The hydrodynamics of hovering in Antarctic krill

Lay Abstract Antarctic krill, one of the most important species of the Southern Ocean ecosystem, are denser than water and must swim continuously to avoid sinking. They swim by beating their five pairs of swimming legs in a sequential pattern from back to front. Hovering by continuous swimming is costly in energy, and we hypothesize that the observed sequential stroking pattern provides an efficient means for krill to remain in the water column. Our goal was to measure the flow around a swimming Antarctic krill to understand the swimming mode and the induced water motion. We used four high‐speed cameras filming at 400 frames per second to measure the three‐dimensional flow produced by a krill hovering in an aquarium, allowing us to estimate the energy required. An additional estimate was made based on a theoretical model that is usually applied to helicopter hovering. The detailed velocity measurements provided information about the flow induced by the motion of the individual legs and the added benefit of the leg moving into water that was already flowing due to the motion of the previous leg. The water motion underneath the krill appeared as a time‐varying jet consistent with that observed for other multilegged swimming animals that use the sequential stroking pattern. This suggested that Antarctic krill are operating in a similar regime of high energy efficiency.