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_&[infin] ~ 1 &[minus] 5 Bl/s) and in periodic vortex streets (U_&[infin] ~ 2 &[minus] 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_&[infin] ~ 2 Bl/s). In laminar flows knifefish mainly used their ribbon fin, with wave frequency, speed and acceleration increasing with U_&[infin]. 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 an oscillating body is more challenging than a periodic vortex shedding for an animal located downstream, which may have consequences on inter- and intra-specific interactions.

VideoS1.mp4 [(00:05 s) Knifefish swimming in laminar conditions. (00:16 s) Fish swimming behind a large and small fixed cylinders. (00:36 s) Fish swimming in laminar, behind the small fixed cylinder and behind the oscillating cylinder. (01:15s) Fish showing passive movements in the wake of an oscillating cylinder. (01:31 s) 3D printed fish model in both the wake of a fixed cylinder and the wake of an oscillating cylinder. (01:49 s) Instantaneous vorticity fields of the ribbon fin of a knifefish swimming in laminar conditions, behind the fixed cylinder and behind the oscillating cylinder].

DataSet_OrtegaJimenezSandfordKnifefish.xls [Datasets for each fish (N=7) including all kinematic variables used for statistical analysis]

RawData_OrtegaJimenezSandfordKnifefish..xlsx [3D digitized points in Cartesian system XYZ in cm of seven ghost knifefish swimming in laminar and unsteady flow conditions. Point 1 (head), Point 2 (tail tip), Point 3 (Left-pectoral fin tip), Point 4 (Right-pectoral fin tip), Point 5 (Left-pectoral fin base), Point 6 (Right-pectoralfin base), Point 7 (Ribbon fin tip at the midline), Point8 (trough tip formed by the ribbon fin wave), Point 9 (back-middle point), Point 10 (Ribbon fin base at the midline).Each fish (n=7) is order sequentially for each condition. LAMINAR CONDITIONS: -1.4 Bl/s (sheet 1to 7), 1.4 Bl/s (sheet 8 to14) 1.8 Bl/s (sheet 15 to 21), 2.5 Bl/s (sheet 22 to 28), 3 Bl/s (sheet 29 to 35), 3.6 Bl/s (sheet 36 to 42), 3.9 Bl/s(sheet 43 to 49), 4.3 Bl/s(sheet 50 to 55), and 4.6 Bl/s. (sheet56 to 57). SMALL CYLINDER CONDITIONS: 1.8 Bl/s (sheet 58 to 64), 2.5 Bl/s (sheet 65 to 71), 3 Bl/s (sheet 72 to 78 and 3.6 Bl/s (sheet 79 to 85). LARGE Cylinder Conditions: 1.8 Bl/s (sheet 86 to 92), 2.5 Bl/s (sheet 93 to 99). OSCILLATING CYLINDER CONDITION: 1.8 Bl/s (sheet 100 to 106). Digitalization software is described in: Hedrick, T. L. (2008). Software techniques for two- and three-dimensional kinematic measurements of biological and biomimetic systems. Bioinspir. Biomim. 3, 034001].