- BBC World, @bbc_ciencia
When it spots its prey, it aims, shoots and waits.
This expert spitting fish shoots its deadly jets of water at small insects, which fall from leaves or branches near the surface of the water and are devoured by the hunter.
The archerfish technique has been filmed and studied in detail by a team of scientists showing that as the fish create each spurt, they modify the flow of water during the cast to fine-tune their aim.
And they do this by changing the opening of the mouth.
«I’ve never seen anything like this where the mouth changes its diameter,» Stefan Schuster, a researcher at the University of Bayreuth in Germany who has studied archers, told the BBC.
«The most common thing is to adjust the pressure.»
But the pressure, which the archerfish applies by squeezing its gills, is not the secret to its ballistic accuracy.
In his work, Schuster found no evidence of pressure adjustments or chemical additions or rapid movements in the water, which could allow the fish to control the stability of the stream and accumulate an accelerated droplet at the tip.
a lethal drop
«The fish just shoot water and are absolutely still when they spout,» Schuster said.
«They do it just with the diameter of the mouth. It’s not a simple maneuver, the diameter is constantly changing.»
This makes the study published in the journal current biology Be the first evidence of an animal actively manipulating the dynamics of a stream of water.
According to his observations, the ability of these animals to spit accurately is similar to that of humans.
Schuster and his colleague Peggy Gerullis trained two archerfish to shoot targets from 20cm to 60cm and filmed them.
The dams were small spheres that allowed scientists to calculate the forces involved in the spit.
The prize for the most accurate shots was usually a fly. «You can easily train a fish to shoot anything you want,» Schuster said.
«They are happy as long as something edible falls nearby.»
Using their detectors, scientists can see that the crucial droplet of water at the tip of the jet, which knocks prey down, forms just before impact.
«This means that the physics that the fish uses is much more complex than previously thought,» Schuster explained.
The precise spitting technique, the researchers suggested, may inspire the design of hose nozzles.