Sediment as a possible source of food for corals

Reef building corals are generally believed to require clear, nutrient-poor water. Yet, paradoxically, coral reefs are among the most productive marine ecosystems in the world. This paradox is commonly explained by the coral's ability to utilize a number of food resources, including zooplankton, phytoplankton, dissolved organic matter and photosynthetic products derived from their endosymbiotic algae, the zoox-anthellae. Sedimentation is a known stressor for corals, inhibiting most feeding modes in various ways. However, evidence for enhanced sediment deposition on certain corals, induced by their morphologies, as well as for sediment ingestion by some corals, has led us to examine the use of sediment as a possible source of food for corals in addition to the other, known food sources. Our experiments with fluorescently labelled sediment show transfer of labelled organic matter from the sediment into the cells of the solitary coral Fungia borrida Dana 1846. The results provide the first direct evidence for the ability of a coral to digest the sediment's organic fraction. These results may indicate a positive role of sediment, which up until now was considered to have only deleterious effects on corals.