Final answer:
Coral reefs are ocean ridges formed by marine invertebrates that secrete a calcium carbonate skeleton. They have a mutualistic relationship with photosynthetic unicellular algae in shallower waters, while corals in deeper waters obtain energy by capturing prey. Coral reefs support a diverse community of organisms and contribute to species richness.
Step-by-step explanation:
Coral reefs are ocean ridges formed by marine invertebrates living in warm shallow waters within the photic zone of the ocean. The coral organisms are colonies of saltwater polyps that secrete a calcium carbonate skeleton. These calcium-rich skeletons slowly accumulate, forming the underwater reef.
Corals found in shallower waters have a mutualistic relationship with photosynthetic unicellular algae, which provides them with nutrition and energy. Some corals living in deeper and colder water do not have a mutualistic relationship with algae; these corals obtain energy and nutrients by capturing prey with stinging cells on their tentacles.
Coral reefs have high biodiversity and support a wide variety of organisms such as microbes, algae, invertebrates, and fishes. Their high productivity helps provide great species richness.