What a Coral Reef Is
A coral reef is a structure built by living things over a very long time. The builders are corals, which are animals, not plants or rocks. A coral colony is made of many small soft-bodied animals called polyps, each usually only a few millimeters across. Most reef-building corals take calcium and carbonate from seawater and use them to make a hard skeleton of limestone. When polyps die, their skeletons stay behind, and new polyps grow on top. Over thousands of years this builds the reef.
Many reef-building corals have a partnership with microscopic algae living in their tissues. The algae use sunlight to make sugars through the process of plant photosynthesis and share part of this food with the coral, and in return the coral gives them shelter. This is why reef-building corals are generally found in clear, shallow, sunlit water, and why tropical seas are their home.
Where and How Big
The Great Barrier Reef lies off the northeast coast of Queensland, Australia, in the Coral Sea. It is often described as stretching roughly 2,300 kilometers (about 1,400 miles) along the coast. It is not a single wall of coral. It is a vast system of thousands of individual reefs, along with many small islands, sand cays and channels, covering an area commonly given as around 340,000 square kilometers. It was listed as a World Heritage Site in 1981, and large parts of it are within a marine park.
How the Reef Is Structured
The reef is more varied than the picture of a wall suggests.
- Fringing reefs grow close to the shore of the mainland or islands.
- Platform reefs are patch-like reefs that rise from the sea floor in the open water.
- Cays are low islands formed from sand and coral fragments piled up by waves.
- Lagoons and channels lie between reefs and hold calmer water, seagrass and sandy bottoms.
- Deeper areas and the reef slopes host different communities than the sunlit reef tops.
The reef is home to a huge variety of life, including many fish, sea turtles, rays, sharks, mollusks and sponges. It is a good example of an ecosystem in which many species depend on a living structure and on one another through food webs. Because the number of species is so large, reefs are often cited as an example of why species biodiversity matters.
Pressures on the Reef
Scientists who monitor the reef identify several pressures, and they interact with each other.
Warming seas and bleaching. When the water stays too warm for too long, corals can expel the algae living in them. Without the algae, the coral loses most of its color and food source, which is called bleaching. Bleached coral is not necessarily dead. If the heat eases soon enough, the algae can return and the coral can recover. If the stress is severe or lasts long, many corals die. Mass bleaching events have affected the reef several times in recent decades. The warming linked to the greenhouse effect is considered the main long-term threat to coral reefs worldwide.
Tropical cyclones. Storms can break coral through strong waves. This is a natural pressure, and reefs have recovered from storms in the past, but recovery takes years and is harder if other stresses are present.
Crown-of-thorns starfish. These starfish eat coral. In outbreaks they can destroy large areas. Outbreaks have been linked in research to nutrient-rich runoff, though the causes are still being studied.
Water quality. Runoff from land can carry sediment, nutrients and chemicals to the reef. This water flows to the sea as part of the water cycle, and it can cloud the water and encourage algae that compete with coral.
Ocean chemistry. The ocean absorbs part of the carbon dioxide people release, which makes seawater slightly less alkaline. This shift along the pH scale is called ocean acidification. It can make it harder for corals and other shell-building animals to build skeletons.
Fishing and coastal development. Local activities can affect fish populations and nearby habitats, which is why management usually combines protected zones with rules on fishing and shipping.
Common Myths
Myth: Coral is a rock or a plant. Coral is an animal. The hard material is a skeleton the animals build.
Myth: Bleached coral is dead coral. Bleached coral is stressed and starving, but it may recover. Death usually follows prolonged or severe bleaching.
Myth: The whole reef is dead, or it has already disappeared. Neither is accurate. The reef is large and varied. Condition differs from area to area, and recent surveys have shown periods of both recovery and loss. Describing the overall health honestly means acknowledging damage without claiming it is gone.
Myth: The reef is one single organism. It is a huge system made of countless colonies of many coral species and other organisms. It is the largest reef system in the world and is often called the largest structure built by living things, but it is not one animal.
Myth: It is the only living thing visible from space. The reef is large enough that parts of it show up in satellite images, but the popular line that it is the only living thing visible from space is not a precise fact. What can be seen depends on conditions and equipment.
Myth: Reefs are fragile and nothing can help them. Corals can recover when conditions allow, and managers use measures such as protected zones, better water quality and monitoring. But no local action can fully offset global warming, so the long-term outlook depends on wider changes as well. Scientists generally describe the future as uncertain and dependent on how much the planet warms.
Why Reefs Matter
Coral reefs cover only a tiny fraction of the ocean floor but support a large share of marine species. They provide food, shoreline protection from waves, and income from tourism and fishing for many communities. For readers, the Great Barrier Reef is a convenient case study of how living structures, ocean chemistry and climate are connected.