study of chlorophyll density in corals is informative. The 
chlorophyll is in the algae which are symbotic. 
So 'branches' below are coral branches.

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.

The coral responds to both low light intensity and to 
nutrient enrichment by increasing the amount of chlorophyll 
per square centimeter.

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 .This increase in algae   also 
caused an increase in chlorophyll  due to increase shading. 
In total darkness there is a loss of chlorophyll and of 
zooxanthellae.

some plants on the forest floor  often have leaves 
perpendicular to the sunlight, another method of optimizing 
sunlight reception used by the coral.