Decapods, an order of crustaceans including crabs, lobsters, and shrimp, are characterized by having ten legs. The name Decapoda is derived from the Greek words for “ten” and “foot.” These ten legs are attached to the cephalothorax, the animal’s fused head and mid-body section. Understanding the functions of these five pairs of thoracic legs clarifies their anatomy.
Defining the Ten Legs: Pereopods
The five pairs of appendages comprising the ten legs of decapods are collectively known as pereopods, or walking legs. They are attached to the five thoracic segments and are numbered P1 through P5, starting from the front. Pereopods are strong, segmented limbs primarily used for locomotion and support across various substrates. They are also utilized for manipulating food and grooming the body.
The Five Specialized Pairs and Their Roles
The First Pair: Chelipeds (P1)
The first pair of pereopods (P1) is the most specialized and is named chelipeds. These appendages are modified into large claws or pincers (chelae), used for defense, aggressive interactions, and capturing prey.
In species like lobsters and some crabs, the two chelae can be different sizes, resulting in a “crusher” claw and a smaller “cutter” claw. The crusher generates force to break hard shells, while the cutter is used for finer manipulation, feeding, and grooming. This asymmetry is often more pronounced in males, playing a role in courtship and displays.
Pereopods 2 through 5 (P2-P5)
The remaining four pairs of legs (P2-P5) primarily serve as walking legs. In many shrimp and prawns, the second and third pairs (P2 and P3) are also chelate, possessing small pincers used for gathering food and cleaning. These smaller chelae often have specialized setae that help filter debris.
The final two pairs (P4 and P5) are typically simple, non-chelate walking legs. In some groups, like hermit crabs, the fifth pair (P5) is reduced and tucked inside the shell, where it is used for specialized tasks such as cleaning the gills.
Appendages Not Included in the Five Pairs
Decapods possess several other appendages that are not considered part of the ten legs.
Maxillipeds
Located immediately in front of the pereopods are three pairs of appendages called maxillipeds. These are attached to the thoracic segments but function as mouthparts, primarily assisting in handling and directing food toward the mouth.
Pleopods and Uropods
The abdomen, or pleon, bears five pairs of appendages known as pleopods, or swimmerets. Unlike the pereopods, pleopods are used for swimming, generating a current of water over the gills for respiration, and in females, for carrying and brooding eggs. The final pair of abdominal appendages are the uropods, which, along with the telson, form the tail fan used for rapid backward movement.