A crab scuttling sideways across a beach is one of the most recognizable movements in the animal kingdom. The shuffle is so iconic that “crabwalk” has become part of everyday language. But scientists have long debated a deceptively simple question: Did crabs figure out this trick once, or did different groups independently arrive at it? A new study has an answer, and it turns out sideways walking appears to have evolved just once, a single behavioral shift that may have helped fuel the explosion of crab species across the planet.
Published in the journal eLife, the study filmed 50 living crab species and mapped their walking styles onto a family tree spanning hundreds of millions of years. Sideways movement appears to have originated in the ancestor of a massive group called true crabs. Based on previously published divergence estimates, that shift may have occurred around 200 million years ago near the start of the Jurassic period. According to the researchers, it may have helped crabs expand into a huge range of habitats, from deep-sea floors to tropical forests, and contributed to true crabs growing into roughly 7,900 species alive today.
Some lineages later reverted to walking forward again, multiple times independently. Those reversions tended to occur in species that had found other ways to avoid predators, like hiding inside clam shells or camouflaging themselves with seaweed. Once crabs no longer needed the escape advantage of darting sideways, some gave it up.
How Researchers Studied Crabs Walking Sideways
Scientists at Nagasaki University in Japan set up circular arenas, large plastic pools between roughly 2.5 and 4.5 feet across, filled with whatever environment each species naturally lives in. Each crab was given time to settle down, then released in the center and filmed for 10 minutes. Researchers tracked two landmarks on each crab’s shell frame by frame to determine which direction the animal moved relative to where its body was pointing.
Every movement was classified as forward, sideways, or backward, with backward movements rare enough to be excluded. A scoring system called the Forward-Sideways Index produced a value ranging from +1 for a purely forward-walking crab to -1 for a purely sideways-walking one. Of the 50 species studied, 35 were sideways movers and 15 were forward movers, with very little overlap between the two groups. Crabs, it seems, are fully committed to one direction or the other.
Crabs Walking Sideways Evolved Just Once on the Family Tree
Walking styles were then mapped onto an evolutionary tree of crabs built from genetic data covering 344 species. Statistical models pointed to a single origin of sideways walking at the base of a group that includes the vast majority of living true crabs. Earlier-branching lineages retained the forward-walking habit of their ancestors, and the probability that the common ancestor of all true crabs walked forward was estimated at 91 percent.
Model estimates put the number of independent reversions from sideways back to forward walking at around 10, while forward-to-sideways transitions happened roughly four times, mostly representing the original gain. One notable example: snow crabs likely underwent a secondary reversion back to sideways walking after their close relatives in the spider crab family had already switched to forward movement.
Why Crabs Walking Sideways Gave Them a Survival Edge
Sideways-walking crabs can bolt left or right with roughly equal swiftness, making their escape route unpredictable to a predator. Experiments with crab-like robots have backed this up: sideways movement turns out to be faster and more efficient than forward walking for a body built like a crab’s.
This escape advantage may help explain the species diversity gap between sideways-walking crabs and their closest relatives. Eubrachyura, the focal group where sideways movement dominates, contains about 7,468 described species, compared to just 110 in its nearest sister group and around 46 in the next closest relative.
If the external dating estimates hold, the origin of sideways walking lines up with the early Jurassic, right after the mass extinction that closed out the Triassic. That extinction would have opened ecological real estate: shallow marine habitats were expanding and competition was reduced. A crab that could escape predators by darting unpredictably in multiple directions would have had a serious edge. The researchers caution that separating the role of this behavioral shift from external factors like post-extinction opportunity will require further study.
The Crabs That Gave Up Sideways Walking
According to the authors, maintaining the sideways-walking body plan may come with tradeoffs, including changes to nerve wiring and joint flexibility that limit behaviors like burrowing and mating. When the pressure to escape predators was lifted by some other strategy, forward walking crept back in.
Many animals have converged on looking like a crab, a phenomenon called carcinization, but the study’s behavioral data show they don’t necessarily move like one. Porcelain crabs move mostly backward. King crabs and coconut crabs walk forward. Across the entire sweep of animal evolution, walking perpendicular to where your head points appears to be essentially a one-off invention, and only in true crabs has it stuck around long enough to reshape an entire group’s place in the natural world.
