Why do fish look down when they swim

Just as you may look down the sidewalk while walking, fish look down while swimming, a new study by an international collaboration led by Northwestern University has confirmed.

The study is the first to combine simulations of the brain, native environment, and spatially varying swimming behavior of zebrafish in a single computer model. By analyzing this pattern, the researchers concluded that this quirk – looking down while swimming forward – is an adaptive behavior that evolved to help the fish stabilize, such as when swimming against the current.

As the water moves, the fish are constantly trying to stabilize themselves to stay in place, rather than being swept away by a moving current. Focusing on other fish, plants, or debris can give the fish a false sense that it is moving. The stable river bed below them, however, gives fish more reliable information about their swimming direction and speed.

“It’s like sitting on a train car that isn’t moving. If the train next to yours starts pulling away from the station, it can make you think you’re moving too,” said Emma Alexander of Northwestern, who conducted the study. “The visual signal from the other train is so strong that it overrules the fact that all your other senses are telling you that you are standing still. This is exactly the same phenomenon that we study in fish. There are many signals misleading motion above them, but the most abundant and reliable signals come from the bottom of the river.”

The study will be published on November 2 in the journal current biology.

Alexander is an Assistant Professor of Computer Science at Northwestern’s McCormick School of Engineering, where she directs the Bio Inspired Vision Lab.

Return “to the source”

To conduct the research, Alexander and his collaborators focused on zebrafish, a well-studied model organism. But, although many labs have tanks full of zebrafish, the team wanted to focus on the fish’s native environment in India.

“It was recently discovered that fish react more strongly to movement below them than above them. We wanted to dig into this mystery and understand why,” explained Alexander. “Many of the zebrafish we study grow up in lab tanks, but their original habitats shaped the evolution of their brains and behaviors, so we had to go back to the source to study the context in which the organism has grown.”

Armed with photographic equipment, the team visited seven sites across India to collect video data from shallow rivers, where zebrafish naturally live. The field crew encased a 360-degree camera in a waterproof dive case and attached it to a remote-controlled robotic arm. Then they used the robotic arm to dip the camera in the water and move it around.

“It allowed us to put our eyes where the eyes of the fish would be, so it’s seeing what the fish see,” Alexander said. “From the video data, we were able to model hypothetical scenarios where a simulated fish moved arbitrarily in a realistic environment.”

‘Wait for me!’

Back at the lab, the team also tracked the movements of the zebrafish inside an LED ball. Because fish have a wide field of vision, they don’t need to move their eyes to look around like people do. So the researchers played motion stimuli through the lights and observed the fish’s responses. When patterns appeared on the bottom of the tank, the fish swam with the moving patterns – further evidence that the fish were taking their visual cues by looking down.

“If you play a video with moving stripes, the fish will move with the stripes,” Alexander said. “It’s like they’re saying ‘wait for me!’ In the behavioral experiment, we counted their tail beats. The more they waggled their tails, the more they wanted to follow the moving stripes.”

The team then extracted data from their videos and combined it with data on how movement signals are encoded in the fish’s brain. They fed the datasets into two pre-existing algorithms used to study optical flow (or the movement of the world through our eyes or camera lenses).

Ultimately, they found that in both scenarios – in the wild and in the lab – zebrafish look down as they swim forward. The researchers concluded that fish look down to understand the movement of their surroundings, then swim to counteract it, to avoid being swept away.

“We tied it all together in a simulation that showed it’s actually adaptive behavior,” said Alexander, who led the computer part of the study. “The surface of the water is constantly moving, and other fish and plants are passing by. Fish are better off omitting this information and focusing on the information below them. Riverbeds have a lot of texture, so the fish see strong features that they can follow.”

Build better robots

Not only does this information provide insight into fish behavior, it could also inform the design of sophisticated machine vision systems and bio-inspired robots.

“If you were making a robot modeled after a fish and you just looked at its anatomy, you might think ‘the eyes are pointing to the side, so I’m going to point my cameras to the side,'” Alexander said. “But it turns out the eyes point sideways because they balance multiple tasks. We think they point sideways because it’s a trade-off: they look up to hunt and down to swim. “

The study is titled “Optical Flow in Natural Zebrafish Habitats Supports Spatial Bias in Visual Self-Motion Estimation.”