crabs’ sideways – By tracking how 50 crab species walk and overlaying that movement onto a DNA-based crab family tree, researchers report that sideways locomotion appeared just once—about 200 million years ago—and the lineage that inherited it later became the most species-rich
When a crab scuttles sideways, it looks effortless. But scientists say the signature gait may be the product of a single, rare evolutionary breakthrough—one that appeared around 200 million years ago and then spread through one major branch of the crab family tree.
The finding. published April 21 in eLife. comes from behavioral ecologist Yuuki Kawabata of Nagasaki University in Japan and colleagues. who collected 50 crab species from across the country.. The team drew their sample from tidal pools. ocean depths. aquariums and local fish markets—then recorded each crab’s movement in a pool. marking whether it moved primarily forward or sideways.
Kawabata’s group then compared those observations with an evolutionary history of crabs built from the DNA of hundreds of species by other researchers. That combination allowed the researchers to pinpoint where sideways walking first appeared on the lineage.
What emerged was stark: all crabs that move sideways trace back to one group of ancestors that lived roughly 200 million years ago. From there, the evolutionary story turns into a question of why.
The branch that inherited sideways locomotion went on to become far more diverse than its forward- and backward-moving relatives.. The researchers report that Eubrachyura—the group that originated with the first sideways-walking ancestor—has nearly 7,500 modern species.. By comparison, just 156 species exist in the two other crab groups that move primarily forward and backward.
Kawabata argues that sideways movement could have functioned as a “key innovation,” a trait that helps animals spread rapidly through varied environments. The team also suggests a practical advantage: the sideways scurry may have enabled quicker escapes from predators that ambush other crabs.
But the study also emphasizes how hard such a shift would have been.. It didn’t just require changes to muscles and ligaments that physically allow a sideways posture.. The researchers say it also involved a rewiring of neural activity that touches many aspects of crab life—how they forage. burrow. socialize and mate.. Kawabata calls the scale of that change striking. saying. “It’s almost impossible for that kind of key innovation to occur.”
Other researchers who were not involved in the work see strength in the approach.. Andrés Vidal-Gadea. a neuroethologist at Illinois State University in Normal. says the wide range of species sampled makes the analysis robust.. At the same time, he cautions that the neural shift may not have been as complex as it first appears.
He points to a possible simplification: sideways-walking crabs might have needed fewer nerve cells to control their muscles than earlier 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 could also matter.. According to the team’s theory. the first sideways-walking crabs began after the Triassic–Jurassic extinction. an event that killed about three-quarters of all species as Pangaea broke apart and sparked widespread volcanic eruptions.. The breakup of the supercontinent also expanded shallow marine habitats that crustaceans rely on. potentially opening up new ecological niches that crabs could exploit with the sideways gait.
Crabs weren’t the only “crablike” animals in the distant past—researchers note that the crablike body plan arose in at least three other crustaceans.. Yet Kawabata says none of these “false” crabs ever evolved sideways locomotion.. “The crablike body form may be needed for moving sideways, but not the opposite.”
Taken together, the results frame the sideways walk as something more than a quirky habit.. If sideways locomotion indeed emerged once and then unlocked major ecological opportunities. it would help explain why one lineage of crabs became exceptionally species-rich—while other crablike relatives went down different evolutionary paths.
crabs sideways locomotion evolution eLife Yuuki Kawabata Nagasaki University phylogenetic tree neural rewiring Triassic–Jurassic extinction Eubrachyura