Crabs’ Sideways Walk Evolved Once 200 Million Years Ago

The sideways walk characteristic of most crabs evolved only once, in a common ancestor that lived about 200 million years ago. That finding comes from a study published April 21 in eLife that examined the movement of 50 crab species collected across Japan and mapped the results onto a crab evolutionary tree constructed from DNA data.

Researchers recorded each crab’s locomotion in a controlled pool, noting whether the animal moved primarily forward or sideways. The analysis showed that all crabs exhibiting sideways movement descend from one group of ancestors dating to roughly 200 million years ago.

The lineage that adopted this form of locomotion, known as Eubrachyura, now contains nearly 7,500 modern species. By comparison, the two groups of crabs that move forward and backward together include only 156 species. The marked difference in diversity suggests that sideways walking may have functioned as a key innovation that contributed to the group’s evolutionary expansion.

Sideways locomotion required shifts in muscles and ligaments as well as a fundamental rewiring of neural activity. The changes affected multiple behaviors including foraging, burrowing, social interaction and mating. One researcher not involved in the study noted that sideways-walking crabs ultimately required fewer nerve cells to control leg movement than earlier forms.

“Instead of every joint in the leg of a crab having to play a more or less equal role, it boiled down to two main joints that did pretty much 90 percent of the work,” the neuroethologist said.

The first sideways-walking crabs appeared in the aftermath of the Triassic–Jurassic extinction, which eliminated about three-quarters of all species. The breakup of the supercontinent Pangaea at that time also expanded shallow marine habitats favorable to crustaceans.

These new ecological niches may have allowed the sideways-walking lineage to spread rapidly across land and sea environments. Other crustaceans later evolved a crablike body plan on at least three separate occasions. None of those “false” crabs developed sideways locomotion.