Startle a fish, and it'll turn tail and flee. However, repeat the exercise a few more times and you'll see that far from being uncontrolled, the fish's departure is a highly choreographed manoeuvre. Bending its body into a tight C shape, the fish then beats its tail to make its escape in less than 0.06 s. According to Eric Tytell, from the University of Maryland, scientists have studied the movements and neural circuits that control the regulated departure for more than 30 years. But there was a hole in our understanding of the fish's escape routine. No one had measured the way the fish interact with their environment. Curious to find out more about the hydrodynamics of the escape response, Tytell and George Lauder from Harvard University teamed up to film bluegill sunfish as the fish fled a threat().
Filming fish as they swam in a flow tunnel, the pair tracked the jets and eddies generated by the fish's bodies with a thin plane of laser light reflected off microscopic spheres suspended in the water. Keen not to disturb the water's flow as they startled the fish, Tytell rigged up a flat plate to generate a pressure wave in the water and trigger an escape response. Having spooked the fish, he filmed its reactions with the plane of laser light situated at three different levels on the fish's body to reveal the resulting fluid movements. Tytell admits that the experiments ran surprisingly smoothly,and he had collected all of the escape sequences that he needed to analyse within a week. Returning to Maryland, Tytell spent months analysing the fluid flows around the fish's bodies before building a model of the complex hydrodynamics generated as the fish turned.
The first thing that struck Tytell was the jet of water generated by the fish's tail as it curled its body into a tight C. This was closely followed by a second jet of water generated at the centre of the C shape, but directed in the opposite direction from the first jet, that continued to develop through to the end of the escape sequence. According to Tytell the first stage of the escape response, as the fish curled up into a C, was thought to be preparatory and not to contribute to the propulsion; but the second jet was clearly generating thrust as the startled fish fled. In the final stages of the escape, as the fish's tail swept to the side at the end of the first tail beat, the fish generated a third jet pulling water in towards its body, which the team suspects counteracts the fish's momentum as it turns.
Tytell admits that he and Lauder were surprised that the early stages of the escape were propulsive, although there had been theoretical studies that had predicted that the first phase was more than preparatory. What is more, it suggests that the Mauthner cells (which trigger the fish's sharp bend into a C) directly contribute to thrust generation, rather than just preparing the fish to make a speedy get away.
