Fish eye tracking changes what we know about swarm behavior

Have you ever watched a school of fish gliding effortlessly through the water? Thousands of synchronized swimmers dart to and fro in perfect harmony, shifting directions in an instant as if guided by an unseen force.

It’s a mesmerizing spectacle – one that has fascinated scientists and nature lovers alike.

But have you ever pondered how these swimmers coordinate so effortlessly, despite having a limited view of their surroundings? What hidden cues influence each individual fish’s split-second decisions within the collective?

How do they perceive and respond to the fluid, ever-changing shoal dynamics around them?

These questions represent some of the captivating mysteries of collective animal behavior – a phenomenon that extends far beyond fish schools.

Scientists from the Max Planck Institute of Animal Behavior and the Cluster of Excellence “Collective Behaviour” set out to investigate.

The team has developed a non-invasive, 3D eye tracking method, which gives us a rare glimpse into the eye movements of fish based on video recordings.

Complexities of collective animal behavior

To appreciate the complexities of collective behavior, we must view the world as the fish does. Making sense of the retinal view of the fish in 3D is a crucial step in this process as we need to be able to follow the fish’s gaze to determine which factors lead it to make a decision about its direction of movement.

Every shift in the school’s collective movement is an outcome of decisions made by each individual fish.

What influences a fish’s decision to change its course? Which visual feature of a neighboring fish helps an individual with its navigation? How does the school react to this one fish’s movement?

The answers to these questions lie in understanding the interactions among individuals in synchronized groups. This collective behavior is also seen in flocks of birds, locust swarms, and large herds migrating across continents.

Tools of discovery

Applying computer vision to camera footage from both the lab and the field, the researchers tracked the position and body posture of each individual in a group.

The vast amount of data collected was analyzed every few milliseconds, allowing scientists to detect subtle shifts in movement and compare them with the responses of surrounding fish.

A crucial aspect of this research involved reconstructing each individual’s visual field, which can provide insights into their perception and its impact on subsequent movement decisions. This method marks a significant leap in methodology in this domain of research.

Reconstructing fish vision

The visual field of a freely-swimming fish isn’t easily reconstructed. It needs to consider not only the eye position but also the body posture of the fish.

The researchers prioritized developing a method that required no interventions such as fitting the fish with an eyepiece.

“Our new, non-invasive method meets all these requirements,” said Liang Li, who played a key role in developing the technology.

“Using the camera images, we reconstruct firstly the 3D body posture of the fish, secondly, the precise position of the eye – which, like for humans, can move in the eye socket – and thirdly, we reconstructed their retinal view, to see what they see.”

Analyzing fish behavior in 3D

This new method marks a significant improvement, as it allows the behavior of fish to be analyzed in 3D. Traditional techniques based on 2D images couldn’t capture the three-dimensional nature of fish schools.

The method only requires a minimum of two cameras and the fish are left undisturbed, swimming freely in the tank. The more cameras added to the system, the more precise the analysis becomes.

“Understanding how animals perceive their environment and interact with social partners is critical for unravelling the mechanisms underlying collective behaviour,” noted Li.

Tracking a fish’s field of vision

This revolutionary approach has already been tested with goldfish, where researchers tracked the field of vision of a goldfish that was following another goldfish that swam ahead.

“The reconstruction of the retinal view revealed that goldfish dynamically adjust their eye movements so that the fish swimming ahead remains constantly in the center of their retina,” explained study lead author Ruiheng Wu.

Furthermore, researchers observed that the eyes moved in opposite directions, a phenomenon known as “negative synchronization,” rather than working in parallel. This enables a fish to observe the movement of two other fish, swimming on either side, at the same time.

In the future, the scientists would like to investigate whether the eyes of predatory fish align while the fish is focusing on prey.

This fascinating new methodology opens up intriguing new research possibilities in the realm of collective animal behavior.

The full study was published in the journal Communications Biology.

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