Metachronal swimming in Antarctic krill: gait kinematics and system design

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. However, the design and operation of this propulsion system in response to various hydrodynamic, energetic, and behavioral needs have not been well investigated. We examine free-swimming Antarctic krill (Euphausia superba) as a model species and identify three distinct behavioral swimming gaits. The pleopod kinematics of these gaits, hovering, fast-forward swimming, and upside-down swimming, are quantified via image analysis of high-speed video. Pleopod stroke amplitude and frequency were found to vary significantly among these swimming modes. In order to increase swimming speed, krill were found first to increase stroke amplitude and secondarily to increase beat frequency. The kinematics of these distinct swimming modes provide insight as we consider multi-appendage metachronal swimming from a design standpoint. The ratio of the distance between adjacent appendage bases and appendage length is identified as a key parameter in metachrony, the value of which is constrained to a narrow range for a wide variety of species.