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Tuesday, September 15, 2009

Sea Things #15: Decorator Crabs

Sea Things is a regular feature on my blog where I profile a different sea creature. Look for it weekly, or something close to weekly.

Decorator crabs are actually a whole group of crabs, not just one type. However, they can be difficult to identify since they are often... well, decorated.

We first came across these interesting crabs in 2007 in Bonaire. We were told to watch out for them on our night dive at Town Pier, since they really would look like sponges that move. Basically, the crabs would take a piece of sponge (or, lacking that, some trash or often a discarded flip flop or sandal), and cover themselves with it. They move around with this piece of sponge on their back to hide from predators. It's as if they're saying "Don't mind me, I'm just a sponge. Definitely not a crab. Nope, just a sponge." It's cute.
Most of the decorator crabs that we've seen since have been significantly smaller than the Bonaire crab (above, on a discarded tire under Town Pier). Usually, we see the little ones that are only about two inches long. They decorate themselves with algae and other bits of things from off of the reef. Other decorator crabs are more specific, and only decorate themselves with very specific things.

So, how do the crabs put things on themselves? Well, they simply use their little pincers to cut off pieces of algae or whatever, and then rough up the end of the algae. When they put it on their shells, little microscopic hooks hold them on. Apparently, whatever it has decorating it will often stay alive, continue to grow and sometimes even reproduce.

There isn't a diver hand signal for decorator crabs, besides the general "crab" signal. This is to hold your hands up like crab pincers and open and close them. They're tough to spot because they're well-hidden. One diver summed it up like this:
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.

Is there a creature that you would like to see featured in Sea Things? If so, shoot me an email and if I can, I'll write about it. Photos on this post are courtesy of Todd Krebs.

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