Long distance or turbo: how tuna fish their fins

Tuna fish are highly specialized long-distance swimmers, who undertake long-distance hikes through the ocean in search of food. At the same time, the animals are quick and agile when hunting or fleeing. Bluefin tuna fishes use a very special trick.

Tuna fish change the angle of their fins, either to stabilize or to swim a quick maneuver.

"I remember that we were very enigmatic when we first discovered these strange structures in and around the back fin, they looked like the pieces of a great puzzle, and we had no idea what that was going to be."

Vadim Pavlov is a studied biologist and self-trained engineer, as he says. At the Hopkins Marine Station at Stanford University, he investigates how the fin shape of fish is adapted to their swimming behavior. But he had never seen anything like the vascular system at the base of the dorsal fin of a tuna in any other fish: a chamber filled with fluid, through which fine channels pass through the fin.

"It was only when we realized that these structures looked like the biological analogue of a typical hydraulic system that all the parts of the puzzle came together: muscles that compress a fluid, vessels that control and direct the fluid flow, and the fin rays like And the pressure of the tuna is the same as the pressure of the tuna, and the pressure is converted into mechanical energy, which was very impressive, so we quickly had the idea that the hydraulics help to change the position and shape of the fins.

Angle of fins changes

The abdomen and back fins of a tuna stabilize the body in the water in the vertical, similar to the keel of a yacht. If the fish floats evenly against the current, a relatively small area is sufficient. Tuna fish cut the fins backwards when they swim long distances to reduce the water resistance. In order to understand when the hydraulic system is being used, Vadim Pavlov and his colleagues made video analyzes of tuna, which live in a large water basin.

"They use the hydraulics quite obviously when they want to swim a fast maneuver and change the direction, for example, if they have to hunt or escape a hunter, then they have to set up their fins to take full advantage of their stabilizing effect."

The angle at which the fins protrude from the body varies by up to 14 degrees, which can also be seen with the naked eye. Since the raised fins produce a higher water resistance, it would be a waste of energy to allow them to be erected during long, even swimming phases. The hydraulics thus provide the fish with a possibility to change the fin position quickly and thus either to save energy or to create more stability.

Lymph system helps the hydraulics

Vadim Pavlov and his colleagues have used lymphatic fluid as the control fluid in the vessels. "Up until now, only the lymphatic system has played a role in the immune response when infections need to be combated, and that it helps in regulating the water balance in our tissues Lymphatic system, which means we have found a previously unknown function of the lymph system. "

As a result of their discovery, the researchers also looked for appropriate structures for close relatives of the tuna. And indeed: At the base of the dorsal fins of certain mackerel species and bonitos, they also found a chamber that could be the fluid reservoir for a hydraulic control. Since these fish are also long-distance specialists, the researchers suspect that the special type of fin control is an adaptation to this way of life.

Vadim Pavlov believes that the system could also be a model for technical applications such as diving or flying robots. According to the tuna model, one could make their control fins more flexible and thus improve the maneuverability.