The archerfish’s water arrows are powerful enough to dislodge insects from leaves and branches – now physics has revealed just how impressive the missiles really are

As a boy, Stefan Schuster would regularly visit his local Zoological Gardens in the German city of Stuttgart for one purpose: to see the archerfish at feeding time. He had read that these denizens of mangrove-lined estuaries in South and South East Asia have a peculiar behaviour for fish. They hunt land-based animals.

With splotches of black across their silvery backs, archerfish blend into the dappled shade cast by the dense foliage above their watery environment. Hidden, they move slowly towards any insects, spiders, or small lizards resting just above the water’s surface, carefully positioning themselves for the attack.

Then, after firmly closing their gills, they spit a fine jet of water at their unsuspecting prey, knocking it from its perch. Dinner is served, wriggling and helpless on the water’s surface.

That’s in the wild. But in captivity archerfish are usually spoon-fed; their predatory instincts wasted. Despite his efforts, therefore, young Schuster never witnessed an archerfish hunt.

But he never lost hope. Many years later, after receiving his first research position at the University of Freiburg in Germany, he decided to purchase some pet archerfish. “There was no scientific interest involved at all,” he says. He just had a spare aquarium at the time.

Not only had Schuster seen the archerfish shoot, he was the target

Remembering his childhood travails, he was expecting to have to wait months to see the fish shooting. It actually took a matter of seconds. “I just put them into the tank and they fired right away into my nose,” Schuster recalls.

Not only had he seen the archerfish shoot, he was the target. Delighted with a childhood dream coming true, he hardly noticed the tinge of pain: the water had stung, like a tiny flick to the nose.

But that minor sting is actually surprisingly difficult to explain. Ever since these fish were first discovered over 250 years ago, biologists have wondered how the archerfish’s jet of water packs such a powerful punch. Only in recent years have laboratories around the world finally uncovered the secrets of nature’s most adept archers.

In 1764, news of an unusual insect-hunting fish native to Batavia (modern-day Jakarta) reached naturalists in Amsterdam. In a short letter, one resident on the island wrote, “With a surprising dexterity, it ejects out of its tubular mouth a single drop of water, which never fails striking the fly into the sea, where it becomes its prey.”

 How could such a small fish project such a powerful missile?

Ichthyologists and naturalists in England weren’t completely sold on such reports, however. They were too bizarre to believe. Animals of the air – such as birds and bats – will often hunt fish, of course, but residents of the water rarely hunt for prey above the surface. Archerfish not only defied this ecological dogma, but did so without ever leaving their aquatic realm.

As the fish were shipped from mangrove to aquaria in Europe, however, the truth quickly became apparent. The original accounts were correct. From then on the question wasn’t if, but how they did it. How could such a small fish – the seven species of archerfish in the Toxotes family rarely grow longer than 10cm – project a missile powerful enough to dislodge well-anchored insects from their perches – or to sting the face of a human, for that matter?

In the 20th Century, biologists took a closer look at the fish’s anatomy for clues. First, they dissected, counted, and weighed the muscles within the jaw. Could they account for the jet’s force?

Not quite. To produce the jet of water, a study published in 1976 concluded, the amount of stress the muscles would be exposed to would be far beyond anything recorded by biologists before. Something else was going on.

What about a catapult mechanism? In such a scenario, energy from slow muscle contraction can be gradually stored in other, more rigid, parts of an animal’s body, then released all at once – which is what happens when a human archer releases the bow’s string and fires an arrow. This mechanism is also at the core of the chameleon’s projectile tongue and the high jump of insects such as grasshoppers, crickets, and planthoppers.  

 Biologists were stumped. It would take physics to explain the archerfish mystery 

One problem: even after several thorough examinations of the fish’s skeleton, no such energy storage element came to light. There was simply no piece of bone that seemed like it could double as a catapult.

The theory was well and truly put to bed in 1986 when researchers measured the amount of electrical activity the muscles in the archerfish’s jaw produced before and during emitting their water jet. If a catapult mechanism existed, some of the fish’s muscles would contract – and produce an electrical charge – generating energy to be stored elsewhere and then released suddenly as the archerfish spits. But the muscles didn’t contract this way. The catapult idea was tossed in the bin.

Biologists were stumped. And for good reason. What they didn’t know was that their anatomical-based approach to the problem was doomed to fail from the very start. It would take a different branch of science to solve the mystery. A team of physicists was needed.  

In 2012, Alberto Vailati, professor of fluid dynamics at the University of Milan, and his colleagues took a look. But not at the fish – they were more interested in the jets of water they fired. In their lab, they carefully tracked the hydrodynamics of the water during its flight from fish’s mouth to a fake fly above the surface. And in the process, they were finally able to uncover where the water’s mysterious power came from.

Using a high-speed camera, able to take a snapshot every 1,000th of a second, the researchers found that the speed of the water leaving the fish’s mouth increased over time. That’s to say, the water leaving the mouth last travelled through the air more quickly than the water that left the mouth first. The water jet’s tail was playing catch-up with the head. 

I cannot think of a weapon developed by a human being which increases its velocity when it approaches the target

It was a key observation. Under normal circumstances a jet of water won’t simply fly through the air as one continuous long cylinder – instead it tends to reduce its surface area by separating into smaller water droplets. You can see this process in action when you fire a water pistol. The phenomenon should render the archerfish’s weapon useless, because its target would simply be hit by a stream of tame water droplets.

But because the water droplets at the rear are travelling faster than those in front, they push them along. Then they merge.

“The tip of the jet acts as a water balloon which gets injected continually, progressively, by the tail of the jet,” says Vailati. “In this way, you have the development of a single large droplet at the tip of the jet.”

 

Both the speed and size of the super droplet increase as it nears its target. Momentum is maximised. “I cannot think of a weapon developed by a human being which increases its velocity when it approaches the target,” says Vailati. “That’s the most effective way to hit the target.”

The archerfish’s secret apparently lay in the way it controlled water

In technical detail, the final impact can pack a punch of 3000 watts per kilogram. Vertebrate muscle maxes out at around 500 watts per kilogram. “In order to achieve what the fish does with this hydrodynamic amplification,” Vailati says, “the mass of the muscle inside the mouth would [need to] be at least six times larger in reality.”

In other words, the archerfish’s secret apparently lay in the way it controlled water – not in any special properties of its muscles or skeleton. Problem solved, then? No more archerfish arcana? Not completely. In 2014, the biologists were back.

After reading the Vailati study, Schuster still thought a vital piece of the puzzle was still missing. He noticed that each archerfish used in the study was trained to target fake prey only 12cm above the water’s surface. But in the wild, archerfish are known to have a large shooting range, extended up to 2m.

How do they aim their self-accelerating bullet of water over such a large range? Again, he looked to high-speed cameras for answers. He took a series of snapshots, each 1000ths of a second apart, to study the fish’s mouth as it shot out a water jet.  

Poring through these images, Schuster and his colleagues noticed something very subtle, and unexpected. When producing a jet of water, the fish’s mouth is in constant motion, changing the size of the opening as the water is ejected.

Insects, spiders, or lizards will always feel the full force of the archerfish’s jet 

To begin with, the mouth gradually becomes larger, allowing an unrestricted release of water. But then, the mouth starts to close again, and the opposite occurs. “The opening is smaller, and the liquid has to flow faster,” says Vailati, commenting on Schuster’s work. Like putting your finger over the end of a hosepipe, the water travels faster and further. 

By modulating this opening and closing mechanism, archerfish can fine-tune the speed of the jet’s tail. If the fish waits longer to begin closing its mouth, the fast-travelling tail of the jet is released later, so it will take longer to catch up to the tip.

That’s useful in order to form the powerful super droplet later, to target prey that is farther away. To target prey that is closer, in contrast, the archerfish simply closes its mouth earlier so the super droplet can form sooner. Regardless of how far away they sit, insects, spiders, or lizards will always feel the full force of the archerfish’s jet. 

Archerfish have crafted something more powerful than is physically possible by their nature alone. By exploiting the physical laws outside of the body, they don’t require any specialised apparatus such as large muscles or a catapult mechanism.

They use tools. Just as our ancestors, primate relatives, and some species of crows craft their own wood and stone implements for grooming, hunting, or feeding, archerfish have learned to fashion weapons. They whittle arrows from water. Their material is also the world they swim in. They have unlimited ammo. This allows archerfish to be very trigger-happy, commonly firing in sequential rounds to ensure prey capture. 

The parallels between this fish and our own species don’t end at tool use. When the archerfish was first described back in the 18th Century, it was dubbed the “jaculator” fish, in reference to the action of throwing, pitching, or hurling. This antique alias may have been very appropriate. Archerfish spit just like we throw.

Imagine throwing a rock at a target. Now move the target further away. Throw again. If you’re a good shot, the swing of your arm and the timing of the rock’s release from the hand will be modified according to the target’s new position. Speed and precision have to be scaled up. The orchestration of muscles is fine-tuned.

For our ancestors, this approach would have been rewarded with an increase in successful hunts. Hitting more distant targets harder and more often would have required a greater cognitive backing. This has led some researchers to propose that the development of throwing played a large part in the ballooning evolution of our hominin ancestors’ brains – more neurons, more brainpower, more food. 

Throwing stones is one thing that remains uniquely human – and archerfish spit like we throw

Today, this evolutionary legacy permits spectacular feats of sporting ability, such as high-speed pitching in baseball, bowling in cricket, and three-pointers in basketball.

It’s a defining feature. Accurate, powerful throwing remains a cornerstone of our species. “Not many people consider this the highlight of human culture,” says Schuster, “but throwing stones is one thing that remains uniquely human.” 

Yet archerfish achieve similar feats with a liquid. Just as we can adjust the strength of our throws, they can finely tune their jets to hit targets of varying distance. “It requires temporal precision of the archerfish in just the same way,” says Schuster.“It is analogous to the celebrated, uniquely human capability of directed and precise throwing of stones.”

And archerfish blow our skills out of the water. A thrown stone will slow down as it approaches the target, a victim of gravity and air resistance. But an archerfish’s super droplet does the opposite – it accelerates. 

In order to achieve the same abilities as these fish, humans would have to throw many stones one after the other in quick succession, each one slightly faster than the one before, says Vailati, “so that you have a giant super stone hitting the target.”

Schuster thinks that the archerfish’s unique hunting style may have catalysed greater intelligence in these fish – just as throwing is posited to have done for our ancestors. Any neurons that have evolved for spitting can be used for other tasks, from adjusting the archerfish’s aim according to the refraction of light into water, to gauging where their prey will land on the water’s surface so that the hunter can be confident of claims its prize first. 

The neurons may even a role to play in social learning. Without firing one jet of water, captive archerfish can learn new tricks – such as hitting moving aerial prey – simply by watching others. “By studying them you cannot escape seeing that they are really clever fish,” Schuster says. “It’s uniquely fishy.”