Being knocked off your perch by a jet of water may not be a dignified death, but it is an effective way for an archerfish to secure lunch. However, once an insect has been dislodged, the race to be first at the landing site is on, leaving little time for archerfish to select an approach strategy. Caroline Reinel and Stefan Schuster from the University of Bayreuth, Germany, explain that many fish use a specialised response to escape predators – they curl their bodies into a C shape before beating the tail hard to flee – and archerfish seem to use the same strategy to ensure their speedy arrival when the fly splashes down (p. ). However, it wasn't clear whether the C-start alone was sufficient to set the fish off at the optimal speed to intercept a falling fly, or whether it was necessary for the fish to fine-tune their approach with fin beats to optimise the retrieval trajectory.
Intrigued, Reinel and Schuster began investigating the fish's pursuit strategy. Firing dead flies off a platform above a tank of water with a puff of air in random directions and from various heights, the duo filmed the reactions of the fish residents at high speed to capture every detail of the victor's departure as they competed to reach the tasty morsel. Then, having selected 306 unambiguous fly intercepts, Reinel focused on the manoeuvres of the first fish to arrive at the landing site, and painstakingly analysed the first 40 ms of the winner's departure.
After measuring the angle of each fish as they pushed off from the C-start, Reinel could see that the winner had already set the correct bearings toward the flies' future impact sites when they set off. Then she analysed the progress of the fishes' snouts in 10 ms intervals as they swam toward the fly and was impressed to see that the first fish had already reached the optimal speed to beat the others to the landing site during the first 10 ms after uncoiling. Explaining that hunting archerfish already know how far their quarry have to fall – because they have to know the height of the fly to successfully knock it off its perch – Schuster adds that the fish are able to use this information, coupled with a glimpse of the fly's descent, to set the speed at which they must travel to ensure interception. However, once they have unleashed the C-start, the fish do not adjust their speed during the early stages of pursuit, although Schuster notes that this does not preclude the fish from adding a final burst of acceleration to outcompete other fish as they converge on the hapless fly. He was also impressed that the fish seemed to be able to take into account the amount of time that it takes for them to trigger a C-start, which reduces the time that they have to home in on the fly and forces them to swim faster.
Having shown that the archerfish's C-start is extraordinarily adaptable, allowing hungry fish to select an accurate intercept course during the victim's descent, Schuster suspects that the fish could help us to learn more about the process of making a decision. ‘The underlying circuitry would be the ideal substrate to study fundamental aspects of decision-making’, says Schuster.
