Fish Look Down When They Swim, Now Scientists Think They Know Why



New research shows that when fish swim, they look down to navigate. “The whimsical trait allows them to gauge direction and speed — and not get lost. It evolved to improve stabilization in strong currents,” said one scientist. Picking up visual cues from the sides could cause them to be swept away by fast moving water

“It’s like sitting in a train car that doesn’t move. When the train next to you starts to move away from the station, it can trick you into believing that you are moving too. The other train’s visual cue is so strong that it overrides the fact that all of your other senses are telling you to sit still. This is exactly the same phenomenon that we are studying in fish. There are many misleading clues to movement above them, but the most common and reliable signals come from the bottom of the river,” said lead author Dr. Emma Alexander from Northwestern University.

The study, published in the journal Current Biology, is based on the humble zebrafish, which has long been used as a model for human health in medical research. It combined simulations of his brain, the native environment in India and swimming behavior into one computational model.


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Sergeant major fish (Abudefduf saxatilis), also known as pintano, swim past a coral reef in this photo. The research also shows that the stripes on the fish could be used to help understand how they move in the water. Photo by KHALED DESOUKI/AFP VIA GETTY IMAGES.

“It was recently discovered that fish are more responsive to movement below them than to movement above them. We wanted to get to the bottom of this mystery and understand why. Many of the zebrafish we study are raised in laboratory tanks, but their natural habitats shaped how their brains and behavior developed, so we had to go back to the source to examine the context in which the organism evolved.” Alexander said.

The colorful creature, named for its stripes, is native to freshwater rivers and rice fields in India. The US team visited seven sites to collect video data of shallow rivers and placed a 360-degree camera in a waterproof dive case. A remote-controlled robotic arm dipped the device into the water and moved it around.

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

Back in the lab, the movements of the zebrafish were tracked in a sphere made of LEDs (light-emitting diodes). Fish have a large field of vision. They don’t have to move their eyes to look around like humans do. So the researchers played movement stimuli over the lights and observed the reactions of the fish. As patterns appeared on the bottom of the tank, the fish swam with the moving patterns – showing they took their visual cues from looking down.

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

Optical flow algorithms, demonstrated in the wild and in the lab, make zebrafish look down as they swim forward. It helps them understand the movement of their surroundings and then swim to counteract it – to avoid being swept away.

“We put everything together into a simulation that showed that it is indeed an adaptive behavior. The water surface is constantly moving and other fish and plants are moving past. Pisces are better off omitting this information and focusing on the information below. Riverbeds have many textures, so fish see powerful features to track.” Alexander said.

The discovery could lead to the development of artificial vision systems and sophisticated bio-inspired robots.


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Tuna fish swim in a farm cage off the coast of Turkiye’s western Izmir province. Understanding fish movement helps in innovation. Photo by MAHMUT SERDER ALAKUS/ANADOLU AGENCY/VIA IMAGES

“If you build a fish-inspired robot and just look 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 that the eyes point sideways because they’re balancing multiple tasks.

“We think they’re facing sideways because it’s a trade-off — they’re looking up to hunt and down to swim.”

Produced in collaboration with SWNS Talker.

The Western Journal did not review this story prior to publication. As such, it may not meet our normal editorial standards. It is provided as a service to our readers by The Western Journal.

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