Community composition and nutrient contributions of coral exosymbionts in Moorea, French Polynesia

Coral reefs are some of the most biodiverse and productive ecosystems on the planet, despite being located in nutrient poor waters. Because of their oligotrophic nature, understanding the processes that govern coral reef nutrient cycling is important for our capacity to predict changes in ecosystem structure and function. One important process is nutrient recycling through consumers, where consumers ingest food and then excrete waste back onto the reef in the form of ammonium, which is both necessary for and readily used by corals. Here we examine how nutrient contributions, abundance, and community composition of “coral exosymbionts”-the small fish and invertebrate communities that live between the branches of coral colonies, relate to coral species identity, and size. First, we surveyed a size gradient of two species of coral, Pocillopora verrucosa and Acropora retusa, to determine exosymbiont abundance and composition. We then measured excretion rates of the most abundant exosymbiont families and applied them to our community data. Acropora retusa and Pocillopora verrucosa had distinctly different exosymbiont communities with Pocillopora verrucosa having higher abundance and diversity of species. Size of coral was positively correlated with exosymbiont abundance and ammonium contributions. Despite differences in exosymbiont community composition, both species of coral were shown to receive similar ammonium contributions from their respective exosymbiont communities. Our results explore the nutrient contributions of coral exosymbionts, 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.