Ever since Stefan Schuster heard about Archer fish as a small boy, he's been hooked! Schuster is fascinated by the fish's ballistic expertise; they can knock an insect off its perch at a range of 50 cm with a single jet of water. He's also amazed by their rapid retrieval response. Having dislodged its target, the successful assassin has to compete with bystanders to reach the tasty morsel first. What impressed Schuster was the speed and accuracy of the fish's reaction, precisely estimating the insect's point of impact in a fraction of a second. Were the fish keeping an eye on the victim as it fell, or had they instantaneously calculated the tumbling insect's flight path? Schuster needed to get his own private school of Archer fish to find out which approach they used. After investing in a 600 litre tank, and five fish, Schuster and his colleagues, Sam Rossel and Julia Corlija, videoed the fish's reactions as a dislodged fly began falling, and realised that the fish instantaneously calculate the prey's point of impact, based on two simple ballistic parameters measured within the first 100 ms of the fall (p. 3321).
Anyone who's ever watched a game of cricket or baseball understands the `outfielder problem'; how to track a projectile moving in three dimensions as it falls toward you. Humans solve the problem by tracking the image of a soaring ball on the retina, but would that work for a fish that has to compensate for optical distortions? Schuster suspected that the Archer fish had come up with an alternative solution. Because they reorientate almost instantaneously towards the point of impact, Schuster knew that the fish must somehow calculate where the fly will land, either by extrapolating the trajectory based on the first instant of the fall, or by calculating the position where the fly will land, based on a few simple ballistic parameters.
The team set up a tank where they could tantalise the fish with flies that never reached the water, to see what information the fish needed to get them to the impact zone in time. First they videoed the fish as they rushed towards a free falling fly. Next they attached the fly to a thin filament, so that it fell for a few moments before being jerked back from the hungry mouths. If the fish needed to track the fly's entire descent, the aborted fall would have stopped the fish from rushing, but they kept on going, even though the fly was left suspended in midair. Finally, the scientists tested the fish by making the fly-target slide horizontally across a glass plate above the fish at the speed it would have begun falling. Amazingly, the fish still converged at the position where the fly should have landed if it fell downwards, so the fish weren't extrapolating, they were calculating!
Schuster believes that the fish estimate the insect's height and initial velocity within 100 ms of dislodging the fly. But as the successful archer is competing with fish that are equally as quick on their fins, it is always the closest fish that catches this outfield trophy.
- © The Company of Biologists Limited 2002