12 OCTOBER 2006
How's a hungry fish supposed to make a living in the shallow water below tropical mangrove trees? The branches just above the water may be crawling with meat, in the form of insects and lizards that probably are giving little thought to the hungry fish below.
But fish don't have arms, so the meat is usually safe.
But indifference is unwise if the waters happen to contain archer fish -- the water pistol of the tropics, the spritzer of the seas. This bizarre bit of biology shoots a stream of water so accurately it can dislodge an insect or small lizard from a branch a full meter above the water.
Movie: Courtesy T. Schlegel.
Archer fish live in the Indian and Pacific Oceans, from Sri Lanka to northern Australia, says Stefan Schuster, associate professor of biology at the Institute for Zoology in Nurnberg, Germany.
It's fair to say that Schuster is fascinated by these freaky fish, which can grow to 25 centimeters in length. In test tanks, the archer fish can hit a target with an accuracy of 1 to 2 millimeters, Schuster says. Usually, one spritz is enough to knock the prey into the water, but the fish can fire at five shots a second if necessary, Schuster has found.
Photo: Courtesy V. Runkel, S. Schuster, University of Erlangen
A machine-gun water pistol
Such long-range accuracy is unique to archer fish, Schuster says. "Shooting at this level of precision is not known" in any other fish. However, some of the labyrinth fish, native to Southeast Asia, can strike food with a water jet across a distance of a few centimeters: "They have at least have the concept that there is something edible in the air."
Schuster found that archer fish can start hunting early, when he noticed a two-centimeter fish in Thailand shooting insects. In the lab, his research group trains fish to shoot at black spheres by rewarding a successful strike with a food pellet. Fish that are just three centimeters long can already hit targets from 20 to 30 centimeters. The spray is pumped by the gills through a groove in the top of the fish's mouth.
Archer fish often try to hunt solo, so they don't have to share their meal. As you can see from the video, freeloading fish often try to filch free-falling food. The lurking fish start heading toward the prey's landing point just 0.1 second after the stream strikes the target, the biologists have found.
It takes energy to shoot a stream of water, and fish, unlike people, don't benefit from wasting energy. You might think a critter as dumb as a fish would just spray the same amount of water at every target, but Schuster and his doctoral students, Thomas Schlegel and Christine Schmid, have just reported that these fish adjust the volume of their shot according to the size of the prey.
This adjustment is quite sophisticated. Naturally, it takes more force to dislodge a larger animal, but how much more? To answer, you must understand that insects and small lizards grip smooth surfaces via van der Waals forces. For reasons we can't easily explain, the total adhesion from van der Waals forces increases according to the prey's length, not its surface area or volume.
Shoot first, ask questions later
Using high-speed video, the team measured how far the target moves after being struck, which indicates how much energy the stream is carrying. Their data showed that the volume of each shot tracked target length (not area or volume) quite closely.
To be sure of scoring lunch, the fish shoot about 10 times as much water as the minimum needed to knock the prey lose, but the trend was still clear: the increase in force paralleled the length of the target.
The archer fish is an energy-conscious fish. The lab-trained fish had no incentive to adjust the stream size to the target because they got rewarded with exactly the same amount of food whenever they hit the target, Schuster says. "We did not expect that they would adjust the shot to target size, because they did not have to. A weak shot was good enough. Over two years of training, there was no need to adjust the force, but they did it anyway."
Evolution, or perhaps the simple need for lunch, has given the archer fish another means of saving energy -- focusing the stream on the target. Schuster says that high speed analysis of the impact shows that "the shot has the form of a water jet. I don't understand why, but it is focused at the time of impact. It makes a really sharp impact, like a bullet."
— David Tenenbaum
• Archer fish Shots Are Evolutionarily Matched to Prey Adhesion, Thomas Schlegel, Christine J. Schmid and Stefan Schuster, Current Biology Vol 16 No 19.