How Is a Coral Reef Formed

Click on this coral polyp.

A coral reef is made of millions of tiny animals called coral polyps. These coral polyps are nocturnal animals. Corals produce children that are called planulae. These children then drift across the ocean to new shorelines where they settle and begin to grow. This is the main way that Hawaiian coral reefs are spread from place to place.

The coral reefs are formed layer by layer. The delicate polyps sit inside their calcium cups, each building a little more calcium carbonate onto the coral head. Multiply this process by billions of polyps and together they add to the reef.

Most sea plants are called algae. Some algae secrete skeletons of calcium carbonate. These plants called coralline (COR-al-een) algae, are important builders of Hawaiian coral reefs because they act like binders. Some of the algae form brown, pink, and red crusts, while other coralline algae grow in small branching clumps.

The most important reef-building plant is probably the microorganism called zooxanthellae (zo-zan-THEL-y). These plants live in the tissues of reef-building coral polyps. They are too tiny for the naked eye to see. Coral reefs wouldn't be able to develop without these algae because the corals could not be able to grow fast enough or make enough foundation material to build reefs. These plants use the light from the sun to develop oxygen and food that the polyps use. Coral polyps produce waste that the zooxanthallae need to survive. This means that these plants and animals help each other survive, and together they let a coral reef grow and work as a system.