Archer fish are known for striking down prey in vegetation surrounding their habitats. These fish do so by propelling water in the form of a jet stream that strikes its prey with enough force to knock it off the vegetation and into the surrounding water. It was originally believed that the fish were capable of producing such strong jet forces because of some internal structure or mechanism but new studies show that the force is actually amplified outside of the fish’s body. During the propagation of the jet, the fish modulates the stream to create a gradual increase in the accumulation of water at the head of the jet (i.e., mass is directed to accumulate at the head of the jet as it moves). This increase in mass towards the head of the jet also increases its velocity. The force that these modulations create when the jet hits the prey outside of the water is nearly six times greater than the force of the water that leaves the fish’s mouth.Edit Summary
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“Archer fish knock down insects anchored to vegetation by hitting them with a precisely aimed jet of water. The striking force of the jet at the impact is such to overcome the strong anchoring forces of insects. The origin of the effectiveness of such hunting mechanism has been long searched for inside of the fish, in the unsuccessful attempt to identify internal structures dedicated to the amplification of muscular power. Here we perform a kinematic analysis of the jet emitted by two specimens of Toxotes jaculatrix. We estimate that at the impact the jet conveys a typical specific power of about 3000 W/kg, which is well above the maximum specific power of the order of 500 W/kg deliverable by a vertebrate muscle. Unexpectedly, we find that the amplification of muscular power occurs outside of the fish, and is due to a hydrodynamic instability of the jet akin to those occurring in Drop-on-Demand inkjet printing. The investigated fish are found to modulate the velocity of the jet at the orifice to favor the formation of a single, large, water drop that hits the prey abruptly with a large momentum. The observed mechanism represents a remarkable example of use of an external hydrodynamic lever that does possibly not entail the high evolutionary cost needed for the development of highly specialized internal structures dedicated to the storing of mechanical energy”(Vailati et al. 2012: 1).
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