Fish Look Down When They Swim, Study Finds

When you’re worried about stumbling, you look down at your feet when you walk. Similarly, fish look down when they swim, a new study finds.

When water moves around them, fish have to find ways to stabilize themselves so they don’t get swept away. If they focus on other fish, plants, or other objects in the water, they might wrongly believe they are moving. Instead, when they look down and fixate on the river bottom, they’re able to more correctly gauge their movement and speed.

It’s like when you’re sitting in a train that isn’t moving, but the train next to you starts to pull out of the station, says study corresponding author Emma Alexander, an assistant professor of computer science at Northwestern University. You might think you’re moving too.

“The visual cue from the other train is so strong that it overrides the fact that all of your other senses are telling you that you are sitting still,” Alexander says. “That’s exactly the same phenomenon that we are studying in fish. There are many misleading motion cues above them, but the most abundant and reliable signals are from the bottom of the river.”

Researchers were intrigued by a recently discovered behavior where fish respond more strongly to motion below them than above them, Alexander tells Treehugger. To completely understand that behavior, they turned to zebrafish, which are often used for research and are well-studied. They had a team following the fish in a lab and another in India to analyze the fish in their natural habitat.

“We observed the behavior under tightly controlled conditions in the lab, and found by observing motion signals in the natural environment that the behaviors we had observed in the artificial setting were advantageous in the habitat that drove their evolution,” Alexander says.

Watching Zebrafish Swim

Researchers recorded videos in seven sites around India in shallow rivers where zebrafish are found. They placed a camera inside a waterproof case and attached it to a remotely controlled robotic arm. They submerged the camera in the water to see what the fish were doing.

In the lab, they tracked fish movements inside a tank. When they played moving lights along the bottom of the tank, the fish swam along with the patterns, changing their direction when the light movement changed.

“We found that when the lights were below them, swim responses were strong—the fish really cared that those stripes were passing them by. But when the lights were above them, they were more likely to ignore them,” Alexander says.

“This matched our computational model, which showed that motion signals from the riverbeds in their native environment were abundant and reliable while motion signals from beside or above them were scarce and often misleading.”

The results were published in the journal Current Biology.

Fish Inspiring Robots

Analyzing the video and data, researchers found that in both the wild and the lab situations, zebrafish looked down when they were swimming forward. They came to the conclusion that the fish looked down in order to understand the motion around them and then they were able to avoid being swept away in the current.

Understanding that ability not only helps researchers learn more about fish, it may also give them information when designing some robots.

“These findings reveal the underlying design principles shaping fish motion responses, which can help us build artificial vision systems that benefit from the lessons of hundreds of millions of years of evolution behind them,” Alexander says.

“The study provides a general framework for understanding spatially biased motion responses so that behaviors can be understood in the context of specific environments and neural processing strategies. To put it another way, if a model like ours predicted a different motion response from what has been observed, it would probably mean that there's something about the animal's habitat or brain that scientists don't yet understand.”