Coral Reefs: The Living Cities Built by Tiny Animals

What Is a Coral Reef?

A coral reef is an underwater ecosystem built by living animals. It looks like rock, and much of it is rock, but that rock was made by corals: small creatures that take calcium and carbonate from seawater and turn them into hard limestone skeletons. Over hundreds and thousands of years, generation after generation of corals adds new layers on top of the old ones, and the result is a reef that can stretch for kilometres and rise tens of metres from the seabed.

Coral reefs cover less than one percent of the ocean floor, yet they are home to roughly a quarter of all known marine species. Fish, sea turtles, sharks, octopuses, sea stars, sponges, clams, shrimps and countless smaller organisms live on, in and around them. Because of this crowded variety of life, coral reefs are often called the rainforests of the sea.

The Builders: Coral Polyps

A coral is not a plant and not a stone. It is an animal, related to jellyfish and sea anemones. All of these belong to a group called cnidarians, which have soft bodies and tentacles armed with tiny stinging cells.

One coral, thousands of animals

What we usually call "a coral" is actually a colony made of many tiny individuals called polyps. Each polyp has a tube-shaped body, often only a few millimetres wide, with a mouth at the top surrounded by a ring of tentacles. The polyps in a colony are connected by a thin layer of living tissue, so they share food and work together almost like one large organism.

The reef-building corals are known as stony corals or hard corals. Each polyp produces a cup-shaped skeleton of calcium carbonate beneath its body. When the polyp grows and divides, the new polyps build their own cups next to and on top of the old ones. Different species grow in very different shapes: branching corals look like antlers or small trees, brain corals form rounded domes covered in winding grooves, table corals spread out like flat plates, and massive corals form solid boulders.

How corals eat

Corals get food in two ways. At night, many polyps stretch out their tentacles and use stinging cells to catch tiny drifting animals called zooplankton. But most of their energy comes from a partnership with microscopic algae living inside their tissues.

A Partnership Powered by Sunlight

Inside the cells of most reef-building corals live single-celled algae commonly called zooxanthellae. Like plants, these algae use sunlight to make sugars through photosynthesis. They share much of this food with the coral; in many corals the algae supply most of the energy the coral needs. In return, the coral gives the algae a safe home, a place in the sunlight, and waste products such as carbon dioxide and nitrogen compounds that the algae use as nutrients.

This partnership is the secret behind the success of coral reefs. Tropical seawater is often clear precisely because it contains very few nutrients, and few living things can thrive in such "empty" water. By recycling nutrients between them, the coral and its algae make a living in a place where most organisms would starve. The algae also give many corals their colours, from brown and green to gold.

Because the algae need light, reef-building corals live mainly in shallow water, usually less than about 50 metres deep, where sunlight can reach them.

Where Coral Reefs Grow

Reef-building corals are fussy about where they live. Most need:

• Warm water, roughly between 20 and 30 °C. That is why most reefs lie in the tropics, between about 30 degrees north and 30 degrees south of the equator.
• Clear water, so sunlight can reach the algae. Muddy water from rivers blocks light and can smother polyps.
• Salty water with a stable salinity. Large amounts of fresh water from rivers make reefs rare near big river mouths.
• A hard surface where young corals can attach, such as rock or old reef.

Reefs are common along the eastern coasts of continents, where warm currents flow from the tropics, and rarer along western coasts, where cold currents rise from the deep. The richest reefs in the world are found in the Coral Triangle, a region of the western Pacific that includes the waters of Indonesia, the Philippines, Malaysia, Papua New Guinea, the Solomon Islands and Timor-Leste. Around three-quarters of all known reef-building coral species live there. Other famous reef regions include the Red Sea, the Caribbean, the Indian Ocean around the Maldives, and the coast of north-eastern Australia.

Three Main Types of Reefs

Scientists usually divide coral reefs into three main types, a system first described by the naturalist Charles Darwin, who studied reefs during his voyage on HMS Beagle in the 1830s.

1. Fringing reefs grow right along the shore, attached to the coast or separated from it by a narrow, shallow channel. They are the most common type.
2. Barrier reefs run parallel to the coast but farther out, separated from land by a wide and deeper lagoon.
3. Atolls are rings or horseshoes of reef, often topped with low sandy islands, that surround a central lagoon with no large island in the middle.

Darwin's explanation of atolls

Atolls puzzled early explorers: how could a ring of shallow-water coral rise from the deep open ocean? Darwin suggested an answer in his 1842 book on coral reefs. He proposed that an atoll begins as a fringing reef around a volcanic island. Over a very long time the volcano slowly sinks, while the corals keep growing upward toward the light. The fringing reef becomes a barrier reef with a lagoon, and when the island finally disappears beneath the waves, only the ring of reef remains: an atoll.

Darwin could not prove his idea in his lifetime. More than a century later, in the early 1950s, scientists drilled deep into Enewetak Atoll in the Marshall Islands. They passed through more than 1,200 metres of reef limestone before reaching volcanic rock, which strongly supported Darwin's theory.

The Great Barrier Reef and Other Giants

The largest coral reef system on Earth is the Great Barrier Reef off the coast of Queensland, Australia. It stretches for more than 2,300 kilometres and is made of around 2,900 individual reefs and hundreds of islands. It is so large that it can be seen from space, and it was named a UNESCO World Heritage Site in 1981. It is not a single wall of coral but a huge maze of reefs, lagoons, channels and islands.

The Mesoamerican Barrier Reef, which runs along the coasts of Mexico, Belize, Guatemala and Honduras, is the largest reef system in the Atlantic Ocean. The Maldives, in the Indian Ocean, is a country made almost entirely of atolls, with its islands sitting only a little above sea level.

Life on the Reef

A reef is like a busy city where every resident has a job. Some of the best-known inhabitants include:

• Parrotfish, which scrape algae off coral with beak-like teeth. They also grind up bits of coral skeleton and pass it out as fine sand. A single large parrotfish can produce many kilograms of sand in a year, and much of the white sand on tropical beaches was made this way.
• Clownfish, which live among the stinging tentacles of sea anemones. A layer of mucus protects them, and the anemone gives them shelter from predators.
• Cleaner wrasses and cleaner shrimps, which run "cleaning stations" where larger fish line up to have parasites and dead skin picked off them.
• Giant clams, which, like corals, host algae in their tissues and can grow larger than a metre across.
• Sea turtles, reef sharks and moray eels, which hunt, graze or hide among the reef's cracks and caves.

Every hole and branch of a reef provides shelter, which is why so many species can live in such a small area. Grazing fish and sea urchins also play an important role: by eating seaweed, they stop it from overgrowing and smothering the corals.

How Corals Reproduce

Corals can reproduce in two ways. In asexual reproduction, a polyp splits or buds to form new polyps, which is how a colony grows. A broken branch can also settle on the seabed and grow into a new colony.

In sexual reproduction, many corals release eggs and sperm into the water. On the Great Barrier Reef, many species do this together in a spectacular event called mass spawning, which happens on a few nights after a full moon in late spring. The water fills with clouds of tiny pink, orange and white bundles that float to the surface. Fertilised eggs develop into swimming larvae that drift for days or weeks before settling on a hard surface and starting a new colony. Scientists think that the timing is controlled by water temperature, the cycle of the moon and the length of the day.

Why Coral Reefs Matter to People

Coral reefs are not only beautiful; they are also extremely useful to humans.

• Food: reef fish and shellfish are an important source of protein for many millions of people in coastal communities.
• Coastal protection: reefs act as natural breakwaters. They absorb most of the energy of incoming waves, protecting beaches, homes and harbours from erosion and storms.
• Tourism: divers and snorkellers travel from all over the world to see reefs, providing jobs and income for many countries.
• Medicine: reef animals such as sponges and sea snails produce unusual chemicals, and scientists study them in the search for new medicines.
• Science: coral skeletons grow in yearly layers, a bit like tree rings. By drilling cores from old corals, scientists can learn about past sea temperatures and climate over hundreds of years.

Threats to Coral Reefs

Despite their size and age, coral reefs are fragile. Large areas of reef have been damaged in recent decades, and scientists consider them among the ecosystems most at risk from climate change.

Coral bleaching

When the water becomes too warm for too long, the partnership between corals and their algae breaks down. The stressed corals expel the algae from their tissues, and because the algae give corals most of their colour, the coral turns pale and its white skeleton shows through the clear tissue. This is called coral bleaching. A bleached coral is not dead yet, and it can recover if temperatures return to normal quickly. But if the heat lasts too long, the coral starves and dies. Mass bleaching events affecting reefs around the world were recorded in 1998, 2010 and from 2014 to 2017, and a fourth global bleaching event was declared in 2024.

Ocean acidification

The ocean absorbs a large part of the carbon dioxide that humans release into the air. When carbon dioxide dissolves in seawater, the water becomes more acidic, and there are fewer carbonate ions available. This makes it harder for corals to build their skeletons and can weaken existing reef structures.

Local pressures

Reefs also suffer from problems closer to home: pollution and mud washed from land, overfishing, destructive fishing with explosives or poison, careless anchoring, and coastal construction. Outbreaks of the crown-of-thorns starfish, which eats coral polyps, can strip large areas of reef, especially where natural predators have been removed.

Protecting and Restoring Reefs

Many countries have created marine protected areas where fishing and other activities are limited so that reefs can recover. Scientists and volunteers also grow coral fragments in underwater nurseries, a method often called coral gardening, and then plant them back onto damaged reefs. Researchers are studying corals that naturally survive higher temperatures, in the hope of helping reefs cope with warmer seas.

Everyday actions matter too. Visitors can avoid touching or standing on corals, choose responsible tour operators and avoid buying souvenirs made from coral or other reef animals. In the long run, however, scientists agree that the most important step for the future of reefs is to reduce the greenhouse gas emissions that are warming the oceans.

Surprising Facts About Coral Reefs

• Not all corals live in the tropics. Some corals live in cold, dark water hundreds of metres deep and have no algae at all. Large cold-water reefs grow off the coast of Norway, for example.
• Corals can glow. Many corals contain fluorescent proteins that shine in bright greens, reds and oranges when lit with blue or ultraviolet light.
• Reefs are very old. Reef-building organisms have existed for hundreds of millions of years, and some mountains are made of ancient fossil reefs. Parts of the Dolomites in northern Italy, for example, were once tropical reefs.
• Today's reefs are young. Because sea levels changed greatly during the ice ages, the living surface of many modern reefs, including the Great Barrier Reef, grew on top of older foundations after the end of the last ice age.
• Some colonies are extremely old. Large massive corals grow only a few millimetres to a couple of centimetres a year, so a boulder coral several metres across may have been growing for hundreds of years.

From a single tiny polyp to a structure visible from space, coral reefs show how small living things, working together over immense spans of time, can shape the face of the planet.