The robber crab is the world's largest land-living arthropod, weighing up to 4 kilograms, and it steals anything it can carry away in its formidable pincers. That's not the only amazing thing about the crab, say biologists. The crab evolved the ability to smell on land, just like insects do. "This is a really cool study," comments Leslie Vosshall, a molecular biologist at Rockefeller University in New York.
When the ancestors of robber crabs first walked out of their watery environment to live on land, their sensory equipment needed a makeover. The olfactory receptors on the antennae of marine crabs detect soluble, water-loving molecules. A landlubbing crab must zero in on molecules that waft through the air.
Bill Hansson, a chemical ecologist at the Swedish University of Agricultural Sciences in Alnarp, Sweden measured the electrical activity of olfactory neurons in the short antennae of robber crabs at Australia's Christmas Island while puffing scents over these sensory organs. Results of the tests indicate that the electrical activity in response to certain odors is the same in robber crabs as in insects.
This translates into similar senses of smell. Like insects, robber crabs are able to detect carbon dioxide, likely an adaptation for locating decaying meat, which releases copious amounts of the gas. Robber crabs also seem to have a high sensitivity for dimethyldisulphide, and specifically dimethyltrisulphide, which are the same carcass odors that attract insects to animal remains. The researchers report their results in today's issue of Current Biology.
Hansson calls the robber crab's development of a bug's nose "a great example of convergent evolution." He adds, "The robber crabs have reinvented their smell for land so remarkably that they can even smell water."