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Science | Animal World
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Archerfish

How a Fish Shoots Through the Air–Water Boundary and Still Hits Its Target

Did You Know a Fish Can Aim at Something It Sees in the Wrong Place?

An insect sits on a leaf above the water.

An archerfish below the surface looks up, forms a narrow jet of water with its mouth and knocks the insect into the water.

The difficult part is not merely making the jet.

Light bends when it crosses the air–water boundary. The insect’s apparent direction underwater is shifted by refraction. Gravity bends the water jet downward. Wind can deflect it. Target distance changes how the jet must develop. The fish itself may approach from different positions.

The fish hunts across two physical worlds at once: it sees through water but strikes through air.

And it does not solve the problem by reciting Snell’s law.

Its nervous system and motor system calibrate perception and action so that a target distorted by optics can still be hit by a jet governed by fluid dynamics.

That makes the archerfish an extraordinary bridge between animal behaviour and Physics.

One Hunting Act Contains Several Scientific Problems

• Where is the prey really located?
• How large is it?
• How far above the water is it?
• What angle should the fish choose?
• How strong and long should the jet be?
• How will gravity and wind alter the shot?
• Where will the insect land after it begins to fall?
• How quickly must the fish accelerate to reach that landing point before competitors?

Read a 2024 study of motor adaptation in archerfish shooting →

Someone Learned to Ask the Fish Better Questions: Stefan Schuster

Animal physiologist Stefan Schuster and collaborators have spent years turning the archerfish’s spectacular hunting behaviour into a precision research system.

Instead of treating the fish as a curiosity, they trained archerfish to shoot at controlled targets at different heights and distances, recorded jets with high-speed cameras and analysed what happened after prey began to fall.

The work revealed that the fish actively shapes its jet, adjusts timing to target distance and makes rapid predictive starts after prey is dislodged. Those starts use information gathered within fractions of a second to aim the fish toward where the prey will land—not where it is when the fish begins moving.

spectacle → measurement → prediction → mechanism.

The useful human lesson is simple: a behaviour that looks like “talent” becomes scientifically interesting when we decompose it into decisions that can be measured separately.

Big Question: How does an archerfish convert a refracted visual image into a controlled water jet, then predict where falling prey will be?

This Learning Manual begins with Primary ideas of light, water and animal adaptation, opens into Secondary refraction and forces, and reaches JC-level sensory processing, motor adaptation, hydrodynamics and predictive behaviour.

Quick Answer

Archerfish in the family Toxotidae hunt aerial prey by firing water jets. Accurate shooting requires coordination among vision, body position, mouth movements and learned or adaptive motor control.

• Refraction changes the apparent direction and size of aerial targets viewed from underwater.
• Visual processing extracts target features and distance information.
• Mouth and tongue geometry forms a narrow water channel.
• Timed mouth opening and closing shapes jet flow.
• Jet hydrodynamics allows later water to catch earlier water, concentrating force near the target.
• Motor adaptation lets fish adjust when physical conditions change.
• Predictive C-starts launch the fish toward the later landing point of falling prey.

What You Will Learn

• Why an aerial target appears shifted underwater.
• How Snell’s law enters a biological problem.
• Why aiming vertically is not the whole solution.
• How the fish creates a water jet.
• Why jets are actively shaped instead of behaving like simple squirts.
• How target distance changes the shot.
• How archerfish adapt to altered physical conditions.
• How visual learning helps estimate object size.
• How the fish predicts where falling prey will land.
• How Singapore mangroves provide a real habitat for archerfish.

Part 1 — Why Does the Insect Look Displaced?

Light travels at different speeds in air and water. When a ray crosses the boundary at an angle, its direction changes.

For a fish looking upward, rays from an aerial insect bend when they enter water. The fish’s eye therefore receives light arriving from a direction different from the original path in air.

what the eye receives is optically real but geometrically displaced.

Part 2 — Snell’s Law Is Hiding in a Mangrove Hunt

The relationship between incoming and refracted angles can be written:

n₁ sin θ₁ = n₂ sin θ₂

The refractive index of water is greater than that of air, so light entering water bends toward the normal.

The archerfish does not need symbolic algebra, but its behaviour has to remain calibrated to the same physical relationship.

Part 3 — Why Not Simply Shoot Straight Up?

If the fish were exactly beneath the target and shot vertically, refraction would be much less troublesome because light traveling along the normal is not bent sideways.

But wild archerfish do not always shoot from directly below prey. They attack at a range of horizontal and vertical positions. Behavioural studies show that they can compensate across changing viewpoints.

A strategy that works only at one angle would be too restrictive.

Part 4 — Size Is Distorted Too

Refraction changes more than direction. The apparent size of an object depends on viewing geometry.

Experiments show that archerfish can learn size constancy: they can judge the actual size of aerial targets from novel viewpoints instead of memorising a list of familiar images.

the fish learns a relationship, not merely a picture.

Part 5 — How Does a Fish Make a Water Gun?

Archerfish form a narrow groove between the tongue and the roof of the mouth. By rapidly compressing the oral cavity and controlling mouth opening, they eject a directed jet.

That is only the beginning. High-speed studies show the fish can modulate the mouth aperture through time to control how the jet develops.

Part 6 — The Jet Focuses Near the Target

A simple squirt slows and breaks apart. Archerfish instead produce a jet in which later fluid can move faster than earlier fluid. The rear portion catches up, concentrating mass and momentum near the front.

The fish changes the timing so that this focusing occurs near the distance of the target.

target farther away → jet must remain coherent longer → timing changes.

Part 7 — The Fish Controls the Nozzle Dynamically

Research on trained archerfish found that mouth opening changes during the shot. Those changes alter water velocity through time.

This active control is important because a fixed nozzle would not automatically optimise force for many target distances.

Part 8 — Gravity Still Acts on the Jet

Once water leaves the fish’s mouth, gravity accelerates it downward. The trajectory is not perfectly straight.

Target height therefore matters. The longer water spends in the air, the more gravity can change its path.

The fish’s motor system must remain calibrated to this relationship alongside refraction and jet breakup.

Part 9 — Wind Creates a Different Kind of Error

Refraction changes what the fish sees. Wind changes what the water does after it leaves the fish.

A 2024 experiment introduced airflow above trained archerfish. Shots initially missed, then gradually shifted until accuracy improved. When the airflow was removed, the fish temporarily erred in the opposite direction.

That aftereffect is strong evidence for motor adaptation: the fish had changed its internal calibration rather than merely getting lucky.

Part 10 — Adaptation Is Not the Same as Evolutionary Adaptation

Here, motor adaptation means an individual fish adjusts its movements through experience over minutes or trials.

Evolutionary adaptation means inherited population change across generations.

The same word appears in two scientific contexts, so the timescale must be made explicit.

Part 11 — Hitting the Insect Is Only Half the Hunt

Once prey falls, several fish may race toward it. The shooter can lose the meal it worked to dislodge.

Archerfish therefore use extremely fast C-shaped body turns called C-starts. Within roughly a tenth of a second after prey begins falling, they can extract enough motion information to launch toward the future point of impact.

they do not chase where the prey is; they accelerate toward where it will be.

Part 12 — The Fish Estimates Target Height

Predicting where a falling insect will land requires information about how high it started. Research from Stefan Schuster’s group shows that archerfish use an independent estimate of target height when configuring predictive starts.

That turns an apparently simple feeding behaviour into a rapid three-dimensional geometry problem.

Part 13 — Archerfish Can Learn Visual Categories

Because archerfish naturally report a visual decision by firing a shot, researchers can train them to select images on screens above the tank.

Studies show that they can perform visual search, discriminate object categories and extract visual features despite having a brain organised very differently from a primate brain.

A 2025 study found information about object category already present in a small population of neurons in the optic tectum, an early visual-processing area.

Part 14 — Why This Does Not Mean the Fish “Understands Physics” Like a Human

The fish behaves in ways consistent with physical relationships. That does not prove it represents Snell’s law symbolically or consciously reasons through equations.

Neural circuits can learn and encode useful sensorimotor relationships without language or explicit mathematics.

accurate physical behaviour ≠ human-style conceptual explanation.

Follow One Photon

1. Light reflects from an insect above water.
2. The ray reaches the water surface.
3. It bends toward the normal as it enters water.
4. The ray reaches the archerfish eye.
5. Retinal and neural circuits encode the target.
6. The brain combines target information with body position and previous calibration.
7. A motor command sets the shot direction.

Follow One Water Packet

1. Water is compressed in the mouth.
2. The opening between tongue and palate forms a narrow channel.
3. Early water exits at one speed.
4. Later water can exit faster.
5. The faster rear water catches the front portion.
6. The jet becomes concentrated near the target distance.
7. Impact force knocks the prey loose.

Think Like a Scientist: How Do We Separate Optics From Motor Control?

• Change target angle while keeping distance constant.
• Change target height while holding apparent size constant.
• Introduce airflow to deflect the jet without altering visual refraction.
• Measure shot errors trial by trial.
• Look for gradual correction and aftereffects.
• Use high-speed cameras to measure mouth motion and jet velocity.
• Present computer-generated targets to isolate visual variables.

Good experimental design changes one part of the perception–action chain while holding others as stable as possible.

Observation vs Inference

• Observation: after airflow begins, shots miss sideways.
• Observation: over repeated trials, errors become smaller.
• Observation: after airflow stops, shots temporarily miss the opposite way.
• Inference: the fish adapted its motor command to an expected perturbation.

Common Misconceptions and Better Models

Misconception	Better model
The fish sees the insect exactly where it is.	Refraction alters the apparent direction and size of aerial targets.
It avoids refraction by always shooting straight up.	Archerfish shoot successfully from many positions and viewpoints.
The water jet is a simple constant-speed squirt.	The fish actively changes mouth dynamics so the jet focuses near the target.
Accuracy proves the fish knows equations.	Sensorimotor systems can encode physical relationships without symbolic reasoning.
After prey falls, the fish simply chases it.	Predictive C-starts direct the fish toward the future landing point.
Motor adaptation and evolutionary adaptation are the same.	One is individual recalibration; the other is inherited population change.
Archerfish never miss.	They are highly capable but not perfectly accurate; errors are measurable and informative.

Checkpoint Questions

1. Why does an aerial insect appear displaced underwater?
2. What does Snell’s law describe?
3. Why is shooting vertically not a complete explanation?
4. How does the fish form a water jet?
5. Why can later water catch earlier water?
6. How does target distance alter jet control?
7. What does gravity do to the shot?
8. How did airflow experiments reveal motor adaptation?
9. What is an aftereffect?
10. What is a predictive C-start?
11. Why does target height matter after prey begins falling?
12. Why does accurate behaviour not prove human-style understanding?
13. How can a researcher use archerfish to study visual cognition?

Answer Key

Open after attempting the questions

1. Light bends when crossing from air into water, shifting apparent direction.
2. The relationship between incidence angle, refraction angle and refractive indices.
3. Wild fish attack from a variety of angles and still compensate.
4. It forms a tongue–palate channel and compresses water while controlling mouth aperture.
5. The fish can make later parts of the jet faster, causing focusing.
6. Longer travel requires different timing so the jet remains coherent and forceful at impact.
7. It bends the water trajectory downward.
8. Fish gradually corrected their shots and showed opposite errors after airflow removal.
9. A temporary error in the opposite direction after a learned perturbation is removed.
10. A rapid launch aimed toward the future point where prey will land.
11. Height affects fall time and therefore landing prediction.
12. Neural calibration can encode useful relations without language or equations.
13. The fish reports visual choices by shooting at controlled screen targets.

Can You Explain WHY?

• Why does the air–water boundary create a problem for both seeing and shooting?
• Why would a fixed jet work poorly across many target distances?
• Why is an aftereffect stronger evidence than one lucky accurate shot?
• Why does a falling insect require prediction rather than simple pursuit?
• Why can Physics be present in animal behaviour even if the animal cannot state a formula?
• Why is the archerfish useful to scientists studying vision?

Singapore Field Connection

Archerfish are part of Singapore’s mangrove fauna. NParks records both the banded archerfish Toxotes jaculatrix and spotted archerfish Toxotes chatareus at Sungei Buloh Wetland Reserve.

Mangrove waterways are ideal places to understand the hunting problem because insects occupy overhanging vegetation while fish remain in brackish water below.

Read NParks’ “Archers of Sungei Buloh Wetland Reserve” →

Try It With Light, Not With Fish

You do not need to make an animal perform for an experiment. The optics can be modelled safely with a cup or clear tank of water.

1. Place a coin or mark beneath or behind a transparent container.
2. View it from several angles through water and air.
3. Record apparent position changes.
4. Draw the normal at the boundary.
5. Predict whether the ray bends toward or away from the normal when entering water.
6. Separate what your eye reports from the actual object position.

Primary Science / PSLE Bridge

• Animals have adaptations for obtaining food.
• Light can change direction when it passes between materials.
• Forces change motion.
• Animals use sensory information to respond to their environment.
• Structure and function are connected.
• Observation and fair tests help explain behaviour.

Go Beyond Primary Science

Simple idea	Higher-resolution route
Light bends	Snell’s law, refractive index, virtual image geometry
Fish squirts water	Pressure, nozzle geometry, unsteady jets, momentum transfer
Fish adjusts aim	Motor adaptation, internal models, sensory feedback
Fish judges target	Visual constancy, optic tectum, object categorisation
Prey falls	Projectile motion, predictive interception, C-start biomechanics
Mangrove predator	Ecological niche, competition, predator–prey dynamics

Deep Science Window — The Jet Is an Actively Programmed Fluid

Archerfish do not merely choose direction. By adjusting mouth aperture over milliseconds, they alter the velocity profile inside the jet so that water converges near the target.

This means the biological motor programme controls not only body motion but the later dynamics of a fluid after it leaves the body.

Deep Science Window — An Internal Model Can Be Inferred From Error

If a fish gradually compensates for a steady perturbation and then makes an opposite error when that perturbation vanishes, researchers infer that the nervous system changed its prediction of the physical world.

This logic is also used in human motor-control research. We often learn what a system predicts by watching the structured errors it makes.

Deep Science Window — Evolution Built a Research Instrument

Most fish cannot easily indicate which picture they think is correct in a laboratory task. Archerfish naturally produce a precise directional shot, turning behaviour into a measurable output.

That lets neuroscientists study visual attention, object recognition and decision-making in a vertebrate whose neural architecture differs substantially from ours.

Evidence Boundaries

• Refraction compensation ≠ symbolic Physics. Accurate sensorimotor behaviour does not prove equation-like reasoning.
• Highly accurate ≠ infallible. Archerfish miss and individual performance varies.
• One species ≠ every archerfish. Most experiments use particular Toxotes species.
• Laboratory target ≠ full wild hunting ecology. Natural prey, currents and competition add variables.
• Motor adaptation ≠ evolutionary adaptation. Timescales and mechanisms differ.
• Predictive C-start ≠ conscious calculation. Fast neural circuitry can generate predictive behaviour.
• Jet mechanism ≠ fixed nozzle. Mouth dynamics change during a shot.

Manual Summary — KNOW → CONNECT → EXPLAIN → APPLY → CHECK

KNOW

Know refraction, refractive index, water jet, motor adaptation, C-start, visual constancy and predictive interception.

CONNECT

Connect refracted light to visual estimates, estimates to motor commands, mouth dynamics to jet force and falling prey to predictive movement.

EXPLAIN

Explain how one hunt coordinates optics, fluid dynamics, perception and fast motor control.

APPLY

Compare archerfish with diving birds, human throwing, robotic targeting or other animals crossing optical boundaries.

CHECK

Ask which part of the explanation belongs to Physics, which to physiology and which to learned calibration.

Where to Go Next

• Animal World | Bodies, Behaviour, Evolution and Living Systems
• eduKate Learning Manual: Whale Evolution
• The Living World

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Teaching Guide for Parents, Tutors and Teachers

For the people who teach because somebody depends on them.

The learner-facing manual should feel first like a hunting mystery and only later like a Physics lesson. The fish gives a reason to care about refraction before the formula appears.

Why Begin With “Seeing the Target in the Wrong Place”?

It creates a truthful perceptual contradiction. The image is not false—the ray really arrives at the eye from that direction—but the target’s geometric location in air differs from the underwater line of sight.

The Central Reasoning Model

light bends → target estimate must be calibrated → mouth shapes jet → gravity/wind perturb path → impact dislodges prey → prey falls → fish predicts landing point.

Why Stefan Schuster Is Here

His research programme models decomposition. “The fish is amazing” becomes a set of measurable problems: distance, jet timing, target size, falling trajectories, competition and rapid decision-making.

Teach in This Order

1. Show the hunt.
2. Introduce the air–water optical boundary.
3. Draw a refracted ray before giving Snell’s law.
4. Add the tongue–mouth jet channel.
5. Explain distance-dependent jet focusing.
6. Add gravity and airflow.
7. Use adaptation and aftereffects.
8. Finish with predictive C-starts and visual cognition.

Questions That Reveal Understanding

• If refraction shifts the image, why does the fish not always miss?
• Why must a long-distance jet be shaped differently?
• Why is an opposite error after airflow removal scientifically useful?
• What variables determine where a falling insect lands?
• Which claim in your explanation is directly observed and which is inferred?

The strange claim must become more true as the explanation deepens, not less. Every tangent—neuroscience, fluid dynamics, visual learning—must return home to one act: a fish knocking food out of the air.

Research Sources and Further Reading

• eLife (2024) — Motor adaptation in archerfish shooting
• Archerfish vision: visual challenges of a unique hunter
• Archerfish actively control the hydrodynamics of their jets
• Forces delivered by archerfish shots
• Archerfish learn to compensate for optical distortions
• Rapid depth perception and predictive C-starts
• Scientific Reports (2025) — Object identity in the archerfish visual system
• NParks — Archers of Sungei Buloh Wetland Reserve

eduKate Learning Manuals are written so that a learner can begin simply, a parent can teach confidently, and both can keep going until the school model opens into real Science.