A ll reef-forming, or hermatypic, corals are a symbiotic associa-tion between a cnidarian ani-mal host and unicellular dinoflagel-late algae, called zooxanthellae (Muscatine 1980). Zooxanthellate corals are predominately found in nutrient-poor coastal waters of tropi-cal and subtropical seas (Muscatine and Porter 1977). The zooxanthellae live within the host animal cells and are photosynthetic, providing a reli-able source of fixed carbon. The animal, in turn, provides nutrients for the zooxanthellae through its catabolic pathways (Davies 1984). Stability in symbiotic associations requires control of population growth between the symbionts. With the exception of stress-induced bleaching events (see Buddemeir and Fautin 1993 for a review), the population density of zooxanthellae per unit surface area of most symbiotic corals is remark-ably constant, averaging approximately one million cells per square centimeter (Drew 1972, Kawaguti and Nakagama 1973). For example, one species common in the Red Sea,