Like a major league pitcher throwing a baseball, archerfish aim a powerful jet of water at their prey—and they do it by changing the shapes of their mouths, a new study says. Previously, researchers knew simply that the rain forest fish squirted water at their intended victims, knocking the bugs or small animals off their perches and into the water, where they’re quickly gobbled up. (Also see “Fish-Eating Spiders Can Catch Prey 5 Times Their Size.”)
Now, new research published September 4 in Current Biology reveals that the archerfish are far more skilled at creating and using these water jets than anyone had guessed—and that the animals may even manipulate water like a tool. “It’s really a remarkable study,” said Alberto Vailati, a physicist at Italy’s University of Milan who has studied archerfish but was not involved in the new research. “It’s very interesting to see such a simple animal perform a very complex task.”
Testing the Waters
Archerfish include several different species in the genus Toxotes, which live in Thailand and other parts of Southeast Asia. Their ability to squirt water jets at prey has made the fish popular in aquariums, which is how researcher Stefan Schuster was first introduced to them. A physicist at the University of Bayreuth in Germany, Schuster studies the nerve circuitry that helps control relatively simple behaviors. His tiny apartment didn’t have room for a large aquarium, though, so he brought his new pets—which belong to the species Toxotes jaculatrix—into the lab. Watching the fish in action, he realized that his new officemates were the model organism he had been looking for. (See “Do You Know Where Your Aquarium Fish Come From?“) Archerfish, Schuster learned, shoot a surprisingly powerful blast of water. As the University of Milan’s Vailati describes it, “If you get hit in the face with this jet, it stings like an insect bite.” Shooting such a strong, targeted stream of water isn’t as easy as it looks. To generate enough force to hit an insect and knock it over, the fish would need to concentrate the force of the water into one giant burst. Schuster and colleague Peggy Gerullis were determined to find out how the archerfish accomplished this, a process that ultimately took them four years. The team started by training a group of nine archerfish to squirt at insect prey in a specific spot in the tank so that they could measure the force and velocity of the stream, as well as record it using a high-speed camera. Tool-Using Archerfish? After analyzing hundreds of hours of data, Schuster and Gerullis finally had their answer. When squirting water, archerfish continually change the shapes of their mouths so that the water stream will successfully aim and fire at prey, the study found. By doing this, the fish essentially alter the properties of moving water. (See National Geographic’s pictures of fish schools.) Most important, the water at the end of the stream is shot out at a faster speed than the water at the beginning. This means that all the squirted water slams into the victim in a short burst, giving it maximum force. Both Schuster and Vailati believe that, because the fish actively and deliberately influence the hydrodynamics of the water, it qualifies as tool use. (Also see “First Pictures: Wild Fish Uses Tool.”) “It’s analogous to a human throwing a stick,” Schuster said. “If they just threw the stick, it wouldn’t count as using a tool because they didn’t change it. But if they sharpened it or removed branches, that would be a tool.” These results may be the start of a sea change in what we think about fish intelligence. Follow Carrie Arnold on Twitter and Google+.