Crabs, members of the crustacean family, are recognized for their distinctive sideways scuttling across beaches and seafloors. This unique mode of travel, known as lateral locomotion, often prompts the question of whether they are physically capable of moving in any other direction. The answer lies in their evolutionary history, body structure, and the mechanics of their walking legs. This sideways gait is not simply a preference but a direct result of their unique anatomy, allowing them to navigate their complex environments with speed and efficiency.
The Standard Sideways Gait
The primary reason most crabs move laterally is rooted in the structure of their pereiopods, which are their four pairs of walking legs. Unlike the limbs of most forward-moving animals, the joints within a crab’s legs are structured like simple hinges designed to flex and extend outward, perpendicular to the animal’s direction of travel. This specific joint arrangement limits the range of motion in the anterior-posterior direction, making a true forward stride mechanically difficult and inefficient.
Because the crab’s body is typically broader than it is long, the flattened, wide shape constrains the degree to which the legs can swing forward or backward. Therefore, the path of least resistance and greatest efficiency for the limbs is a coordinated, alternating lateral push that propels the body to the side. The legs on the side toward the direction of travel pull the body, while the legs on the opposite side push it.
This coordinated lateral movement allows the crab to achieve a highly stable and rapid gait. The eight walking legs work in a synchronized pattern, with one set providing a propulsive force and the other recovering for the next step. This design maximizes the area of contact with the ground, which is particularly beneficial on uneven or shifting surfaces like sand and rocks.
When Crabs Move Forward or Backward
While the sideways scuttle is the dominant gait, some species have adapted to move in other directions. Certain crabs, like the Australian soldier crab (Mictyris longicarpus), routinely walk forward and do not possess the extremely flattened body shape common to most sideways-moving species. These species often utilize a metachronal gait, where a wave of leg movements travels along the body, similar to the walking pattern of lobsters.
Spider crabs, such as Libinia emarginata, show a preference for forward movement and have specific skeletal and muscular adaptations to facilitate this. Even the common sideways-walking species can often shuffle slowly in a forward or backward direction, but this movement is slower and requires more effort than their lateral movement. This limited non-lateral movement is achieved by slightly rotating the leg at the base joint, but it remains a secondary and less efficient option for locomotion.
Why Lateral Movement is a Survival Advantage
Lateral movement is an evolutionary advantage, providing superior speed and stability in its natural habitat. Moving sideways allows the animal to maintain a lower profile and squeeze quickly into narrow crevices and under rocks for shelter or ambush. This ability to navigate confined spaces is important in intertidal zones and rocky shorelines where crabs commonly reside.
The lateral gait enables an immediate burst of speed for rapid escape from predators. A sideways sprint allows the crab to orient its eyes, which are positioned on stalks, toward the threat while simultaneously moving away from it. This combination of speed and constant visual awareness is a survival mechanism that has been refined over time.