Seawater partial pressure of CO2 had no impact on the densities of Symbiodinium per unit area of coral skeleton in P. damicornis (Table 2). Light level and temperature level did have a significant impact on areal Symbiodinium densities (p<0.0001). Symbiodinium densities were higher in low light (792480±27864 s.e. cells per cm2) compared to high light (617026±19628 s.e. cells per cm2) and higher at low temperature (825897±25589 s.e. cells per cm2) compared to high temperature (599125±21861 s.e. cells per cm2). Light level and temperature level had a significant interactive effect; low light interacting with low temperature had a greater than additive effect on Symbiodinium densities compared to the other three combinations of light and temperature (Figure 2 a). There was no significant interaction between temperature and pCO2, light and pCO2 or all three factors (Table 2).