Reefs

The Great Barrier Reef Marine Park is the world's largest group of coral reefs, covering an area of 344,400km². It extends for over 2300km along the coast of Queensland. The continental shelf provides the platform from which the reefs on the Great Barrier Reef develop. In the north, the continental shelf lies close to the coast, within 20km at its narrowest point near Cape Melville, and gradually widens to about 400km off the coast in the south.

How coral reefs form

This picture taken through a microscope shows a soft coral polyp (Lobophytum compactum) Green shows the polyp tissue, while the red represents the zooxanthellae.

Reefs are masses of limestone made from skeletons of millions upon millions of tiny marine animals and plants. Colonies of tiny, living coral polyps grow on a reef's surface. These animals are the main reef builders. They extract dissolved limestone from the water and, with the help of single-celled plants (called zooxanthellae) living inside them, lay it down as hard limestone around the lower half of their bodies. The polyps can pull their whole bodies inside these limestone cups for protection, if necessary. The combined skeletons of many colonies of polyps form the large corals that can be seen on reefs. When coral polyps die, their limestone skeletons and the remains of other animals and plants are added to the framework of the reef.

Crustose coralline algae

Coralline algae

Coralline algae, a common group of reef plants, which form limestone crusts, cement them all together.

New coral polyps continue to grow on the surface and the reef slowly enlarges.

The Great Barrier Reef is the largest structure built by living organisms on the Earth today.

There are three main types of reefs found on the Great Barrier Reef:

• Ribbon;
• Platform; and
• Fringing reefs.