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

Krill are shrimp-like crustaceans with a high degree of mobility with reported swimming modes including fast forward swimming (FFW), hovering (HOV), and upside down swimming (USD). Krill propel themselves by paddling their five sets of pleopod appendages in a coordinated metachronal wave that generates a fluid wake jet. In this study, 3D velocity and vorticity fields are measured for FFW and USD swimming modes in Antarctic krill (Euphausia superba) using high-speed tomographic Particle Image Velocimetry (tomo-PIV). The results give novel insight to bio-locomotion of krill and the related fluid flow in this intermediate Reynolds number regime. The specimens are moving rapidly in each quantified mode: 2.1 body lengths per second (BL/s) for FFW and 2.8 BL/s for USD. When comparing FFW and (previously reported) HOV results, the krill body velocity has a near eight-fold increase, whereas the flow field data reveal only two-fold increase in the wake velocity around the pleopods during the power stroke. Also, lift-force-generating vortices are observed at the distal tips of the pleopods in each mode, which further supports the hypothesis that vortex formation generates lift force to complement the drag-based propulsion of the pleopod stroke.