UC Santa Barbara

Coral reefs host high biodiversity and productivity despite residing in nutrient poor waters. Our capacity to predict coral reef structure and function relies on our understanding of the tightly coupled processes that govern the cycling of essential nutrients. One such process is consumer-driven nutrient recycling, where consumers ingest food and then excrete unassimilated nutrients (primarily nitrogen and phosphorous) back onto the reef. Ammonium, the dominant form of nitrogen excreted by most marine animals, is particularly important for coral growth and is readily taken up by corals. We investigated how exosymbionts, the communities of small fish and invertebrates that live in the interstitial space between coral branches, may serve as a source of ammonium for corals. Additionally, we explored how the contributions of exosymbionts varied based on identity, coral taxa, and coral size as well as the presence of small predatory fishes. We surveyed exosymbiont communities from two common species of coral, Pocillopora verrucosa and Acropora retusa, in Moorea, French Polynesia. We collected exosymbionts and performed incubations to quantify their contributions of regenerated ammonium to the two coral species. Although both coral species hosted distinct exosymbiont taxa, they received similar ammonium contributions from their respective communities. Exosymbiont abundance and ammonium excretions also correlated positively with coral size. Ammonium contributions from exosymbionts decreased in the presence of a hawkfish predator, especially those from pomacentrid fishes who we identified as key nutrient recyclers based on their high excretion rate. However, hawkfish alone contributed high levels of ammonium which could make up for much of the losses from exosymbionts. Our study provides a novel perspective at how exosymbionts can mediate coral nutrient dynamics, revealing yet another aspect of their mutualistic interaction with coral hosts, and adding to the overall body of knowledge of coral reef nutrient cycling processes.