Crabs are crustaceans found in diverse aquatic and terrestrial environments around the globe. They exhibit a wide range of sizes, from a few millimeters to a leg span of 13 feet, adapting to various habitats including oceans, freshwater bodies, and land. Their bodies are encased in a hard outer shell, known as an exoskeleton, which provides protection and structural support. This outer covering is composed primarily of highly mineralized chitin.
The Limbs of a Crab
Crabs possess multiple pairs of legs, classifying them as decapod crustaceans. The term “decapod” literally translates to “ten legs,” reflecting the typical number of limbs most crabs exhibit. This arrangement commonly includes five pairs of legs. While this is the general rule, some species present variations; for instance, porcelain crabs may appear to have only eight legs, and king crabs typically feature six walking legs along with two claws, totaling eight specialized appendages. These differences highlight evolutionary adaptations within the broader crab family.
Each of a crab’s legs is a segmented and jointed structure, encased within its exoskeleton. This hard outer covering provides support and protection, and must be shed during growth.
The limbs are formed from distinct sections, moving from the body outwards, with each segment connected by a joint. For example, the merus is a significant segment, and the dactyl is the final segment that typically makes contact with the ground during locomotion. The segmented structure grants crabs a wide range of motion.
Muscles and tendons within these segments facilitate flexible movements. The foremost pair of these ten legs is transformed into specialized pincers or claws, termed chelipeds. The remaining four pairs are used for movement.
How Crabs Use Their Legs
Crabs use their legs for various purposes, with each pair often specialized for particular tasks. The chelipeds, or claws, serve multiple functions beyond locomotion. These appendages are used for defense against predators, capturing prey, and sometimes in communication or mating rituals. The size and strength of these claws can vary significantly between species, with some having a larger “crusher” claw and a smaller “pincer” claw for different handling needs.
The four pairs of walking legs, also known as pereiopods, are used for the crab’s characteristic movement. Crabs are known for their sideways scuttling, a gait efficient due to the outward bending of their leg joints. This movement pattern, often described as an “alternating tripod gait,” allows them to move quickly and stably across various surfaces. While sideways movement is most common, some crab species can walk forward, backward, or diagonally.
For certain aquatic species, the rearmost pair of legs is adapted for swimming. These legs are often flattened and paddle-shaped, resembling oars, enabling efficient propulsion through water. Crabs like the blue crab are adept swimmers, using these modified legs to generate thrust at rates of 20 to 40 revolutions per minute. Beyond swimming, these specialized legs can also be chemosensitive, assisting in detecting food items in their environment.