Fish 'look down' when they swim, and we finally know why

Fish ‘look down’ when they swim, and we finally know why

Some fish watch the depths closely while swimming, new research suggests, for the same reason we pay attention to where we put our feet.

They don’t take measurements, of course, but according to a new study, being biased by stimuli falling on the lower parts of the eye serves an important purpose for fish, helping them to monitor their own movement in the water by movement.

To figure this out, the researchers built a computer model that incorporates simulations of a zebrafish’s brain, natural habitat, and swimming behavior.

Analysis of this pattern suggests that “constantly looking down” is an adaptive behavior for zebrafish, the researchers report. It may have evolved to help stabilize them in a current.

Self-stabilization can be difficult in flowing water, and small fish often need to maneuver just to hold their position. This constant readjustment is informed in part by visual cues. If your background is moving, for example, it might be time to stabilize.

But those visual cues are tricky underwater. On land, we have lots of stationary objects like trees and buildings to help us gauge motion. Underwater, fish are surrounded by unreliable reference points, the relative movement of which can be confusing.

“It’s like 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’re also moving,” said lead author Emma. Alexander, a computer scientist from Northwestern University.

“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 team studied zebrafish in the lab, using LEDs in their tanks to create moving patterns.

These fish do not move their eyes to look around like we do. They don’t really need it, their eyes already offering a wide enough field of vision. But they start swimming when they see movement below them, the study found.

“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!'”

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The researchers also studied shallow waterways in India where wild zebrafish live, as this landscape has shaped the evolution of zebrafish behavior.

They placed 360-degree cameras in seven-stream waterproof housings, then remotely controlled a robotic arm to move the cameras, simulating the field of view of wild zebrafish.

“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.”

The researchers fed the data from these experiments into algorithms to study optical flow, or the apparent motion of the landscape through the visual field. They found that, both in the lab and in the wild, zebrafish use information from their lower visual field to determine their movement.

“We tied it all together in a simulation that showed that, in fact, it’s adaptive behavior,” Alexander said.

This study focused on zebrafish, and while a similar pattern may apply to other shallow-water fish, we need more research to confirm this, Alexander told ScienceAlert. In other habitats, this type of visual bias may not help at all.

“In deep ocean waters, a very different set of stimuli is available,” Alexander said, “and we expect this lower field bias to no longer be advantageous.”

Even in the same habitats, some fish may move or process visual information differently.

While this research is interesting, it could also have practical applications through biomimicry, such as helping us develop better robots and machine vision.

“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 was published in Current biology.

#Fish #swim #finally

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