MCHENRY, M.J.: Force generation by undulatory swimming at intermediate Reynolds numbers in tunicate larvae (Botrylloides, sp.)

Despite recent advances in computational modeling and flow visualization, the hydrodynamics of swimming animals remains a subject of debate. This is certainly the case for undulatory swimmers operating at intermediate Reynolds numbers (Re), where both inertial and viscous forces are important. In the present study, propulsive forces were measured in both tethered and freely swimming ascidian ‘tadpole’ larvae (Botrylloides sp.), which swim at Re~100. These 2mm long larvae were tethered to micro-capillary tubes with light suction. By measuring the stiffness of this tether, the total force generated by the larval body was calculated from tether deflection measurements. Forces measured in tethered larvae were greatest in the lateral direction and were found to vary between 1 and 40 MU N among individuals. These measurements were compared to theoretical predictions for lateral forces by applying blade element methods to the axial kinematics of the tail. Predicted forces were also compared to the centripetal forces generated by freely swimming larvae recorded in three dimensions with high-speed video. These measurements will provide the basis for a more comprehensive treatment of the dynamics of helical swimming in larvae of ascidians and other marine invertebrates.