Video: Fish spits water like we throw spears

Jia You is a news intern at Science

Archerfish boast an unusual talent: They hunt by spitting powerful streams of water out of their mouths at unsuspecting above-water insects. Now, a new study reveals that this ability is even more remarkable than researchers thought: Like a true sharpshooter, the fish can use their mouths to adjust the focus of their water jets and maximize harm on prey at different distances. The finding hints that archerfish are smarter than previously thought, and it may inspire new technologies for inkjet printing and even cutting objects with water.

Dwelling in river estuaries and mangroves, archerfish (Toxotes jaculatrix) hunt during the day to avoid competition with halfbeaks, omnivorous fish with swordlike mouths. But that’s also when relatively few insects pass by the water, forcing the fish to be efficient shooters. Previous research has shown that the fish can accurately size up their prey from below the water and adjust for distortions caused by the refraction of light, an ability that requires complex calculations.

Observing wild archerfish in Thailand, biologists Peggy Gerullis and Stefan Schuster of the University of Bayreuth in Germany found that these 25-centimeter-long fish can squirt water at prey as far as 2 meters away, which requires the jet to only become fully focused right before hitting the target for maximum impact. To understand how the fish accomplish this feat, the researchers spent a year training archerfish in their lab to hit targets ranging from 20 to 60 centimeters in height from a precise location, so they could fully capture the spitting motion and the jet’s propagation.

With high-speed cameras, the team discovered that the fish’s water-squirting mechanism is much more complex than a gun’s firing mechanism, they report online today in Current Biology. Like a bullet passing through a barrel, water compressed by an archerfish’s gill covers first travels through a tube formed by the fish’s tongue and a ridge in the roof of its mouth. But instead of heading straight out into the air, the water then enters an enlarged cavity, which stabilizes the stream and reduces splattering over long distances. The same mechanism is found in the mammalian penis, which allows the male to focus its urine, Schuster says. 

But that’s not the most surprising part. When the researchers observed how the fish’s mouth moves during the squirting, they found that the fish doesn’t simply spit the water out. Instead, its mouth movements vary depending on the prey’s distance. The process, which takes place over just a few milliseconds, involves two steps: First, the fish opens its mouth to a fixed maximum width. Then, its mouth starts closing before water travels through, so that the mouth is only partially open when the jets emerge. Depending on its distance from the target, the fish varies the speed of its mouth opening and closing to change the speed and stability of the outgoing stream. Slower opening and closing stabilizes the jet and lessens its acceleration, allowing the water to focus over a longer distance and hit a prey with maximum impact.

The study shows that archerfish use water as a tool, an ability people don’t usually associate with fish, says Herwig Baier, a neurobiologist at the Max Planck Institute of Neurobiology in Munich, Germany, who was not involved with the research. The precision with which the fish controls the timing of its movement is comparable to the temporal control that humans exhibit when throwing a spear, he says.

In humans, the ability to throw powerfully is hypothesized to have played an important role in our brain’s evolution. Schuster suspects a similar line of reasoning applies to the archerfish: The fish’s sophisticated control of its mouth movements could have increased the number of neurons it has and enhanced other cognitive skills as well. For example, in his observations of wild archerfish in Thailand, Schuster found that the fish shoot at artificial objects that do not resemble natural prey. “They like trying out new things, regardless of whether they are rewarding,” he says.

But so far, there’s no evidence that shows archerfish have larger brains than fish of comparable size, Baier says. Rather, the fish’s exceptional water-shooting ability could be an adaptive behavior, comparable to how other fish jump out of water at precise moments to catch prey, he says.

Many unanswered questions remain regarding how the fish spits water out, says biologist Frank Fish of West Chester University in Pennsylvania, who was not involved with the research. It’s still unclear whether the fish’s mouth movements change the pressure of its jet as well, he notes, or what role its tongue plays in shaping the jet.

Next, Schuster is partnering with engineers to build a nozzle that can vary in width similar to how the archerfish’s mouth functions, to test out how changes in the diameter could yield different jet focuses. If they do build such a nozzle, the technology could have applications in inkjet printing and water jet cutters, industrial tools that use high-pressure water jets to cut materials, says physicist Alberto Vailati of the University of Milan in Italy, who was not involved with the research. In inkjet printing, for example, varying the width of the jet could reduce satellite ink drops—an important advance for biomedical researchers using inkjet printing to build tissues. “The biophysics approach is very promising,” he says.

(Video credit: Ingo Rischawy/Schuster lab, University of Bayreuth)

Posted in Biology, Plants & Animals