Coral reefs are the largest geological structures on earth built by living organisms! The little architects responsible for these structures are small coral polyps that secrete the calcium carbonate structure.
Coral polyps are also tiny farmers, providing shelter and nutrients to single-celled algae called zooxanthellae. Zooxanthellae resides inside polyp cells and in return for protection, these algae cells provide oxygen and help the coral reef system thrive!
Sketch and fact contributed by Karli Chudeau
[Edited by Adrian Perez]
Source: Goreau, T. F., Goreau, N. I., & Goreau, T. J. (1979). Corals and coral reefs. Scientific American, 241(2), 124-137.