Unraveling the Mystery of Crab's Sideways Walking! Was the Crab's Sideways Walking Actually a One-Time Great Invention? Nagasaki University and Others Explore the Origins of Evolution

The "Sideways Walk" of Crabs Wasn't Just a Quirk

When you spot a crab on the beach, there's a question that everyone has pondered at least once.

Why do crabs walk sideways?

From a human perspective, moving forward seems more natural. Dogs, cats, and birds generally move in the direction they are facing. But crabs are different. They move swiftly sideways while keeping their bodies facing us. This movement is somewhat humorous and has been a subject in picture books, animations, and jokes for a long time.

However, recent research is reinterpreting this "sideways walk" not just as a visual curiosity but as a significant event in evolutionary history.

According to research published by a group from Nagasaki University and others, it's highly likely that the sideways walk of crabs was acquired only once, about 200 million years ago, by a common ancestor that connects to many of today's crabs. This means that the many crabs seen on beaches and riverbanks today did not each invent sideways walking independently. Instead, they may have inherited this mode of movement from a distant ancestor across generations.

200 million years ago was the era when dinosaurs began to rise, and the configuration of Earth's continents and marine environments were vastly different from today. The behavior born in such an ancient world still influences the appearance and lifestyle of today's crabs. When you think about it, that small sideways step suddenly takes on a grand temporal scale.

Observed 50 Crab Species and Classified Their Walking Patterns

The focus of this research was on the group known as "true crabs," or Brachyura. Many species that come to mind when thinking of crabs fall into this category.

The research team observed the actual walking patterns of 50 crab species. They placed individuals of each species in a circular plastic arena and recorded their movements for 10 minutes using a standard video camera. The observation environment was made to resemble their natural habitats as closely as possible.

They then analyzed the primary direction in which the crabs moved. The results were largely divided into two groups. Out of the 50 species, 35 primarily moved sideways, while 15 moved forward.

What's important here is that it can't be simply concluded as "all crabs walk sideways." While many crabs do walk sideways, there are indeed groups that walk forward. This is why the research team was able to investigate when and how sideways walking evolved by comparing it with phylogenetic trees.

Observing behavior alone doesn't reveal the evolutionary path. Therefore, the research team combined existing phylogenetic tree data constructed using 10 genes from 344 crab species. In other words, they overlaid the evolutionary relationships of "which crab is closely related to which" with behavioral data on "how each walks."

As a result, it was shown that sideways walking likely arose not multiple times independently but only once at the root of an evolutionarily advanced group of crabs called Eubrachyura, from an ancestor that walked forward.

"Crab-like Shape" Emerged Multiple Times, but "Sideways Walking" Was Rare

In discussing crab evolution, the term "carcinization" is well-known. This refers to the phenomenon where non-crab crustacean lineages evolve to resemble the body shape of crabs.

A stout body, a short folded abdomen, and legs spread out to the sides. Such "crab-like shapes" are thought to have appeared independently multiple times in crustacean evolution. It's as if nature repeatedly arrived at the same blueprint, often cited as an example of convergent evolution.

However, this research shows that shape and behavior do not necessarily evolve in the same way.

Crab-like body shapes have emerged repeatedly. On the other hand, the behavior of sideways walking was a significant change that may have occurred only once in true crabs. This is very intriguing. While body shapes can evolve in similar directions multiple times, behavioral innovations like modes of movement may not emerge so easily multiple times.

Behavior is linked to various factors, including the skeleton, muscles, nervous system, development, living environment, and relationships with predators. The seemingly simple act of walking sideways actually requires the body's structure and neural control to work in harmony. That's why, once acquired, it was maintained for a long time, while it rarely emerged independently multiple times.

Why Was Sideways Escaping an Advantage?

So, what advantages did sideways walking bring to crabs?

Researchers are focusing on escape from predators. Crabs that can move quickly sideways can escape almost equally well to either side. This is troublesome from the perspective of the pursuer. It's hard to predict whether the prey in front will go right or left.

In human sports, lateral movements that throw off the opponent's predictions are powerful. It's similar to how in soccer or basketball, not just forward and backward speed, but also lateral changes of direction are important. For crabs, sideways walking might have been a feint to survive on beaches and rocky areas.

Furthermore, crabs have wide bodies. Their legs extend to the sides of their bodies. With such a structure, moving sideways tends to be more efficient than moving forward. Sideways walking may have been a mode of movement that effectively linked body shape and locomotion performance.

Of course, sideways walking wasn't all-powerful. The research also shows examples of some crab lineages reverting to forward walking. This suggests that there are costs or constraints associated with sideways walking.

For example, for species that live hidden, burrow in sand, or seek protection inside or near other organisms, the importance of quickly escaping sideways may decrease. For crabs that chose lifestyles other than escaping predators, the necessity to stick to sideways walking might have diminished.

Evolution is not as simple as "once you acquire something good, you use it forever." If the environment or ecology changes, traits that were once advantageous might change in a different direction. The way crabs walk is a good example of this flexibility.

The Origin of Sideways Walking Coincided with Major Earth Changes

Another intriguing aspect of this research is the estimated period when sideways walking emerged.

The research team estimates the origin of sideways walking in true crabs to be about 200 million years ago, around the early Jurassic period. This period corresponds to just after the mass extinction at the end of the Triassic. It was a time when the global biota underwent significant turnover, creating new ecological niches.

During the same period, the breakup of the supercontinent Pangaea was progressing, and shallow marine environments were expanding. Shallow sea areas are places where diverse organisms can easily partition habitats. If environments like rocky areas, sandy grounds, tidal flats, and coral reefs spread complexly, the diversification of organisms adapting to them is also likely to progress.

Sideways walking in crabs may not have been a magical ability that led to success on its own. Rather, it might have become an important tool to take advantage of new opportunities created by changes in Earth's environment.

Evolutionary success occurs when internal innovations and external environments coincide. The ancestors of crabs with the ability to escape quickly sideways encountered the spread of shallow seas and changes in predation pressure. That combination may have led to the current diversity of crabs.

Over 7,900 Species of True Crabs and Their "Successful Walking Style"

According to researchers, there are over 7,900 known species of true crabs. This is a very large diversity compared to closely related groups. Crabs have expanded into various environments, not just the sea, but also freshwater, land, and deep sea.

How much did sideways walking play a role in this background? This research provides important clues to that question.

Of course, it cannot be definitively stated that "crabs succeeded solely because of sideways walking." The diversification of organisms involves numerous factors, including climate, terrain, predators, food, reproductive strategies, and body structure. Sideways walking is just a part of it, and further verification is needed to determine how much it actually increased survival and reproductive success.

The researchers themselves state that future work needs to refine the timeline reflecting fossil records and measure the actual adaptive advantages of sideways walking. For example, performance tests could consider how escape speed and ease of direction change differ between sideways and forward walking, and which is more advantageous in environments with predators.

Nonetheless, this research has shown that modes of animal movement can be major turning points in evolution. Like bird flight, fish swimming, and mammalian running, the method of movement determines where an organism can go, what it can eat, and what it can escape from. Sideways walking for crabs might also have been a key to expanding their world.

On Social Media: "I Love This Kind of Research" and Reactions to "Sideways Walking for 200 Million Years"

 

This research, while a topic of specialized evolutionary biology, has garnered relatively friendly reactions on social media. The reason is clear: the theme is the sideways walk of crabs, something everyone has seen.

On X, news accounts and research-related accounts introduced it with angles like "Crabs have been walking sideways for 200 million years," receiving favorable reactions such as "I love people doing this kind of research" and humorous takes like "I've been walking sideways for 200 million years." The responses are more about the fun of science seriously answering familiar questions than the details of the research content.

On Reddit, questions like "Why do crabs walk sideways?" have been repeatedly posted, with simple explanations exchanged about leg joint orientation, energy efficiency, and body structure. This research can be seen as adding an evolutionary history perspective to such general questions, addressing "When was sideways walking born, and how was it passed down?"

What's interesting about the reactions on social media is that the sideways walk of crabs is perceived not just as a science news item but with meme-like enjoyment. The surprise of "Have crabs always walked sideways?" or "Did their ancestors also move sideways?" represents moments when scientific discoveries slightly change everyday perspectives.

Scientific news can sometimes be buried under complex terminology and specialized statistics. However, the question "Why do crabs walk sideways?" is something that both children and adults can intuitively share. That's why this research is easy to reach a wide audience. The narrative that a common action of a familiar creature hides a 200-million-year history is likely what captivates people's interest.

Delving into "Why?" Reveals the Story of Evolution

The sideways walk of crabs is fascinating just by observing the creature in front of you. But tracing the origin of that movement quickly expands the story to the ancient seas.

An ancestor that once walked forward acquired sideways movement at a certain period. That change might have been a small difference by chance. However, that difference enhanced the ability to escape predators, aided expansion into new environments, and was eventually passed down to diverse crabs.

On the other hand, not all crabs continued to walk sideways. As lifestyles changed, some lineages returned to forward walking. This reflects that evolution is not a linear progression but a trial-and-error process adapted to the environment.

What this research shows is that behavior also carries the history of evolution. Not only bones and shells that easily fossilize but also behaviors like walking, escaping, and feeding are deeply involved in the diversification of organisms.

Next time you see a crab on the beach, you might look at its sideways walk a little differently. The crab in front of you is not just escaping sideways. It is tracing the evolutionary path that began 200 million years ago with its legs.

Source URLs

ScienceDaily. Confirmed the possibility that crab sideways walking originated from a common ancestor about 200 million years ago, observed the behavior of 50 species, with 35 walking sideways and 15 walking forward.
https://www.sciencedaily.com/releases/2026/05/260501052844.htm

eLife Press Release. Confirmed the research background, author comments, the possibility that sideways walking aids in escaping predators, and its relationship with environmental changes.
https://elifesciences.org/for-the-press/81901af4/study-suggests-crabs-iconic-sideways-walk-evolved-from-common-ancestor

Reviewed Preprint in eLife. Used to confirm details such as the paper title, authors, research summary, behavioral analysis of 50 species, single origin of sideways walking, and secondary return to forward walking.
https://elifesciences.org/reviewed-preprints/110015

Introduction of the research by Nagasaki University. Confirmed the research summary in Japanese, publication date, paper information, and positioning of the research team.
https://www.nagasaki-u.ac.jp/ja/science/science450.html

Introduction of the research by Nagasaki University in English. Used to confirm paper information, author information, and English summary of the research announcement.
https://en.nagasaki-u.ac.jp/6581/

EurekAlert! Reprint of research release. Used to confirm paper publication date, DOI, research subjects, and article title.
https://www.eurekalert.org/news-releases/1125115

NHK News-related post on X. Confirmed the spread on Japanese SNS and how it was introduced as "possibly started walking sideways about 200 million years ago."
https://x.com/nhk_kabun/status/2049258095990501709

Nagasaki Newspaper-related post on X. Confirmed the news spread on Japanese SNS and its introduction as a study by the Nagasaki University team.
https://x.com/nagasaki_np/status/2050109107999514834

Related post on Reddit. Referenced as an example where general users have previously exchanged questions about crab sideways walking from the perspective of leg structure and efficiency.
https://www.reddit.com/r/explainlikeimfive/comments/f4qpoc/eli5_why_do_crabs_walk_sideways/