The relationship between torque and body shape of maneuvering swimmers*

Animals in nature rarely swim in a straight, steady-state manner. Therefore, understanding how animals swim requires that we also understand how they maneuver. However, in a laboratory setting, it is challenging to capture and measure maneuvering animals, so limited data are available beyond steady forward swimming. To address this gap, we examined how two different species jellyfish (a primitive, non-vertebrate swimmer) and zebra fish (a higher order, vertebrate fish), complete turns ranging from 15 to 60 degrees. Using high-speed video and particle image velocimetry we quantified the kinematics and hydrodynamics of turning in multiple individuals from both species. We found that for the jellyfish the portion of the medusan bell located at the inside of the turn always initiates bell contraction. These asymmetric bell contractions set up pressure fields around the bell to generate imbalanced forces.  Similarly, the zebra fish setup a pressure field that produces a turning torque before changing their body shape to minimize the moment of inertia and continue the turn passively. The temporal relationship between torque production and body shape have similar trends across turns, individuals and species.