Crabs that inherited this trait are much more species rich, suggesting that this contributed to their success
A crab scurries sideways along a beach in the United Arab Emirates. Scientists have attributed this distinctive gait to a single ancestor who lived around 200 million years ago.
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The iconic sideways walk of crabs may have evolved only once, in an ancestor who roamed the Earth about 200 million years ago.
This conclusion, published on April 21 in eLifecomes from researchers who tracked the movement of 50 species of crabs and mapped the results onto a crab family tree. The lineage that inherited lateral locomotion became by far the most species-rich group of crabs on the planet, suggesting that this trait may have been a key factor in their evolutionary success.
The lateral walking of crabs is almost unique in the animal kingdom, but its origin has long eluded researchers. To get to the bottom of it, behavioral ecologist Yuuki Kawabata of Nagasaki University in Japan and his colleagues collected 50 crabs – each of a different species – across the country, drawing from tide pools, ocean depths, aquariums and local fish markets.
The team recorded the movement of each crab in a pool, noting whether it moved primarily forward or sideways, then mapped those results onto a crab evolutionary tree constructed from the DNA of hundreds of species by other researchers. This allowed Kawabata and his colleagues to see where in the crab’s history lateral walking first appeared.
What they discovered was striking: All the laterally moving crabs were descended from a group of ancestors that lived around 200 million years ago. And the group that inherited lateral movement is by far the most diverse: Eubrachyura, originating from this first lateral-moving ancestor, has nearly 7,500 modern species, compared to just 156 in the two groups that move forward and backward.
The lateral movement “could have been a key innovation,” says Kawabata, allowing these crabs to spread quickly through diverse ecosystems. Sideways running may have given them a head start on their relatives, providing them with a quick escape from ambushing predators, the team suggests.
This evolution did not come easily. It was not only necessary muscles and ligaments move but also a rewiring of neuronal activity affecting many aspects of the crabs’ lives: how they feed, dig their burrows, socialize and mate. What’s amazing, Kawabata says, is that such a significant change has occurred. “It’s almost impossible for this kind of key innovation to happen.”
The breadth of species sampled makes it a robust study, says Andrés Vidal-Gadea, a neuroethologist at Illinois State University in Normal, who was not involved in the research. And while the evolution is surprising, he adds, it may actually be a simplification: Crabs that walked sideways needed fewer nerve cells to control their muscles than crabs that moved sideways. previous generations did.
“Instead of each joint in a crab’s leg having to play a more or less equal role, it came down to two main joints that did roughly 90 percent of the work,” says Vidal-Gadea. “That immediately simplifies the problem.”
The timing may have also helped. The first side-walking crabs began to move across land and sea following the Triassic-Jurassic extinctiona mass extinction that killed about three-quarters of all species when the supercontinent Pangea broke apart and caused massive volcanic eruptions. The Pangea rift also expanded the shallow marine habitats in which crustaceans thrive, freeing many niches for crabs to take advantage of with their new lateral skills, the team theorizes.
Other species would eventually evolve to also fill these new niches; the crab-shaped body plan appeared In at least three other crustaceans. Yet none of these “false” crabs ever evolved lateral locomotion, Kawabata says. “The crab-like body shape may be necessary for moving laterally, but not vice versa.”
