The Archerfish

This little fish, about five inches long, may seem pretty ordinary. But it has evolved a remarkable method of catching its insect prey.

Banded Archerfish

There are seven species of Archerfish in the genus Toxotes, all of them native to Indonesia, the Philippines, Australia, and the southern Pacific islands. Most are just three or four inches long, though the largest, the Large-Scaled Archerfish (T chatareus) reaches up to 16 inches. In habitat, they are found mostly in brackish estuaries and mangrove swamps, but also sometimes venture into freshwater rivers and streams.

The species seen most often in public aquariums is the Banded Archerfish, T jaculatrix. They are approximately hand-sized and triangular in shape, with a silvery body and dark crossbands. Although they are occasionally seen on pet dealer lists, they do not breed easily in captivity and are not often available except as wild-caughts.

Although they are attractive fish, they are popular public aquarium exhibits because of their unusual way of hunting prey: the Archerfish have built-in squirt guns. When they spot an insect on a leaf or branch overhanging the water, they creep up to the surface, take careful aim, and suddenly squirt a jet of water that knocks the prey off its perch and into the stream, where the fish can easily grab it.

When the first reports began to arrive in Europe from Java of an odd fish that could shoot at its prey, biologists simply did not believe it and dismissed the story as a hoax. But when live specimens began to arrive, they found it was true. The fish were able to fire their water “arrows” with remarkable precision. They also found that this was a learned skill: young fish were not very accurate with their shots, and often had to make several attempts before they were successful. Older experienced fish, on the other hand, rarely missed. They were even capable of “shooting” a flying insect in mid-air.

For centuries, biologists did not really know how this odd “weapon” worked. At first, it was believed that the fish was just rapidly pushing a jet of water out through its lips at the target. In the roof of the Archer’s mouth is a groove that the fish can cover with its tongue to produce what is, in effect, the barrel of a water pistol. By opening its gill flaps to let in water and then suddenly closing them, the Archerfish produces a sharp increase in pressure which drives water through this channel and out its mouth.

But some basic physics rules out this simple explanation. When a jet of water travels through the air, its surface tension always breaks the stream into a series of smaller blobs or droplets, and air resistance slows these down significantly as they travel. Thus, the range of the water jet and its force on impact is limited. Archerfish, however, are able to hit targets up to five feet away (some twenty times the length of their body), with sufficient power to knock even large insects off their feet. Clearly, something else must be going on.

So some scientists postulated that there was some sort of spring at work, which would store energy as tension and then, at the moment of use, release it and use that energy to propel the water through the air. This is how most other biological “engines”—such as the shooting tongue of a chameleon—work. But when the Archerfish was dissected and examined, there was no structure found in its mouth that could serve such a function. Nor were the muscles in its jaws and cheeks anywhere near strong enough to produce such a powerful jet.

It wasn’t until 2012 that the mystery of the Archerfish’s “arrows” was solved, by a team of physicists in Italy led by Alberto Vailati. Using high-speed cameras, they shot dozens of hunting fish and studied them frame by frame. And what they found was amazing.

By looking at the water jet, the scientists were able to confirm that it was indeed made up not of a single continuous stream of water, but of a train of smaller droplets. There was something unusual, however: this configuration changed over time. As the water left the fish’s mouth, it was strung out in droplets like pearls on a necklace. But as it traveled through the air, the droplets in the rear of the stream were accelerating faster than those at the front. At a particular point on the way to the target, all of the water droplets would meet in mid-air and merge together to form one giant blob of water that was large and powerful enough to pummel the insect target, knock it off its perch, and stun it.

But there was something even more interesting: another team in Germany, led by Stefan Schuster, found that even when the Archerfish was shooting at insect prey located at different ranges, the water droplets would still always merge just before impacting the target. Somehow, the fish was able to adjust the water stream while aiming, giving it the proper timing so it always converged at precisely the correct instant to have the maximum effect, whether it was shooting at close or long range. Now, the mystery was how the fish accomplished this.

To figure out how this was done, Schuster had to look at one of the Archerfish’s unique characteristics—its tube-shaped snout. It had always been assumed that this was a simple aiming device, like the barrel of a gun. But, watching the high-speed video, it was discovered that the fish’s mouth was actually the secret to the “water arrow’s” power.

As anyone who has ever put their thumb over the mouth of a garden hose knows, one of the basic principles of hydrodynamics holds that when the same flow of water is put through a smaller outlet, it moves faster. The Archerfish, Schuster discovered, was also taking advantage of this law of physics. As it shot its stream of water, the fish would change the shape of its mouth opening, making it wider at the beginning and narrower at the end. This had the effect of accelerating the droplets at the rear of the stream more rapidly than those at the front, allowing them to catch up in mid-air and merge together to form the “impact blob”. Even more remarkably, by moving its mouth in subtly different ways, the fish was able to adjust this rate of acceleration for different ranges, using exactly the correct speed to allow the droplets to always merge at the right instant, just before impact, no matter how far they had to travel. At short range, the rearmost droplets are accelerated rapidly so they catch up quickly: at long range, they move more slowly and take more time to merge together.

And because the faster-moving rear droplets push against the slower-moving ones in front of them as they merge, the final blob of water is moving at its fastest speed just before impact, increasing its effectiveness.

All of this involves some complex math and sophisticated hydrodynamics. So does this mean that the Archerfish is the Einstein of the fish world? Well, no—none of this is happening consciously. It is rather like a human throwing a rock at a moving target: there are lots of complicated math and physics that need to be solved, but all of it is done unconsciously and is carried out by subtle muscle movements and adjustments that lead effortlessly to the desired result. The Archerfish does the same thing. Just as humans learn over time to throw a baseball and get better at it, the fish learn as they get older and more experienced how to better adjust their aim at insect prey. But for humans, the equivalent accomplishment would be to throw a dozen small rocks in an instant, adjusting the speed of each one to make them progressively faster so they all landed at the target at precisely the same time.

Not bad for a fish.

 

 

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