In some cases, being passive gives fish an advantage

Some of the fastest fish in the ocean, including jackfish and barracudas, are part of a group known as the carangiforms. These fish have powerful tail fins, or caudal fins, that help propel them to immense speeds. Understanding the mechanics of these caudal fins can help researchers design more efficient underwater robots.

Nayak et al. used computational modeling to study the caudal fin dynamics of carangiform swimmers, examining the impact of the body-tail joint in determining their hydrodynamic performance.

Carangiforms are not only fast but also efficient. These fish have had to develop creative strategies to conserve energy while maintaining their impressive thrust.

“One possible strategy is to reduce active muscle input and allow the surrounding fluid forces to drive the motion of the caudal fin,” said author Muhammad Khalid. “Such passive or semi-active behavior can contribute to efficient thrust generation when properly synchronized with the flow.”

The authors used computational modeling to evaluate a proposed two-phase swimming motion with an active “heaving” phase followed by a passive “pitching” phase. They used their model to compare the performance with an approach where both phases are active. They found that the passive approach, featuring a caudal fin with nonlinear stiffness, outperformed the active model. In particular, they found that a phase of approximately 90 degrees between synchronous passive pitching and active heaving yielded the best performance.

“Our analysis shows how favorable phase synchronization leads to constructive interaction between the shed vortices and the caudal fin motion, thereby improving thrust generation,” said Khalid.

The authors plan to continue their work by investigating the dynamics of the caudal fin with multiple swimmers in schooling configurations.

Source: “Dynamical characteristics of the body-caudal fin joint of a carangiform swimmer and its influence on hydrodynamics,” by Dev Pradeepkumar Nayak, Muhammad Saif Ullah Khalid, and Ali Tarokh, Physics of Fluids (2026). The article can be accessed at https://doi.org/10.1063/5.0324024 .