The mixed media of the coral sea floor: The coral reef is a unique ecosystem that is home to hundreds of species of reef-building organisms. Corals, sponges, and anemones are among the most complex and successful life forms on earth. Corals live in a symbiotic relationship with photosynthetic dinoflagellates in the genus Symbiodinium. The endosymbiont, or coral host, provides the coral with carbohydrates and some amino acids, and in return, the coral provides shelter, food and energy for the dinoflagellate. The endosymbiont is therefore essential for the survival of the species.