The Underwater Sharpshooter

Banded archerfish.

Source: James St. John/Wikimedia Commons

Stefan Schuster, a professor of animal physiology at the University of Bayreuth in Germany, has spent much of the last two decades diving into the extraordinary abilities of archerfish. These small fish, native to the mangrove-lined estuaries of Southeast Asia and Australia, are well-known for one peculiar behavior: their unique method of hunting land-based prey.

Archerfish spit jets of water to dislodge insects and other small animals resting on twigs or leaves above the water’s surface. The fish are remarkably accurate shots, able to bring down prey up to 3 m (10 ft) above the water’s surface. (Watch a video about the behavior here.)

And according to Schuster and others who work with them, archerfish will happily shoot at just about anything.

“You can train them to shoot at artificial objects that do not fall into the water and reward them with something else,” he says. “This makes many experiments on the shooting behavior possible. And everyone in the lab has the impression that it is actually fun for them to contribute to the experiments!”

Spit take

For a study, a few years back, Schuster and his colleague Peggy Gerullis trained archerfish to fire their water jets from fixed positions in their tanks. They discovered that the fish open and close their mouths to subtly adjust the shape and speed of their jets, depending on the distance of the target.

Archerfish shooting.

Source: Pearson Scott Foresman/Wikimedia Commons

During the analysis of the high-speed videos of two trained fish, the researchers noticed something strange. The archerfish were stationary when they released their jets. But right before the fish shot, they started moving their pectoral fins in a forward direction. These movements appeared to be linked to the shooting.

So Schuster and Gerullis analyzed their videos again, this time with their eyes on the fins. They also reached out to fellow archerfish researcher Caroline Reinel, who looked for the fin movements in videos from experiments with untrained archerfish shooting freely. She found that the fin movements were tightly coordinated with every archerfish shot.

“We all found it impressive that every single fish was doing this rapid, forward flap of the pectoral fins,” says Schuster. “We think it’s an important and previously overlooked component of archerfish shooting.”

A little help from my fins

In a paper published in the Journal of Experimental Biology, Schuster, Gerullis, and Reinel describe these characteristic rapid fin maneuvers and demonstrate that they are synchronized with shooting.

The researchers found that, slightly before each shot, when the fish is stationary, its pectoral fins begin to flap forward rapidly. The onset and duration of this forward flapping motion seemed to depend on the height of the target.

Schuster and his colleagues say the fin movements likely play a role in the archerfish’s unique ability to fire powerful, long-distance water jets. The timing of the fin maneuvers relative to the expected recoil forces from the jet suggests they are needed to keep the shooting fish stable.

“This is just one of the behavioral specializations that make archerfish fascinating,” says Schuster. “It is probably the sum of their abilities that makes these fish so special.”

Banded archerfish.

Source: I, Chrumps/Wikimedia Commons

A bevy of behavioral adaptations

In nature, archerfish are surrounded by numerous competitors. If one archerfish is successful in dislodging terrestrial prey, and it falls to the water surface, the shooter must make rapid and accurate decisions to get there before any other fish.

“If all the archerfish could do was down prey, the prey would be lost,” says Schuster. Other fish, some better equipped to detect water surface waves, could potentially beat the shooter to the spot where the prey fell.

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According to Schuster, every archerfish shot requires a series of complex calculations: Not only do the fish have to aim their water jets while compensating for refraction and distance, but they also have to determine exactly where their prey will land and get there first.

Schuster is investigating these high-speed decisions in archerfish and has discovered that, based on watching the initial movement of the falling prey, the fish make a rapid stop that turns them toward where the prey will land and gives them the speed to arrive simultaneously with the prey.

“This means that as soon as something starts falling, the fish are already on their way and at the right spot before the other fish even notice that something has happened,” says Schuster. “And they make his decision in almost no time, just 40 ms is enough.”

Even with these recent discoveries, Schuster says our knowledge of archerfish is still limited.

“In the last 20 years, archerfish have always been surprising,” he says.

“There are just some types of animals that have to be able to do amazing things in order to survive. If you look closer and closer, you’ll always find more.”