Sea Angel Swimming at Intermediate Reynolds Numbers

Sea angels are shell-less holoplanktonic marine snails (i.e. gymnosomatous pteropods) which swim by flapping a pair of wing-like parapodia. Sea angels swim using these low aspect ratio, highly flexible wings in order to migrate, search for food, and avoid predators. However, the fluid dynamics of their swimming is not well understood. Here we study the locomotion of a subtropical sea angel Pneumoderma atlantica collected in Bermuda. This species has a body length up to 13.1 mm, wing span up to 5.2 mm, and beats its wings at frequencies up to 4.3 Hz. It swims upwards in sawtooth trajectories at an intermediate Reynolds numbers of about 400. Using high speed cameras, we measure the 3D wing and body kinematics and compare them to a polar species of sea angelClione antarctica, which inhabits water almost twice as viscous as the subtropical species. We also present time-resolved 2D micro-PIV measurements of the swimming of P.atlantica and show that this species generates lift by performing a version of the cylindrical overlap-and-fling maneuver in which its wings are pulled close to its body at the end of each power and recovery stroke.