How Can an Archerfish Hit an Insect That Looks to Be in the Wrong Place?

Seen From Under Water, an Insect Above the Surface Is in the Wrong Place

When an archerfish shoots, its mouth is in the air but its eyes stay well below the water level [1]. So it must look at its target through the water surface. Light bends there, following a rule called Snell’s law, and the direction in which the fish sees a target above the water is shifted toward the zenith, the point straight overhead [1]. To hit the insect where it really sits, the fish has to correct for that shift [1].

How big is the error? One outreach summary from the University of Notre Dame says the fish’s perceived angle to the prey could be off by up to 25 degrees, and that the fish copes by adjusting its spitting angle before it releases the jet [3]. Archerfish eyes also have special adaptations for seeing at the boundary between air and water [2]. The eyes are not the whole story, though.

The Water Jet Does Not Fly Straight, and Wind Can Push It

The fish makes its jet with its own built-in tube. The roof of its mouth has a deep groove, and pressing the tongue against it turns the groove into a tube. When the fish suddenly squeezes its gill covers, water is forced from the throat into the tube, and the tip of the tongue acts as a valve that regulates the flow [2].

The jet then travels through open air. Gravity pulls it down, so it does not run straight from the mouth to the target, and wind can push it sideways [1]. Even the height of the target changes the shot [1]. So the fish faces three problems at once: bent light, a falling jet, and moving air.

In 1764 the Fish Was Said Never to Miss, but It Misses Quite Often

A 1764 letter to the Royal Society of London by John Schlosser described a fish from Southeast Asia that hunts insects above the water by squirting water from its mouth “without ever missing,” as the authors of a 2024 study retell it. They add that the archerfish may not be as accurate as claimed, and that in fact it misses quite often. They still call its hunting one of the most remarkable strategies in nature [1].

Even so, under changing conditions the fish hits its target with a high success rate [1]. One explanation is that the fish is born with built-in knowledge of the physics. The researchers find that hard to believe, because as a fish grows, its eyes and its body keep changing, and a fixed program would have to keep up with all of that [1]. The opposite idea is that the fish learns by trial and error, correcting its misses as it goes [1]. That idea can be tested.

A Fan Test Showed Archerfish Learning From Their Misses

A team at Ben-Gurion University of the Negev in Israel trained fish to shoot at a food pellet on a metal net 35 to 40 cm (about 14 to 16 inches) above the water. Of 39 fish tried, seven passed the screening, which required shooting at least 20 times per session and staying accurate [1]. The team then blew air sideways over the water at 7.5 meters per second (about 17 miles per hour), so that the wind pushed the jet off course [1].

At first the shots missed. The error grew by roughly 4 to 5 mm on average. Over several shots it shrank and levelled off [1]. When the air was switched off, the fish missed in the opposite direction, as if still expecting the wind, and that error faded after a few more shots [1]. The authors read this as a sign that the fish had built an expectation of the wind and was aiming ahead of it [1].

In a second experiment the fish were made to shoot in the opposite direction while the wind stayed the same. The errors jumped again, which fits an adjustment tied to the fish’s own body rather than to the direction of the wind. The authors say this pattern is consistent with the response to a change in the refraction index or other factors tied to the fish itself [1]. That is a suggestion, not a measurement. The test used wind, not bent light, so it does not directly show how the fish corrects for refraction [1].

The Fish Also Shapes Its Jet So the Water Gathers at the Target's Distance

Aiming is not the only skill. In a 2014 study, Peggy Gerullis and Stefan Schuster report that archerfish adjust how long the water takes to gather into a compact mass at the front of the jet, so that the water forms its largest mass at the distance of the target. The fish do this by changing how the cross-section of the mouth opening changes over time [4].

A Scientific American report on the work says that for a target 60 cm (about 24 inches) above the water, the fish sprayed for a longer time and opened the mouth more gradually than for a target 20 cm (about 8 inches) up [5]. A far target gets a longer, differently timed jet, not just a harder one.

After the Hit, the Fish Heads for Where the Insect Will Land

Shooting is only half of the hunt, because the insect, once knocked off its twig, falls into the water and has to be caught there. A 2002 study by Samuel Rossel, Julia Corlija and Stefan Schuster found that a fish predicts the point where the insect will hit the water and swims in a straight line toward that point, rather than chasing the insect itself [6]. The turn starts about 100 milliseconds, one tenth of a second, after the insect is knocked loose [6]. A later review by Schuster says that, depending on contrast and temperature, such turns can begin as little as 40 milliseconds after the prey starts to move [7].

So the fish solves a second aiming problem. It first aims a jet at something it sees in the wrong place, and then it aims its own body at a spot where nothing is yet.

Try It Yourself: Make a Hidden Coin Appear With Water

You need an opaque mug or bowl, a coin, and a jug of water at room temperature.

1. Put the coin on the bottom of the empty mug.
2. Slowly move your head back and down until the rim of the mug just hides the coin. Keep your head still.
3. Ask someone to pour water in gently, without moving the coin. The coin comes back into view.

Light leaving the coin bends at the water surface on its way to your eyes, so the coin seems to lift. The fish looks the other way, up from the water into the air, but the same bending of light is at work.

For a second try, drop the coin into a clear bowl of water a few inches deep. Look from the side at a slant and try to touch the coin with the tip of a plastic spoon in one quick move. Are you off at first? Does it get easier after ten tries? Wipe up any spills afterward.

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