The iconic "sideways walking" of crabs most likely only appeared once in the history of evolution, originating from a common ancestor about 200 million years ago, and has since become a major feature of the "true crab" species. Related results were published in the journal eLife in the form of a peer-reviewed preprint. Based on the largest comparative analysis of crab movement patterns to date, living observations and phylogenetic data were combined to trace the evolutionary origin of this sideways walking.
Lateral movement is the iconic feature of "brachyura", which is also the most abundant branch of crabs in the order Decapoda. The research team suggests that lateral walking may help crabs avoid predators because this mode of movement allows them to escape quickly in more unpredictable directions. Hiroki Kawahata, corresponding author of the paper and associate professor at the Graduate School of Science and Technology at Nagasaki University in Japan, said that lateral movement may play an important role in the ecological success of true crabs. There are currently about 7,904 known species of true crabs, far more than their sister group Anomura and the closely related group Astacidea. They have spread widely to a variety of habitats such as land, freshwater and even deep sea. Their typical "crab-shaped" body shape has also evolved repeatedly in different lineages. This phenomenon is called "crabification".
Although information on the biology of true crabs is abundant, data on their locomotor behavior is relatively scarce. Most true crabs do primarily walk sideways, but some species also walk primarily forward, which raises several key questions: When did lateral walking appear? How many times has it appeared independently during evolution? And are there lineages that "retreat" from lateral walking to predominantly forward walking?
To answer these questions, the researchers selected 50 species of true crabs to analyze their movement patterns. They used a standard video camera to film each crab species for 10 minutes in a circular plastic arena that simulated a natural environment. Due to operational limitations, only one individual of each species was recorded. The team then combined these behavioral observations with a previously published phylogenetic tree of true crabs. This phylogenetic study reconstructed the evolutionary relationships of most major lineages based on 344 species of true crabs and 10 gene sequences. Since behavioral data and phylogenetic data do not completely cover the same group of species, the research team simplified the evolutionary tree, retaining 44 genera, 5 families and 1 superfamily, and replaced missing species with closely related groups when necessary.
Among the 50 species observed, 35 species mainly walked sideways and 15 species mainly walked forward. When the researchers mapped these locomotion patterns onto an evolutionary tree, they found that sideways walking most likely appeared only once in the evolutionary history of true crabs. Analysis shows that this behavior originated from an ancestor at the base of Eubrachyura, a relatively "higher" group of true crabs in the evolutionary tree, who was originally the main ancestor of the progressive behavior, and continued to be retained in the subsequent evolution of the true crab lineage. Kawahata pointed out that this "single event" contrasts with the phenomenon of "crabification", which occurs repeatedly and independently in decapoda. This suggests that body size and appearance can converge many times, while behavioral changes such as sideways walking are relatively rare.
The research team believes that this change in movement patterns may have brought important survival advantages to true crabs. Lateral movement allows the crab to move at high speeds in both directions, improving its ability to evade predators. But the study also pointed out that the evolutionary threshold for sideways walking may be high, because this mode of movement may interfere with other key behaviors such as burrowing, feeding, and mating. At present, it seems that the stable and typical sideways walking is almost unique to "true crabs". Only a few other animals, such as crab spiders, nymphal leafhoppers, etc., are thought to show similar locomotion to a certain extent.
The study also factored environmental changes into the overall consideration of the crab's success. According to estimates, the lateral walking of true crabs originated about 200 million years ago in the Early Jurassic, shortly after the Triassic-Jurassic mass extinction. This period saw dramatic changes in the Earth's environment, including the beginning of the breakup of Pangea, the significant expansion of shallow marine environments, and the early stages of the so-called "Mesozoic Marine Revolution." Scientists believe that these changes created a large number of new ecological opportunities and provided a stage for the radiation evolution of true crabs.
Kawahata pointed out that to further clarify the relative roles of "novel traits" and environmental changes in true crab diversification, more work is needed, such as trait-dependent diversification analysis, time-scale reconstruction combined with fossil records, and performance tests that directly link lateral walking to adaptive advantages. He said the current findings highlight that the crab's lateral movement is a rare but innovative trait that may play an important role in its ecological success. On the one hand, this innovation can open up new adaptive space, but on the other hand, it is constrained by the phylogenetic history and ecological environment. Powered by direct behavioral observations and a phylogenetic framework, this work expands our understanding of how animal movement patterns arise, diversify, and are conserved over evolutionary time.