Burst-and-coast swimming is not always energetically beneficial in fish (Hemigrammus bleheri)

Burst-and-coast swimming is an intermittent mode of locomotion used by various fish species. The intermittent gait has been associated with certain advantages such as stabilizing the visual field, improved sensing ability, and reduced energy expenditure. We investigate burst-coast swimming in rummy nose tetra fish (Hemigrammus bleheri) using a combination of experimental data and numerical simulations. The experiments were performed in a shallow water channel where the tetra fish swam against an imposed inflow. High speed video recordings of the fish were digitized to extract the undulatory kinematics at various swimming speeds. The kinematics data were then used in Navier-Stokes simulations to prescribe the undulatory motion for three-dimensional geometrical models of the fish. The resulting steady-state speeds of the simulated self-propelled swimmers agree well with the speeds observed experimentally. We examine the power requirements for various realistic swimming modes, which indicate that it is possible to use continuous swimming gaits that require considerably less mechanical energy than intermittent burst-coast modes at comparable speeds. The higher energetic cost of burst-coast swimming suggests that the primary purpose of intermittent swimming may not be to conserve energy, but it may instead be related to a combination of other functional aspects such as improved sensing and the likely existence of a minimum tail-beat frequency. Importantly, using sinusoidal traveling waves to generate intermittent and continuous kinematics, instead of using experiment-based kinematics, results in comparable power requirements for the two swimming modes.