Knifefish turning performance during forward swimming

Rapid turning and swimming contribute to ecologically important behaviors in fishes such as predator avoidance, prey capture, mating, and the navigation of complex environments. For riverine species, such as knifefishes, that are commonly challenged by turbulent flows, turning control may be effective for counteracting adverse locomotive perturbations. Most research on fish maneuvering focuses on fish with traditional fin and body morphologies, which primarily use body bending and the pectoral fins during turning. However, it is uncertain how fishes with uncommon morphologies, are able to achieve sudden and controllable turns. Here we studied the turning performance and the turning hydrodynamics of the Black ghost knifefish (Apteronotus albifrons, N=6) which has an atypical elongated ribbon fin. Fish were filmed while swimming forward at ~2 BL s-1 and feeding from a fixed feeder (control) and an oscillating feeder (75 Hz) at two different amplitudes. 3D kinematic analysis of the body revealed the highest pitch angles and lowest body bending coefficients occurred during steady swimming. Low pitch angle, high maximum yaw angles and large body bending coefficients were characteristic of small and large turns. Asynchrony in pectoral fin use was low during turning, however ribbon fin wavelength, frequency, and wave speed were greatest during large turns.  Digital particle image velocimetry (DPIV) showed larger counter-rotating vortex pairs produced during turning by the ribbon-fin in comparison to vortices rotating in the same direction during steady swimming. Our results highlight the ribbon fin’s role in controlled rapid turning through modulation of wavelength, frequency, and wave speed.

Methods
This dataset was collected using 3D highspeed videography filming at 250 frames per second in the lateral and ventral views. Data analyzed using Matlab and R.