Tuerkayana hirtipes, a true crab species examined in the study Tsubasa Inoue Many crabs are famous for their sideways shuffle, but little is known about how this movement evolved.
Now, scientists have traced the history of their iconic walk to a common ancestor that lived roughly 200 million years ago. The findings, published as a peer-reviewed preprint in the journal e-Life on April 21, hint that the sideways scuttle evolved in “true crabs” just once, and support the idea that the trait was key to their ecological success.
There are around 8,000 species of true crabs, a group called Brachyura, which differ from “false crabs,” such as hermit crabs. The impostors can move forward—but so can some true crabs.
That raises some “interesting questions” about true crabs, says Yuuki Kawabata, a study co-author and behavioral ecologist at Nagasaki University in Japan, in a statement. “When did their sideways locomotion originate, how many times over the years did it evolve and how many times did it revert?”
To investigate, Kawabata and colleagues obtained live crabs representing 50 species from the ocean, tide pools, public aquariums and local fish markets. Each animal was filmed for ten minutes inside a circular arena designed to resemble its natural habitat, with elements such as seawater, fresh water or brackish water. They identified 35 sideways-moving species and 15 that traveled forward.
The team then combined their observations with an evolutionary map of Brachyura created by other scientists. Analysis revealed that the sideways shuffle evolved in a single ancestor at the base of the subgroup Eubrachyura, which then became a highly conserved trait. Today, Eubrachyura includes the most advanced and diverse crabs, with approximately 7,500 species. For comparison, two of the other subgroups—which have forward-walking true crabs—are comprised of around 150 total species.
The side-to-side locomotion “could have potentially acted as a key innovation” that allowed Eubrachyura to spread quickly across different environments, Kawabata tells Sierra Bouchér at Science News. Moving easily in two sideways directions—rather than just one, forward—probably helps crabs quickly escape predators and makes their travel path unpredictable, the researchers posit.
While crabs’ sideways walk may have evolved only once, crustaceans have evolved crab-shaped bodies at least five times. It’s a type of convergent evolution called “carcinization.”
It’s unusual for useful traits to only evolve once in organisms, notes science educator and nanotechnologist Michelle Dickinson, who was not involved in the study, on an episode of the “Sunday Session” podcast. Think of wings, for instance, which evolved separately and at different times in birds, bats and insects, she adds. Crabs’ sideways scurry, however, is pretty unique among animals.
The researchers point out that environmental factors may have played a role in this crustacean innovation. The period when the form of locomotion emerged—the early Jurassic period—was full of major changes. For example, the supercontinent Pangea split up, expanding shallow marine habitats in which crabs could thrive. The authors suspect this created new opportunities for crabs to diversify.
Andrés Vidal-Gadea, a biologist at Illinois State University who was not involved in the research, tells Science News that sideways walking may have been a simplification for the animals, as they required fewer nerve cells to control their muscles than their ancestors.
“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,” he says. “That immediately simplifies the problem.”
Still, more work is needed to understand what drove this change in crabs, the researchers note. “To disentangle the relative roles of innovation and environmental change, we need further analyses of trait-dependent diversification, fossil-informed timelines and performance tests that link true crabs’ sideways movement to adaptive advantages,” Kawabata says in the statement.
Sara Hashemi is a science writer and fact-checker currently based in New York City. Her work has appeared in Sierra, The Body, Maisonneuve magazine and more.
