Most people observe crabs moving with a distinctive sideways shuffle, often called “crabwise.” This lateral motion is a defining characteristic of true crabs (Brachyura). While this movement may appear awkward compared to the forward gait of most land animals, it is an efficient form of locomotion for the crab body plan. This unique movement is rooted deeply in the crab’s internal anatomy and the evolutionary pressures of its coastal environment.
The Mechanics of the Crab Leg Joint
The reason most crabs walk sideways is due to the fixed design of their leg joints, which act much like hinge mechanisms. A crab’s walking legs are composed of six segments. The first joint, the coxa, connects the leg to the main body shell (carapace). The coxal joint and subsequent major joints permit a wide range of motion along the horizontal (lateral) plane, but severely restrict movement along the sagittal (forward-backward) plane.
This anatomical arrangement means that extending a leg forward or backward requires more energy and results in a clumsy, less stable gait. Each leg’s second joint, the basis-merus joint, is situated at a right angle to the direction the crab is facing, forcing the limb to swing out to the side. When walking sideways, the legs on the leading side pull the body while the trailing legs push, creating a smooth and rapid scuttling motion. The wide, flattened body shape further limits the legs’ ability to swing forward without interfering, making sideways movement the mechanically simplest and most energy-efficient option.
Lateral Movement as an Evolutionary Advantage
The sideways gait is not merely a byproduct of anatomical restriction; it represents an evolutionary adaptation contributing to the success of true crabs. This lateral scuttle provides superior stability, which is beneficial when navigating the uneven, wave-swept terrains of rocky shores and sandy beaches. A crab maintains a low center of gravity and a wide stance while moving, helping prevent it from being toppled by surging water or shifting substrate.
The ability to move rapidly and laterally allows crabs to execute quick escapes from predators. The scuttling motion enables sudden bursts of acceleration, allowing the crab to dart into the nearest shelter without needing to turn its entire body first. Furthermore, the wide, flat body shape—known as carcinization—allows the crab to slip into narrow crevices and burrows side-first, providing immediate concealment. This specialized body plan and lateral movement proved advantageous, contributing to the diversification of the Brachyura lineage.
Variations in Crab Locomotion
While the sideways walk is characteristic of most Brachyura, it is not a universal rule across all crab species. Certain crabs have evolved modified joint structures and different body shapes to accommodate alternative forms of locomotion. For example, the ghost crab (genus Ocypode) is known for its speed and can run in multiple directions, including forward, due to adaptations in its leg joints and musculature.
Other species, such as spider crabs (Libinia emarginata) and some box crabs (Mictyris platycheles), primarily walk forward. These exceptions demonstrate that the degree of restriction in the leg joints varies, corresponding to their habitat needs. Spider crabs, which inhabit calmer, rockier environments, utilize a slower, deliberate forward gait for climbing. Swimming crabs (Portunidae) utilize flattened hind legs to propel themselves rapidly backward through the water.