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 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 carcinisation.

"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?"

To address these questions, Kawabata and colleagues first carried out behavioral 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.