Final answer:
Coral reefs are limestone ridges in the ocean built by tiny coral organisms from the phylum Cnidaria, creating habitats for diverse marine life and providing coastal protection.
Step-by-step explanation:
Limestone ridges built by tiny animals are known as coral reefs. Coral reefs are formed by the calcium carbonate skeletons of coral organisms, which are marine invertebrates in the phylum Cnidaria.
These organisms secrete calcium carbonate to form a hard skeleton, which grows and accumulates over time, creating the reef structure. The coral polyps live in symbiosis with algae, which provide them with nutrients and help in the deposition of calcium carbonate.
Coral reefs are important habitats for a multitude of marine life and provide protection from waves and storms for coastlines. The Great Barrier Reef is an example of a well-known reef that has formed over millions of years.