In coral reefs, the cells of the cylindrical coral polyp contain single-celled photosynthetic algae, or zooxanthellae.Coral polyps filter organic material from water and metabolize it, forming carbon dioxide and nitrogenous wastes that zooxanthellae use in photosynthesis.The oxygen and sugars they form in turn are used by coral polyps and dinoflagellates for metabolism.Many corals cannot feed successfully when they lose their zooxanthellae, but zooxanthellae can usually grow and reproduce outside of the corals.For zooxanthellae, this interaction is _______; for corals, it is _______.

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The interaction between zooxanthellae and corals is a form of mutualism because both parties benefit: zooxanthellae receive carbon dioxide and nitrogenous wastes from corals for photosynthesis, while corals get oxygen and sugars in return. However, it is facultative for zooxanthellae, meaning they can survive and reproduce outside of the corals, but it is obligate for many corals, as they cannot feed successfully without their zooxanthellae.

In Coral Reefs, the Cells of the Cylindrical Coral Polyp