials from its environment to blend in. These crabs are so particular about their decorations that a study by zoologist Hugh Bamford Cott found that even when moved to a new location, they immediately began adjusting their camouflage using local materials. They do not decorate themselves to attract mates, but rather to hide and protect themselves from predators. Seaweeds, colonies of bryozoans and hydrozoans are common choices for decoration, while some species prefer colorful corals, sea fans, and sponges. As long as they stay close to their surroundings, their local camouflage is highly effective.
However, some crabs have taken their decorations to the next level by utilizing them not only for camouflage but also to strengthen their defenses in other ways. For instance, certain crabs use anemones, urchins, and hydrozoans that can give any potential predators a nasty sting if they get too close. Conversely, applying specific toxic seaweeds can leave a distasteful (and potentially deadly) flavor in the mouth of anything that tries to take a bite, making these crabs an unappealing option for many of their natural predators.
Some species of crabs take their use of decorations to the next level by turning them into weapons. One such example is the boxer crab, also known as the pom-pom crab, which attaches anemones and arches to its claws and uses them to defend itself when provoked or attacked. The boxing crab waves its adorned claws around, which are armed with stinging cells, to deter predators. Interestingly, the pom-pom crab’s relationship with sea anemones is essential for the survival of both species, as the crab carries the anemone in its claws.
Some decorator crabs use Hydrichthella epigorgia, a hydrozoan similar to coral that consists of numerous tiny polyps working together as a collective organism, for their advantage. While most hydrozoans have feeding tentacles, this species has sticky polyps that trap microscopic food and slowly transfer it to the animal’s mouth. Clever crab species have covered their front legs in H. epigorgia to produce a pair of effective fishing rods.
What happens when decorator crabs don’t have access to their usual materials like sponges, algae, and marine debris? According to a study by Danielle Dixon and her team at the University of Delaware, these crabs will use almost anything in their environment to decorate themselves. In an experiment, the scientists provided individual containers for the crabs and gave them green and red pom-poms resembling Christmas decorations. Surprisingly, the crabs didn’t hesitate to use the pom-poms to add a festive touch to their appearance.
