Coral reefs are primarily formed by what type of organisms?

Coral reefs are primarily formed by coral animals known as polyps, which are tiny, soft-bodied organisms that belong to the phylum Cnidaria. Each polyp secretes a calcium carbonate skeleton, and over time, the accumulation of these skeletons from countless polyps leads to the formation of a coral reef. This process involves the symbiotic relationship between polyps and a group of algae called zooxanthellae, which live within the tissues of the coral and provide essential nutrients through photosynthesis. Thus, while algae and other marine organisms contribute to the ecosystem and health of the reef, the structure itself is fundamentally built by the calcium carbonate exoskeletons produced by the coral polyps. Understanding this central role of polyps helps clarify the biological and geological processes that create and sustain coral reef ecosystems.