Why do fish swim with their heads down?

Why do fish swim with their heads down?

Is it because the head is too big and the center of gravity is too forward?

Or is it because you are not confident about your appearance and just want to be low-key?

Actually, neither is true.

A new study confirms that they just don't want to be disturbed by the things around them and just want to "live more steadily"...

This is actually the same reason why people might look down at the sidewalk when walking.

The related research paper, titled "Optic flow in the natural habitats of zebrafish supports spatial biases in visual self-motion estimation", has been published in the scientific journal Current Biology.

"Don't bother me"

The study is the first to integrate multiple simulations of the zebrafish brain, native environment, and spatially varying swimming behavior into a single computational model.

The results of the model analysis show that this "quirk" of looking down while swimming forward is just an adaptive behavior of fish .

Fish have evolved this to keep their bodies stable, as when swimming against a current.

When the water is moving, fish will try hard to stay stable so that they can stay in place and not be swept away by the moving water.
If they swim in the water while looking around, they will give the illusion of moving.

If they swim with their heads down and looking at the riverbed, they can better judge the direction and speed they are swimming at the time.

In this regard, Emma Alexander, the first author and corresponding author of the paper and assistant professor of computer science at Northwestern University, said:

“It’s like being in a stationary train car and then the train next to you starts to pull out of the station, and you’d be fooled into thinking you’re moving too.”

“The visual cue of another train overrides the fact that you’re sitting still.”

That is to say, for swimming fish, there are many misleading motion signals around, and the most reliable signal comes from the river bottom.

Let's go to India

To find out, Alexander's team and their collaborators studied zebrafish, a widely studied model organism.

Although many labs have zebrafish, they wanted to conduct this research in their native environment.

"It has recently been discovered that fish respond more strongly to movement from below than from above . We wanted to dig deeper into this mystery and understand why."

"The native habitat shaped the evolution of their brains and behaviors, and we need to go back to the source and investigate the context in which this organism developed," Alexander explained.

So they came to India with their filming equipment.

They visited seven sites in India and collected video data of shallow rivers where zebrafish naturally live.

They enclosed a 360-degree camera in a waterproof submersible case and attached it to a remote-controlled robotic arm, which they used to lower the camera into the water and move it.

“ We put the ‘eyes’ where the fish’s eyes should be, so we see what the fish sees ,” Alexander said.

“From the video data, we were able to simulate hypothetical scenarios and simulate fish moving arbitrarily in a real-world environment.”

Zebrafish: Wait for me!!

Back in the lab, Alexander's team and their collaborators also tracked the movement of zebrafish inside an LED ball.

Because fish have a large field of vision, they don't need to move their eyes to look around like people do.

So they gave motion stimulation through lights and then observed the fish's reactions.

It was observed that when a pattern appeared on the bottom of the fish tank, the fish would swim along with the moving pattern.

This suggests that fish get their visual cues by looking downward.

“If you play a video with moving stripes, the fish will move with the stripes.”

"It's like they're saying, 'Wait for me,' and we counted the beats of their tail wags, and the harder they wag, the more they're trying to keep up with the moving stripes."

They extracted data from the videos and combined it with data on how motion signals are encoded in the fish's brains.

They then fed the dataset into two algorithms used to study optical flow, or the motion observed by our eyes or camera lenses.

Ultimately, they found that in both the wild and the lab, zebrafish looked down as they swam forward.

Figure | The swimming of zebrafish larvae is driven by signals from the lower visual field. (Source: This paper)

Therefore, Alexander et al. came to the conclusion mentioned at the beginning of the article:

Fish look downward to interpret motion cues from their surroundings and then swim to avoid being swept away by moving water.

The surface of the water is constantly moving, as are other fish and plants. Fish are best served to ignore this information and focus on signals further below.

The riverbed has a lot of texture, so fish can see strong features that they can follow.

In addition, the research team also stated that this research result not only allows us to have some understanding of the behavior of fish, but also can provide information for the design of artificial vision systems and complex bionic robots.

"If you were making a robot inspired by a fish and you just looked at its anatomy, you might think 'its eyes are facing one way, so the camera has to face one way too'."

But it turns out that fish looking their eyes to the side are a balancing act, looking up for hunting and down for swimming .

References:

https://www.cell.com/current-biology/fulltext/S0960-9822(22)01617-7