Spitting fish 'adjust for distance' when shooting

The jets of water that archer fish use to shoot down prey are "tuned" to arrive with maximum impact over a range of distances, according to a study.

High-speed cameras were used to analyse fishes' spitting performance in detail.

As they create each jet, the fish tweak the flow of water over time, causing a focussed blob of water to gather just in front of the target, wherever it is.

The ability comes from precise changes to the animal's mouth opening, which may prove useful in designing nozzles.

Senior author Prof Stefan Schuster, from the University of Bayreuth in Germany, explained that jets of water and other fluids are used to cut or shape materials in industries ranging from mining to medicine.

He believes his new fish-based findings could improve the technology.

"I've never seen anything in which they use a nozzle that changes its diameter," he told the BBC. "The most standard approach is adjusting the pressure."

But pressure, which the archer fish apply by squeezing their gill covers together, is not the secret to their ballistic precision.

Prof Schuster and his PhD student Peggy Gerullis found no evidence for pressure adjustments, nor for chemical additives or flicking movements in the water, which might account for the fishes' ability to control the stability of the water jet, and focus the accelerating blob at its tip.

"The fish add nothing - they only shoot water, and they keep absolutely still during release of the jet," Prof Schuster said.

"They just do it with the mouth opening diameter. It is not a simple manoeuvre... The diameter is continuously changing."

That makes the new study, published in Current Biology, the first evidence of an animal actively manipulating the dynamics of a water jet.

Prof Schuster and Ms Gerullis trained two archer fish to hit targets at distances from 20cm to 60cm, under bright lights to help with filming.

The targets were small spheres, which allowed the team to calculate the forces involved.

Accuracy, of course, was rewarded - usually with a small fly. "You can easily train a fish to shoot at anything you want," said Prof Schuster. "They are perfectly happy as long as something edible falls down."

The tricky part was organising the angles.

"To be ready to monitor to the right spots with reasonable spatial resolution, you have to convince the fish somehow to fire from a defined position. That was the hardest part of the study, actually."

With patience, the researchers collected enough measurements to reveal that the all-important blob of water at the jet's tip, which allows archer fish to dislodge their prey, forms just before impact - no matter the target distance.

To accomplish this, the animals fine-tune not just the speed, but the stability of the water jet.

"It means that the physics the fish is using is much more complicated than previously thought," Prof Schuster explained.

Dynamic jet control must now be added to an already impressive list of this fish's abilities.

Other research has explored questions ranging from how archer fish compensate for the distortion of their vision by the water surface, to how they learn to hit moving targets by copying their companions, to exactly how they produce a water jet that catches up on itself to form their distinctive, watery missile.

Prof Schuster believes that their spitting accuracy may have evolved in a similar way to human throwing, which some theorists argue sparked an accompanying expansion of our cognitive abilities.

His team has also done fieldwork in Thailand, where they observed that the fish hunt in daylight, when their insect targets are few and far between. So having a good range, and not missing, are a big advantage for survival.

That power and precision requires brain power.

"People have calculated that to double [throwing] range requires roughly an 8-fold increase in the number of neurons involved in throwing," Prof Schuster said.

So are these fish evolving into the cleverest animals under water?

"I don't think they will develop into humans. [But] they have many strange abilities that you wouldn't expect from fish.

"Maybe we can show by looking more closely at the brain, that shooting might have played a similar, prominent role in driving these abilities, as it's thought that throwing played in human evolution.

"That's just a crazy idea of mine."

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