Swimming isn't such a drag: How the coalescence and flexibility of shrimp pleopods minimize drag during metachronal swimming

Metachronal swimming is characterized by the sequential beating of closely spaced flexible swimming legs (pleopods), phase-shifted in time. The stiffness and increased surface area of pleopods during the power stroke have been shown to maximize thrust. However, their characteristic bending and associated fluid flow effects during the recovery stroke have received far less attention despite being antagonistically important in reducing drag. By combining measurements of shrimp pleopod stiffness with kinematics and particle image velocimetry, we explore the relationship between the mechanical properties of pleopods and organism-fluid interactions. Unrecognized in previous works, we show that pleopods bend almost horizontally and shed no observable tip vortices during the recovery stroke. At Re = 1000, shrimp essentially trade inertial drag forces for much weaker viscous forces. Other species exhibit similar pleopod bending, suggesting a standard stiffness coefficient for efficient metachronal swimming. Further, due to their proximity, three out of five pleopods coalesce at any time during a complete cycle, effectively reducing the drag of three legs to only one. Considering appendage stiffness opens new avenues for the design of novel, more capable multi-functional underwater robots.