2000-11 Decorator Crabs

Camouflaging allows animals to share an environment filled with predators. It also engenders some of the most delightfully bizarre creatures in the sea. The fun comes in finding them.

Like all good hunters, underwater naturalists need cunning. Remaining still while searching for small cryptic marine animals dramatically improves success. Many creatures that hole up at your approach become less timid and venture into the open when you are motionless. But the real magic comes from an enhanced ability to see movement. Often a coral head that appears uninhabited at first glance comes to life in subtle ways when closely inspected. It might be a flick of a fin, twitch of an antenna or an emerging head that draws attention to the splendid little creatures, such as decorator crabs, that would be missed by roving eyes.

Little crabs make tasty treats for reef predators. To lessen the carnage many members of the spider crab family hide from hungry mouths by attaching marine growth to their bodies. These little fellows surgically sever bits and pieces from their surroundings, roughen the base with their mouths and fasten them to curved body hooks, known as setae. A variety of flexible decorations is used including hydroids, algae, sponges, bryozoans and tunicates. The hijacked materials typically continue to grow and may even reproduce.

One afternoon while lying on a shallow sea floor, watching a colony of Sailfin Blennies, the almost imperceptible movement of calcareous algae caught my eye. A moment later, the small branch mysteriously moved again. I positioned my mask only inches from the greenery and stared. It took a minute before I detected the broken outline of a tiny decorator crab adorned with algal blades. To get a better look, I gently lifted the crab and placed it on a flat encrusting sponge. In the process, the creature's topknot toppled and fell to its side. Without missing a beat, and to my immense relief, the crab grabbed the fallen sprig with its claw, hoisted it overhead, and in one clean motion, slipped its camouflage costume back into place.