Why do crabs walk sideways?

There are two aspects to this question. The first concerns the mechanics of crab motion—the how—which evolutionary biologists call a proximate question. The second concerns the function of crab motion—the why—which evolutionary biologists call an ultimate question. Before delving into why crabs move sideways, let’s first take a look at how they do it.

To answer how crabs walk sideways, we have to look at their body plan. Crabs have a flat body with five pairs of jointed legs attached to their sides (the first pair is modified into claws). The joint at the junction between leg and body—the shoulder joint—has limited front-to-back mobility. By contrast, the second joint in each leg bends readily, but only sideways and outward, favoring efficient lateral locomotion. An advantage of having the legs move sideways instead of forward and backward is that it prevents them from becoming entangled, which would cause the crab to trip.

But the main adaptive value, or function, of a sidelong gait is that a crab can move with nearly equal speed to either the left or the right. This allows the animal to respond quickly to predators. The ability to move rapidly in either direction, along with the flexibility and potential unpredictability of sideways locomotion, makes this an advantageous escape strategy. Moreover, a crab’s sidelong gait, together with its flattened body shape, enables it to exploit a wide range of ecological niches—from narrow rock crevices to pits in sand and mud.

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Not all crabs walk sideways

Most true crabs—those belonging to the infraorder Brachyura, such as fiddler crabs, snow crabs, and ghost crabs—characteristically exhibit lateral movement. Other crustaceans in the same arthropod order (Decapoda) but belonging to the infraorder Anomura, such as hermit crabs, king crabs, and porcelain crabs, generally move forward or backward. Among the true crabs, members of several families, such as Majidae, Mictyridae, and Raninidae, frequently move forward and backward rather than laterally. The presence of diverse antipredator strategies, including camouflage and group behavior, may have reduced the reliance of these animals on lateral motion to escape predators.