Ready, aim, shoot.
Archerfish spit jets of water to knock off aerial prey.

Volker Runkel and Stefan Schuster/University of Erlangen-Nürnberg

On-the-Fly Thinking

Being a soccer goalie is a tough job, requiring split-second assessments of an incoming ball's speed, direction, and altitude, along with an instantaneous response to intercept the shot. But researchers have now shown that, despite having much less brainpower than humans, fish are capable of this kind of complex tracking and rapid reaction.

Archerfish are freshwater fish that spit jets of water as far as 2 meters to knock potential prey off twigs suspended above the water (ScienceNOW, 7 September 2004). They must act fast and accurately to snare the prey as it lands, or risk losing it to other fish. It's a skill they get better at with practice.

This ability--and the quick thinking it involves--piqued the curiosity of neuroscientist Stefan Schuster and graduate student Thomas Schlegel, both of the University of Erlangen-Nürnberg in Germany, who are interested in how brains make decisions. Schuster and Schlegel placed insects on platforms, out of view of the fish, then blew the insects into the water using bursts of air. To make the task harder for the fish, the scientists sometimes used multiple platforms and other times multiple insects. In each experiment, the researchers measured the time it took for the fish to turn and move to the prey.

No matter how complicated things got, the archerfish stayed on target. They responded just as quickly--sometimes in as little as 40 milliseconds--to unseen quarry blown off the platform by the experimenters as to prey they shot down themselves, indicating that the fish didn't have to know how or when the insects were being attacked to pinpoint their landing spot. Indeed, the fish could take stock of an insect in flight and still retrieve it fast. Archerfish were even able to home in on insects blown off platforms that were either far away or at an angle to their line of vision.

In one test, Schuster and Schlegel simultaneously knocked two insects off a platform in different directions to test whether the fish would get confused and dart to a place in between. Instead, the fish picked one insect, typically the one that would land closest to them, and got to the eventual splashdown as fast as though there had been no distractions, the researchers report in the 4 January issue of Science.

Further studies showed that the motion of the insects activates a six-neuron circuit in the brains of archerfish that's used to make quick escapes from predators. The eyes take in the information needed and relay speed, direction, and altitude data through the shortest path possible to motor neurons that cause the fish to turn and move. Along the way, the hindbrain integrates these data with other factors, such as the fish's position relative to the insect's path.

The work "provides an important demonstration for researchers in human cognitive neuroscience by showing that very intelligent behavior can arise from very simple systems," says cognitive neuroscientist Russell Poldrack of the University of California, Los Angeles. "The ability of these fish is pretty amazing."

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