Analysis of the morphological characteristics of the Black Ghost Knifefish anal fin resulting in highly maneuverable locomotion

Bionic propulsion technologies inspired by the median and/or paired fin of fish offer a promising method of addressing the limitations of traditional propeller systems in terms of low-speed maneuverability and adaptability to complex environments. This study experimentally investigated the morphological characteristics and fin kinematics of the black ghost knifefish to provide direct theoretical and data support for the design and motion modeling of bionic undulating fins. We analyzed the morphological parameters of 18 anesthetized black ghost knifefish specimens of different body lengths and 32 effective motion cases (covering nearly 2000 instantaneous moments) to systematically establish a comprehensive morphological and kinematic database for anal fin propulsion. The anal fin exhibited an arched distribution with a streamlined profile. The ratio of maximum fin height to body height was approximately 0.24, and its growth pattern conformed to the criterion of local minimization of body drag. We found that the fish achieves high maneuverability across conditions by regulating the direction of wave propagation (forward/backward) and undulation mode (unidirectional/bidirectional wave). Subsequently, key kinematic parameters were extracted and analyzed by spatiotemporal Fourier transform. The results indicated that wave frequency serves as the primary control variable for cruise speed, while wave speed, wavelength, and wave number collectively form an interrelated operational range that influence propulsion performance. The wave amplitude along the anal fin followed a typical arched nonuniform distribution, and the bidirectional wave propagation mechanism further enhanced high-maneuverability adaptation. Overall, these findings provide valuable insights into the propulsion mechanism of undulating fins that could narrow the performance gap between bioinspired robotic prototypes and their biological counterparts.