Meeting Abstract
Many members of the plankton rely on multi-oared propulsion. One species that travels 200,000 body lengths/day and form schools generally characterized by synchronized and polarized swimming is the Antarctic krill, Euphausia superba. We have investigated the coordination of krill 3D swimming behavior, biomechanics of locomotory appendages, and structure of hydrodynamic wake. Krill swim by multi-oared propulsion, beating five pairs of swimming legs metachronally. Metachronal swimming, in which adjacent appendages stroke in sequence, is widespread among crustaceans inhabiting the transitional flow realm in which both viscosity and inertia effects are important. The ratio of the distance between adjacent appendage bases and appendage length is identified as a key design parameter. Our drag coefficient model comparing metachronal, synchronous, and intermediate motions revealed metachronal kinematics give the highest average body speed for both linear and quadratic drag laws. Experimentally, we found that krill increase swimming speed first by increasing beat amplitude and secondarily by beat frequency. Our time-resolved tomographic PIV measurements of a hovering Antarctic krill reveal flow being drawn backwards with each pleopod stroke with vortices forming around each pleopod pair during the power stroke. Measurements in the wake of the krill reveal a pulsed jet flow with mean and oscillatory components. This wake signature may form a communication channel with nearby conspecifics. To determine the presence of structure within schools of krill, parameters such as density, polarity, nearest neighbor distance, and nearest neighbor position suggest an anisotropic school structure in which nearest neighbor positions are nonrandomly distributed.
