Archerfish Tunes Its Shot Power According to the Size of the Prey

Archerfishes (Toxotes sp) reunite a group of seven relatively small fish species, with a maximum length of 40 cm, found in fresh, brackish or marine waters from India to the Philippines, Australia and Polynesia.

They are unique in the animal kingdom because of their peculiar way of hunting for insects and other small terrestrial animals from above the water by firing with great accuracy streams of water into the air to their prey, knocking down animals as big as small lizards onto the water's surface. Once fallen into the water, the fish dashes and gulps its prey.

It was thought these shots were all-or-none blasts of fixed force, but recently, a team of scientists from Universität Erlangen-Nürnberg in Erlangen, Germany found archerfish to be more sophisticated, adjusting the water amount they use in one shoot based on the size of their targets.

The team employed high-resolution imaging (at 5,000 frames per second while televisions and movies display pictures at 24 frames per second) of water streams shot during archerfish hunting. They found that archerfish automatically calibrate the force they use to knock down prey according to prey size, and this strategy appears not to be influenced by learning. The process occurred even when the fish were kept for two years without using this hunting technique.

It appeared that the tuning aspect of the archerfish's hunting strategy is purely innate and it has nothing to do with learning. Its behavior follows a rule saying that, in the case of the small animals, an animal's tendency to stick a surface is closely proportional to the animal's size. The archerfishes always calibrated their water sprays so that prey animals are hit with about ten times the force of their organs of animals of that size could hold on.

The new study also showed that the archerfish's hunting technique require a significant amount of energy and the fishes calibrated the force of their water shots by tuning the mass of water in a shot, rather than changing the speed of the shot. Doubling the mass of water shot, the fish doubles the energetic cost of the shot; doubling speed of the shot would require quadrupled energetic cost.