04.11.2022

Why fish look down when they swim

New study of an international team with participation of the University of Tübingen

Simulations show quirky behavior helps fish estimate swimming direction and speed

• Scientists recently discovered that fish look down when they swim
• In a new study, an international team led by Northwestern University explores the mystery behind this behavior
• The researchers combined data from zebrafishes’ brains, native environment and behavior into one computational model
• Through this model, the team concluded that riverbeds provide more abundant and reliable visual cues

Just as you might look down at the sidewalk as you walk, fish look downward when they swim, a new study by a Northwestern University-led international collaboration has confirmed. Professor Aristides Arrenberg from the University of Tübingen’s Werner Reichardt Center for Integrative Neuroscience and Institute of Neurobiology was involved in the research. The study has been published in the journal Current Biology. 

The study is the first to combine simulations of zebrafishes’ brains, native environment and spatially-varying swimming behavior into one computational model. By analyzing this model, the researchers concluded that this quirk — looking down while swimming forward — is an adaptive behavior that evolved to help the fish self-stabilize, as when swimming against a current.

As water moves, fish are constantly trying to self-stabilize in order to stay in place — rather than getting swept away in a moving stream. Focusing on other fish, plants or debris might give the fish a false sensation that it is moving. The stable riverbed below, however, gives fish more reliable information about their own direction and speed. 

“It’s similar to sitting on a train car that isn’t moving. If the train next to yours starts to pull away from the station, it can trick you into thinking you are moving too,” said Emma Alexander, an assistant professor of computer science in Northwestern’s Mc Cormick School of Engineering , who led the study. “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.”

Going ‘back to the source’

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

Aristides Arrenberg and his team 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 explained. “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 in which the organism developed.”

Armed with camera equipment, the research teams of Tod Thiele (University of Toronto Scarborough) and Aristides Arrenberg went to seven sites across India to gather video data of shallow rivers where zebrafish naturally live. The field team encased a 360-degree camera inside a waterproof diving case and attached it to a remotely-controlled robotic arm. Then they used the robotic arm to plunge the camera into the water and move it around.

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

‘Wait for me!’

Back in Arrenberg’s lab, the team also tracked zebrafishes’ movements inside a sphere of LEDs. Because fish have a large field of view, they do not have to move their eyes to look around like people do. So, the researchers played motion stimuli across the lights and watched the fishes’ responses. When patterns appeared on the bottom of the tank, the fish swam along with the moving patterns — more evidence that the fish were taking their visual cues from looking downward.

“If you play a video with moving stripes, the fish will move along with the stripes,” Alexander said. “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 abstracted data from the videos and combined it with data from how motion signals get encoded into the fish’s brain. They fed the datasets into two pre-existing algorithms used for studying optic flow (or the movement of the world across our eyes or camera lenses). 

Ultimately, they discovered that in both scenarios — in the wild and in the lab — zebrafish look down when swimming forward. The researchers concluded that fish look down to understand their environment’s motion and then swim to counteract it — to avoid being swept away.

“We tied everything together into a simulation that showed that, in fact, this is an adaptive behavior,” said Alexander, who led the computational part of the study. 

Building better robots

Not only does this information give some insight into fishes’ behavior, it could also inform designs for artificial vision systems and sophisticated bio-inspired robots.

“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,’” Alexander said. “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.”

Based on a press release by Northwestern University, Illinois, USA/Hochschulkommunikation

Publication:

Emma Alexander, Lanya T. Cai, Sabrina Fuchs, Tim C. Hladnik, Yue Zhang, Venkatesh Subramanian, Nicholas C. Guilbeault, 4,5 Chinnian Vijayakumar, Muthukumarasamy Arunachalam, Scott A. Juntti, Tod R. Thiele, Aristides B. Arrenberg, and Emily A. Cooper: Optic flow in the natural habitats of zebrafish supports spatial biases in visual self-motion estimation. Current Biology, https://doi.org/10.1016/j.cub.2022.10.009

Contact:

Prof. Dr. Aristides Arrenberg
University of Tübingen
Institute of Neurobiology
Werner Reichardt Centre for Integrative Neuroscience
+49 7071 29 88798
aristides.arrenbergspam prevention@uni-tuebingen.de