algae, sponges, small crustaceans, bryozoans
to 5 inches (12.7 cm)
kelp crabs, sheep crabs, dungeness crabs, other crabs; Family: Majidae (spider crabs)
northern California to Baja California
If you see a rock moving in one of our exhibits, look closer. It might be a decorator crab that's camouflaged itself with tiny seaweeds and animals like anemones, sponges and bryozoans. The crab selects pieces of seaweed and small animals from its habitat and fastens them to hooked setae (Velcrolike bristles) on the back of its shell.
As long as the crab stays in the neighborhood, it blends in and looks at home. Crabs that have grown large enough to defend themselves don't decorate their backs; however, plants and animals settle there without help, take hold and grow.
The population of decorator crabs isn't in danger presently; however, oil spills and run-off of pesticides, used oil, paint solvents and other chemicals endanger the crabs' habitats. As stewards of the oceans, we must carefully dispose of hazardous materials like these or, better yet, use enviromentally safe products.
Decorator crabs are an important food source for some fishes, including croakers and cabezon.
A crab's shell doesn't grow, but the crab does. To solve this dilemma, a crab must molt as it grows, shedding its old exoskeleton and forming a new, larger one. The old shell loosens as a new one forms beneath it. When the old shell splits, the soft animal crawls out. Before its new shell hardens, the crab absorbs water and expands to a size larger than before the molt. While the new shell is hardening, the crab hides from predators.
Decorator crabs recycle their living decorations during the molting process—they remove the anemones, sponges and other decorations from their old shell and use them to decorate their new shell.