Crabs Evolved Sideways Scuttle Once 200 Million Years Ago

A study published as a peer-reviewed preprint in e-Life on April 21 traces the sideways scuttle of true crabs to a single common ancestor that lived roughly 200 million years ago. Researchers led by Yuuki Kawabata of Nagasaki University filmed 50 crab species in circular arenas, identifying 35 sideways-moving and 15 forward-moving species. Combining observations with an evolutionary map of Brachyura, they found that sideways locomotion evolved once at the base of the subgroup Eubrachyura, which today includes about 7,500 species—vastly outnumbering forward-walking subgroups with around 150 species total. The trait may have been a key innovation enabling Eubrachyura to diversify rapidly, possibly aiding predator escape and making movement unpredictable. The early Jurassic period, when Pangea split and shallow marine habitats expanded, may have created opportunities for diversification. Andrés Vidal-Gadea of Illinois State University suggests sideways walking simplified neural control, requiring fewer nerve cells. The researchers call for further analysis of trait-dependent diversification and fossil-informed timelines. Notably, while sideways walk evolved once, crab-shaped bodies have evolved at least five times through convergent evolution called carcinization.