Seeing is believing: metabolism provides insight into threat perception for a prey species of coral reef fish

Responding appropriately to predator threat is a critical survival skill for all organisms. Under-responding can result in death, while continually over-responding can waste precious energy reserves and compromise important life history attributes such as growth and reproduction. This trade-off becomes particularly pertinent in predator-rich environments such as coral reefs, yet almost nothing is known of the sublethal physiological responses that coral reef predators elicit in their prey. To address this knowledge gap, we designed equipment and protocols to measure the metabolic responses of a common coral reef fish (juvenile ambon damsel, Pomacentrus amboinensis) to chemosensory and visual stimuli of a common predator (adult yellow dottyback, Pseudochromis fuscus). Pomacentrus amboinensis did not exhibit a metabolic response to the chemosensory predator stimulus, yet there was a consistent and significant metabolic response to the visual stimulus that endured for at least 24 h if the predator remained visible. A complete lack of metabolic response of P. amboinensis to the visual stimulus of a nonpredatory wrasse, Halichoeres argus, revealed an impressive ability of juvenile P. amboinensis to rapidly discriminate between similar-sized predatory and nonpredatory fishes. These divergent metabolic responses of P. amboinensis were not explained by measureable differences in their swimming behaviour in the predator versus nonpredator treatments, yet prey that were exposed to repeat predation attempts (predatory strikes) had the strongest metabolic response to the predator. Our findings demonstrate the capacity of coral reef fishes to interpret predator cues and prioritize threats and actions. Moreover, this study is the first to demonstrate that high predator densities on coral reefs could result in repetitive short-term or even chronic long-term elevations in energy expenditure of prey communities.