Scientists have uncovered new clues about how crabs developed their distinctive sideways movement.
A new study, released as a Reviewed Preprint in eLife, brings together the largest dataset yet on how crabs move. By comparing many species, the researchers traced this unusual walking style back to a shared ancestor that lived roughly 200 million years ago. Editors at eLife describe the findings as valuable and supported by largely convincing evidence, with broad relevance for scientists studying how animals move.
Why Sideways Movement Matters
Sideways walking is a hallmark of ‘true crabs’ (Brachyura), the largest group among crab decapods. This unusual way of moving may offer important advantages. For example, it can help crabs escape predators by making their direction harder to predict.
“Sideways locomotion may have contributed significantly to the ecological success of true crabs,” says senior corresponding author Yuuki Kawabata, Associate Professor at the Graduate School of Integrated Science and Technology, Nagasaki University, Japan. “There are around 7,904 species of true crabs, far exceeding that of their sister group, Anomura, or their closest relatives, Astacidea; they have colonized diverse habitats around the world, including terrestrial, freshwater and deep-sea environments; and their crab-like body shape has evolved repeatedly over time in a phenomenon known as carcinization.
“Despite the rich information available on true crabs, data concerning their locomotor behaviors are sparse. Although most true crab species use sideways locomotion, there are some groups that walk forwards, which raises some interesting questions. When did their sideways locomotion originate, how many times over the years did it evolve, and how many times did it revert?”
Tracking Crab Movement Across Species
To investigate these questions, Kawabata and colleagues studied how 50 species of true crabs move. Each species was recorded for 10 minutes using a standard video camera inside a circular plastic arena designed to resemble its natural habitat. Because of practical limitations, the researchers observed one individual per species.
The team then combined these observations with data from a previously published crab phylogeny that mapped the evolutionary relationships of Brachyura using 10 genes from 344 species across most major lineages. Since the behavioral data did not always align perfectly with the species in that phylogeny, the researchers simplified the evolutionary tree to 44 genera, along with five families and one superfamily. This allowed closely related groups to stand in for species that were not directly included.
A Single Evolutionary Shift
Out of the 50 species studied, 35 primarily moved sideways, while 15 moved forward. When the researchers mapped these behaviors onto the evolutionary tree, a clear pattern emerged. Sideways walking appears to have evolved just once, originating from a forward-walking ancestor at the base of Eubrachyura, a group that includes more advanced crabs. After that point, the trait remained largely unchanged across true crabs.
“This single event contrasts starkly with carcinization, which has occurred repeatedly across decapod species,” Kawabata explains. “This highlights that while body shapes may converge multiple times, behavioral changes such as sideways walking can be rare.”
A Key Innovation for Survival
The researchers suggest that this one-time shift to sideways movement may have played a major role in the success of true crabs. Moving laterally allows crabs to travel quickly in either direction, making it easier to evade predators. At the same time, this type of locomotion is uncommon across the animal kingdom, possibly because it can interfere with other important activities such as burrowing, mating and feeding.
According to the authors, sideways walking may represent a rare evolutionary innovation seen mainly in true crabs, and possibly in a few other groups like crab spiders and leafhopper nymphs.
Evolution and Environmental Opportunity
The study also points out that evolutionary success is not driven by biological innovations alone. Environmental factors can play a major role as well. The researchers estimate that sideways walking in true crabs originated around 200 million years ago (the earliest Jurassic, immediately post-Triassic-Jurassic extinction). This period included major environmental changes such as the breakup of Pangaea, expansion of shallow marine habitats and the early Mesozoic Marine Revolution, all of which likely created new opportunities for species to diversify.
“To disentangle the relative roles of innovation and environmental change, we need further analyses of trait-dependent diversification, fossil-informed timelines and performance tests that link true crabs’ sideways movement to adaptive advantages,” Kawabata adds.
Expanding Our Understanding of Animal Movement
“These current results highlight that sideways locomotion in true crabs is a rare but innovative trait that may have contributed to their ecological success,” Kawabata concludes. “Such innovations can open new adaptive opportunities and yet remain constrained by phylogenetic history and ecological contexts. With direct behavioral observations and a phylogenetic framework, this work expands our understanding of how modes of travel in animals diversify and persist through evolutionary time.”
Yuuki Kawabata carried out this research with co-first authors Junya Taniguchi, Tsubasa Inoue and Kano Kohara from the Kawabata Laboratory. Additional contributors include Jung-Fu Huang, National Kaohsiung University of Science and Technology, Taiwan; Atsushi Hirai, Susami Crustacean Aquarium, Wakayama, Japan; Nobuaki Mizumoto, Auburn University, Alabama, US; and Fumio Takeshita, Kitakyushu Museum of Natural History & Human History, Japan.
