Costumes That Kill

Oct. 31, 2006 — -- Halloween being the day for fabulous costumes, may we introduce one of the great costume makers ever -- the decorator crab.

Decorator crabs live in shallow ocean water. They don't run very fast. They are surrounded by fish who would just love to gobble them up, so over the eons they devised a scheme, says Jay Stachowicz, an evolution and ecology professor at the University of California at Davis. "They cover themselves with various materials and try to look like something else."

This is done instinctively, he says, yet different crabs dress differently.

There are some crabs that cover their bodies with moss, others pluck bits of seaweed, and still others add small ocean animals, trimming them to size with their mouths. They then press what they've gathered onto little hooks that rest on their shells.

"These hairs function just like Velcro," says Stachowicz, "so these crabs effectively invented Velcro well before we did."

The end result? The crabs simply disappear from view. They are everywhere, but they look like bunches of seaweed, or moss or pebbles, or whatever is nearby.

Sometimes a decorator crab uses entire animals as cover. It can carefully scoop up two sea anemones loaded with stinging chemicals and gently place them on its back, tying them down with its "Velcro" hooks so it can walk openly on the ocean floor, knowing that no fish would dare get too close. It's kind of like walking the street with a machine gun strapped to your head; this is a costume that definitely scares predators.

Stachowicz says when these crabs find a poisonous anemone, they approach and, "massage its bottom a little bit to relax it" so it can be removed from its perch and placed on the crab's back.

So, these crabs are not only elegant designers, they make, if you will excuse the hyperbole (and we hope you do -- it's Halloween) costumes that kill.

"That sounds like a great title for my next scientific paper," says Stachowicz, who confesses he likes "crawly creepy things, the slimier the better."