Researchers have provided new insights into the evolutionary origin of sideways walking in crabs.
Their study, published today as a Reviewed Preprint in eLife, presents the largest comparative dataset on crab locomotion to help understand the origins of the animals' iconic walk, tracing it back to a common ancestor around 200 million years ago. eLife's editors say the work is valuable, with largely convincing evidence, and will be of interest to others studying animal locomotion.
Sideways walking is a defining feature of 'true crabs' (Brachyura) – the largest of the crab decapod groups. Among other benefits, this mode of travel may be particularly useful for escaping from predators as it makes the crabs' escape direction unpredictable.
"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 colonised 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 carcinisation.
"Despite the rich information available on true crabs, data concerning their locomotor behaviours 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?"
To address these questions, Kawabata and colleagues first carried out behavioural analyses of 50 true crab species. Using a standard video camera, they recorded for 10 minutes the movements of each species in plastic circular arenas that matched their native environment. Due to logistical constraints, the movements of one representative crab from each species were recorded.
They then combined their behavioural analysis with data extracted from a recently published crab phylogeny* – a study that reconstructed the evolutionary history of Brachyura using sequences of 10 genes for 344 species across most major lineages of true crabs. As their behavioural dataset did not always match the species included in the phylogeny, the team reduced the evolutionary tree to 44 genera (a rank that sits in between a species and a family), five families and one superfamily, allowing closely related groups to represent the observed species when the same species were unavailable.
Of all 50 species covered, the team classified 35 as sideways movers and 15 as forward movers. Their analyses revealed that the change to sideways walking occurred only once from a single, forward-walking ancestor at the base of Eubrachyura (a group comprising more advanced crab species) and then remained highly conserved across true crabs.
"This single event contrasts starkly with carcinisation, which has occurred repeatedly across decapod species," Kawabata explains. "This highlights that while body shapes may converge multiple times, behavioural changes such as sideways walking can be rare."
The single origin and diversity of Eubrachyura are consistent with the idea that sideways walking acted as a key innovation that contributed to the ecological success of true crabs. One likely adaptive advantage of this mode of travel is the ability to move rapidly at similar speeds in both lateral directions to allow for easier escape from predators. Despite this advantage, it has been difficult to evolve across the animal kingdom, possibly because it affects other behaviours such as burrowing, mating and foraging. The authors say it therefore represents a unique event that has occurred only in true crabs, and potentially crab spiders and leafhopper nymphs.
Finally, they note that the innovation of sideways walking may not be the only process contributing to the evolutionary diversification of true crabs. External factors, such as the ecological opportunity provided by mass extinction, are also critical for evolutionary diversification. Indeed, their analysis suggests that the origin of true crabs' sideways walk falls around 200 million years ago (the earliest Jurassic, immediately post–Triassic–Jurassic extinction). This period is marked by Pangaean rifting, expansion of shallow-marine habitats and the early Mesozoic Marine Revolution, all of which typically increased ecological opportunity.
"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.
"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 behavioural 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 study alongside co-first authors Junya Taniguchi, Tsubasa Inoue and Kano Kohara, from the Kawabata Laboratory. Additional authors 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.
* Wolfe, J.M., Ballou, L., Luque, J., Watson-Zink, V.M., Ahyong, S.T., Barido-Sottani, J. et al. 2024. Convergent Adaptation of True Crabs (Decapoda: Brachyura) to a Gradient of Terrestrial Environments. Systematic Biology 73:247–262. DOI: https://doi.org/10.1093/sysbio/syad066 .
##
