Distribution of Labeled Carbon in Reef-Building Corals with and without Zooxanthellae.

The uptake and distribution of carbon-14 in two reef-building corals, Manicina areolata and Montastrea annularis, were studied by radioautographic methods. Experiments on colonies with and without zooxanthellae were run in light and in darkness. We suggest that corals cannot derive any effective nutrition from their zooxanthellae, and that the physiological response of corals to the presence of zooxanthellae is due to the secretion of trace amounts of vitamin-like or hormone-like substances by the algal symbionts, and not to their food value per se.