Decorator and Sponge Crabs

Several types of crabs collectively known as the decorator and sponge crabs camouflage themselves from predators by carrying something on top of their shells. The toothed decorator crabs (Dehaanius) are small crustaceans with hooklike bristles on their backs where algae can be attached. Algae attached to a crab’s shell acts as excellent camouflage. Like all crabs, these have five pairs of walking legs. The first pair is modified as claws, which pick up algae and place the material onto the hooks.

Sponge crabs, members of the Dromiidae family, hold their camouflage in place with two pairs of posterior legs that have been modified for grasping. The legs are turned up and armed with points that the crab sticks through the sponges to hold them in place. Sponges give off noxious chemicals that can cause predators to think twice before attacking.

decorator crabs (Dehaanius)
Decorator crabs (Dehaanius)

When it is time to molt, a crab that is decorated in sponges lifts them off and sets them aside. When the new, larger shell is in place and hardened, the crab picks up the sponges and places them on its back again. Sponges do not grow as quickly as crabs, so after a few molts the crabs have outgrown the spongy hat. To solve this problem, crabs find new sponges, using their front claws to trim them for a proper fit. If a sponge is not available, the sponge crab will wear algae or anything else it can find.