A Garden on a Crab's Back
Discover how decorator crabs attach seaweed and other living materials to their shells to blend into their surroundings.
Play the quiz as a gameA Living Disguise
An underwater rock may seem to move. Look closer, and you might see legs beneath a patch of seaweed. The walker could be a decorator crab. Many kinds of decorator crabs fasten bits of living things to their shells. These decorations help the crabs blend into the places where they live.
A decorator crab's hard shell protects its soft body, but a bare shell may stand out against seaweed-covered rocks. Covering the shell with local materials solves that problem. A crab wearing green seaweed can be harder to notice among green plants. This disguise is called camouflage. It works because the crab resembles its surroundings.
The crab uses its claws to pick up a piece of seaweed. It may trim the piece with its mouthparts before pressing it against special hooked hairs on its shell. The hairs catch and hold the piece, a little like the hooks on a burr that sticks to a sock. The decoration is attached to the outside; it is not part of the shell.
Decorations are not all the same. One crab may carry thin pieces of seaweed, while another wears bits of sponge, a simple animal that lives in water. The pieces may keep growing after they are attached. A decorated shell can look like a tiny underwater garden, but the crab is carrying that garden wherever it goes.
As a crab grows, it eventually molts, or sheds its old shell. Its new shell is larger, but it starts without the old shell's decorations. Some decorator crabs transfer pieces from the old shell to the new one. Others gather fresh materials. Decorating again helps a crab blend in as it grows.
Imagine a close-up photograph with this caption: "Hooked hairs hold seaweed on a decorator crab's shell." The caption would explain how the seaweed stays in place, even when the crab walks.