In Hunting, Archer Fish Can Hit Prey Near And Far With Same Force

By on September 3, 2015
An archer fish (Toxotes chatareus) fires a stream of water from its mouth at an insect target overhead (not pictured). The water at the end of the jet can be seen gathering into a ball; the archer fish can control when the balling occurs depending on the distance to the intended target. This effect keeps impact forces consistent even when targets are at very different distances from the fish. (Credit: Nickolay Hristov, Issac Banks, and Morgan Burnette)

Scientists at Wake Forest University have made new discoveries relating to the hunting strategies of archer fish, which spit water at prey as a predatorial move. The findings could help advance human technologies, the researchers say, as well as shed light on the remarkable abilities archer fish have.

Previous research had answered a few questions about archer fish, namely that they spit streams of water at prey that form into oval-shaped globules just before impact. This occurs regardless of distance, so scientists studying the fish in the past had guessed that the forces of a nearby hit could be similar to those used by the fish to hit a target farther away. But they stopped short of proving it.

Wake Forest researchers have put the guess to the test by employing high-speed cameras to record archer fish hunting prey at up to 1,000 frames per second. They used a compression load cell to measure exactly what forces were at play and found that the distance makes no difference — the forces are the same.

“We need to slow down movements and behaviors so we can see what is going on. Often when these behaviors happen so fast, we can’t make sense of them without assistance,” said Morgan Burnette, a doctoral student at the university studying biology and leader of the investigation. “For reference, Hollywood movies are shot at 24 frames per second, so we’re talking about 20 to 40 times better temporal resolution.”

It is amazing how well archer fish can control the streams they shoot out. After all, they are each only about 3 inches long, and so the distances they overcome are very large by human standards. Burnette says that they can hit insects or other prey up to six body lengths away. If a human standing six-feet tall were to attempt such a feat, they would have to deal with a target 36 feet away.

The six-body-length limit was the farthest that Burnette tested the fish’s shooting abilities. The lower was just one body length away from the fish. And this set up another one of the study’s interesting finds.

“The individuals we had perform this task overwhelmingly (about 80 percent of the time) choose to shoot down the closest target first,” said Burnett. “This was surprising, given that the on-target forces for the distant targets were similar to those that we measured on closer targets, so the fish was choosing to shoot down the closer of the two targets for another reason.”

The findings illustrate just how useful archer fish are for answering the study’s questions. And they play up just how interesting the fish is.

“I think the archer fish is an excellent model to describe this because they’re sending projectiles of water from their mouth to dislodge targets on overhanging vegetation,” said Burnette. “This skill has many different variables that need to be considered,” like acquiring targets across the air and water interface, and accounting for target distance and target size, etc. “So what information does the fish need to collect in order to fire a successful shot?”

Clearly, there’s a lot of calculations going on that humans just can’t see. But there is promise in dissecting those further, Burnette says. The dynamics could be used in computer software to make robotics function better, or even in automated manufacturing processes.

“I think human technology can really benefit from what we learn from animals like the archer fish. It also gives us really great insight into the experience of the animal,” said Burnette. “Fish are not simply mindless automatons, and from the archer fish, we are learning that they are very aware of their surroundings and are using the information from their environment in some very complex ways.”

Top image: An archer fish (Toxotes chatareus) fires a stream of water from its mouth at an insect target overhead (not pictured). The water at the end of the jet can be seen gathering into a ball; the archer fish can control when the balling occurs depending on the distance to the intended target. This effect keeps impact forces consistent even when targets are at very different distances from the fish. (Credit: Nickolay Hristov, Issac Banks, and Morgan Burnette)

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