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In a wide range of coral reef fish species, a sequence of well defined decision rules generates escape behavior to escape a perceived threat. Credit: Stella Hein
The escape response to escape perceived threats is a fundamental behavior observed throughout the animal kingdom, and laboratory studies have identified specialized neural circuits controlling this behavior. Understanding how these neural circuits function in complex natural environments, however, has been a challenge.
A new study conducted by researchers from UC Santa Cruz and NOAA Fisheries has taken up this challenge using an astute experimental plan to record and analyze the evacuation responses of coral reef fish. The results, published on November 12 in Proceedings of the National Academy of Sciences, reveal how a sequence of well-defined decision rules generates evasion behavior in a wide range of coral reef fish species.
"We adopted an approach used in laboratory studies in a complex natural environment and found that the same behavioral mechanisms seemed to apply." A set of simple rules is combined in different ways to generate a rich suite of behaviors that allow to achieve this fundamental goal: avoid being killed, "said first author Andrew Hein, an assistant researcher at the Institute of Ocean Sciences at the University of Santa Cruz and research ecologist at the laboratory of NOAA fishing in Santa Cruz.
The coral reef fish of the study feed on algae in the shallow reef flats, where they are vulnerable to predators such as moray eels and reef sharks. To simulate a threat, researchers used a widely used visual stimulus, called "imminent stimulus," a black dot that grows slowly, then quickly, creating the illusion of an object coming close to it. A waterproof tablet computer installed on a coral reef of Mo'Orea, French Polynesia, played imminent stimulus, while video cameras recorded the reactions of fish that swam in the area in front of the tablet.
The researchers then used computer vision technology to analyze the video. Automated tracking and a method known as "ray tracing", originally developed by video game designers, allowed them to reconstruct what each fish saw when they decided to flee or no threat. They found that fish initiated evasion maneuvers in response to the perceived size and rate of expansion of the threat stimulus using a decision rule that corresponds to the dynamics of known loom-sensitive neural circuits.
"This same behavioral pattern that neuroscientists have identified in laboratory studies seems to work in a more complex natural environment," Hein said. "But we have also found something new: sensitivity to the impending stimulus is adjusted up and down according to the location of the other fish.If an individual is closest to the stimulus, he is much more likely to run away that there is another peach between her and the threat ".
A third factor in the escape response was the location of a safe place to shelter, offered by a mountainous coral along the experimental area. The initial response to the stimulus is to quickly turn away from the threat, but almost immediately, the fish then turned to the shelter and swam directly to it.
"When you look at their paths, they look like spaghetti – they are all different – but the analysis shows that they are all generated by the same set of behavioral rules," Hein said.
Explore further:
The social life of fish can be the key to safeguarding coral reefs
More information:
Andrew M. Hein et al., "Conservative Behavioral Circuits Govern High-Speed Decision Making in Wild Fish Shoals" PNAS (2018). www.pnas.org/cgi/doi/10.1073/pnas.1809140115
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