Explainer: What are corals?

Take a plunge into Australia’s Great Barrier Reef, and you’ll come face to face with a what looks like a gorgeous rocky garden. This dazzling array of corals is the largest living structure on Earth. Some, like staghorn coral, look like leafless bushes sprouting from the ocean floor. Others, such as honeycomb coral, resemble rocks dotted with tiny holes. Then there’s brain coral, which, you guessed it, look like brains. And that’s just a small sample of the brilliant diversity to be found in a reef.

Despite their appearance, corals are animals. These invertebrates are close relatives of jellyfish and sea anemones. Most corals live in colonies of thousands of tiny individuals called polyps growing together. Each polyp sports a cylindrical body topped with a ring of tentacles surrounding its mouth.

“You can think of them as upside-down jellyfish,” says Peter Cowman. At Queensland Museum in South Brisbane, Australia, this marine biologist studies the evolutionary history of corals.

Like their jellyfish cousins, coral polyps use tentacles to feed and defend themselves. These appendages are packed with specialized stinging cells called nematocysts. Each contains a coiled, barbed thread. When touched, the cell “fires” this thread to inject venom into its target. Most corals hunt at night, snagging tiny zooplankton, floating eggs and small fish.

Many corals have a more laid-back way of feeding, too. Inside their tissues, they house algae called zooxanthellae (Zoh-uh-ZAN-thuh-lay). (This is an example of symbiosis, where two creatures benefit by associating together.) These single-celled organisms are photosynthetic. They use light and carbon dioxide to make sugars and proteins. Most of these nutrients are offered to the coral. “In return, the coral gives them a safe apartment to live in,” says Cowman. “They’re the solar-powered roommates.”

Corals that rely on light-powered algae live in shallow, warm waters. It’s here where you find coral reefs. These structures are made by a type of coral called hard or stony corals. “They are the engineers of the reef system,” says Cowman.

Stony corals produce hard skeletons of aragonite, a mineral made of calcium carbonate. Over hundreds to thousands of years, generations of polyps can build up massive reef systems.

Soft corals, like their name implies, don’t make these stony skeletons. Instead, they have bendy structures that move with ocean currents. They can often be found living among reefs, though. “The soft corals are like the decorators,” says Cowman. “They look like underwater swaying trees and fans.”

Some corals — both soft and hard — can live in deep water, up to 6,000 meters (20,000 feet) below the water surface. Stony corals found at these depths are often smaller than their shallow-water counterparts. Most tend to live as single polyps. Some species do create reefs, but they’re less of a team effort. Called “mounds,” these deep reefs grow over the skeletons of old or dead corals on the ocean floor. 

Reefs at risk

The first true corals appeared about 500 million years ago. But many of today’s reef-building corals can be traced back to the end of the Cretaceous Period some 66 million years ago. That’s when an asteroid impact wiped out about 80 percent of all animals (including the nonbird dinosaurs).

“The mass extinction had an effect in the oceans as well,” says Cowman. Existing reef-builders died out. Their extinction allowed stony corals to take their place. Today, corals are found in all of the world’s oceans.

But many are at risk from human activities. These dangers include pollution and destructive or heavy fishing. One of the biggest threats is climate change. High water temperatures and too much sunlight can stress corals, leading to a phenomenon called coral bleaching.

At higher temperatures, the algae produce toxic compounds. “When the water gets too hot, the coral gets stressed and they evict their zooxanthellae,” says Cowman. These microbes are also responsible for a coral’s bright colors. In their absence, the coral’s tissues turn transparent. All that remains visible is the coral’s white skeleton.

Bleaching doesn’t kill the coral immediately. If conditions improve, the coral may recover and even take the algae back in. But if coral go too long without their algal partners, they may get sick or die. “If they can’t get the zooxanthellae back in again, they [will] actually starve to death,” says Cowman.

This bleaching affects coral worldwide. Since 2023, the world’s corals have been experiencing the largest bleaching event on record.

Rainforests of the sea

Many scientists and conservationists are concerned about corals’ future. Coral reefs are among Earth’s most diverse ecosystems. Though they cover less than one percent of the seafloor, coral reefs support at least one-fourth of all marine species. “On land, you think of rainforests as having all that diversity,” says Cowman. “The rainforests of the sea are the coral reefs.”

Corals are important for people, too. Humans rely on many reef-dwelling species for food. Reefs can also buffer coasts against damage from strong waves or storms, says Cowman. Chemical compounds from reef animals have been found to help fight pain and treat cancer. And reefs support tourism around the globe. The food, products, jobs and coastal protection provided by corals are estimated to be worth $2.7 trillion per year.

Says Cowman, “as humans, we rely on reefs a lot in our daily lives, even if we don’t think of it.”