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Dec 07

The Amazing Archer Fish

What is so amazing about the Archer Fish?

The only fish that can shoot a surface to air missile to hit targets above the water, knocking its prey down and eating it.

archer fish hits insect with jet of water

Click picture for animation

 Once that they spot their prey, they shoot it down by squirting a precisely aimed jet of water with their mouth, so that the prey falls into water where it gets readily devoured.

 It takes just on tenth of a second after the fish has squirted its prey to anticipate the spot an insect will fall so swim towards it and devour it.

It does this using the narrow groove in the roof of its mouth. The creature presses its tongue against this groove to form a narrow channel, then contracts its gill covers to force a powerful jet of water through the channel.

Physicists from the University of Milan looked at the different trajectories of spit squirted by archer fish. They discovered jets of water consistently hit insect targets at a faster speed than they left the animal. The head of the jet is slower than the back of the jet. The back of the jet stream catches up with the head of the jet stream at the point of impact giving a harder impact than from a continuous uniform jet stream.

Archer fish are remarkably accurate in their shooting; adult fish almost always hit their prey on the first shot. They can bring down insects and other prey on perches up to 3 m above the water’s surface. This is partially due to their good eyesight, but also their ability to compensate for the refraction of light as it passes through the air-water interface and take into account the trajectory caused by gravity, when aiming for their prey. They aim higher than a straight line to the target.

 

 The preferred angle they spit at prey is 74° from the horizontal, but can still aim accurately when spitting at any angle between 45° and 110°.

 When an archer fish selects its prey, it rotates its eye so that the image of the prey falls on a particular portion of the eye in the ventral temporal periphery of the retina, and its lips just break the surface, squirting a jet of water at its victim. The resulting jet of water can be up to 5 m long, but their accuracy allows them to shoot insects only 1–2 m away depending on body size. The fish can alter the power of the shot for prey of different sizes. If the first shot does not knock the victim into the water, the archer fish will keep trying.

 Young archer fish start shooting when they are about 2.5 cm long, but are inaccurate at first and must learn from experience. During this learning period, they hunt in small schools. This way, the probability is enhanced that at least one jet will hit its target.

archer fish jumps out of the water

Archer fish are surface dwellers remaining near the surface of the water. They are also opportunistic and will often leap out of the water and grab an insect in their mouths if it happens to be within reach. They can leap up to distance of 12 inches. They use their ability to visualise where the refracted image of the insect is to target their leap. If after hitting an insect with several jets, the insect is not dislodged, then it will resort to leaping at it.

 

 

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