Simplified Coral Anatomy

The hard skeleton of coral is formed by the secretion of calcium carbonate by the polyp. The cup-like skeleton deposited by an individual polyp is called a corallite. Polyps gather food particles with the nematocysts (stinging, venomous cells) in their tentacles, and feed from sugars produced by photosynthesizing zooxanthellae, a type of algae. The coral tissue protects these algae from herbivorous grazers, and the algae in turn use many of the polyps’ waste products such as carbon dioxide, nitrogen, and phosphorus. Illustration by Laura Torresan, USGS