Colonies of the massive stony coral Faviafavus were exposed to different flow speeds and levels of light, and to the addition of zooplankton prey. The relative importance of each factor in controlling polyp expansion behavior was tested. The coral polyps fully expanded when they were exposed to low light intensity (0-40 micromol m(-2) s(-1)) and high flow speed (15 cm s(-1)), regardless of prey presence. They also partially expanded under low and medium light (40-80 micromol m(-2) s(-1)) at medium flow speed (10 cm s(-1)). The corals expanded their polyps only when they were exposed to light levels below compensation irradiance (Icom: light level at which photosynthesis = respiration), which was determined to be about 107 +/- 24 micromol m(-2) s. The results presented here indicate that high flow, low light, and the presence of planktonic prey induce coral expansion. There is a hierarchy of response to these stimuli, in which light level and flow speed are dominant over prey presence. Coral response to these three factors is probably due to the relative importance of gas exchange and zooplankton prey.