A new international study led by Northwestern University has confirmed that fish look down when they swim, just like you might look down at the ground when you walk.

This is the first study to create a unified computer model of a zebrafish’s brain, natural surroundings, and spatially variable swimming behavior. By looking at this model, the researchers came to the conclusion that the fish’s odd habit of looking down while swimming forward is an adaptive behavior that evolved to help the fish stay stable, like when swimming against a current.

Fish self-stabilize in moving water to avoid being swept away. If a fish looks at other fish, plants, or trash, it might think it’s moving when it’s not. However, fish have a more consistent sense of their swimming direction and speed thanks to the riverbed’s stability.

“It’s similar to sitting on a train car that isn’t moving. If the train next to yours starts to pull to away from the station, it can trick you into thinking you are moving too,” says lead author Emma Alexander. “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. 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.”

The findings were published in the journal Current Biology today.

Alexander and her colleagues focused their research on zebrafish, a well-studied model organism. However, despite the fact that many laboratories have tanks filled with zebrafish, the researchers chose to concentrate on the fish’s natural habitat in India.

“It was recently discovered that fish respond to motion below them more strongly than motion above them. We wanted to dig into that mystery and understand why,” Alexander adds. “Many zebrafish that we study grow up in lab tanks, but their native habitats shaped the evolution of their brains and behaviors, so we needed to go back to the source to investigate the context for where the organism developed.”

The crew traveled to seven locations in India using camera equipment to film the shallow waterways where zebrafish exist in nature. The field team mounted a 360-degree camera to a remotely operated robotic arm after enclosing it in a waterproof diving casing. The camera was then lowered into the water and maneuvered using the robotic arm.

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

The scientists also monitored the movements of zebrafish inside an LED ball inside the facility. Fish, unlike humans, do not have to move their eyes to look around since they have a vast field of view. The scientists therefore played motion stimuli while the lights were on and observed the fishes’ reactions. When patterns developed on the tank’s bottom, the fish swam along with them, indicating that the fish were absorbing visual cues from looking downward.

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

The team then took information from the videos and put it together with information about how motion signals are stored in a fish’s brain. They applied the datasets to two previously developed methods for analyzing optic flow (or the movement of the world across our eyes or camera lenses).

In the end, they found that zebrafish do look down when swimming forward, both in the wild and in the lab. The researchers came to the conclusion that fish look down to see how their surroundings are moving and then swim against it so they don’t get swept away.

“We tied everything together into a simulation that showed that, in fact, this is an adaptive behavior,” adds Alexander, who led the computational part of the study. “The water surface is constantly moving, and other fish and plants are moving by. Fish are better off omitting that information and focusing on the information below them. Riverbeds have a lot of texture, so fish are seeing strong features they can track.”

This information not only tells us something about how fish act, but it could also help us design artificial vision systems and high-tech robots that are based on living things.

“If you were making a fish-inspired robot and you just looked at its anatomy, you might think ‘the eyes are pointing sideways, so I’m going to point my cameras sideways,” adds Alexander. “But it turns out that the eyes are pointing sideways because they are balancing several tasks. We think they point sideways because it’s a compromise — they look upward to hunt and downward to swim.”

Source:10.1016/j.cub.2022.10.009

Image Credit: Scott Eisen/Getty Images for BCH