Hydrodynamics of active metachronal swimming modes in Antarctic krill, Euphausia superba

Abstract Krill propel themselves by paddling their five sets of pleopod appendages in a coordinated metachronal wave that generates a fluid wake jet and corresponding forward thrust. In this study, three-dimensional velocity and vorticity fields are measured for fast forward (FFW) swimming and upside-down (USD) swimming modes in Antarctic krill ( Euphausia superba ) using high-speed tomographic Particle Image Velocimetry (tomo-PIV). The results provide novel insight into the bio-locomotion of krill and the related fluid flow in the intermediate Reynolds number regime. The specimens move rapidly in each quantified mode: 2.1 body lengths per second (BL s −1 ) for fast forward (FFW) swimming and 2.8 BL s −1 for upside-down (USD) swimming. When comparing fast forward (FFW) swimming and (previously reported) hovering (HOV) flow results, the krill body speed has a near eight-fold increase, whereas the flow field data reveal only a two-fold increase in the fluid velocity around the pleopods during the power stroke and in the wake jet. Also, vortices are observed at the distal tips of the pleopods, which further supports the hypothesis that the pleopods generate lift force to complement the drag-based propulsion of the pleopod stroke.