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Ocean Watch Susan Scott |
Dresser crabs are
abundant but hard to spotHere's a good question from my e-mail: "Have you ever heard of a dresser crab? It selects sponges and items to adorn itself for camouflage purposes. If you know anything about it or can direct me, I would appreciate it."
I never heard of a dresser crab, but I know about some crabs that dress up. These crustaceans, called sponge, collector and decorator crabs, carry things on their backs as camouflage.
Given how often a person sees these creatures, it would be logical to think they are rare. They are not. Hawaii hosts 30-some species. It's just that their get-ups are so good, they're hard to spot even when sitting right in front of your face.
Sponge crabs are the easiest to find because they're the biggest. Hawaii hosts about a dozen species. The largest of these is the sleepy sponge crab, which grows to about 8 inches wide. These well-named crabs move slowly and carry a big sponge.
Sponge crabs hold onto their sponge hats with two pairs of rear legs that bend up over their backs. These gymnastic appendages have pointed tips that the crab sticks into its sponge like hairpins.
Molting might seem a problem for a crab with a sponge dependency, but no. When it's time to cast off its old shell, the crab simply lifts off its sponge and sets it aside. When the new, larger shell forms, the crab re-dons its squishy beret.
The sponges on the backs of crabs are living and grow right along with their hosts, but not as fast. Therefore, after a few molts, these crabs have grown too big for their sponges. When this happens, the crab shops for a new one, cutting it with sharp claws for a tailored fit.
When sponges are in short supply, sponge crabs don whatever they can find. Hawaii's sponge crabs have been seen carrying rubber slippers and pieces of rope, wood and metal. If researchers offer nothing better to a captive sponge crab than a piece of paper, the crab will cut itself a neat hat and pin it in place with those needlelike legs.
One diver found a sponge crab off Ewa Beach wearing a crown-of-thorns starfish. The crab had trimmed the arms of the starfish for a perfect fit.
This might seem risky, given that these starfish bear poisonous spines, but sponge crabs eat crown-of-thorns starfish. Likely, the Ewa Beach individual was about to eat its hat.
Sponges provide more enduring camouflage and offer chemical protection, as well. In a tank, an octopus will not attack a crab bearing a sponge. Remove the sponge, though, and the crab is history.
Other Hawaii crabs that dress up are called collector or decorator crabs and belong to the spider crab family. Spider crabs have triangular bodies with pointy heads and get their name from their long, slender legs.
More than 20 kinds of collector crabs live on Hawaii's reefs, but finding one is tough. Not only are these creatures only a half-inch to 1 inch long, but they cover themselves with seaweed, soft corals and anything else in the vicinity, making them nearly invisible.
Spider crabs hang onto this mismatched attire with tiny hooks on the surface of their legs and shells.
All this is probably more than my reader wanted to know, but I think a column about cross-dressing crustaceans is a fine way to greet 2004. It's going to be a good year.
See the Columnists section for some past articles.
Marine science writer Susan Scott can be reached at http://www.susanscott.net.