Scientists discover why fish look down when they swim

EVANSTON, Ill. — Fish look down when they swim because that’s how they navigate, according to new research. The quirky trait enables them to estimate direction and speed and not get lost.

Researchers from Northwestern University say the skill evolved to boost stabilization in strong currents. Taking visual cues from the sides could lead to fast flowing water sweeping them away.

“It’s similar to sitting on a train car that isn’t moving. If the train next to yours starts to pull to away from the station, it can trick you into thinking you are moving too,” says Northwestern’s Emma Alexander, the leader of the study, in a media release.

“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.”

The study, published in the journal Current Biology, is based on observations of the humble zebrafish — a common friend of medical researcher looking to model human health. It combined simulations of the animal’s brain, their native environment in India, and swimming behavior into one computational model.

“It was 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 explains. “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 for where the organism developed.”

The colorful creature, named after its stripes, is native to fresh water rivers and paddy fields in India. The team visited seven sites to gather video data of shallow rivers, placing a 360-degree camera inside a waterproof diving case. A remote-controlled robotic arm plunged the device into the water and moved it around.

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

zebrafish habitat
The researchers collected video data a forested stream with a sandy substrate and low-to-medium flow. (CREDIT: E. Alexander/Northwestern University)

Back in the lab, the zebrafish’s motions were tracked inside a ball of LEDs (light emitting diodes). 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 their responses.

When patterns appeared on the bottom of the tank, the fish swam along with the moving patterns — showing they 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 says. “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.”

Optic flow algorithms showed that, both in the wild and in the lab, zebrafish look down when swimming forward. It helps them understand their environment’s motion and then swim to counteract it — avoiding being swept away.

“We tied everything together into a simulation that showed that, in fact, this is an adaptive behavior,” the study leader explains. “The water surface is constantly moving, and other fish and plants are moving by. Fish are better off omitting that information and focusing on the information below them. Riverbeds have a lot of texture, so fish are seeing strong features they can track.”

The discovery could lead to the development of 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 concludes. “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.”

South West News Service writer Mark Waghorn contributed to this report.

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