Next time you find yourself in an aquarium, pay attention to the fish's fins. The middle fins in particular show a mesmerising variety in number, shape and position on the fish's body. But what is the purpose of such a variation? David Matthews and George Lauder of Harvard University aimed to address this question by investigating the interactions between the middle and tail fins and their effect on a fish's movement. Biologists and engineers have previously attempted to navigate these tough waters, so what new angle did the pair introduce?
Matthews and Lauder used a simplified fish-like robotic model: a thin plastic foil mimicking the proportions and form of a fish (a fish body, a tail fin and two middle fins symmetrically arranged on either side of the body) connected to a robotic flapping device introducing a sideways movement with the help of motors. This model was a step closer to real fish than previous mechanical models, in which just the forces produced by the fins were investigated. On top of that, they decided not to control every aspect of the fish's movement, which would overcomplicate the robot-fish, but measure them instead. They attached a force-sensing rod to the front of the foil, where the fish's head would be, and changed the number of flapping cycles the body experienced per second to propel itself forward. Using this setup, they could quantify the foil's energy and movement such as its speed and the maximum amount each model fin flaps respectively, all together determining the swimming performance of this model fish.
With the robot-fish at hand, the authors first asked how the position of the fish's middle fins and the timing of their flapping, i.e. whether they flap in unison with the tail fin or not, affect its movement. They performed experiments with four types of robot-fish: flexible or stiff, and with middle fins near or far from the tail. They then showed that the number of flapping cycles per second has the largest effect on the fish's swimming performance, which is consistent with live fish that flap their fins more often when they want to accelerate. The modelling also revealed that the stiffness of the foil didn't significantly change its movement while the location of the middle fins did. The far-fin foil was always the fastest, and when the middle fins were moving out of sync with the tail fin, it was also the most efficient. The team also observed that the out of sync flapping of the fins was beneficial for force production, which varied less over time, costing less energy for the fish.
To understand these results further, Matthews and Lauder used a visualisation technique to look at the water flow around the fins. They showed that having middle fins is helpful as their flapping results in a strong side-to-side flow at the front of the tail fin – irrespective of the fin position in flexible foils. In contrast, they found that for the stiffer foils, the one with the far middle fins resulted in a direct interaction of its flow with the front of the tail fin, amplifying the production of the propelling force. Thus, they were able to conclude that the patterns of flow amplification are consistent with their statistical model and can help explain why fin position and flapping timing affect force production. As a result, while we previously knew that flow from one fish's body can impact swimming performance in following individuals, we now also know that following fish can independently improve their performance using their secret weapon – middle fins.