Crabs' Sideways Walk Evolved Once 200 Million Years Ago

For the first time, researchers have pinpointed exactly when crabs started walking sideways, and the answer reveals why these quirky creatures conquered the oceans.

A team led by behavioral ecologist Yuuki Kawabata at Nagasaki University filmed 50 crab species in custom arenas designed to match their natural homes. Some species got seawater tanks, others freshwater or brackish pools, all to capture their most natural walking behavior.

The results surprised everyone. Of the 50 species studied, 35 moved sideways while 15 walked forward like most animals.

When the team mapped these movements onto an evolutionary tree, they discovered something remarkable. The sideways shuffle evolved just once in a common ancestor during the early Jurassic period, roughly 200 million years ago.

That single innovation changed everything. The subgroup that inherited this trait, called Eubrachyura, now includes about 7,500 species. Meanwhile, the two other true crab subgroups that kept walking forward? They total only 150 species combined.

The sideways advantage is clear. Moving easily in two perpendicular directions instead of one helps crabs escape predators faster and makes their escape routes unpredictable.

The timing matters too. When sideways walking first appeared, the supercontinent Pangea was breaking apart, creating vast new shallow marine habitats perfect for crabs to colonize.

Why This Inspires

What makes this discovery so fascinating isn't just the evolutionary detective work. It's proof that a single creative solution can unlock extraordinary success.

Biologist Andrés Vidal-Gadea from Illinois State University suggests the sideways walk might have actually simplified life for crabs. Instead of coordinating every joint equally, two main joints now do 90 percent of the work, requiring fewer nerve cells and less complex muscle control.

This kind of convergent simplicity is rare in nature. While useful traits like wings evolved separately multiple times in birds, bats, and insects, crabs figured out their signature move just once and never needed to reinvent it.

The research, published in the journal eLife, opens new questions about how physical innovations drive ecological success. The team plans to study how sideways movement directly improves survival and reproduction across different environments.

Sometimes the strangest adaptations become the most powerful advantages.