Black corals are colonial organisms that show a high heterogeneity in their colony morphology and data currently available suggests that they are feeding on zooplankton. This morphological diversity and the fact that zooplankton is composed of many groups of organisms, displaying different sizes and life history traits, likely drives variation in their feeding behavior. The aim of this study was to determine the diet of shallow-water black corals from the Great Reef of Toliara (Madagascar), using the stable isotope ratios of carbon, nitrogen and sulfur. Tissues of 12 black coral species and their potential food sources were sampled and analysed during the dry and wet seasons. Results revealed inter-specific variability in the stable isotope composition between colony morphologies. Plankton taxonomic composition varied among size fractions, while only the isotopic composition of the smallest size fraction differed from the larger ones. The model estimated that a combination of plankton, detritus and BPOM, were important contributors to the diet of black corals, without major distinction between day and night time or dry and rainy season. Results suggest that even if certain food sources were isotopically different between time and season, most black corals studied seem to show an opportunistic feeding behavior. As hypothesized, they also showed that colony morphology plays an important role in their feeding behavior.
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