Are Pleopods Just “More Legs”? The Functional Morphology of Swimming Limbs in Eurythenes Gryllus (Amphipoda)

Abstract

Amphipods, like most swimming crustaceans, employ a drag-based mechanism to produce thrust. The propulsors are paddle-shaped pleopods that move parallel to the direction of motion. These paired abdominal limbs generate both the propulsive thrust and the respiratory currents that bathe the thoracic gills. This study addresses the basic kinematics of motion and the pleopodal skeletomusculature of the deep-sea scavenger Eurythenes gryllus. The limb beat cycle consists of a power stroke where the three pleopod pairs, with their setal fan outstretched, swing sequentially through an arc parallel to the body axis, and then return anteriorly in a collapsed and bent configuration. The joint connecting the body to the muscular peduncle is complex, allowing promotion and remotion along the main body axis. Several hard plates for extrinsic muscle attachment are surrounded by arthrodial membrane. The extrinsic musculature is proportioned accordingly, with a large mass of muscles controlling the power stroke and a few long muscles generating the recovery stroke forces. The intrinsic musculature within the peduncle and annular rami serves two functions: (1) many long, thin muscle fibers within the peduncle function as support muscles responsible for shape changes; (2) large, strap-like muscles control flexion for the recovery stroke and abduction-adduction of the exopod for the power stroke. Several ancillary skeletal structures enhance swimming efficiency: reinforcement in the pleonal wall for muscle attachment, coupling hooks between the pleopod pair to effectively create a single paddle, the complementary shapes of the exopod and endopod, and an exopodal hook that facilitates the complete collapse of the pleopods on the recovery stroke. The functional design of the pleopod maximizes efficient recovery stroke motion while still providing strong remotion during the recovery stroke.