·Reef building corals are called hermatypic cnidarians ¨corals¨ and live in large colonies. These Corals are tiny animals with a tubular body and an oral gap fringed with tentacles. All cnidarians are carnivorous; therefore the tentacles 'polyps' of the coral are equipped with nematocysts, poisonous cells to sting its prey. Most corals feed at night on small marine organisms ranging in size from zooplankton to small fish. The majority of the coral's energy requirements are provided by photosynthetic organisms that live in its tissue, called zooxanthellae. Zooxanthellae supply the coral polyps with oxygen and food, and are responsible for the color of the corals. In return, the corals provide a protected living area for the zooxanthellae. The coral 'reef' slowly builds itself from calcium carbonate, which is the by-product of hermatypic coral growth.