Why do coral reefs grow in shallow, sunlit water?

Coral reefs grow in shallow, sunlit water primarily to support the symbiotic relationship between reef-building coral polyps and zooxanthellae.12

Zooxanthellae are photosynthetic, single-celled algae that live within the tissues of the coral. Because these algae require sunlight to perform photosynthesis, they thrive in the well-lit photic zone of the ocean. This partnership is mutually beneficial: the zooxanthellae provide the coral with nutrients and significantly increase the speed at which the coral can secrete calcium carbonate to build its rigid skeleton.1324

While reef-building corals can construct reefs at depths of up to 40 metres, they grow most profusely in shallow, sunlit waters where high light intensity and warm temperatures—typically above 18–22 °C—facilitate the rapid accumulation of the reef structure. In deeper or darker waters, this essential photosynthetic process cannot be sustained, limiting the ability of these corals to form complex reef ecosystems.1562