Propulsive design principles in a multi-jet siphonophore.

Coordination of multiple propulsors can provide performance benefits in swimming organisms. Siphonophores are marine colonial organisms that orchestrate the motion of multiple swimming zooids for effective swimming. However, the kinematics at the level of individual swimming zooids (nectophores) have not been examined in detail. We used high-speed, high-resolution microvideography and particle image velocimetry of the physonect siphonophore Nanomia bijuga to study the motion of the nectophores and the associated fluid motion during jetting and refilling. The integration of nectophore and velum kinematics allow for a high-speed (maximum ∼1 m s-1), narrow (1-2 mm) jet and rapid refill, as well as a 1:1 ratio of jetting to refill time. Scaled to the 3 mm nectophore length, jet speeds reach >300 lengths s-1 Overall swimming performance is enhanced by velocity gradients produced in the nectophore during refill, which lead to a high-pressure region that produces forward thrust. Generating thrust during both the jet and refill phases augments the distance traveled by 17% over theoretical animals, which generate thrust only during the jet phase. The details of velum kinematics and associated fluid mechanics elucidate how siphonophores effectively navigate three-dimensional space, and could be applied to exit flow parameters in multijet underwater vehicles.