Scientists Just Solved the Mystery of Why Crabs Walk Sideways
Researchers from Nagasaki University traced sideways walking in true crabs (Brachyura) to a single evolutionary event 200 million years ago, analyzing 50 species and phylogenetic data to confirm it emerged once in a Jurassic ancestor. The study suggests this movement style contributed to crabs' ecological success, allowing them to evade predators by moving unpredictably in lateral directions.
Scientists have determined that true crabs (Brachyura) developed their sideways walking style just once, around 200 million years ago, during the Jurassic period. A study published as a reviewed preprint in *eLife* analyzed 50 crab species, combining video observations of their movement with evolutionary data from 344 species. The research found that sideways locomotion likely originated in a single forward-walking ancestor near the base of Eubrachyura, a group of more advanced crab species, and persisted throughout their evolution. The team, led by Yuuki Kawabata of Nagasaki University, observed that 35 of the 50 species primarily walked sideways, while 15 moved forward. By mapping these behaviors onto an evolutionary tree, they concluded sideways walking evolved only once, contrasting with the repeated evolution of crab-like body shapes (carcinization). This suggests behavioral changes like sideways movement are rarer than physical adaptations. The study proposes that sideways walking may have enhanced crabs' survival by allowing rapid lateral escapes from predators. True crabs, numbering around 7,904 species, thrive in diverse environments—terrestrial, freshwater, and deep-sea—due in part to this movement advantage. The findings highlight how a single evolutionary trait can significantly influence a group's ecological success. Researchers used circular plastic arenas to film crab movement for 10 minutes per species, pairing observations with a previously published crab phylogeny. The analysis simplified the evolutionary tree to include 44 genera and five families, using closely related groups to fill gaps where species data was missing. The study underscores that while body shapes may evolve repeatedly, behavioral traits like sideways walking are often fixed early in a lineage.
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