Finlet dynamics in tuna give insights into control mechanisms for enhanced propulsion

Tunas exhibit remarkable morphological adaptations that enable exceptional kinematic performance, such are finlets, which are a series of small non-retractable fins located on the upper and lower edges of the fish’s body. Despite their importance, the underlying hydrodynamic functions and mechanisms governing finlet motion remain inadequately characterized, presenting substantial challenges in this line of research.

To investigate the control mechanisms of finlet pitching motion, Huang et al. developed three distinct models of tuna finlets. By integrating numerical simulations with empirical measurements, the researchers proposed an active control model that emulates the neuromuscular control observed at the finlet root.

“Tunas are fascinating creatures with exquisite morphology to achieve remarkable kinematical performance, just like sports cars,” author Wei-Xi Huang said. “Due to the challenge of many aspects of their hydrodynamic functions, we wanted to establish an adequate model to uncover the pitching mechanism of the finlets.”

Prior observations of free-swimming tuna indicate that finlet pitching generally conforms to local flow dynamics, suggesting a passive model might describe this behavior. However, this research introduces new insights into the active control mechanisms involved in finlet locomotion.

“We have, for the first time, found quantitative evidence of active control of tuna finlets from a fluid mechanics perspective,” Huang said.

Moving forward, the researchers aim to simulate tuna swimming at higher Reynolds numbers within turbulent flow conditions, an essential progression given that most existing studies have focused on reduced Reynolds numbers in laminar regimes. This research not only lays a robust foundation for future studies on tuna locomotion but also has significant implications for the design of bio-inspired underwater vehicles.

Source: “An active torque model for regulating tuna finlets,” by Jun-Duo Zhang, Peng Han, Chun-Xiao Xu, Hyung Jin Sung, and Wei-Xi Huang, Physics of Fluids (2025). The article can be accessed at https://doi.org/10.1063/5.0231384 .