The Brine Shrimp's Butterfly Stroke
  We investigate the fluid dynamics of brine shrimp larvae swimming in this gallery of fluid motion video. Time resolved particle image velocimetry was performed using nano-particles as seeding material to measure the time dependent velocity and vorticity fields. The Reynolds number of the flow was roughly 8 and the Womerseley number (ratio of periodic forcing to viscous forcing) was about 5. Vorticity dynamics reveals the formation of a vortex ring structure at the tip of each arm at the beginning of the power stroke. This two vortex system evolves dramatically with time as the stroke progresses. The outer circulation is noted to weaken while the inner circulation strengthens over the power stroke. The gaining strength of the inner vortex correlates with the acceleration and forward movement of the larvae. 
Brennan Johnson, Deborah Garrity, Lakshmi Prasad Dasi
physics.flu-dyn