Scientists have discovered new clues about how crabs developed their unique lateral movements.
The new study was released as a reviewed preprint. e-lifehas compiled the largest dataset to date on how crabs move. By comparing many species, researchers were able to trace this unusual walking pattern back to a common ancestor that lived about 200 million years ago. eLife editors say the findings are valuable, broadly relevant to scientists who study how animals move, and supported by nearly convincing evidence.
Why lateral movement is important
Sidewalking is a characteristic of the “true crabs” (bracula), the largest group of decapod crabs. This unusual method of transportation may have important advantages. For example, it helps crabs escape from predators by making their direction less predictable.
“Lateral migration may have contributed significantly to the ecological success of real crabs,” says senior corresponding author Hiroki Kawabata, associate professor at Nagasaki University’s Graduate School of Science and Technology. “There are approximately 7,904 true crab species, far outnumbering their sister group Anomura and their close relative Astacidea. Crabs colonize a variety of habitats around the world, including terrestrial, freshwater, and deep-sea environments, and their crab-like body shape has repeatedly evolved over time in a phenomenon known as carbonization.”
“Despite the wealth of information available about true crabs, data on their locomotor behavior are sparse. Most true crab species move sideways, but some groups walk forward, which raises some interesting questions. When did their sideways movement begin? How many times over the years did it evolve, and how many times did it change back?”
Tracking crab movements across species
To investigate these questions, Kawabata and his colleagues studied how 50 species of real crabs move. Each species was recorded for 10 min using a standard video camera in a circular plastic arena designed to resemble its natural habitat. Due to practical limitations, the researchers observed one individual per species.
The team then combined these observations with previously published crab phylogenetic data that mapped Brachyura’s evolutionary relationships using 10 genes from 344 species across most major lineages. Because behavioral data did not always perfectly match species in their phylogeny, the researchers simplified the evolutionary tree to 44 genera with five families and one superfamily. This allowed closely related groups to substitute for species not directly included.
single evolutionary change
Of the 50 species studied, 35 moved primarily laterally and 15 moved forward. When the researchers mapped these behaviors onto an evolutionary tree, a clear pattern emerged. Sidewalking appears to have evolved only once, descending from a frontwalking ancestor at the base of Eubracula, a group that includes more advanced crabs. Since then, this trait has changed little across real crabs.
“This single event stands in clear contrast to repeated carbonizations across decapod species,” Kawabata explains. “This highlights that while body shape may converge many times, changes in behavior such as sideways walking may be rare.”
Key innovations for survival
The researchers suggest that this one-time shift to lateral migration may have played a major role in the success of real crabs. Lateral movement allows the crab to move quickly in either direction, making it easier to escape from predators. At the same time, this type of locomotion is rare throughout the animal kingdom, likely because it can interfere with other important activities such as burrowing, mating, and feeding.
According to the authors, sideways walking is a rare evolutionary innovation found primarily in real crabs, and could possibly also be seen in some other groups, such as crab spiders and leafhopper nymphs.
Evolution and environmental opportunities
The study also points out that evolutionary success is not driven solely by biological innovation. Environmental factors can also play a big role. Researchers estimate that true crabs’ lateral walking began about 200 million years ago (the earliest period of the Jurassic period, just after the Triassic-Jurassic extinction period). This period included major environmental changes such as the breakup of Pangea, the expansion of shallow marine habitats, and the early Mesozoic marine revolution, all of which likely created new opportunities for species diversification.
“Disentangling the relative roles of innovation and environmental change requires further analysis of trait-dependent diversification, fossil-based timelines, and performance tests linking lateral movement in true crabs to adaptive advantages,” Kawabata added.
Expand your understanding of animal movement
“These results highlight that lateral movement in true crabs is a rare but innovative trait that may have contributed to their ecological success,” Professor Kawabata concluded. “Such innovations can open up new opportunities for adaptation, but remain constrained by phylogenetic history and ecological context. Through direct behavioral observations and a phylogenetic framework, this study expands our understanding of how animal locomotion patterns diversify and persist through evolutionary time.”
Hiroki Kawabata conducted the study with co-lead authors Junya Taniguchi, Tsubasa Inoue, and Kano Obara from the Kawabata lab. Other contributors include Jung-Fu Huang from National Kaohsiung University of Science and Technology in Taiwan. Jun Hirai, Susami Town Crustacea Aquarium, Wakayama, Japan. Nobuaki Mizumoto, Auburn University, Alabama, USA. and Fumio Takeshita, Kitakyushu Museum of Natural History and Human History, Japan.
