Corals which use symbiotic algae called zooxanthellae occur everywhere in the photic zone of reefs, but they're distributed according to their requirements for light.
A new paper from a team of Mexican marine biologists published in the Proceedings of the Royal Society of London today says that their vertical distribution is related to the type of symbiotic zooxanthellae they contain.
To prove their point, the scientists worked on both the reef itself and in the lab to study the physiology of the symbionts inside two main types of Pacific coral found in the Gulf of California.
They found that each coral species contains a distinct species or form of zooxanthellae which is specifically adapted to cope with a certain amount of light, and therefore forces the coral to occupy a certain vertical position on the reef.
For more details read the paper: Iglesias-Prieto R, Beltran VH, LaJeunesse TC, Reyes-Bonilla H, Thome PE. (2004) - Different algal symbionts explain the vertical distribution of dominant reef corals in the eastern Pacific. Proc R Soc Lond B Biol Sci. 2004 Aug 22;271(1549):1757-63.