Why do crabs walk sideways? It’s a question humans have asked for thousands of years, and science finally has the answer.
It looks awkward. It looks inefficient. And yet crabs have been doing it for an almost incomprehensible length of time and thriving because of it. Now, scientists have finally traced the origin of this peculiar movement back to its roots. And the answer is both deeper and stranger than most people would expect.

Why Do Crabs Walk Sideways? The Science Explained

A new study published in the journal eLife brings together the largest comparative analysis of crab movement ever assembled. Researchers from Nagasaki University in Japan, along with collaborators from Taiwan, Australia, and the United States, combined direct observations of living crab species with evolutionary data to trace the history of sideways walking through deep time.

Their conclusion: sideways locomotion in true crabs evolved exactly once, roughly 200 million years ago, in a single common ancestor.
Every crab you’ve ever seen moving sideways along a beach, through a reef, or across the ocean floor inherited that movement from that one animal. One evolutionary moment, and it stuck.

Why Did It Happen Then?

The timing turns out to be significant. Two hundred million years ago places us at the very beginning of the Jurassic period, immediately following one of Earth’s major mass extinction events, the end-Triassic extinction.

That catastrophe wiped out enormous numbers of species and reshaped marine ecosystems around the planet. In the chaos, new ecological opportunities opened up. Shallow marine habitats expanded. The early Mesozoic Marine Revolution was underway.
Into that transformed world stepped an ancestor of what we now call true crabs a creature that, for whatever reason, began moving differently from everything around it. And that difference turned out to be an advantage.

So What’s the Actual Benefit?

The research team’s analysis suggests that sideways walking likely gave early crabs a significant edge against predators. The movement allows for rapid, unpredictable bursts of speed in multiple directions the kind of erratic, hard-to-follow motion that makes a chasing predator’s life very difficult.
Think of it less like an awkward shuffle and more like a built-in evasion system. A crab moving sideways can change direction almost instantaneously without needing to turn its body. For an animal that lives in open habitats where predators are a constant threat, that’s an enormous survival advantage.

“Sideways locomotion may have contributed significantly to the ecological success of true crabs,” said senior author Yuuki Kawabata of Nagasaki University.

And the numbers support that. True crabs, the group that walks sideways, are one of the most ecologically diverse and widespread groups of marine animals on Earth. They’ve colonized beaches, coral reefs, deep ocean floors, mangroves, rivers, and even land. Something clearly worked.

The question of why crabs walk sideways has finally been answered after 200 million years.

Not Every Crab Got the Memo

Here’s a detail that makes this story even more interesting: the research found at least six cases where certain crab lineages reversed course and returned to forward walking.
Spider crabs, soldier crabs, and pea crabs, for example, do not move sideways the way their relatives do. Evolution, apparently, is not always a one-way street. Some lineages found forward walking useful again, perhaps for different ecological reasons, perhaps because of body shape changes that made sideways movement less practical.
This back-and-forth in evolutionary history is a reminder that adaptations are not permanent fixtures. They’re responses to circumstances. When circumstances change, so can the adaptation.

A Striking Contrast With Body Shape

The researchers also drew a fascinating comparison to another well-known phenomenon in crab biology called “carcinization,” the repeated, independent evolution of a crab-like body shape in many different crustacean lineages. Carcinization has happened over and over again across millions of years in species that are not closely related.
Sideways walking is the opposite story. While body shape has converged toward the crab form many times over, the movement style evolved only once and was then inherited, not reinvented.
As Kawabata noted, “This highlights that while body shapes may converge multiple times, behavioral changes such as sideways walking can be rare.”

The Bigger Lesson

This study is a small but satisfying example of science answering a question that humans have probably wondered about since the first time someone watched a crab on a beach thousands of years ago.
It also illustrates something important about evolution: the things that look strange or inefficient often have a very good reason behind them. The crab’s side

Read also: Why is 2026 the Year of the Ocean’s Great Awakening?

Sources: eLife Journal / ScienceDaily / Phys.org / Discover Magazine