Light intensity and herbivory are two major factors which affect plants' physiology and population dynamics. Corallina pilulifera, is a turf-forming red alga which is often dominant in rocky intertidal and subtidal shores. In the study site in southeast kyushu, C. pilulifera demonstrated a wide range of colour variation in fronds, which appeared to result from the variation of light availability. The accumulation of pigment would affect the growth of the alga through the process of photosynthesis. Hermit crabs also seem to affect the dynamics of algal population, by cutting their fronds and removing the sediment accumulated beneath the algae. The former action generates a negative effect on algal growth, while the latter generates a positive effect. Thus, in this study we investigated (1) the relationship between light intensity, colour variation and growth, and (2) the effect of hermit crabs on the algal population. An attempt was made to experimentally manipulate sedimental removal, frond cutting and crab density.