Coral reefs are actually colonies of small polyps that each live within a hollow cylinder of calcium carbonate, which it excretes. Their tentacles extend above the top of this cylinder and sweep in small organisms and other food scraps. On the outside of the hollow cylinders single-celled green algae live. These algae use the carbon dioxide and nitrogen wastes given off by the corals to produce sugars and oxygen through the process of photosynthesis. The sugar and oxygen are then used by the coral for growth. In addition, the algae are protected by the structure of the reef from animals that might eat them. a.) Mutualism
b.) Commensalism
c.) Parasitism
d.) Predator-Prey