Crabs exhibit a distinctive form of movement. Rather than moving directly forward, crabs typically scuttle sideways. This unique locomotion is a highly evolved survival strategy that provides significant advantages in their coastal environments. Their body structure is uniquely adapted for this method of movement.
The Unique Anatomy of Crab Legs
Crabs possess a hard outer shell, an exoskeleton, which protects them from predators and environmental elements. Most crabs have ten legs, organized into five pairs. The first pair are prominent front claws, called chelipeds or pincers, used for grasping objects, catching food, and defense. The remaining eight legs, four pairs, are dedicated to locomotion and are called walking legs.
These walking legs are attached to the sides of the crab’s wide, flat body rather than underneath. Each leg is composed of multiple segments that articulate through hinge-like joints. Unlike human knees that bend forward, a crab’s leg joints are structured to bend primarily outward, which naturally influences their range of motion. This specialized joint design and leg placement lay the groundwork for their characteristic sideways scuttle.
The Mechanics of Sideways Walking
A crab’s broad and flattened body shape is a primary factor influencing its sideways movement, as this design is not well-suited for efficient forward motion. The legs are positioned laterally, and their joints bend outwards, making sideways movement the path of least resistance. This anatomical arrangement allows each leg ample room to move without obstructing another, facilitating a faster and smoother gait. This coordinated movement of their legs resembles rowers working in sync, propelling the crab laterally.
The sideways scuttle is an energy-efficient way for crabs to navigate their surroundings. This specialized locomotion provides agility, allowing crabs to dart quickly under rocks or disappear into burrows. Such rapid sideways dashes are a survival strategy for escaping predators and maneuvering through tight spaces in their coastal habitats. The efficiency and stability of this sideways movement are important for their survival and thriving in diverse environments.
Beyond Sideways: Other Crab Movements
While the sideways scuttle is the most recognized form of crab locomotion, not all crabs move exclusively in this manner. Many crab species possess the ability to move forward, backward, and even diagonally, though these movements may not be their most efficient or preferred methods. The specific body shape plays a significant role; crabs with bodies that are longer than they are wide, such as certain raninid crabs or the spider crab Libinia emarginata, can move predominantly forward.
Some crabs are also known for their speed and specialized movements. Ghost crabs, for instance, are known for running very quickly, reaching speeds of up to 10 miles per hour on land. When moving at their highest speeds, ghost crabs may lift their rearmost legs, relying on only the first and second pairs for rapid propulsion. Aquatic crab species have adapted their rearmost legs into flattened, paddle-like structures, enabling them to swim effectively. Crabs can also burrow into sand or mud, using leg motions and body flexing to create shelter.