How do crabs walk sideways?

Crabs walk sideways primarily due to their specific body plan and the structure of their legs. Crabs have flat bodies with five pairs of jointed legs attached to their sides, with the first pair modified into claws. The shoulder joint, where the leg meets the body, has limited front-to-back mobility, while the second joint in each leg bends readily in a sideways and outward direction. This mechanical arrangement favors efficient lateral locomotion and prevents the legs from becoming entangled, which would otherwise cause the crab to trip.1

Beyond the mechanics, this sidelong gait serves as an effective escape strategy. It allows the crab to move with nearly equal speed to either the left or the right, enabling a quick response to predators. Furthermore, this movement style, combined with their flattened body shape, allows crabs to navigate and exploit a wide range of ecological niches, such as narrow rock crevices or pits in sand and mud. While most true crabs exhibit this lateral movement, it is not universal among all crustaceans or even all crab families.1