Explainer: The Hidden World of Corals

Take a swim in Australia’s Great Barrier Reef, and you will see what looks like a vibrant underwater garden. This dazzling collection of corals is the largest living structure on Earth. The variety is impressive. Some corals, like staghorn coral, look like leafless bushes growing from the ocean floor. Others, such as honeycomb coral, resemble rocks filled with tiny holes. Then there is brain coral, which looks exactly like a human brain. This is only a small sample of the incredible diversity found in a reef ecosystem.

Despite their plant-like appearance, corals are actually animals. These invertebrates are close relatives of jellyfish and sea anemones. Most corals live in large colonies made of thousands of tiny individuals called polyps. Each polyp has a cylindrical body topped with a ring of tentacles around its mouth. You can think of them as upside-down jellyfish, explains Peter Cowman, a marine biologist at the Queensland Museum Tropics in Townsville, Australia. He studies the evolutionary history of corals.

Like their jellyfish cousins, coral polyps use tentacles to feed and defend themselves. These arms are packed with specialized stinging cells called nematocysts. Each cell holds a coiled, barbed thread. When something touches the cell, it fires the thread to inject venom into the target. Most corals hunt at night, catching tiny zooplankton, floating eggs, and small fish.

Many corals also have a more relaxed way to get food. Inside their tissues, they host algae called zooxanthellae. This relationship is called symbiosis, where two different organisms help each other survive. These single-celled organisms are photosynthetic. This means they use sunlight and carbon dioxide to make sugars and proteins. Most of these nutrients are shared with the coral host. In return, the coral provides a safe home for the algae. Cowman describes them as "solar-powered roommates."

Corals that rely on these light-powered algae usually live in shallow, warm waters. These areas are where we find coral reefs. These massive structures are built by a type of coral known as hard or stony corals. Cowman states that they are the engineers of the reef system.

Stony corals produce hard skeletons made of aragonite. Aragonite is a mineral composed of calcium carbonate. Over hundreds to thousands of years, many generations of polyps build up massive reef systems. Soft corals, as their name implies, do not create these stony skeletons. Instead, they have flexible structures that move with ocean currents. Although they do not build rigid reefs, soft corals often live among hard coral reefs. Cowman notes that soft corals act like decorators, looking like swaying trees and fans underwater.

Some corals, including both soft and hard types, can live in deep water. They can be found up to 6,000 meters (20,000 feet) below the surface. Stony corals at these depths are often smaller than those in shallow water. Most deep-water species live as single polyps. While some species do create reefs, they are less of a collective effort. Called "mounds," these deep reefs grow on top of the skeletons of old or dead corals on the ocean floor.

The first true corals appeared about 500 million years ago. However, many of today’s reef-building corals can be traced back to the end of the Cretaceous Period, about 66 million years ago. This era ended when an asteroid impact killed about 80 percent of all animals, including non-avian dinosaurs.

This mass extinction significantly affected ocean life. Cowman explains that existing reef-builders died out, allowing stony corals to take their place. Today, corals are found in all of the world’s oceans. But many species are at risk from human activities. These dangers include pollution, destructive fishing practices, and heavy fishing loads. One of the most significant threats is climate change.

High water temperatures and excessive sunlight can stress corals. This leads to a phenomenon called coral bleaching. At higher temperatures, the algae within the coral produce toxic compounds. When the water gets too hot, the coral becomes stressed and evicts its zooxanthellae. These microbes are also responsible for the bright colors of coral. Without them, the coral’s tissues turn transparent. This leaves only the white skeleton visible.

Bleaching does not kill the coral immediately. If environmental conditions improve, the coral may recover and even take the algae back in. However, if corals go too long without their algal partners, they may become sick or die. Cowman warns that if they cannot regain their zooxanthellae, they will starve to death. This bleaching event affects coral populations worldwide. Since 2023, the world’s corals have been experiencing the largest bleaching event on record.

Many scientists and conservationists are deeply concerned about the future of corals. Coral reefs are among the most diverse ecosystems on Earth. Although they cover less than one percent of the seafloor, coral reefs support at least one-fourth of all marine species. Cowman draws a parallel to terrestrial ecosystems. He states, "On land, you think of rainforests as having all that diversity. The rainforests of the sea are the coral reefs."

Corals are also vital for human societies. Humans rely on many reef-dwelling species for food. Reefs buffer coastlines against damage from strong waves or storms. Additionally, chemical compounds derived from reef animals have been found to help fight pain and treat cancer. Reefs also support tourism industries around the globe. The food, products, jobs, and coastal protection provided by corals are estimated to be worth $2.7 trillion per year.

Cowman emphasizes the importance of these ecosystems. He notes, "As humans, we rely on reefs a lot in our daily lives, even if we don’t think of it."