Extracoelenteric zooplankton feeding is a key mechanism of nutrient acquisition for the scleractinian coral Galaxea fascicularis

Internal and external feeding on zooplankton may provide scleractinian corals with important nutrients. However, the latter process has never been properly quantified. To quantify the dynamics of zooplankton capture, digestion and release for a scleractinian coral, we performed detailed video analyses of Galaxea fascicularis feeding on Artemia nauplii. A highly dynamic process of prey capture, digestion and release was observed. A single G. fascicularis polyp (N=3) captured 558±67 and released 383±75 Artemia nauplii over a 6 h interval. On average, 98.6% of prey captured was not ingested. Instead, prey items were clustered into aggregates that were digested externally by mesenterial filaments. In addition, we employed carbon, nitrogen and phosphorus analysis of zooplankton before and after digestion by G. fascicularis colonies (N=6). For total organic carbon, 43.1% (0.298±0.148 µg Artemia–1) was lost after 6 h of digestion. For total organic nitrogen, total organic phosphorus and orthophosphate (PO43–), these values were 51.3% (0.059±0.028 µg Artemia–1), 50.9% (0.009±0.004 µg Artemia–1) and 84.6% (0.0019±0.0008 µg Artemia–1), respectively. For extracoelenteric zooplankton feeding alone, total estimated nutrient inputs for G. fascicularis colonies were 76.5±0.0 µg organic carbon, 15.2±0.0 µg organic nitrogen, 2.3±0.2 µg organic phosphorus and 0.5±0.8 µg inorganic phosphorus per cm2 coral tissue per day. These values exceed calculations based on intracoelenteric feeding by up to two orders of magnitude. Our results demonstrate that extracoelenteric zooplankton feeding is a key mechanism of nutrient acquisition for a scleractinian coral. These results are of importance to coral aquaculture and our understanding of benthic–pelagic coupling on coral reef