1461 Scleractinian corals derive their nutritional needs, in part, from photosynthates translocated by their symbiotic zooxanthellae (Muscatine, 1990). The effects and mechanisms of autotrophy have been well documented (Muscatine, 1980). Corals nevertheless are true heterotrophs, ingesting a wide range of food such as dissolved and particulate organic matter (Anthony, 1999; Anthony and Fabricius, 2000), sediment (Rosenfeld et al., 1999), bacteria (Farrant et al., 1987; Sorokin, 1991) and zooplankton (Lasker et al., 1983; Lewis, 1992; Ferrier-Pagès et al., 1998; Sebens et al., 1996). The relative importance of phototrophy vs. heterotrophy is still partially understood. In shallow waters and high-light-adapted corals, photosynthesis alone may meet all the nutritional needs (Falkowski et al., 1984) while in deep living corals or in some species depending mostly on predation, photosynthesis may only supply a small fraction of this energy demand (Szmant- Froelich and Pilson, 1984; for a review, see Barnes and Chalker, 1990). Corals living in turbid environments are also more dependent on heterotrophy than those living in clear waters (Anthony and Fabricius, 2000). The major role of feeding is to provide the symbiosis with essential nutrients such as nitrogen and phosphorus (Rahav et al., 1989; Cook et al., 1994). Both host and algal symbionts respond quickly to food availability (Fitt, 2000). At the algal level, Dubinsky et al. (1990) as well as Titlyanov et al., (2000a,b, 2001) showed an enhancement of the areal pigmentation and zooxanthellae density in fed corals, leading to an increase in the areal photosynthesis. At the animal level, heterotrophy tends to increase the amount of tissue synthesis (Jacques and Pilson, 1980; Sebens and Johnson, 1991; Kim and Lasker, 1998). An enhancement in skeletal growth has also been observed, suggesting that corals allocate a high proportion of the energy brought by food to calcification processes (Jacques and Pilson, 1980; Witting, 1999; Ferrier- The Journal of Experimental Biology 207, 1461-1469 Published by The Company of Biologists 2004 doi:10.1242/jeb.00911 We investigated the effect of zooplankton feeding on tissue and skeletal growth of the scleractinian coral Stylophora pistillata. Microcolonies were divided into two groups: starved corals (SC), which were not fed during the experiment, and fed corals (FC), which were abundantly fed with Artemia salina nauplii and freshly collected zooplankton. Changes in tissue growth, photosynthesis and calcification rates were measured after 3 and 8 weeks of incubation. Calcification is the deposition of both an organic matrix and a calcium carbonate layer, so we measured the effect of feeding on both these parameters, using incorporation of 14 C-aspartic acid and 45 Ca, respectively. Aspartic acid is one of the major components of the organic matrix in scleractinian corals. For both sampling times, protein concentrations were twice as high in FC than in SC (0.73 vs 0.42·mg·P –1 ·cm –2 ·skeleton) and chlorophyll c 2 concentrations were 3–4 times higher in fed corals (2.1±0.3·μg·cm –2 ). Cell specific density (CSD), which corresponds to the number of algal cells inside a host cell, was also significantly higher in FC (1.416±0.028) than in SC (1.316±0.015). Fed corals therefore displayed a higher rate of photosynthesis per unit area (P g max = 570±60·nmol·O 2 ·cm –2 ·h –1 and I k =403±27·μmol·photons m –2 ·s –1 ). After 8 weeks, both light and dark calcification rates were twofold greater in FC (3323±508 and 416±58·nmol·Ca 2+ ·2·h –1 ·g –1 ·dry·skeletal·mass) compared to SC (1560±217 and 225±35·nmol·Ca 2+ ·2·h –1 ·g –1 dry·skeletal·mass, respectively, under light and dark conditions). Aspartic acid incorporation rates were also significantly higher in FC (10.44±0.69 and 1.36± 0.26%RAV·2·h –1 ·g –1 ·dry·skeletal·mass, where RAV is total radioactivity initially present in the external medium) than in SC (6.51±0.45 and 0.44±0.02%RAV·2·h –1 ·g –1 dry·skeletal·mass under dark and light conditions, respectively). Rates of dark aspartic acid incorporation were lower than the rates measured in the light. Our results suggest that the increase in the rates of calcification in fed corals might be induced by a feeding-stimulation of organic matrix synthesis. Key words: coral, feeding, photosynthesis, calcification, organic matrix, 14 C-aspartate, 45 Ca. Summary Introduction Interactions between zooplankton feeding, photosynthesis and skeletal growth in the scleractinian coral Stylophora pistillata Fanny Houlbrèque*, Eric Tambutté, Denis Allemand and Christine Ferrier-Pagès Centre Scientifique de Monaco, Avenue Saint-Martin, MC-98000 Monaco (Principality) *Author for correspondence (e-mail: fhoulbreque@centrescientifique.mc) Accepted 2 February 2004