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

To address the limitations of traditional propeller systems in low-speed maneuverability and adaptability to complex environments, bionic propulsion technologies inspired by the Median and/or Paired Fin (MPF) mode of fish offer a promising alternative. This study experimentally investigates the morphological characteristics and fin kinematics of the Black Ghost Knifefish, aiming to provide direct theoretical and data support for the efficient design and motion modeling of bionic undulating fins, thereby narrowing the performance gap between bio-inspired robotic prototypes and their biological counterparts. Detailed experimental analyses were conducted based on morphological parameters from 18 anesthetized specimens of different body lengths and 32 effective motion cases (covering nearly 2000 instantaneous moments), systematically establishing a comprehensive morphological and kinematic database for anal fin propulsion, thereby revealing the morphological characteristics and high-maneuverability locomotion traits of the Black Ghost Knifefish's anal fin. The results reveal that the anal fin exhibits an arched distribution with a streamlined profile. The ratio of maximum fin height to body height is approximately 0.24, and its growth pattern conforms to the criterion of local minimization of body drag. The fish achieves high maneuverability across various conditions by regulating the direction of wave propagation (forward/backward) and the undulation mode (unidirectional/bidirectional wave). Key kinematic parameters were extracted and analyzed via spatiotemporal Fourier transform. The results indicate 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 coordinately influences propulsion performance. The wave amplitude along the anal fin follows a typical arched non-uniform distribution, and the bidirectional wave propagation mechanism further enhances its high-maneuverability adaptation. This study provides valuable insights for understanding the propulsion mechanism of undulating fins and improving the performance of bio-inspired systems.