A modified joint might have made all the difference to scurrying crabs as they diverged from their plodding lobsterlike brethren.
Comparing leg shape, size, and motion among three living crustaceans from increasingly ancient origins allowed Andrés Vidal-Gadea, at Louisiana State University in Baton Rouge, and his colleagues to get at the mechanics of crab walking: specifically, how some crustaceans changed their forward march into a sideways scuttle.
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Forward-walking crayfish were the most primitive of the three lineages, and sideways-striding shore crabs the most recent. The portly spider crab, which plods sideways only 20 percent of the time, falls between. It belongs to one of the first groups of tailless crabs, which evolved about 320 million years ago.
“These guys [spider crabs] walk completely differently from sideways walking crabs,” says Vidal-Gadea. “Anatomically they look like forward-walking lobsters.”
In forward-walkers, each limb’s movement is limited by the leg ahead or behind it. Yet as crabs’ fourth leg joints evolved to be more flexible, their limbs could glide side-to-side, the team suggests in the March Arthropod Structure & Development. And crabs took off, moving equally fast in two directions.
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Previous work suggests that over evolutionary time certain crustaceans tucked their vulnerable tails—housing the meaty abdomen—under the body. The front claws shrunk to balance the loss of hind weight. In the absence of formidable pinchers, speed saved crab lives.
With RoboLobster, a lobsterlike robot, already in use, this research may inspire a new generation of mechanical crawlers, says Vidal-Gadea.