Hydrodynamic roles of oral arms and tentacles in jellyfish locomotion

Jellyfish have emerged as compelling models for bioinspired underwater propulsion due to their highly efficient swimming and simple body architecture. While the vortex ring dynamics by jellyfish bell contractions have been extensively characterized, the hydrodynamic functions of anatomical structures such as the oral arms and tentacles remain poorly understood. In this study, we investigate how these trailing appendages affect swimming performance and vortex dynamics in Aurelia aurita. We conducted comparative experiments on jellyfish with and without oral arms and found that removing the oral arms led to a significant increase in swimming speed. Using particle image velocimetry (PIV), we visualized the vortex dynamics in both intact and modified jellyfish. The results reveal that vortex rings generated during bell contractions impinge on the oral arms, introducing additional drag and reducing propulsion efficiency. We further investigated the hydrodynamic role of tentacles using PIV to show that tentacles can substantially influence vortex formation and alter the trajectory of the vortex ring. Experiments using a biomimetic flapping fin, with and without tentacle-like filaments, demonstrated that tentacles can enhance thrust production while reducing drag during the relaxation phase of the swimming cycle. This work advances our understanding of fluid-structure interactions in jellyfish and informs the design of bioinspired propulsion strategies in unsteady aquatic environments.