🌅 Why Do Crabs Walk Sideways?

SOURCE

Taniguchi et al. (2026)

WHAT TO KNOW

• A new study by researchers at Nagasaki University in Japan presents the largest dataset yet on how crabs move, tracing the species’ sideways scuttle to a single ancestor that lived roughly 200 million years ago. The researchers analyzed the movement of 50 crab species and mapped the results onto a crab evolutionary tree, finding sideways walking likely originated in one forward-walking ancestor and had remarkable persistence thereafter, appearing in most modern crab species today. The study was released as a preprint, meaning it has yet to be peer-reviewed.

WHY IT MATTERS

• Sideways walking is a hallmark trait of “true crabs,” the largest group among crab decapods, which also includes most modern crab species. Biologically speaking, there’s no obvious reason why crabs move sideways, however, the researchers believe it could offer important advantages, like helping crabs escape predators by making their movements harder to predict. The authors also believe timing may have played a role in the emergence of sideways walking as it began just after the Triassic-Jurassic extinction, when major environmental changes, like the break up of Pangea and expansion of shallow marine habitats, likely created new opportunities for species to diversify.

CONNECT THE DOTS

• The discovery that sideways walking evolved just once among crabs runs counter to the phenomenon of carcinization, i.e., the repeated evolution of crab-like bodies among different non-crab crustaceans. Carcinization is itself an example of a phenomenon called convergent evolution, resulting in different animal groups independently evolving the same traits. Convergent evolution is the reason both bats and birds have wings, and why primates, koalas, and giant pandas all have opposable thumbs.