The hydrodynamics of hovering in Antarctic krill

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. Negatively buoyant pelagic animals such as the Antarctic krill ( Euphausia superba ) must maintain their position in the water column through a constant downward transfer of momentum to the surrounding fluid. Antarctic krill, which operate in a low‐to‐intermediate Reynolds number regime, hover by beating their pleopods (or swimming legs) in a metachronal wave from back to front. The objective of this paper is to examine how hovering in Antarctic krill is facilitated by the flow produced by a metachronal stroke pattern. A high‐speed tomographic particle image velocimetry system was used to measure both the flow around the pleopods and in the wake. The flow measurements and actuator disk theory were used to estimate the energy required for hovering in Antarctic krill. Lift‐generating tip vortices were found on the pleopod exopodites. These vortices, as well as pleopod spacing and exopodite kinematics, integrate the design and kinematics of the appendages with the resulting flow to make the metachronal swimming system used by the krill an effective tool to generate lift for hovering. The Strouhal number ( St ) of most drag‐based paddlers, such as the Antarctic krill, was found to fall within the range of 0.2< St <0.4. Whereas it is known that an efficiency peak for lift‐based locomotion lies in this St range, it is hypothesized here that a similar efficiency peak exists for metachronal drag‐based locomotion.