Crabs are crustaceans belonging to the order Decapoda, a group that also includes shrimp and lobsters. The name Decapoda translates from Greek as “ten feet,” which defines the standard appendage count. Most true crabs possess ten primary appendages attached to their cephalothorax, the fused head and chest section of their body. This count consists of eight walking legs and a single pair of modified appendages known as claws, or chelipeds.
The Standard Decapod Count: Differentiating Claws and Legs
The ten appendages of a true crab are scientifically referred to as pereiopods, which are arranged in five paired sets. These five pairs of thoracic legs are not uniform in function or appearance, which often causes confusion regarding the number of legs. The first pair of pereiopods is the chelipeds, the large claws used for defense, mating displays, and securing prey. Although functionally distinct from walking legs, these claws are anatomically considered modified legs.
Following the chelipeds are the remaining four pairs of pereiopods, which perform the primary function of locomotion. These eight appendages are the walking legs that allow the crab to scuttle sideways or move forward across the substrate. This anatomical arrangement of one pair of claws and four pairs of walking legs is the standard blueprint for the Brachyura infraorder, which encompasses all true crabs.
Specialized Functions of Crab Appendages
Crab appendages have evolved to perform a variety of specialized tasks necessary for survival. For instance, in species like the blue crab (Callinectes sapidus), the final pair of walking legs is adapted for swimming. These back legs are flattened into a paddle-like shape, scientifically called the dactyl, which allows the crab to propel itself through the water. This adaptation is characteristic of the Portunidae family of swimming crabs.
Crabs also possess appendages near their mouth that aid in feeding, known as maxillipeds. These three pairs of thoracic limbs are situated around the mouthparts, manipulating and passing food to the mandibles for chewing. The maxillipeds can also form an operculum, a protective covering that holds food securely against the mouth.
Crabs possess a defense mechanism called autotomy, allowing them to voluntarily shed an appendage when threatened or injured. This self-amputation occurs at a pre-determined fracture plane, minimizing blood loss and permitting escape. The lost limb can then regenerate over several molts.
Notable Exceptions and False Crabs
The ten-appendage rule for true crabs has several common exceptions that frequently cause public confusion. King crabs, such as the Alaskan King Crab, belong to a different infraorder, the Anomura, which includes hermit crabs. King crabs visibly display only eight appendages: one pair of large chelipeds and three pairs of walking legs. Their fifth and final pair of pereiopods is significantly reduced in size and tucked beneath the carapace, making it functionally insignificant for locomotion.
Hermit crabs, also Anomurans, adhere to the decapod count of ten. However, their final two pairs of legs are smaller and specialized to secure the crab within its shell. These small, often unseen legs prevent them from appearing to have ten legs like a true crab. The horseshoe crab is not a crustacean at all. Horseshoe crabs are chelicerates, making them more closely related to spiders and scorpions, and they possess six pairs of appendages on their cephalothorax.