Fish
Archerfish
Toxotes jaculatrix · also called banded archerfish, spinner fish
The archerfish is a small fish that hunts the air. Hovering just below the surface of a mangrove creek, it looks up at an insect resting on a leaf, takes aim through the distorting water surface, and fires a bolt of water that knocks the prey down to be eaten.
Overview
Archerfish live in the brackish estuaries and mangrove forests of the Indo-Pacific, and also range up into fresh river water and out onto coastal reefs. They favour shaded, still or slow water overhung by vegetation, where insects and spiders are within shooting range. They are usually seen in small groups near the surface, silver with bold dark bars, the body flattened from side to side and the mouth turned up for looking and shooting upward.
The water jet
An archerfish’s spit is a piece of precision hydraulics. The fish forms a tube by pressing its tongue against a grooved channel in the roof of its mouth, then slams its gill covers shut to drive a pulse of water through it. By varying the timing, it makes the tail of the jet move faster than the head, so the stream gathers into a single heavy droplet at the moment it strikes — concentrating the force to dislodge an insect from its grip on a leaf up to two or three metres above.
Solving the physics
Two physical problems make this hard, and archerfish solve both:
- Refraction. Because light bends at the water surface, a submerged fish sees insects displaced from their true position, by an amount that depends on the angle. Archerfish move to advantageous positions and learn to aim at where the target actually is.
- Ballistics. The jet is pulled down by gravity on the way up, so the fish must aim slightly beyond and above the target, adjusting for distance.
They also react astonishingly fast to the falling prey, starting to turn and swim toward the predicted landing spot within about 40 milliseconds of the hit — beating tankmates to the prize.
Intelligence
Archerfish are a favourite model for studying cognition in fish. They learn by watching — untrained fish acquire the difficult skill of hitting moving targets simply from observing skilled fish — and they can be trained to discriminate human faces, spitting reliably at one learned face among dozens of similar ones.
Status
The banded archerfish is Least Concern and common throughout its wide range. It is collected for the aquarium trade, where its hunting behaviour makes it a popular display fish, and it depends on healthy mangrove and estuarine habitat — the loss of which to coastal development and aquaculture is the main long-term concern for the species.
Archerfish: common questions
How does an archerfish shoot water?
It presses its tongue against a lengthwise groove in the roof of its mouth, turning the two into a narrow barrel, then suddenly clamps its gill covers shut to force a pulse of water down that channel and out through pursed lips. The fish shapes the jet so that water at the back of the stream travels faster than the front, causing the jet to bunch into a heavy blob just as it reaches the target — delivering a concentrated impact that can dislodge an insect clinging to a leaf well above the surface.
How does an archerfish aim through the water surface?
Light bends as it passes from air into water (refraction), so an insect on a leaf appears to be in a different position than it really is, and the illusion changes with the viewing angle. Archerfish learn to correct for this, manoeuvring to a favourable angle beneath the target and adjusting their aim to hit the insect's true location, not its apparent one. They also account for gravity's pull on the falling water jet.
Can archerfish learn from each other?
Yes, and it is one of the clearer cases of social learning in a fish. Hitting a small, moving target above the water is a hard skill, but archerfish that have only ever watched trained tankmates shoot at moving targets can then do it themselves on the first try, without practice. They also recognise individual human faces in experiments, spitting at a learned face and ignoring new ones.
Do archerfish only spit for food?
Spitting is the signature behaviour, but archerfish are flexible hunters. They readily grab insects that land on the water, leap clear of the surface to pluck prey from low leaves, and take aquatic invertebrates and small fish. Young archerfish spit poorly and rely more on these methods; the spitting skill improves with size and practice, and larger fish often steal the prey that a smaller fish's shot knocks down.