The Sideways Secret: How Crabs Evolved Their Iconic Walk 200 Million Years Ago

The Mystery Behind the Crab's Sideways Shuffle

If you've ever watched a crab scuttle across a beach, you've witnessed one of nature's most recognizable — and peculiar — forms of movement. But why do crabs walk sideways at all? According to a groundbreaking new study, the answer stretches back roughly 200 million years to a single pivotal moment in evolutionary history.

Researchers have determined that the sideways walking style seen in most modern crabs originated just once, from a single common ancestor, and has remained a defining trait of the group ever since. The findings, published as a Reviewed Preprint in eLife, are based on the most comprehensive dataset on crab locomotion ever assembled.

A Single Evolutionary Turning Point

The study examined movement patterns across 50 species of true crabs, known scientifically as Brachyura — the largest group within crab decapods. Each species was filmed for 10 minutes inside a specially designed circular arena crafted to simulate its natural environment. Of the 50 species observed, 35 moved primarily sideways, while the remaining 15 walked forward.

When researchers mapped these behavioral patterns onto an established evolutionary tree built from genetic data spanning 344 species, a striking conclusion emerged: sideways locomotion evolved only once. That single shift occurred at the base of a subgroup called Eubrachyura — which encompasses the more advanced true crabs — and is estimated to have taken place approximately 200 million years ago, during the earliest part of the Jurassic period, just after the Triassic-Jurassic mass extinction event.

"This single event contrasts starkly with carcinization, which has occurred repeatedly across decapod species," said senior corresponding author Yuuki Kawabata, Associate Professor at Nagasaki University's Graduate School of Integrated Science and Technology. "This highlights that while body shapes may converge multiple times, behavioral changes such as sideways walking can be rare."

Why Sideways? The Survival Advantage

So what made this unusual form of movement so successful that it stuck around for two centuries? The researchers believe it comes down to one key advantage: survival.

Moving laterally allows crabs to dart quickly in either direction without needing to turn their bodies around. This unpredictability makes them significantly harder for predators to track and catch. In evolutionary terms, that edge can make all the difference.

Despite this advantage, sideways locomotion remains rare across the broader animal kingdom. Scientists suspect this is because lateral movement can create complications in other essential activities — including burrowing, feeding, and mating. Outside of true crabs, only a handful of other creatures, such as crab spiders and leafhopper nymphs, have adopted a similar style of movement.

An Explosive Period for Ocean Life

The timing of this evolutionary shift is itself significant. The period around 200 million years ago was one of dramatic global transformation. The supercontinent Pangaea was beginning to fragment, shallow marine environments were expanding, and the early Mesozoic Marine Revolution was underway — all conditions that would have opened up rich new ecological opportunities for species capable of exploiting them.

The researchers argue that this environmental backdrop likely amplified the advantage conferred by sideways movement, helping true crabs diversify and colonize a remarkable range of habitats — from ocean floors and freshwater systems to terrestrial environments.

Today, there are approximately 7,904 known species of true crabs, a number that far outpaces that of their closest relatives. Kawabata notes that this extraordinary diversity may be no coincidence.

"Sideways locomotion may have contributed significantly to the ecological success of true crabs," he explained.

Behavior vs. Body Shape: A Rare Kind of Innovation

One of the most compelling aspects of this research is what it reveals about the nature of evolutionary innovation. The crab-like body form — a broad, flattened shape with reduced abdomen — has evolved independently numerous times across different crustacean lineages, a process known as carcinization. Yet sideways locomotion appears to have emerged only once.

This distinction is important: it suggests that behavioral adaptations can be even more evolutionarily constrained than physical ones. Once a behavioral trait takes hold and proves successful, it may persist largely unchanged across millions of years and thousands of species.

What Comes Next

Despite the clarity of these findings, the researchers acknowledge that many questions remain. Disentangling the relative contributions of biological innovation and environmental opportunity will require further study — including trait-dependent diversification analyses, fossil-calibrated timelines, and performance experiments that directly link sideways movement to measurable survival benefits.

"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 added.

A New Window Into How Animals Move

Ultimately, this research does more than solve a quirky riddle about crabs. It offers a broader framework for understanding how movement strategies evolve, persist, and shape the long-term fate of entire animal lineages.

By combining direct behavioral observation with rigorous phylogenetic analysis, the team has demonstrated that a single, ancient shift in the way an animal moves can echo across hundreds of millions of years — and thousands of species — with remarkable staying power.

The study was led by Kawabata alongside co-first authors Junya Taniguchi, Tsubasa Inoue, and Kano Kohara, with additional contributions from researchers at institutions in Taiwan, Japan, and the United States.