Relationship between caudal fin closing motion and acceleration capability of Rudarius ercodes balistiform locomotion

Fish leverage the interaction of multiple fins to improve their swimming ability. Acceleration performance, in particular, is a key aspect of locomotion that directly affects survival through predator evasion and prey capture. Most of these studies have focused on body and caudal fin (BCF) swimmers, which are specialized for high-speed locomotion. In contrast, relatively few studies have examined median and paired fin (MPF) swimmers, and fins' functional roles under different locomotor modes remain insufficiently understood. Therefore, MPF swimmers may employ acceleration strategies distinct from those of BCF swimmers. Understanding the effects of fin interaction on acceleration performance in MPF swimming can contribute to a unified understanding of how such interactions influence swimming performance across fishes with different locomotor modes. A type of MPF swimmer is the balistiform, which uses its dorsal and anal fins as the main propulsion organs. We observed the closing of the caudal fins in Rudarius ercodes in balistiform locomotion during acceleration. We hypothesized that R. ercodes increased their average acceleration (the time taken to reach the experimentally observed velocity) by closing their caudal fins. We performed water tank observations to capture swimming behavior and three-dimensional computational fluid dynamics (3D-CFD) analysis to clarify the impact of caudal fin opening and closing on acceleration capability in balistiform locomotion. For example, parameters such as swimming speed and caudal fin spreading angle were measured in the swimming observations. In contrast, the fluid dynamic analysis computed the vortex structures, propulsive efficiency (the ratio of input energy contributing to thrust), and the cost of transport (the energy required to travel a unit distance). Our analysis showed that closing the caudal fin increased the average acceleration by 30%, increased the cost of transport, and decreased the dimensionless Froude efficiency. We also clarified the role of the caudal fin in MPF locomotion straight-line swimming. Our findings will help us better understand how fin interactions affect fish's swimming ability.