Spitting Fish Adjust for Distance When Shooting

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

The researchers used high speed cameras to analyse the fishes’ performance when hunting prey. They found that as they create each jet, the fish tweak the flow of water to cause a focussed blob of water to gather at the front of the target. This comes from changes to the animal’s mouth opening.

Senior author Professor Stefan Schuster from the University of Bayreuth in Germany said that jets of water and other fluids are used to cut or shape materials in industries ranging from mining to medicine. He believes that these findings could improve technology.

How Do They Do It?

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

But the fish don’t use pressure to control the water. Instead, they squeeze their gill covers together, changing the diameter of their mouth.

"The fish add nothing - they only shoot water, and they keep absolutely still during the 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."

How Did The Study Work?

The researchers trained two archer fish to hit targets at distances from 20cm to 60cm. The targets were small spheres, which allowed the team to calculate the forces involved. Accuracy was rewarded with a small fly. The hardest part of the study was making the fish shoot from an organised position.  "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,” he said.

Through this, along with high tech filming, they revealed that the all-important blob of water at the tip of the jet forms just before contact, no matter what the distance.

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

So could archer fishes be the cleverest fish underwater?

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