Decorator crab can be recognized by the ‘clothes’ it wears </MCC HEAD>

Story last updated at 8:28 PM on Wednesday, February 7, 2007

Decorator crab can be recognized by the ‘clothes’ it wears

By Carmen Field
Kachemak Bay Research Reserve

Kachemak Bay has an amazingly diverse array of crabs, with nearly 30 species identified to date. Of these, one of the most intriguing is the graceful decorator crab (Oregonia gracilis). Looking a bit like a fat-bodied spider, this crab is most easily recognized by the “clothes” it wears — a collection of marine debris or harvested organisms that allow the crab to remain hidden on shallow mudflats or among nearshore rocks and seaweeds.

Photo by Kevin Co, Kachemak Bay Research Reserve
This decorator crab resides in one of the Kachemak Bay Research Reserve aquariums and is shown here with its original ornamentation of multi-colored sponges found in deep water. After arriving in the reserve's classroom tank, it stripped off these sponges and redecorated itself with hydroids and brown algae.

The pear-shaped body covering, or carapace, and eight walking legs of this spider crab family member are brown, tan or gray and typically decorated with algae, hydroids, anemones, sponges, bryozoans or pieces of mussel shell. Hooked bristles on the crab’s exoskeleton act as holdfasts to anchor decorating material to the crab (they do not glue algae and marine animals to their shell as once thought). Theories on why decorator crabs cover their shells with marine life include camouflage from predators, physical protection (by stinging animals like hydroids or anemones worn on their shells), and food storage.

The carapace of these crabs features a long, slender rostrum between the eyes and one long spine behind each eye. The carapace length, measured from rostrum tip to the rear of the shell, is generally 2-3 inches, and the carapace width is roughly half of its length. Two slender, pink or orange-tipped pincers, or chelipeds, grasp food. In males, the pincers are longer than the legs; in females, they are shorter than the legs.

Like all crabs, decorator crabs must molt as they grow. A molting crab will exit its old exoskeleton through a split along the rear of the carapace. The shed skeleton is left completely intact except for the split. The resulting soft-shelled crab fills with water and/or air to stretch its pliable new skeleton to a larger size that it can then grow into. At this point the crab is vulnerable and has limited mobility so will seek shelter for the time it takes the exoskeleton to harden — usually a minimum of 48 hours. After each molt, a crab must decorate its new exoskeleton in order to remain camouflaged.

During the 1989 Exxon Valdez oil spill, graceful decorator crabs in China Poot Bay were observed changing their ornamentation to blend with unusual surroundings. When a Styrofoam-filled boom — stretched across China Poot Bay’s mouth to protect a Dungeness crab nursery from drifting oil — broke from tidal pressure, countless tiny Styrofoam balls were released into the water.

These balls washed into the intertidal zone where many decorator crabs lived, causing most of the crabs to redecorate their bodies and replace the seaweed and marine organisms with the minute white balls. Subsequent tides then washed the Styrofoam away, leaving the white decorator crabs somewhat stranded in terms of visibility. While gulls and crows enjoyed a decorator crab feast at low tide that day, they most likely ingested an unhealthy dose of Styrofoam balls, too.

From the Bering Sea to Monterey Bay in central California and Japan in the eastern Pacific, graceful decorator crabs are opportunistic scavengers. They feed on brown seaweeds, detritus, and dead or living marine animals they capture from the ocean floor or rock crevices with their long pincers. Pacific halibut are known to prey on these crustaceans, and other bottom fish probably eat them, as well.

If you have questions about Kachemak Bay, contact reserve staff at 235-4799 or visit the Web site at www.kbayrr.org.

Carmen Field is an education specialist and naturalist at the Kachemak Bay Research Reserve.