Why do fish look down when they swim?

Some fish swim looking at the seabed and for a long time this was unknown in the scientific world, however, new research has shown why this happens.

Despite seeming strange, this has to do with the same behaviour that humans present when they turn to look at their feet, to be sure of the ground they are stepping on.

According to a new study, being predisposed to the undersides of the eye is intended to help fish monitor their own movement in the water.

In order to carry out the experimental part of the study, the scientists built a computer model, with simulations of the brain of a zebrafish, as well as its native habitat and swimming behaviour.

Constantly looking down is linked to adaptive behaviour, which suggests it may have evolved to help them stabilize in a current, according to research published in the journal Curren Biology.

Why fish look down

Achieving stability can be difficult for small fish in flowing water, having to perform different movements to achieve a stable position. By keeping their eyes down, the fish have visual cues that allow them to perform this readjustment if the bottom is moving.

However, those visual cues are not the same underwater as they are on land. The fish have unreliable reference points, whose movement becomes confusing.

"It's similar to sitting in a train carriage that doesn't move. If the train next to you starts to move away from the station, it can also trick you into thinking you're moving," said lead author Emma Alexander, a computer scientist at Northwestern University.

“The visual cue from the other train is so strong that it overrides the fact that all your other senses are telling you to stand still. That is exactly the same phenomenon that we are studying in fish. There are many misleading signs of movement above them, but the most abundant and reliable signals are from the bottom of the river."

Use of LED lights

As part of the experimental phase, the researchers studied the zebrafish in the laboratory, implementing the use of LED lights at the bottom of the tanks, in order to create movement patterns.

Unlike humans, these fish do not move their eyes to observe their surroundings, due to the wide field of vision they have. However, according to scientists, they start swimming when they see movement patterns below them.

The researchers also conducted studies in the shallow streams of India, the habitat of wild zebrafish. These aquifers are considered key places to determine the evolution of the behaviour of these animals.

Natural habitat

“It was recently discovered that fish respond to movement below them more strongly than movement above them. We wanted to delve into that mystery and understand why," says Alexander. "Many zebrafish we studied grow in laboratory tanks, but their native habitats shaped the evolution of their brains and behaviours, 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 connected it to a remote-controlled robotic arm. They then used the robotic arm to dip the camera into 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.

"We put everything together in a simulation that showed that this is, in fact, adaptive behaviour," says Alexander. “The surface of the water is constantly moving, and other fish and plants are moving. Fish are better off skipping that information and concentrating on the information below them. Riverbeds are very textured, so fish see strong features that they can track."

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