pereopods

Pereopods are the thoracic walking legs of crustaceans, especially the malacostracan group that includes shrimps, lobsters, and crabs. They arise on the thorax (the pereon) and, in most decapods, there are five pairs. These legs are primarily used for locomotion along the substrate and for manipulating objects on the seafloor or in aquaria. In many species, the first pair of pereopods is enlarged and chelate, forming a claw or pincers for grasping prey, defense, or handling large items. The remaining pairs are typically slender and suited for walking or stabilization.

Anatomically, pereopods are segmented limbs. In decapods the leg is composed of multiple podomeres that provide flexible joints, enabling precise movement and control. Among the pairs, variation in size and robustness often reflects ecological adaptation: robust first legs in crabs and other benthic crustaceans versus more uniform, slender legs in some shrimps.

Function and diversity: Pereopods support life across a range of crustaceans, from shore-dwelling crabs to open-water shrimp. While walking is their primary function, they can also assist in feeding, digging, grooming, or holding mates in some species. The exact morphology and degree of specialization of the pereopods vary with taxon and lifestyle.

Relation to other appendages: Pereopods are distinct from pleopods, which are abdominal swimming legs, and from maxillipeds, which are feeding appendages on the head. The term can also be found as pereiopods in some literature. Pereopods are a key diagnostic feature in many crustacean groups, helping distinguish thoracic walking limbs from other limb types.