Decorator Crab draped with seaweed
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 near shore rocks and seaweeds 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.