How coral reefs are built and who lives on them

A coral reef is a structure that living animals have built over a long period, mostly stony corals whose skeletons stack up as new colonies grow on top of older ones. The reef is more than rock. It is a habitat that shelters fish, invertebrates and algae, and it grows only where conditions suit the corals. Those conditions are usually warm, clear, sunlit water in shallow seas, which is one reason reefs are fragile when the water changes.

Reefs come in several forms. Fringing reefs grow close to a shoreline, barrier reefs sit farther out and are separated from land by a lagoon of deeper water, and atolls are ring-shaped reefs that surround a lagoon, often formed around islands that have since sunk. Each form grows according to the land, depth and water movement at its location, but all of them depend on the same basic process of coral growth.

How coral colonies build the structure

Each stony coral colony is made of many small animals called polyps. Each polyp sits in a cup of hard calcium carbonate that it builds from minerals in seawater, and the colony grows by budding new polyps. When a polyp dies, its skeleton remains, and new polyps grow on top of it. Over many generations this process builds up the reef, and the skeleton of each colony becomes part of the structure.

Growth is not the only process at work. Storms, boring animals and grazers break coral skeletons into rubble, and coralline algae, a type of red algae that deposits hard material, cement the fragments together. This cementing helps the reef stay solid enough to resist waves, and it is one reason a healthy reef can recover its shape after storm damage that would otherwise break it apart.

The partnership that powers the corals

Most reef-building corals depend on tiny single-celled algae called zooxanthellae that live inside their tissues. The algae use sunlight to make sugars through photosynthesis, which supply much of the energy the coral needs. In return the coral gives the algae shelter and the nutrients they need. The algae are also the source of much of the brown and golden color seen in corals.

This partnership works only within a narrow range of conditions. Water that stays too warm for too long, or that becomes too murky to let enough light through, can stress the corals. When stressed, corals may expel their algae and turn pale, a condition called bleaching. Corals can recover if conditions improve in time, but prolonged or repeated bleaching can kill them, and once a skeleton dies it is quickly colonized by algae and other organisms.

Who lives on and around the reef

The reef supports a wide range of animals, and fish are the most visible. Parrotfish and some sea urchins graze algae from hard surfaces, keeping them clear so that new corals can settle and grow. Predators such as groupers and moray eels hunt smaller fish and crustaceans among the crevices. Cleaner wrasses work from fixed stations, where larger fish visit to have parasites removed.

Invertebrates fill much of the space in the gaps between coral and under rock. Sponges, sea stars, sea cucumbers, crabs, shrimp, clams, snails and worms all live on the reef, and many are small enough to be overlooked. Some reef fish depend on particular corals for food or shelter. Many juvenile fish stay among branching coral until they grow large enough to move into more open water, which makes the health of the coral directly important to their survival.

Why reef systems are so easily damaged

Reef-building corals grow slowly, so damage can take years to repair, and some losses are never fully reversed. Corals are sensitive to water temperature, sediment and nutrient runoff from land. Sediment can smother coral tissue and block the light that the algae need, while excess nutrients can encourage algae to overgrow corals. Overfishing can upset the balance between grazing animals and algae, and rising sea temperatures are a major stress for many reefs.

Physical contact also does harm. Anchors, dragged fins, hands and boat hulls can break branches or crush the living surface, and a broken branch can leave tissue open to infection. Because corals grow so slowly, the loss of a single branch may represent many years of growth. Careful behavior on the reef matters for this reason alone.

What divers and snorkelers can do

Individual choices make a difference. Keep fins above the reef, avoid standing on or touching corals, and use mooring buoys where they are provided rather than dropping an anchor onto the reef. Do not buy coral, shells or other reef material as souvenirs, and follow the rules set by local marine parks, which often limit fishing, anchoring and access in sensitive areas. If you notice damage, such as broken coral or an abandoned anchor line, tell your operator or the park so that managers can respond.

Knowing how a reef is built makes it easier to see why small impacts matter. A coral branch that took many years to grow can be broken in a moment, and the animals that depend on it lose their shelter with it. A careful, slow approach protects both the reef and the animals that live there.