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.