Published in the journal\u00a0Ocean, the study led by Ze-Jun Liang and colleagues from Northwestern Polytechnical University\u2019s Ocean Institute in China provides the most detailed analysis to date of the knifefish\u2019s anal fin morphology and kinematics. Their findings challenge conventional assumptions about fish locomotion and lay the groundwork for a new generation of bio-inspired propulsion systems.<\/p>\n
\u201cTraditional propeller-based systems struggle with low-speed maneuverability and stability in complex environments,\u201d said corresponding author Peng Xu. \u201cBy contrast, the knifefish achieves precise control using undulations of its anal fin\u2014a flexible, elongated structure that generates traveling waves to produce thrust without body bending. Understanding this mechanism is key to overcoming the limitations of current underwater vehicles.\u201d<\/p>\n
The researchers studied 18 live specimens, capturing nearly 2,000 instantaneous motion cases with high-speed cameras. They found that the anal fin follows an arched, streamlined profile, with a maximum fin height-to-body height ratio of approximately 0.24\u2014a design that minimizes body drag. More importantly, the fish achieves its signature agility by dynamically controlling the direction of wave propagation and the undulation mode.<\/p>\n
Unlike most fish that rely on a single wave traveling from head to tail, the knifefish can generate waves that move forward, backward, or even in opposite directions simultaneously. When two counter-propagating waves meet, they create a \u201cnode\u201d where forces cancel, allowing the fish to hover or make rapid direction changes without turning its body.<\/p>\n
\u201cWhat\u2019s striking is that the fish can maintain a rigid body posture while swimming, which reduces drag and simplifies the engineering challenge of building robotic systems,\u201d said co-author Yi-Wei Fan. \u201cThis ability to independently control wave parameters offers a new paradigm for propulsion\u2014one that decouples thrust generation from body bending.\u201d<\/p>\n
Using spatiotemporal Fourier transform analysis, the team extracted key kinematic parameters, including wave frequency, speed, wavelength, and amplitude. Their results show that wave frequency is the primary control variable for cruising speed, while wave speed, wavelength, and wave number form an interrelated operational range that collectively governs propulsion performance.<\/p>\n
\u201cWe observed a clear functional relationship between swimming speed and undulation parameters,\u201d explained co-author Dong-Yang Chen. \u201cWave frequency emerged as the most reliable predictor of speed, while amplitude and wave number remained relatively stable across different swimming conditions. This suggests that the fish modulates its motion by fine-tuning frequency, much like a musician adjusting tempo.\u201d<\/p>\n
The study also revealed that the undulation amplitude along the fin follows an asymmetric, arched distribution\u2014smaller at both ends and larger in the middle\u2014which contributes to efficient thrust generation. This non-uniform pattern contrasts sharply with the simplified constant-amplitude models used in most existing robotic prototypes.<\/p>\n
\u201cMost bio-inspired undulating fin designs to date have relied on idealized rectangular fins and constant-amplitude undulations,\u201d said lead author Ze-Jun Liang. \u201cOur findings show that real biological systems are far more sophisticated. By incorporating the morphological and kinematic synergies we\u2019ve identified, we believe robotic propulsion efficiency and maneuverability can be significantly improved.\u201d<\/p>\n
The researchers note that the black ghost knifefish is an ideal model not only for its hydrodynamic prowess but also for its neurobiological significance. As a weakly electric fish, it relies on a rigid body to minimize electric field distortion during electrolocation. Its propulsion system must therefore generate thrust without compromising sensory capabilities\u2014a constraint that aligns closely with the needs of sensor-laden autonomous underwater vehicles.<\/p>\n
Looking ahead, the team plans to apply these insights to the development of a new class of undulating-fin robots. Their next steps include translating the kinematic database into control algorithms and testing prototype designs in real-world aquatic environments, including turbulent flows and confined spaces.<\/p>\n
\u201cOur ultimate goal is to create underwater vehicles that can operate with the same efficiency and agility as the knifefish,\u201d said Xu. \u201cThis would open up new possibilities for inspection, exploration, and search-and-rescue missions in complex underwater environments where traditional propellers fall short.\u201d<\/p>\n
The study was supported by the Postdoctoral Innovation Fund of Northwestern Polytechnical University Taicang Yangtze River Delta Research Institute, the National Natural Science Foundation of China, and other research programs.<\/p>\n","protected":false},"excerpt":{"rendered":"Published in the journal\u00a0Ocean, the study led by Ze-Jun Liang and colleagues from Northwestern Polytechnical University\u2019s Ocean Institute…\n","protected":false},"author":2,"featured_media":394030,"comment_status":"","ping_status":"","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[7],"tags":[85,46,141],"class_list":{"0":"post-394029","1":"post","2":"type-post","3":"status-publish","4":"format-standard","5":"has-post-thumbnail","7":"category-science","8":"tag-il","9":"tag-israel","10":"tag-science"},"_links":{"self":[{"href":"https:\/\/www.newsbeep.com\/il\/wp-json\/wp\/v2\/posts\/394029","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.newsbeep.com\/il\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.newsbeep.com\/il\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/il\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/il\/wp-json\/wp\/v2\/comments?post=394029"}],"version-history":[{"count":0,"href":"https:\/\/www.newsbeep.com\/il\/wp-json\/wp\/v2\/posts\/394029\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/il\/wp-json\/wp\/v2\/media\/394030"}],"wp:attachment":[{"href":"https:\/\/www.newsbeep.com\/il\/wp-json\/wp\/v2\/media?parent=394029"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.newsbeep.com\/il\/wp-json\/wp\/v2\/categories?post=394029"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.newsbeep.com\/il\/wp-json\/wp\/v2\/tags?post=394029"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}