Archer fish. Credit: Courtesy of L. Zinnato
Archer fish have been observed to knock down prey with a precise jet of water that is six times as powerful as the fish could generate with its own muscles. This allows the archer fish to take down aerial prey from inside the water.
The scientists published their findings in the journal PLoS ONE. This behavior of Toxotes jaculatrix was first described in the 18th century. The fish live in mangrove forests and estuaries where there are many insects above water. This impressive strategy allows the archer fish to fetch food that would usually be out of its reach. The water jet can target and dislodge a single insect so that it falls into the water for the fish to consume.
It’s still a mystery how T. jaculatrix is able to do so in less than a second. Scientists have hypothesized that the existence of a special organ in the fish’s body was the source. In this study, scientists put archer fish into an aquarium to measure how accurate their water jet was. The fish was able to spout water at targets up to 15.3 cm (6 in) above the water. A high-speed camera was used to capture each state of the jet, from its formation to its acceleration.
The first section of the water leaving the fish’s mouth travels at about 2 meters (6.6 feet) per second. As the fish continues to expel water in the jet, this speed increases. This adds more power and momentum to the head of the jet. The resulting jet can deliver a significant impact. Surface tension holds the head of the jet together in the air, allowing the head to balloon out as it becomes larger and more powerful.
Similar dynamics have been used to develop inkjet drop-on-demand printing. “Power gets progressively transferred from the muscles to the tail of the jet, and in turn, to the head of the jet,” the researchers explained in their paper.
Reference: “How Archer Fish Achieve a Powerful Impact: Hydrodynamic Instability of a Pulsed Jet in Toxotes jaculatrix” by Alberto Vailati, Luca Zinnato and Roberto Cerbino, 24 October 2012, PLoS ONE.
DOI: 10.1371/journal.pone.0047867
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