Archer Fish Right on Target

Archerfish, which knock their prey into the water with a well-aimed blast of water, tailor the power of their shot to the size of their meal, German scientists have found. Writing in this months edition of the journal Current Biology, Thomas Schlegel and his colleagues from the Universitat Erlangen-Nurnberg used high-speed photography capable of capturing 5000 frames per second to work out how much force the fish was using to knock prey of its perch. There were two key findings: The larger the prey the more force the fish used. The effect was even seen in fish that had previously grown up in a tank where firing any amount of water resulted in their receiving a food reward. The researchers think that the effect is down to the simple principle that the larger something is, the greater the adhesive force it uses to cling on to a surface. The second finding was that the fish minimise the amount of energy they use to retrieve their lunch. For larger targets they increase the amount of water they fire, rather than the pressure or speed at which they fire it. This means that they pack double the punch for double the energy, rather than double the punch for quadruple the energy, which would be the result if they altered the pressure or volume (because kinetic energy varies with the square of the speed but only linearly with mass). As a result they burn off the least energy possible for each shot they take meaning they make the most out of meal times.