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Study Finds Crabs’ Sideways Walk Evolved Once Around 200 Million Years Ago

A reviewed preprint pairs behavior from 50 species with a genetic family tree to map a single shift in locomotion.

Some of the true crab species included in the study 'Evolution of sideways locomotion in crabs'. Clockwise from top left: Two photographs of Tuerkayana hirtipes; Cardisoma carnifex (by Tsubasa Inoue); and Ocypode sinensis (by Junya Taniguchi) (CC BY 4.0) Credit: Tsubasa Inoue and Junya Taniguchi (CC BY 4.0)
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Overview

  • The analysis places a single shift to sideways movement at the base of Eubrachyura, a major branch of true crabs, with the trait conserved thereafter.
  • Researchers filmed one representative of each of 50 species in standardized arenas and classified 35 as sideways movers and 15 as forward walkers.
  • They mapped these behaviors onto a multi-gene crab phylogeny and, to align taxa, collapsed the tree to 44 genera, five families and one superfamily.
  • The team estimates the shift occurred about 200 million years ago, shortly after the Triassic–Jurassic extinction when shallow-sea habitats expanded.
  • The authors suggest lateral gaits may help crabs evade predators but note limits of one individual per species and call for fossil-informed and performance tests.