Effects of the Allelopathic Macroalga <i>Galaxaura divaricata</i> on the Foraging Behaviors of Reef Fishes Across Herbivorous and Nonherbivorous Feeding Guilds

Macroalgal overgrowth-induced phase shifts pose a significant threat to coral reef ecosystems. While herbivores can help control macroalgae biomass in reefs, some allelopathic macroalgae can resist herbivores from foraging. This resistance can lead to a decrease in herbivory levels. However, the impact of these allelopathic effects on other feeding groups of reef fish remains unclear. We examined the presence of the allelopathic macroalga: <i>Galaxaura divaricata</i> on the foraging behaviors of various reef fish species from different feeding guilds, including ten herbivorous and ten nonherbivorous species, in designed tank experiments. Most herbivorous fishes exhibited reductions of more than 60% in feeding on palatable macroalgae when exposed to <i>Galaxaura</i>. Nevertheless, the extent of feeding reduction differed among herbivorous species and feeding guilds, suggesting species-specific variation in tolerance. <i>Galaxaura</i> had varying effects on the feeding quantity of shrimp among different species of nonherbivorous fish. The presence of <i>Galaxaura</i> did not significantly affect the feeding quantity of most Labridae, Balistidae, and Pomacanthidae fishes, but did physically reduce the feeding quantity of some large carnivorous fishes (e.g., Haemulidae, Lutjanidae, and Serranidae). There was a high variation in the effect on the feeding quantity of opportunistic corallivorous fishes. Our results indicate that allelopathic macroalgal dominance can influence not only herbivorous fishes that regulate algal biomass, but also nonherbivorous fishes. Such changes in foraging may alter trophic interactions in coral reef ecosystems. This also suggests that maintaining functional diversity in fish feeding guilds and preserving specific herbivorous functions are crucial for the resilience of coral reefs against the phase shift caused by macroalgal overgrowths.