1. Introduction
The primary production of coral reefs is largely attributed to benthic primary producers rather than planktonic ones because of the much larger benthic biomass (Atkinson, 2011). Scleractinian corals are one of the major benthic primary producers in coral reefs because they have endosymbiotic algae (zooxanthellae) in the host animal tissue. The algal cell density in corals is approximately on the order of millions per 1 cm2 of the coral surface area and is affected by environmental changes such as seawater temperature (Fagoonee et al., 1999) and light intensity (Masuda et al., 1993). The areal photosynthetic and calcification rates of corals are often influenced by the symbiotic algal density (Gattuso et al., 1999, Marubini and Davies, 1996 and Sawall et al., 2011). Thus, the density of zooxanthellae in corals is an important parameter to evaluate coral physiology and coral reef biogeochemistry.