Archer Fish Shoots Prey With Water Jet
October 25, 2012

Archer Fish Uses Powerful Water Jet To Catch Its Next Meal

Brett Smith for — Your Universe Online

[ Watch the Video: Archer Fish Shooting at Prey ]

[ Watch the Video: Archer Fish Firing at Prey ]

In the sweltering mangrove swamps of India and Polynesia, small insects that wander onto a low-hanging leaf or too close to the end of a branch are vulnerable to an attack from a high-speed, liquid projectile intended to knock them off the tree and into the water below where an archer fish waits to pounce on its targeted prey.

Biologists have long wondered how this small fish is able to generate the powerful water jet that contains more force than its muscles could possibly generate. New research from a team of Italian scientists seems to have solved that mystery by showing how water dynamics outside the fish´s body may be responsible for powering this deadly projectile.

With the fish´s prey typically anchored to a surface with the force of ten times its body weight, the archer fish needs to be able to strike its target hard enough so that it is knocked into the water below, according to the team´s report that was recently published in the open-access journal PLOS ONE.

Previous research has examined the fish´s use of collagen fibers that other animals like the salamander use to store energy and abruptly release it in an attempt to catch prey with their tongues. This mechanism was ruled out for archer fish, as its biomechanics would not translate into the amount of force necessary.

To examine the archer fish behavior in detail, the team used a combination of high-speed video recording and kinematic analysis to analyze two different specimens. They were able to clearly see the structure of the water projectile as it was being generated and while it flew through the air.

“The jet appears as being composed of a thin tail and a bulged head, with the volume of the head of the jet progressively increasing during the flight,” they wrote in their report.

Through their kinematic analysis, the biologists were able to determine that the fish draws in a small amount of air into its orifice before spitting. This causes the water jets initial velocity to be greater than zero and adds to the force of impact.

They were also able to show that the fish modulates the velocity of the water jet so it strikes the target with the force of a single large water drop. This technique allows the fish to effectively strike its prey without the high evolutionary costs of developing specialized structures, according to the researchers.

"The origin of the effectiveness of the jet squirted by archer fish has been searched for inside of the fish for nearly 250 years. The striking finding of our work is that a large amplification of muscular power occurs outside of the fish and leads to a very powerful impact of the jet with the prey" said lead author Alberto Vailati, a researcher at the University of Milan.

Archer fish are first able to fire off predatory water jets when they reach a length of 2.5 cm. These young fish shoot inaccurately at first, but become better with experience.