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Escape responses of coral reef fish obey simple behavioral rules

The escape response to evade perceived threats is a fundamental behavior seen throughout the animal kingdom, and laboratory studies have identified specialized neural circuits that control this behavior. Understanding how these neural circuits operate in complex natural settings, however, has been a challenge. A new study overcame this challenge using a clever experimental design to record and analyze escape responses in coral reef fish. The results reveal how a sequence of well-defined decision rules generates evasion behavior in a wide range of coral reef fish species. The coral reef fish in the study feed on algae in shallow reef flats, where they are vulnerable to predators such as moray eels and reef sharks. To simulate a threat, the researchers employed a widely used visual stimulus called the looming stimulus, a black dot that grows in size slowly and then rapidly, creating the illusion of a rapidly approaching object. A waterproof tablet computer deployed on a coral reef in Mo'orea, French Polynesia, played the looming stimulus, while video cameras recorded the responses of fish that swam into 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 casting," originally developed by video game designers, allowed them to reconstruct what each fish was seeing as it decided whether or not to flee from the threat. They found that fish initiated escape maneuvers in response to the perceived size and expansion rate of the threat stimulus using a decision rule that matched the dynamics of known loom-sensitive neural circuits.

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