The 200-Million-Year Secret Behind Crabs' Sideways Shuffle

Many have likely witnessed the rapid sideways movement of crabs on the coast. But why do they dart left or right instead of moving straight ahead? This seemingly peculiar gait holds the secret to a crab's survival strategy, a product of evolution believed to have begun around 200 million years ago. The key lies in the structure of a crab's leg joints. This anatomy makes it easier for the legs to move sideways than forward. Combined with their broad, flattened body shape, crabs can achieve longer and faster strides by moving laterally. This ability is crucial for escaping predators, allowing for quick evasion without wasting time turning their bodies. A study titled "Evolution of Sideways Locomotion in Crabs" delved into the origins of this behavior. Researchers observed 50 species of living crabs in various settings, including the field, laboratories, private collections, and even fish markets. They recorded the movement of each crab, dividing them into two groups: 35 species that walked sideways and 15 that moved forward. No intermediate forms of locomotion were found. This behavioral data was then integrated with a phylogenetic tree based on genomic data. The results indicated that all sideways-walking crabs originated from ancestors that moved forward. In some lineages, species later reverted to forward locomotion due to adaptation to specific ecological conditions. Researchers estimate that the ancestors of sideways-walking crabs emerged around 200 million years ago, in the early Jurassic period, following the Triassic-Jurassic extinction event. This era was characterized by the breakup of the supercontinent Pangea, the expansion of shallow marine habitats, and increased competition between predators and prey in the oceans. Under such conditions, sideways locomotion likely provided crabs with a survival advantage.