Corals are the major building blocks in reef formation. their importance is dependant on the coral-zooxanthellae relantionshipp. The zooxanthellae help the corals produce limestone in an efficient manner. It is not necessary for corals to be the dominant benthic organisms. Incorporated in the reef structure are larger molluscs together with calcareous plants cemented by sand-sized sediments. The most common reef building coral in the Great Barrier Reef is those that belong to the Acropora genus. Acropora are a type of branching coral.