Archerfish may look like peaceful swimmers, but they spit water at unsuspecting prey with remarkable power and precision.
The fish turn out to be the first known tool-using animal to change the hydrodynamic properties of a free jet of water, according to a new study published in the journal Current Biology.
Even humans who impressively spit things like watermelon seeds cannot compete with the accuracy of these water-shooting fish. Archerfish are found in the open ocean and in water bodies of Thailand, India, Australia, the Philippines, and certain other countries.
A better comparison, where humans are concerned, has to do with our talent for throwing objects with our hands.
"One of the last strongholds of human uniqueness is our ability to powerfully throw stones or spears at distant targets," co-author Stefan Schuster said in a press release. “This is really an impressive capability and requires — among many fascinating aspects — precise time control of movement."
"It is believed that this ability has forced our brains to become bigger, housing many more neurons to afford the precision," continued Schuster, who is a researcher at the University of Bayreuth. "With the many neurons around, they could be used for other tasks apart from applying them for powerful throws. It is remarkable that the same line of reasoning could also be applied to archerfish."
For the study, Schuster and co-author Peggy Gerullis trained the relatively big-brained fish to hit targets ranging in height from approximately 8 to 24 inches from a fixed position. The researchers then monitored various aspects of jet production and propagation (wave movement) as the fish did their thing.
The target training sessions revealed that the time needed before water masses up at the jet tip isn't fixed. Archerfish instead make adjustments to ensure that a nice drop of water forms just before impact.
The fish achieve this by modulating the dynamics of changes in the cross-section of their mouth opening, the researchers report. The timing adjustments that archerfish must make to powerfully hit their targets over an extended range are comparable to the "uniquely human" ability of powerful throwing.
A primary urge led to the evolution of the fish's skill, and that is hunger.
"The predominant impression from our field work in Thailand over several years is that there is very little to actually shoot at, so it's important for the fish to be efficient," says Stefan Schuster of the University of Bayreuth in Germany. “It pays to be able to powerfully hit prey over a wide range of distances."
Usually the fish shoot water at a surprised victim, such as a little lizard on a twig above the water. The stunned lizard is forced off the twig and into the water, where the fish gobbles it up.
Coming soon to a store near you could be water nozzles modeled after the fish's natural mechanism for controlling water. Adjustable jets are big business in many industries, including medicine.
The scientists must first overcome some challenges that clearly the fish resolved through their evolution some time ago.
"The biggest problem is how to modify the abrasive properties of a jet," Schuster says. "Usually this is done by modulating the release pressure or by varying the abrasives added to the jet. We are not aware of someone actually using a dynamically adjustable valve."
Photo: Archerfish shooting water at insect prey. Credit: Ingo Rischawy, Schuster Lab, University of Bayreuth