How Coral Reefs Are Built

Coral reefs are limestone structures built over time by small marine animals, the stony corals. Most corals live in colonies, with numerous individuals, called polyps, living together on top of their limestone skeletons. Each coral is basically a ring of tentacles surrounding a central mouth that sits in a cuplike hole in the underlying skeleton. At night, a coral extends its tentacles to feed on drifting particles and plankton (tiny floating organisms). A coral's skeleton is created by cells in the base and walls of the polyp, which secrete calcium carbonate (limestone). Only the surface of a reef is alive. The underlying structure is made of limestone rockold coral skeletons.

All reef-building corals have tiny algae called zooxanthellae that live inside their tissues and help them create their limestone skeleton. In a mutually beneficial partnership called symbiosis, the algae receive protection and can use the coral's waste products as fuel for photosynthesis (the use of energy from sunlight to combine carbon dioxide and water to make food and grow). Coral polyps, in turn, get an efficient means to eliminate wastes and a source of oxygen, a by-product of the algae's photosynthesis. Working together, the coral and algae create the framework of the coral reef. The reef's complex structure provides a home for many other creatures, including sponges, fish, octopuses, crabs, starfish, and sea anemones.

The shallow, clear waters of the tropics provide optimal conditions of sunlight, warmth, wave energy, and food for coral reef growth. However, if these environmental conditions change, the health and survival of a reef can be put at risk.