Primitive and modern swimmers solve the challenges of turning similarly to achieve high maneuverability

Turning maneuvers by aquatic animals are essential for basic life functions such as finding food or mates while avoiding predation. However, turning by animal swimmers requires resolution of a fundamental dilemma based in rotational dynamics: the torque powering a turn is favored by an expanded body configuration, yet minimizing the resistance to a turn (the moment of inertia) is favored by a contracted body configuration. How do animals balance these opposing demands to achieve high maneuverability? Here, we show that two animals spanning the evolutionary history of aquatic locomotion—the radially symmetric Aurelia aurita jellyfish with low tissue complexity, and the bilaterally symmetric Danio rerio zebrafish with advanced neuromuscular complexity—each exhibit a small, rapid shift in body kinematics preceding turning maneuvers. Although small in absolute magnitude, the high fluid accelerations achieved by this motion generate powerful pressure gradients that govern subsequent turning dynamics. This solution allows these animals to initially maximize torque production before the major body curvature change that subsequently minimizes the moment of inertia. The effectiveness of this solution for rotational motion, coupled with the pervasive demands of turning, suggests a key purpose for the near universal capability of swimmers to rearrange their mass by flexible bending. The subtleties of this pattern have previously been obscured by the conventional focus on parameters governing linear, unidirectional swimming, but they may prove critical for explaining the evolution of efficient aquatic locomotion employed by animals in their natural environments.