What is coral made from?

The beautiful, rock-like structures we see in the ocean are actually created by millions of soft-bodied animals called coral (Deals) polyps. These organisms, relatives of jellyfish and sea anemones, build hard, protective homes for themselves by extracting calcium carbonate from the surrounding seawater. As new polyps build on top of the empty homes of their ancestors, the colony grows. What we recognize as a single piece of coral is a massive, interconnected structure formed from these countless individual limestone skeletons.

This incredible building process is powered by a crucial partnership. Living within the tissues of most reef-building polyps are microscopic algae called zooxanthellae. Through photosynthesis, these algae provide the coral with essential nutrients and energy, which accelerates skeleton growth. This symbiotic relationship is also responsible for the stunning and vibrant colors of healthy coral reefs. When corals are stressed by environmental changes like warming water, they expel the algae, leading to a phenomenon known as coral bleaching.

Over thousands of years, the slow and steady work of these tiny creatures results in the creation of vast underwater ecosystems. Coral reefs are among the largest biological structures on Earth, so immense that some, like the Great (Review) Barrier Reef, are even visible from space. Every intricate branch and colorful formation is a testament to the collective architectural efforts of these minuscule marine animals.