Robo-tuna reveals how foldable fins help the speedy fish manoeuvre

A tuna-shaped robot takes advantage of the real fish’s secret for speed and nimbleness: selectively folding or sticking out its fins, a feature that could improve aquatic robot designs.

Tuna are some of the ocean’s fastest swimmers, thanks in part to the way they retract or fold their fins to reduce drag. Zhonglu Lin at Xiamen University in China and his colleagues investigated how such fins could make robots more agile.

They built a tuna-shaped robot that was half a metre long. It could be controlled with a motor in its head, another in its dorsal fin on its back and a third in the caudal fin at the end of its tail. The researchers filmed their robot as it swam across a pool, testing the effects of the dorsal fin being flat or erect while the robot accelerated, turned or steadily swam forward.

They found that folding or unfolding the dorsal fin significantly impacted factors like speed, efficiency and linear acceleration. When the robot tuna turned, erecting the dorsal fin increased its velocity by almost 33 per cent. But if they kept the fin erect when the robot was swimming steadily forward, its motion was up to 13 per cent less efficient and the robot used more energy.

Lin says that these findings match how tuna in nature erect their dorsal fins to execute rapid and precise movements, such as when capturing prey, and then fold them to continue swimming. “Underwater vehicles can improve their balance and navigation, along with agile movement at high speeds, by designing similar flexible control systems,” he says.

“Understanding the swimming performance for tuna at such a high level is inherently interesting as something our best human swimmers cannot achieve,” says Frank Fish at West Chester University in Pennsylvania.

But Fish adds that the caudal fin may play a larger role than the dorsal fin in the tuna’s swimming skills. His own studies of these animals have shown this to be especially true when they turn, he says. “We measured the turning performance of Pacific bluefin tuna and found it far greater than the robotic performance,” says Fish. This may mean that tuna-inspired robots could be improved with closer study of the caudal fin, too.