Several factors may affect heterotrophic feeding of benthic marine invertebrates, including water flow rate and polyp context (i.e., the presence of neighbouring polyps). We tested the interactive effects of water flow rate and polyp context on zooplankton feeding by the scleractinian coral Galaxea fascicularis. Single polyps and colonies were incubated in a flow cell for 30 minutes with an ambient Artemia nauplii concentration of 10,000？ and water flow rates ranging from 1.25 to 40？cm？ . Water flow rate and polyp context showed significant main and interactive effects on feeding rates of G. fascicularis polyps. More specifically, feeding rates were optimal at flow rates of 1.25？cm？ for single polyps and 5 to 10？cm？ for polyps inhabiting colonies. The presence of epizoic acoelomorph flatworms may have negatively affected the observed feeding rates, especially at high flow. Our results demonstrate that water flow affects coral feeding and thus heterotrophic nutrient input at both a polyp and colony level. These findings are of relevance to our understanding of how biotic and abiotic factors interact on coral heterotrophy and may serve to optimise coral aquaculture. 1. Introduction Heterotrophy is vital to coral health, as it supplies the holobiont with essential nutrients including amino acids and fatty acids [1]. For scleractinian corals, profound effects of heterotrophy on the physiology of the coral host and its symbiotic dinoflagellates have been documented. Zooplankton feeding has been found to enhance coral calcification, organic matrix synthesis, and photosynthetic rates [2, 3]. Up to 100% of the daily metabolic carbon requirements can be supplied by zooplankton, both during bleaching episodes [4] or when high prey concentrations are used in aquaculture [5]. These findings fit well with the long-term effects of zooplankton feeding on corals, which show that heterotrophy can be a limiting factor to growth [1, 6]. Several factors may affect coral feeding rates, including bleaching status [4], prey density [7], symbiotic organisms such as epizoic flatworms [8], water flow rate [9–16], and colony size [12, 16]. Water flow is a key parameter in this respect, as sessile organisms including corals depend on water movement to provide them with prey items [17]. Increased flow rates will increase the encounter rate or flux of food particles [10, 14, 18, 19], but will also increase the kinetic energy of particles approaching coral polyps. A higher kinetic energy of food particles may constrain the capture abilities of coral polyps, as has been documented for
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