Some fish closely monitor the depths below while swimming, new research shows, for much the same reason we pay attention to where we put our feet.
Of course, they don’t take steps, but according to a new study, the bias to stimuli falling on the lower parts of the eye serves an important purpose for fish, helping them track their movement in moving water.
To understand this, the researchers built a computational model incorporating simulations of a zebra’s brain, native 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 them stabilize in a current.
Self-stabilization can be difficult in flowing water, and small fish often need to maneuver just to keep 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 these visual cues are tricky underwater. On land, we have many stationary objects such as trees and buildings to help us measure motion. Underwater, fish are surrounded by unreliable reference points, whose relative motion could cause confusion.
“It’s similar to sitting in a train carriage 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 moving too,” said lead author Emma Alexander. in computer science from Northwestern University.
“The visual signal from the other train is so strong that it overrides the fact that all your other senses are telling you that you are sitting still. This is exactly the same phenomenon we study in fish. There are many misleading signs of movement above these, but the most abundant and reliable signals are from the bottom of the river.’
The team studied zebrafish in the lab, using LEDs in their tanks to create animations.
These fish don’t move their eyes to look around like we do. It doesn’t really need to, as their eyes already provide a fairly large field of view. But they start swimming when they see patterns of 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 streams in India where wild zebra live, as this landscape shaped the evolution of zebra behavior.
They placed 360-degree cameras in waterproof cases in seven streams and then remotely controlled a robotic arm to move the cameras, simulating the field of view of a wild zebra.
“It allowed us to put our eyes where the fish’s eyes would be, so we see what the fish see,” Alexander said. “From the video data, we were able to model hypothetical scenarios where a simulated fish was moving arbitrarily within a realistic environment.”
The researchers fed data from these experiments into algorithms to study visual flow, or the apparent motion of the landscape in the visual field. They found that, both in the lab and in the wild, zebras use information from their lower visual field to determine their movement.
“We put everything together in a simulation that showed that, in fact, this is an adaptive behavior,” Alexander said.
This study focused on zebrafish, and while a similar model could apply to other shallow-water fish, more research is needed to confirm this, Alexander explained to ScienceAlert. In other such habitats the visual bias may not help at all.
“In deep ocean waters, a very different set of stimuli is available,” Alexander said, “and we expect that this lower-field bias would no longer be beneficial.”
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 thanks to biomimicry, such as helping us develop better robots and artificial vision.
“If you were building a fish-inspired robot and you were just looking at its anatomy, you might think, ‘The eyes point sideways, so I’m going to point my cameras sideways,'” Alexander said.
“But it turns out that the eyes point sideways because they’re balancing 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.
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