Interactions between zooplankton feeding, photosynthesis and skeletal growth in the scleractinian coralStylophora pistillata

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Abstract
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 14C-aspartic acid and 45Ca, 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 c2 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 (Pgmax= 570±60 nmol O2 cm–2 h–1 and Ik=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 Ca2+ 2 h–1 g–1 dry skeletal mass) compared to SC (1560±217 and 225±35 nmol Ca2+ 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.