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Why are corals able to form reefs in shallow sunlit water?

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coral reef The Great Barrier Reef, off the coast of Queensland, Australia, is the world's largest coral reef.

Reef-building corals are able to thrive in shallow, sunlit waters primarily due to a symbiotic relationship with zooxanthellae, which are microscopic, plantlike dinoflagellate algae living within the coral tissues.1

These algae require sunlight to perform photosynthesis, a process that provides significant benefits to the coral host. Specifically, the zooxanthellae help accelerate the formation of the coral's rigid calcium carbonate skeleton by efficiently removing metabolic waste products, such as carbon dioxide, that would otherwise inhibit skeletal growth.1

Because light intensity decreases rapidly with depth, reef-building corals are restricted to shallow zones where sufficient radiant energy is available to support these algae. This reliance on light is a critical factor in why these organisms construct their massive, limestone-based structures in the upper layers of the ocean, typically between the low-water mark and 11 metres, though they can exist at greater depths where light penetration remains adequate.123