Median fins of fish-like swimmers play a crucial role in linear acceleration and maneuvering processes. However, few research focused on untethered robotic fish experiments. Imitating the behaviour of real tuna, we developed a free-swimming bionic tuna with a foldable dorsal fin. The erection of dorsal fin, at proper conditions, can reduce head heave by 50%, enhance linear acceleration by 15.7%, increase turning angular velocity by 32.78%, and turning radius decreasing by 33.13%. Conversely, erecting the dorsal fin increases the wetted surface area, resulting in decreased maximum speed and efficiency during steady swimming phase. This finding partially explains why tuna erect their median fins during maneuvers or acceleration and fold them afterward to reduce drag. In addition, we verified that folding the median fins after acceleration does not significantly affect locomotion efficiency. This study supports the application of morphing median fins in undulating underwater robots and helps to further understand the impact of median fins on fish locomotion.

Underwater robots do a lot of neat things--take photos of underwater volcanoes, track leopard sharks, and explore shipwrecks--but they could still learn a few things from fish. Tuna are built to cruise across oceans, usually at around 2 mph. But they can crank up to 45 mph at the drop of a snack (Michael Phelps races at around 5 or 6 mph, for comparison). And tuna are agile, too, able to whip after fast-turning squids or sardines. They owe their agility, in part, to a newfound hydraulic system that allows them to raise and lower some specialized fins.