Acroporid corals with clade C Symbiodinium algae showed a higher growth rate than those associated with clade D algae. In addition, fluorescent protein in juvenile Ursolic-acid polyps was also changed the amount by endo9-methoxycamptothecine symbiotic Symbiodinium clade. Yuyama et al.,showed that the expression pattern of fluorescent protein homolog and some stress responsive genes were different between clade A and clade D symbiosis. In the present study, we exposed aposymbiotic juvenile polyps to monoclonal cultures of Symbiodinium clade C1 and clade D, and we used these polyps as model symbiosis system. Previous laboratory experiments demonstrated that immediately after metamorphosis, juvenile Acropora tenuis polyps could form symbiotic relationships with Symbiodinium algae in clades A and D, but not with those in clade C. Thus, we conducted a longterm laboratory experiment for cultivating corals associated with clade C1 algae. We compared the growth rate of the skeleton and fluorescence of polyps between corals associated with algae in clades C1 and D. In addition, photosynthetic activity of symbiont could produce reactive oxygen species that might affect the coral condition. For further understanding the different effect of each symbiont clade on corals, we also investigated the antioxidant activity. Past studies have shown that corals could acquire increased thermal tolerance by shifting their dominant symbiont algae clade C to clade D. In the present study, we prepared the model system for coral�Czooxanthellae symbiosis by infecting A. tenuis juveniles with monoclonal Symbiodinium culture in clade C1 or D. We discussed here the different effects of symbiotic algae clades C1 and D on physiological properties of juvenile polyps. We found that the green fluorescence of juvenile polyps was different depending on associations with different Symbiodinium clades. When C1 Symbiodinium algae were introduced to juvenile polyps, a bright fluorescence was observed in the early stage of symbiosis. Since such a bright fluorescence was not observed in aposymbiotic corals and corals associated with clade D Symbiodinium algae, this was probably caused by an association with clade C1 algae. Fluorescence proteins exhibit significant hydrogen peroxide scavenging activities. In corals, genes coding for fluorescent proteins change their expression under temperature stress. Heat stress could be attributed to greener of juvenile polyps. Bright fluorescence was observed in corals containing clade C1 algae, suggesting that corals experience oxidative stress when they initially acquire clade C1 Symbiodinium algae. Furthermore, to investigate the correlation of oxidative stress and fluorescence in corals, their antioxidant activitywas measured. Catalase is responsible for deactivating the reactive oxygen species, H2O2, into water and oxygen. It has been reported that catalase activities increase rapidly in coral tissues exposed to high temperature. In this study, catalase activity tended to be higher in corals with clade D algal symbiosis than in those with clade C1 algal symbiosis. Furthermore, 5-month-polyps had a higher level of catalase activity than 1-month-polyps, indicating that catalase activity may be influenced by increase in the number of endosymbiotic algae. Our results suggested that the bright fluorescence of polyps was not caused by oxidative stress from endosymbiotic algae. Another possible function of the green fluorescent protein is to be a key component in the immune system of corals.