Coral Is an Animal Not a Plant

For centuries, the branching, rock-like nature of coral (Deals) reefs led to a simple classification: they were plants, or perhaps strange minerals. The truth is far more dynamic. These immense structures are built by vast colonies of tiny, soft-bodied creatures called polyps. Each individual polyp secretes a protective limestone skeleton (calcium carbonate) at its base. As generations of polyps live and die, their skeletons build upon one another, slowly forming the massive, complex architecture of a coral reef over thousands of years. This collective construction project is undertaken by animals no bigger than a pencil eraser.

The reason for the long-held confusion is understandable, especially given coral's reliance on sunlight, a traditionally plant-like behavior. This is explained by a crucial symbiotic relationship with microscopic algae called zooxanthellae. These algae live within the polyp's tissues, photosynthesizing to produce energy that feeds their host. In return, the polyp provides the algae with shelter (Review) and the compounds needed for photosynthesis. This powerful partnership is not only responsible for the coral's vibrant colors but is also the engine that powers the construction of entire reef ecosystems.