Microheterotrophy in the zooxanthellate coral Stylophora pistillata: Effects of light and ciliate density

We examined the ability of the zooxanthellate coral Stylophora pistillata (Esper, 1797) to feed on microheterotrophs (bacteria and oligotrichous ciliates). The effect of light on the feeding rates was also investigated. Grazing experiments were first conducted by exposing coral colonies to known amounts of H-3-thymidine-labeled bacteria and ciliates and measuring the appearance of radioactivity in coral tissues. A method was developed to obtain clean cultures of H-3-labeled ciliates. Results showed that 7% of the labeled bacteria and 90% of the labeled ciliates were ingested after 4-6 h incubation. Corals were then incubated in medium containing different concentrations of unlabeled ciliates (200, 500, 800 cells ml(-1)). Replicates of each concentration were exposed to one of three light levels (0, 80, 250 mu mol m(-2) s(-1)). Coral feeding rate increased with prey density, from 1.40 to 4.10 x 10(4) ciliates (0.22-0.65 mu g C mg protein(-1) h(-1)) for 200-800 ciliates ml(-1), respectively. However, a plateau was observed after a total ingestion of 4 x 10(4) ciliates (1.7 mu gC mg protein(-1)). The total number of ciliates ingested, as well as the ingestion rates decreased when the light intensity increased. During dark experiments, the maximal amount of carbon ingested was twice as much as that ingested in light experiments. However, heterotrophic nutrition occurred even if the colonies could satisfy their carbon metabolism via photosynthesis. Zooplankton feeding seems therefore to complement autotrophic nutrition. Under high Light, the small amount of microplankton ingested may provide nitrogen, phosphorus, or vitamins to corals, and this food supply may be especially important in tropical waters where inorganic nutrient concentrations are low. Conversely, when light is Limiting, predation may also provide most of the energy necessary for coral maintenance.