The iconic sideways walk of crabs may have evolved just once, in an ancestor that roamed Earth roughly 200 million years ago.
That conclusion, published April 21 in eLife, comes from researchers who tracked the movement of 50 crab species and mapped the results onto a crab family tree. The lineage that inherited sideways locomotion went on to become by far the most species-rich group of crabs on the planet, suggesting the trait may have been a key driver of their evolutionary success.
Crabs’ sideways walk is almost unique in the animal kingdom, yet its origin has long eluded researchers. To get to the bottom of this, behavioral ecologist Yuuki Kawabata of Nagasaki University in Japan and colleagues collected 50 crabs — each a different species — from across the country, drawing from tidal pools, ocean depths, aquariums and local fish markets.
The team recorded each crab’s movement in a pool, noting whether it moved primarily forward or sideways, then mapped those results onto a crab evolutionary tree built from the DNA of hundreds of species by other researchers. That allowed Kawabata and colleagues to see where in crab history sideways walking first appeared.
What they found was striking: All sideways-moving crabs descended from one group of ancestors that lived about 200 million years ago. And the group that inherited sideways movement is by far the most diverse — Eubrachyura, originating with that first sideways-moving ancestor, has nearly 7,500 modern species, compared with just 156 in the two groups that move forward and backward.
Sideways movement “could have potentially acted as a key innovation,” Kawabata says, allowing these crabs to spread rapidly through diverse ecosystems. The sideways scurry may have given them a leg up over their relatives, offering a quick getaway from ambushing predators, the team suggests.
This evolution didn’t come easily. It required not only muscles and ligaments to shift but also a rewiring of neural activity touching many aspects of crab life — how they foraged, burrowed, socialized and mated. What’s astonishing, Kawabata says, is that such a major shift happened at all. “It’s almost impossible for that kind of key innovation to occur.”
The breadth of species sampled makes this a robust study, says Andrés Vidal-Gadea, a neuroethologist at Illinois State University in Normal, who was not involved with the research. And though the evolution is astounding, he adds, it may actually have been a simplification: Sideways-walking crabs needed fewer nerve cells to control their muscles than previous generations did.
“Instead of every joint in the leg of a crab having to play a more or less equal role, it boiled down to two main joints that did pretty much 90 percent of the work,” Vidal-Gadea says. “That immediately simplifies the problem.”
Timing may also have helped. The first sideways-walking crabs started scuttling across the land and seas in the wake of the Triassic–Jurassic extinction, a mass extinction that killed off about three-quarters of all species as the supercontinent Pangaea broke apart and drove extensive volcanic eruptions. Pangea’s rifting also expanded the shallow marine habitats crustaceans thrive in, freeing up many niches for crabs to take advantage of with their new sideways skills, the team theorizes.
Other species would eventually evolve to fill these new niches too; the crablike body plan arose in at least three other crustaceans. Yet none of these “false” crabs ever evolved sideways locomotion, Kawabata says. “The crablike body form may be needed for moving sideways, but not the opposite.”
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