How Coral Reefs Form and Why They Matter

Coral reefs cover only a small portion of the ocean floor, yet they support a remarkable share of marine species. They are built by tiny animals over very long periods, and some reefs are visible from space.

Key takeaways

• Corals are animals, and reefs are built from the limestone skeletons they leave behind.
• Many reef-building corals live in partnership with algae called zooxanthellae.
• Reefs grow best in warm, clear, shallow, sunlit water.
• Reefs shelter a large variety of marine life and protect some coastlines from waves.
• Warming water, pollution and other pressures can cause coral bleaching.

Corals are animals

A coral reef looks like rock or plants, but the living part is made of small animals called polyps. Each polyp is a soft, tube-shaped body with a ring of tentacles around its mouth, related to sea anemones and jellyfish. Polyps use calcium from seawater to build a hard cup of limestone around themselves. When polyps die, their skeletons remain and new polyps build on top, so over hundreds or thousands of years the structure grows.

A partnership with algae

Reef-building corals contain microscopic algae called zooxanthellae living inside their tissue. The algae use sunlight to make sugars through photosynthesis and share much of that energy with the coral, while the coral gives the algae shelter and nutrients. The algae also give corals much of their colour. This partnership explains why most reef-building corals live in shallow, clear water where sunlight reaches them.

Where reefs grow

Most reefs occur in tropical and subtropical seas where water is generally warm. They need clear water so light can penetrate, salty water of normal ocean strength and fairly low levels of sediment. There are also cold-water corals that live in deep, dark parts of the ocean without algae, feeding on particles drifting down.

Types of reefs

Scientists often describe three broad forms. Fringing reefs grow close to the shore. Barrier reefs, such as the Great Barrier Reef off Australia, are separated from land by a lagoon. Atolls are ring-shaped reefs that often form around sunken volcanic islands. Charles Darwin proposed an explanation for how these types are related as islands slowly sink and reefs grow upward.

Why reefs matter

Reefs provide food and shelter for fish, shellfish and many other creatures, including nursery areas for young animals. They can reduce wave energy and protect coastlines, and they support fishing and tourism for many communities. Reef organisms are also studied for compounds that could be useful in medicine.

Threats and bleaching

When water becomes too warm or conditions are stressful, corals can expel their algae and turn pale, an event called bleaching. Corals can recover if conditions improve soon enough, but prolonged or repeated bleaching can kill them. Other pressures include pollution, sediment from land, overfishing, destructive fishing methods and ocean acidification. Reducing local pressures and addressing climate change are both part of reef conservation.

Zones of a reef and who lives there

A reef is not uniform. The reef crest is the shallow outer edge where waves break, home to sturdy corals that can handle the pounding. Behind it, a calmer lagoon may hold branching corals, seagrass and sandy patches. On the outer slope, the fore reef drops into deeper water, with plate-like corals that capture dim light. Each zone supports different animals: parrotfish that graze algae, groupers and sharks that hunt, turtles that feed on sponges and seagrass, and countless crabs, shrimps, worms and molluscs. Small cleaner fish run cleaning stations where larger fish queue to have parasites removed, a classic example of cooperation.

How corals reproduce

Corals reproduce in several ways. Many species release eggs and sperm into the water in synchronised mass spawning, often on a few nights after a full moon in late spring or summer. On the Great Barrier Reef this spectacle has been called an underwater snowstorm. The fertilised eggs develop into tiny swimming larvae that drift for days or weeks before settling on a hard surface and growing into new polyps. Corals can also reproduce asexually, when fragments break off and grow into new colonies, which is useful for reef recovery and for gardeners growing corals in nurseries.

Ocean acidification

Warming is not the only threat. The ocean absorbs a large share of the carbon dioxide that people release, and this makes seawater slightly more acidic. Acidification reduces the availability of the carbonate ions that corals and shellfish use to build their skeletons and shells, which can slow reef growth and make structures weaker. The effects are expected to grow if emissions continue, and scientists are studying how different species respond.

Restoration and protection

People are experimenting with ways to help reefs. Coral gardening involves growing fragments in nurseries and attaching them to damaged reefs. Researchers are studying heat-tolerant corals and whether assisted breeding could help. Marine protected areas limit fishing and damaging activities, giving reefs a better chance to recover. Reducing pollution and sediment runoff from land, and managing fishing sustainably, are practical steps. Restoration can help local areas, but experts stress that reducing greenhouse gas emissions is essential for the long term.

People who depend on reefs

Hundreds of millions of people live near coral reefs and rely on them for food, income from fishing and tourism, and protection from storm waves. For some island nations, reefs are central to culture and the economy. This is one reason that reef health is a matter of livelihoods as well as biodiversity.

Frequently asked questions

Is coral a plant or an animal?

Coral is an animal. It looks plant-like because it stays in one place and hosts algae.

How fast do reefs grow?

Different species grow at different speeds, often only a few millimetres to several centimetres a year, so large reefs take a very long time to form.

Is bleached coral dead?

Not necessarily. A bleached coral is stressed and may recover if the algae return, but it is more vulnerable.

Why do corals spawn at the same time?

Spawning together increases the chance that eggs and sperm meet and overwhelms predators with a flood of food, so more larvae survive. Cues such as water temperature, day length and the phase of the Moon help coordinate the timing.

What can I do to help coral reefs?

Reduce your carbon footprint, choose sustainable seafood, avoid touching or standing on coral when you visit, and support organisations working on reef conservation. Using sunscreen labelled as reef-friendly is sometimes suggested, though the best evidence is for protecting reefs from warming and pollution.

How old are coral reefs?

Corals have existed for hundreds of millions of years, but the reefs we see today are much younger. Most modern reefs have grown during roughly the last ten thousand years, since sea levels stabilised after the last ice age.

Sources and further reading

• Coral reef (Wikipedia)
• NOAA Coral Reef Conservation Program