In the same way that one looks at the sidewalk tiles while walking, fish look down when swimming, a new study by an international team of researchers has confirmed. As they conclude in the journal ‘Current Biology’, this behavior evolved to help fish stabilize when swimming upstream.
As the water moves, the fish are constantly trying to stay in place, instead of 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. However, the stable riverbed below them gives them more reliable information about the direction and speed of your swim.
“It’s similar to sitting in a train car that doesn’t move. If the train next to you starts to move away from the station, it can trick you into thinking that you are also moving, ”explains Emma Alexander, from Northwestern University (USA) and responsible for the study. “The visual signal from the other train is so strong that it overrides the fact that all your other senses are telling you that you are standing still. That is exactly the same phenomenon that we are studying in fish. There are many misleading signals of movement above them, but the most abundant and reliable signals come from the bottom of the river », she stresses.
To conduct the research, Alexander and his collaborators focused on zebrafish, a well-studied organism. But while many labs have tanks full of zebrafish, the team wanted to focus on the fish’s native environment in India.
He lives naturally
“It was recently discovered that fish respond to movement below them more strongly than to movement above them. We wanted to delve into that mystery and understand why,” says Alexander. “Many zebrafish we studied grow up in laboratory 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 in which the organism developed.”
Armed with camera equipment, the team visited seven sites in India to collect video data from shallow rivers, where zebrafish naturally live. The field team encased a 360-degree camera inside a waterproof diving case and attached it to a remote-controlled robotic arm. They then used the robotic arm to submerge the camera in the water and move it around.
“It allowed us to put our eyes where the eyes of the fish would be, to see what the fish see,” says Alexander. “From the video data, we were able to model what-if scenarios in which a simulated fish moved arbitrarily through a realistic environment,” he adds.
Wait for me
Back in the lab, the team also tracked the zebrafish’s movements inside a ball of LEDs. Because fish have a large field of vision, they don’t have to move their eyes to look around like people do. The researchers then played motion stimuli through the lights and observed the fish’s responses. When patterns appeared on the bottom of the tank, the fish swam along with the moving patterns, further evidence that the fish were taking their visual cues by looking down.
“If you play a video with moving rays, the fish will move along with the rays,” says Alexander. “It’s like they’re saying ‘wait for me!’ In the behavioral experiment, we counted the beats of its tail. The more they waggled their tails, the more they wanted to keep up with the moving rays.”
The team then extracted data from their videos and combined it with data on how motion signals are encoded in the fish’s brain. Ultimately, they found that in both settings, in the wild and in the lab, zebrafish look down when swimming forward. The researchers concluded that the fish look down to understand the movement of their surroundings and then swim to counteract it, to avoid being swept away.
“We put it all together in a simulation that showed that this is indeed adaptive behavior,” says Alexander. “The surface of the water is constantly moving, and other fish and plants are moving. Fish are better off ignoring that information and concentrating on the information below them. Riverbeds are very textured, so fish see strong features that they can track.”
This information not only provides insight into fish behavior, but could also inform designs for machine vision systems and sophisticated bio-inspired robots.