Hermatypic, zooxanthellate corals are limited to the euphotic zone in tropical seas. Corals, like any other object, will absorb a variable fraction of the total radiant flux incident upon them, depending on their shape, orientation, and absorbency. The remaining light will be scattered or reflected. An integrating (Ulbricht) sphere was used to measure the fraction of the incident quantum flux that is absorbed by a coral colony introduced into the integrating sphere.

In branched colonies shading among branches reduces the absorbed light per unit area and/or per zooxanthella. Therefore, under low irradiance, it is advantageous for colonies of branched species to flatten out and reduce branching to one plane, perpendicular to the light. Increasing in algal density, will increase the light absorbed by the corals until that density will cause an intercellular "packaging effect" among the zooxanthellae. The in vivo spectral average-chlorophyll a absorption cross section, *a, as found in the coral, decrease with increasing chlorophyll density.

The coral responds to both low light intensity and to nutrient enrichment by increasing the amount of chlorophyll cm^-2. The increase of chlorophyll under light resulted from increasing chlorophyll per algal cell. The response of the coral to nutrient enrichment is mostly due to the increase in the number of algae cm^-2. This increase in algae cm^-2 also caused an increase in chlorophyll cell^-1 due to increase shading. In total darkness there is a loss of chlorophyll and of zooxanthellae.

The corals that have high concentrations of chlorophyll in both nutrient-enriched and shade- adapted colonies absorbed more light compared to colonies with low pigmentation. In the case of low light there is an adaptation of the photosynthetic apparatus of the algae and the quantum yield is high. In contrast, nutrient-enriched colonies have low light-utilization efficiency because of a low rate of photosynthesis. Photosynthetic rates, on a per-cell basis, were inversely correlated with algal densities, indicating possible competition among the algae for CO2.

Eutrophication reduced the contribution of the zooxanthellae to the host. The algae overgrow their host and the skeletal growth rate of the colonies is slowed down. Corals dominate shallow coastal regions in tropical oceans. In these oligotrophic waters they success under a wide range of irradiance levels.