Coral Reef

The Coral Reef biome is a living structure built by animals. Reef-building corals lay down hard skeletons of limestone, and over centuries those skeletons accumulate into the ridges, walls, and mazes that shelter more species than almost anywhere else in the sea. Coral reefs are among the most biologically diverse ecosystems on Earth. An estimated 25 percent of all marine life, including over 4,000 species of fish, depends on reefs at some point in the life cycle, according to the US Environmental Protection Agency, even though reefs occupy a tiny fraction of the ocean floor.

What builds the reef

Coral is an animal, related to jellyfish and sea anemones. A coral colony is made of hundreds of thousands of individual polyps, small soft-bodied animals with a ring of tentacles. Stony corals, the reef builders, pull dissolved calcium from seawater and harden it into calcium carbonate, the same mineral that makes up seashells. That mineral becomes their skeleton. When a diver looks at a coral head, only the thin surface layer is alive. The mass underneath is skeleton that may be decades old, according to the EPA.

Not all corals build reefs. Soft corals such as sea fans and sea whips, known as octocorals, grow more like fleshy plants and lay down no stony skeleton. Fire corals, or hydrocorals, build a hard skeleton and carry stinging cells that burn on contact. Black corals are another branching soft coral, common in deeper water.

The partnership that powers the reef

Reef-building corals survive on a partnership. Inside their tissues live microscopic algae called zooxanthellae. The coral gives the algae a protected place to live, and the algae photosynthesize and pass much of the food they make back to the coral. This arrangement, in which 2 species live together to mutual benefit, is called symbiosis. It is also why reefs grow only in clear, shallow, sunlit water, since the algae need light.

The algae also give coral its color. Coral polyps are nearly transparent, so what a diver sees is the color of the algae inside. When corals are stressed, most often by water that is too warm, they expel their algae and turn stark white. This is coral bleaching. A bleached coral is not yet dead, but it has lost its main food supply, and if the stress lasts it usually starves.

What lives there

A reef packs an extraordinary range of life into a small area. The structure itself, full of holes, ledges, and crevices, gives thousands of animals somewhere to hide. Sponges, oysters, clams, crabs, sea stars, and sea urchins live on and within the reef framework, along with a dense community of fish. Grazing fish such as parrotfish and surgeonfish crop the algae that would otherwise smother the coral, and parrotfish grind up coral rock as they feed, producing much of the white sand that surrounds a reef. Predators including groupers, snappers, moray eels, and reef sharks hunt through the structure. Reefs are also ecologically tied to nearby seagrass meadows, mangroves, and mudflats, and many reef fish spend their early lives in those habitats before moving out to the reef.

Why the biome matters

Reefs support life and people far out of proportion to their size. They provide habitat, feeding grounds, and nursery areas for over 1 million species, and they feed people directly, especially on small islands, according to the EPA. They also shield coastlines, absorbing wave energy and reducing the flooding and erosion that storms and tsunamis would otherwise deliver to the shore. The economic value is large. One estimate puts the total net benefit of the world's reefs at 29.8 billion dollars a year, split among tourism and recreation, coastal protection, fisheries, and biodiversity, according to figures cited by the EPA. Reef organisms are also a source of new medicines.

Where the biome occurs

Reef-building corals need warm, clear, shallow water, so they grow in a band roughly between 30 degrees north and south of the equator, according to the EPA. Corals themselves live throughout the world's oceans, including cold and deep water, but only the shallow, algae-partnered species build the great reefs. In the United States, coral reefs occur in Florida, Puerto Rico, the US Virgin Islands, Hawaii, Guam, American Samoa, and the Northern Mariana Islands, plus deep reefs about 100 miles off Texas and Louisiana in the Gulf of Mexico. Florida's Coral Reef, running along the Keys and southeast Florida, is the only barrier reef in the continental United States.

Threats and conservation

Warming water is the central threat, and its effects are now global. Between January 2023 and March 2025, bleaching-level heat stress hit 84 percent of the world's reefs across 82 countries and territories, in what scientists call the fourth global bleaching event and the most intense on record, according to the International Coral Reef Initiative. The scale of the change is stark. The first global bleaching event, in 1998, affected 21 percent of reefs, a figure that rose to 37 percent in 2010 and 68 percent from 2014 to 2017.

Disease is the other major pressure. Stony coral tissue loss disease, first seen on Florida's Coral Reef in 2014, has since spread through the Caribbean. It affects at least 22 coral species and can kill a colony within weeks to months, and it has driven lasting declines in coral cover and diversity. Ocean acidification, the drop in seawater pH as the ocean absorbs carbon dioxide, makes it harder for corals to build their skeletons at all. Pollution, runoff, sedimentation, destructive fishing, and physical damage from anchors and divers add to the strain. Restoration is underway, including coral nurseries and rescue programs that hold healthy colonies in land-based tanks to preserve genetic diversity, though those efforts have not yet kept pace with the losses.

How divers can help

Divers and snorkelers have direct influence on reef health. Keeping hands, fins, and gear off the coral prevents breakage, since a colony snapped in a moment may have taken decades to grow. Good buoyancy control is the single most useful skill for protecting a reef. Anchoring on sand or using mooring buoys rather than dropping anchor on the structure avoids serious damage. Divers moving between reefs should clean and dry their gear, since stony coral tissue loss disease can spread on contaminated equipment. Choosing reef-safe sunscreen, respecting marine protected areas, and following fishing limits on groupers, snappers, and grazing fish all help the reef hold its balance.