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Corals are colonies of made up of tiny animals called polyps. Hard corals have polyps with 6 tentacles, or multiples of 6 tentacles. Soft coral polyps have 8 tentacles. The oldest coral on the Great Barrier Reef would be a Porites sp. and is probably about 1,000 years old. Old colonies of this species are the size of a small room. These corals grow at about 1 cm in height per year and their skeleton reflects the weather conditions at the time of the growth. Hence large corals which are hundreds of years old can provide information about water temperature and rainfall patterns that pre-date European settlement. Tiny algae called zooxanthellae live in the flesh of most corals. The algae photosynthesise and transfer energy to the coral. Therefore, coral reefs (which are built mostly by these corals) are generally confined to shallow waters because like other plants, the algae require light to survive.

Coral reefs are fragile ecosystems. Because corals need warm, sunlit water to live, they often grow close to the top of the water. Sadly, being so close to land makes them often be damaged by poisons and dirt that can come from boats and the land nearby. Dirt makes the water more cloudy, which makes the sunlight less. Poisons can bleach and kill corals. Also, they are hard for ships to see, but easy to hit, which makes ships often run into the coral, damaging both the boats and coral. Because of this, many countries are trying to lessen the kinds of building that usually happen near beaches that have coral reefs nearby, and be more careful about the boats that go around reefs.[4]

The coral reef builds upwards, growing towards the light much like trees in a rainforest, competing for space and light. Once the coral reef reaches sea level it cannot survive the harsh surface conditions so begins to grow outwards. Creating spectacular formations of coral that spawn further growth and spreading of the reef ecosystem. Through time, animals grow and the sand, rubble and debris of life is broken down by waves and eroding animals, such as worms and sponges. A complex reef ecosystem is built over time. Today’s underwater gullies and caves were formed because of that erosion. These are the same formations scuba divers enjoy exploring today because they are shelters for an abundance of marine life. Coral reefs tend to grow where there is a lot of water movement, bringing nutrients, oxygen and new species. Most reef-building corals cannot grow in waters shallow enough to expose them at high tide or deeper than 50 metres, making them highly sensitive to changing sea levels. The Reef is continually evolving and changing as climate and sea levels change. Healthy, diverse reef ecosystems are more resilient, that is, they are able to adapt to change. The Great Barrier Reef Marine Park Authority is concentrating on maximising reef resilience through research and management.