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 cilia tes). The effect of light on the feeding rates was also investigated. Grazi ng experiments were first conducted by exposing coral colonies to known amo unts of H-3-thymidine-labeled bacteria and ciliates and measuring the appea rance of radioactivity in coral tissues. A method was developed to obtain c lean cultures of H-3-labeled ciliates. Results showed that 7% of the labele d bacteria and 90% of the labeled ciliates were ingested after 4-6 h incuba tion. Corals were then incubated in medium containing different concentrati ons 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 c iliates 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 numb er of ciliates ingested, as well as the ingestion rates decreased when the light intensity increased. During dark experiments, the maximal amount of c arbon ingested was twice as much as that ingested in light experiments. How ever, heterotrophic nutrition occurred even if the colonies could satisfy t heir carbon metabolism via photosynthesis. Zooplankton feeding seems theref ore 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 water s where inorganic nutrient concentrations are low. Conversely, when light i s Limiting, predation may also provide most of the energy necessary for cor al maintenance.