What are corals? Corals themselves are animals. But tropical reef-building corals have tiny plant-like organisms living in their tissue. The corals couldn’t survive without these microscopic algae–called zooxanthellae (zo-zan-THELL-ee). This cutaway diagram of a coral polyp shows where the photosynthetic algae, or zooxanthellae, live—inside the polyp’s tissue. The coral gives the algae a home. In return, the algae provide the coral with food.