Beyond the Karman gait: knifefish swimming in periodic and irregular vortex streets [RESEARCH ARTICLE]

Victor M. Ortega-Jimenez and Christopher P. Sanford Neotropical freshwater fishes such as knifefishes are commonly faced with navigating intense and highly unsteady streams. However, our knowledge on locomotion in apteronotids comes from laminar flows, where the ribbon fin dominates over pectoral fins or body bending. Here, we studied the 3D kinematics and swimming control of seven black ghost knifefish (Apteronotus albifrons) moving in laminar flows (flow speed U~1 – 5 Bl/s) and in periodic vortex streets (U~2 – 4 Bl/s). Two different cylinders (~2 and ~3 cm diameter) were used to generate the latter. Additionally, fish were exposed to an irregular wake produced by a free oscillating cylinder (~2 cm diameter; U~2 Bl/s). In laminar flows knifefish mainly used their ribbon fin, with wave frequency, speed and acceleration increasing with U. In contrast, knifefish swimming behind a fixed cylinder increased the use of pectoral fins and resulted in changes in body orientation that mimicked steady backward swimming. Meanwhile, individuals behind the oscillating cylinder presented a combination of body bending, ribbon and pectoral fins movements that counteract the out-of-phase yaw oscillations induced by the irregular shedding of vortices. We corroborated passive out-of-phase oscillations by placing a printed knifefish model just downstream of the moving cylinder but, when placed one-cylinder diameter downstream, the model oscillated in phase. Thus, the wake left behind...

Source: Journal of Experimental Biology - April 1, 2021 Category: Biology Authors: Ortega-Jimenez, V. M., Sanford, C. P. Tags: RESEARCH ARTICLE Source Type: research