Not all corals dine in: variation in niche partitioning between corals and their Symbiodinium indicates a range of symbioses

Abstract

The symbiosis between corals and Symbiodinium has allowed coral reefs to achieve high levels of productivity and diversity in oligotrophic habitats. By sharing resources, corals and Symbiodinium access novel niche space. Some have argued that the coral-algal interaction ranges from mutualism to parasitism. Trophic niche theory predicts that mutualist symbionts would occupy the same niche space as their host (shared resources) whereas commensal symbionts would occupy a different niche (fewer shared resources). To test this hypothesis, we measured the δ13C and δ15N of separated coral tissue and symbiont cells from 6 coral genera commonly encountered in Hong Kong. Using Stable Isotope Bayesian Ellipses in R (SIBER) analysis, we compared the isotopic niche placement and area of each host and symbiont pair. Our results supported the hypothesis: in some genera (Acropora and Goniopora) the host and symbiont had nearly 100% overlap of their isotopic niches implying shared nutritional resources, while in others (Platygyra and Favites) there was no overlap. Finally, some genera (Pavona and Porites) have partial overlap suggesting a flexible symbiosis. These patterns were driven by nitrogen, not carbon. Indeed, the difference between the δ15N values of host and symbiont was positively correlated with calyx volume, suggesting that smaller polyps evolved to support obligate symbioses while larger polyps enable corals to meet their nutritional requirements through alternative means.