Swimming by medusae Sarsia tubulosa in the viscous vortex ring limit

As organisms swim in their natural environment, they are constantly striving to forage successfully, escape from predation, and search for mates to reproduce. At some stage in their life cycle, most organisms have operated in environments where the Reynolds number (Re) is small and have developed strategies and behaviors to overcome the effects of viscosity. Relatively little is known about these animal-fluid interactions at relatively small (1 < Re < 10), viscous size scales. Swimming organisms have been described analytically using the self-propelled swimmer model, which applies for conditions where the organism is assumed to swim steadily in a noninertial fluid regime or Re <  1. However, for unsteady swimming processes, such as jumping or jet propulsion, these steady models do not account for the impulsiveness of the swimming behavior. The unsteady impulsive Stokeslet and impulsive stresslet models have been used to describe jumping by copepods, but neither model has been applied to jetting organisms. The purpose of this study is to identify which analytical, unsteady model best describes swimming by jetting organisms at small, viscous length scales. We conducted high-speed kinematic and velocity field measurements on 1-mm velar-diameter Sarsia tubulosa, a jetting, ambush-feeding medusa. From our measurements and comparisons using similar criteria established for copepod jumping, we conclude that the impulsive Stokeslet model more accurately describes swimming by small S. tubulosa. Since the hydrodynamic signature of an impulsive Stokeslet does not have strong vorticity bounding the medusa's body, this finding has important ecological implications for the ambush-feeding predator.