These organisms in turn are sustained by the reef itself. The entire reef can be interpreted as a mass of interdependent creatures. We normally think of the hard coral as one animal, but in reality it is a colony of mutually dependent organisms. For example, through a network of connecting stomach extensions, food is shared out all over the colony, with each polyp both contributing to and sharing in the general welfare. The reef itself then, can be thought of as a whole creature with its own ways of capturing food and creating a suitable environment for future generations of inhabitants.
Corals also have a symbiotic relationship with the green algae called zooxanthella.
This algae lives within the tissue of the reef-building coral. The algae contributes to the calcification capability of corals by extracting carbon dioxide from the animal's body fluids. This increases the concentration of the carbonate ion needed for the precipitation of calcium carbonate. This relationship represents the interdependence of the system whereby both parties benefit. The coral contributes to the relationship by providing a supply of nutrients to the zooxanthella.