The hydrodynamics of hovering in Antarctic krill

Abstract

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.