Swimming across scales: shrimp kinematics and hydrodynamics across Reynolds numbers

Metachronal swimming is a widespread propulsion method used by diverse and ecologically important animals operating at intermediate Reynolds (Re) numbers. Its ubiquity suggests that selective pressures on performance and efficiency have favored it over other swimming modes. Interspecific comparisons can reveal traits linked to swimming performance, but are often confounded by differences in morphology. Shrimp are an ideal model organism to compare metachronal swimming across scales due to their similar body plans and consistent pleopod morphology. We hypothesize that metachronal swimming will be sensitive to changes in morphology (e.g., setal density) and kinematics (e.g., beat frequency, amplitude) that reflect adaptation to local hydrodynamic conditions. Using three species and a juvenile of the smallest species, we sampled four size scales with pleopod Re ranging from ∼50 to ∼1700. We performed particle image velocimetry on freely swimming individuals, measured key morphological traits, tracked swimming kinematics, and calculated wake momentum. Our results identify both shared strategies and scale-dependent differences in the structure and motion of the pleopods, advancing a unifying framework for understanding the principles of metachronal propulsion.