Coral reef
A coral reef occupies less than 0.1% of the world's ocean area, an expanse about half the size of France. Yet at least 25% of all marine species make their homes there. That lopsided arithmetic defines the coral reef, an underwater ecosystem built by reef-building corals. These are colonies of tiny animals called polyps, held together by calcium carbonate. Corals belong to the animal phylum Cnidaria, the same group as sea anemones and jellyfish. Unlike anemones, they secrete hard skeletons that support and protect them.
Reefs first appeared 485 million years ago, at the dawn of the Early Ordovician, and shallow ones are sometimes called the rainforests of the sea. Fish, mollusks, worms, crustaceans, echinoderms, sponges and tunicates all crowd into them. So how does a structure made of animal skeletons grow into an island? Why does such abundance thrive in water that holds so little food? And what is happening to a living system that has lasted this long?
In 1842, Charles Darwin published The Structure and Distribution of Coral Reefs, which set out an idea he had conceived during the voyage of the Beagle. He argued that the slow rise and fall of the oceanic crust shaped the atolls of the open ocean. His theory ran in three stages. A fringing reef forms around an extinct volcanic island. As the island and the sea floor subside, that reef becomes a barrier reef. Finally the island disappears and an atoll remains. Darwin predicted that bedrock, the remains of the original volcano, would lie beneath each lagoon. Later research supported him.
Polyps, Darwin understood, thrive in agitated tropical water but can live only within a limited depth range, starting just below low tide. Where the land rises, coral lifted above the sea dies. Where the land sinks slowly, the reef grows upward on a base of older, dead coral and keeps pace. If the bottom sinks too fast, the reef can be overwhelmed in a process called coral drowning. Corals that rely on algae living inside their tissues die when the water grows too deep for that algae to photosynthesize.
About 20,000 years ago, the sea stood 120 m lower than it does in the 21st century. Where the Great Barrier Reef now lies, the water was creeping across hills on the Australian coastal plain. By 13,000 years ago the sea was still 60 m below its present level, and many hills had become continental islands. Once water topped most of them, corals overgrew the hills and formed cays and reefs. Sea level there has not changed significantly in the last 6,000 years, and the living reef structure is estimated at 6,000 to 8,000 years old. This reef grew on a continental shelf rather than around a volcano, yet Darwin's principles apply. Development stopped at the barrier stage because Australia is not about to submerge.
Most coral reefs are less than 10,000 years old, because they formed after the Last Glacial Period, when melting ice flooded the continental shelves. Healthy tropical reefs grow horizontally 1 to 3 cm per year and vertically anywhere from 1 to 25 cm per year. At the northwestern end of the Hawaiian Archipelago, growth no longer keeps pace with relative sea level and the reef structure has permanently drowned. That spot is called a Darwin Point.
The fringing reef is the most common type, attached directly to a shore or separated from it by only a narrow, shallow channel or lagoon. Fringing reefs follow coastlines for many kilometres and are usually less than 100 metres wide. The fringing reefs of the Red Sea, which grow along a 2000 km coastline, are described as "some of the best developed in the world." Some of those are 6,000 years old.
Barrier reefs sit farther out, divided from land by a deep channel or lagoon that can be several kilometres wide and 30 to 70 metres deep. Their outer edge formed in open water rather than beside a shoreline. They take far longer to form than fringing reefs, which makes them much rarer. The Great Barrier Reef off Queensland is the largest, with over 2,900 individual reefs and 900 islands. The Mesoamerican Barrier Reef System stretches 1000 km from Isla Contoy, at the tip of the Yucatan Peninsula, down to the Bay Islands of Honduras.
Platform reefs, also called bank or table reefs, can rise anywhere the sea floor comes close enough to the surface for reef-building corals to grow. Unlike fringing and barrier reefs, which extend only seaward, they grow in all directions. Examples include the Swain and Capricorn Group in the southern Great Barrier Reef, 100 to 200 km from the coast. Parts of a platform reef can break the surface and form sandbanks and small islands. Inside atolls, small platform reefs go by the name patch reefs and may span only a few dozen metres. Some platform reefs of the Laccadives are U-shaped, bent by wind and water flow.
Atolls are numerous in the South Pacific, in places such as the Caroline Islands, the Cook Islands, French Polynesia and the Marshall Islands. The entire Maldives consists of 26 atolls. On the surface of reefs, eroded material can pile up into cays, small sandy islands that plants can stabilize. In the Caribbean and on the Great Barrier Reef, cays provide habitable and agricultural land.
The Indo-Pacific region, which includes the Red Sea, Indian Ocean, Southeast Asia and the Pacific, holds 91.9% of all reef area. Shallow-water reefs form only between roughly 30 degrees north and 30 degrees south of the equator. The optimum temperature for most is 26 to 27 C, and few exist below 18 C. Reefs in the Persian Gulf have adapted to 13 C in winter and 38 C in summer, and 37 species of scleractinian corals live around Larak Island. Bermuda hosts the northernmost reef system, thanks to the nearby Gulf Stream. The southernmost reef lies at Lord Howe Island, off the east coast of Australia.
Cold currents keep reefs rare along the west coasts of the Americas and Africa. Freshwater from the Amazon and the Ganges also keeps corals away from stretches of coast. The Raja Ampat Islands in Indonesia's Southwest Papua province offer the highest known marine diversity. In January 2024, the Blake Plateau was announced as the world's largest known deep-water coral reef, a 6.4 million-acre expanse stretching from Miami to Charleston, South Carolina.
Against the barren tropical sea around it, a reef looks like an "oasis in the desert." Darwin's observation gave rise to a riddle sometimes called Darwin's paradox: "How can such high production flourish in such nutrient poor conditions?" Reefs typically produce 5 to 10 grams of carbon per square meter per day.
The tropical sun shines year-round, warming the surface layer until it floats above cooler, deeper water. A stable thermocline, a zone of rapid temperature change, separates the two, and in most of the ocean little mixing happens across it. Organisms that die sink and decompose into nitrogen, phosphorus and potassium. In the tropics those nutrients do not return directly to the surface. Microscopic phytoplankton, the base of the food chain, use up what the surface holds and have little chance of replacement. The poverty of nutrients and plankton is also why tropical water is so clear.
The answer begins with recycling. Lagoons fill with material eroded from the reef and island, and become havens from waves and storms. Turf algae and other producers transfer nutrients to corals. Many corals extend their tentacles at night to catch passing zooplankton, which give the polyp nitrogen. Sponges in the crevices filter feed so efficiently that in the Red Sea they consume about 60% of the phytoplankton drifting past. They later excrete nutrients in a form corals can use.
The rough surfaces of coral matter too. A boundary layer of still water normally wraps around a submerged object like a barrier. Waves breaking on jagged coral disrupt that layer, so the coral can reach passing nutrients. Deep, nutrient-rich water also arrives through internal waves, which move along density layers inside the ocean and can act as nutrient pumps. Cyanobacteria add soluble nitrates through nitrogen fixation.
Reefs also trade with neighbors. Seagrass meadows and mangrove forests supply dead plants and animals rich in nitrogen. In return, reefs shield the mangroves and seagrass from waves and produce the sediment where their roots take hold.
One million microscopic algae live in every cubic centimetre of coral, on average. These algae are dinoflagellates of the genus Symbiodinium, commonly called zooxanthellae. Polyps cannot photosynthesize themselves, so the algae supply glucose, glycerol and amino acids. Corals get up to 90% of their nutrients this way. In exchange, the coral shelters the algae and provides the carbon dioxide they need. Without this partnership, coral growth would be too slow to form significant reef structures, which is why reefs grow faster in clear, sunlit water.
Pigments in the algae give brown corals their color, while reds, blues and greens come from colored proteins made by the coral itself. A coral that loses a significant fraction of its algae turns white, or sometimes a pastel shade. Such a coral is said to be bleached, a condition that can kill it if not corrected.
There are eight clades, or lineages, of Symbiodinium, and most research has covered clades A through D. Clade A lives mostly in shallow water and can make amino acids that absorb ultraviolet radiation, so it resists heat and UV better. Clades B and C appear more often in deeper water, and they are more prone to bleaching. Clade D has been observed to tolerate high temperature, and it survived modern bleaching events better than B and C.
The skeleton grows as polyps deposit calcium carbonate beneath and around themselves, pushing the coral head upward and outward. Waves, parrotfish, sea urchins and sponges grind the skeletons into fragments that settle into the reef or form the sandy floor of lagoons. Coral shapes are named for what they resemble on land, including wrinkled brains, cabbages, table tops, antlers, wire strands and pillars.
Spawning offers a spectacle. In externally fertilized species, polyps across a reef release eggs and sperm into the water at the same time. Timing depends on season, water temperature, and tidal and lunar cycles, and release often occurs at night, three to six days after a full moon. The resulting larvae are called planulae. They may drift for only a few days, though some survive afloat for weeks. At long distances, sounds from existing reefs probably guide them; at short distances, chemical compounds take over.
Corals are not the only architects. Coralline algae deposit limestone in sheets, forming a protective crust where waves hit hardest. In the Hecate Strait of British Columbia, glass sponge reefs have grown up to 7 kilometres long and 20 metres high. They were thought extinct until living reefs were discovered in 1987-1988. Oyster reefs build up as new larvae settle on the shells of old ones. During the Cretaceous, an extinct group of bivalves called rudists became the dominant tropical reef-builders, outnumbering scleractinian corals by the mid-Cretaceous. Before any of them, the sponge-related archaeocyathids were the planet's first reef-building animals.
Over 4,000 species of fish live on coral reefs, and the reason for that variety remains unclear. One hypothesis is a lottery, in which the first fish to settle a territory can usually defend it against latecomers. Another points to competition among adults, and a third to predators thinning the young after they settle. What does seem clear is that hiding places matter most. The great number and variety of refuges are the most important factors behind the high diversity and biomass of reef life.
Shifts happen daily. Cardinalfish and squirrelfish hide through daylight and hunt at night, while damselfish, surgeonfish, triggerfish, wrasses and parrotfish hide from eels and sharks. Aside from humans, mammals are rare on reefs, and visiting dolphins are the main exception. Sea snakes eat fish and their eggs, and hawksbill sea turtles feed on sponges. Sponges themselves are a major presence, with 420 species in Indonesian reefs, 486 in Indian waters and 1500 on the Great Barrier Reef.
Midway Atoll in Hawaii supports nearly three million seabirds of 17 species. That includes 1.5 million Laysan albatross, two-thirds of the global population, and one-third of the world's black-footed albatross. The short-tailed albatross is the rarest, with fewer than 2,200 surviving after excessive feather hunting in the late 19th century.
The benefit to people is enormous. The global economic value of coral reefs has been estimated at anywhere from US$30-375 billion in 1997 and 2003 estimates, up to US$2.7 trillion in a 2020 estimate and US$9.9 trillion in a 2014 estimate. About 500 million people benefit from reef services. Reefs can reduce wave energy by 97%, and many small islands would not exist without them. An estimated 197 million people live below 10 m elevation and within 50 km of a reef. Without the top meter of reefs, flood damages from a 100-year storm would rise by 91% to US$272 billion.
About six million tons of fish are taken from reefs each year. A well-managed reef yields an average of 15 tons of seafood per square kilometre annually. Southeast Asia's reef fisheries alone yield about $2.4 billion in seafood each year. Belize's reefs and mangroves are valued at $395-559 million annually, and Bermuda's reefs at an average of $722 million per year. Destroying one square kilometre of reef over 25 years has been estimated to cost between $137,000 and $1,200,000.
From 2009 to 2018, coral reefs worldwide declined 14%. Shallow tropical reefs have lost 50% since 1950, partly because they are sensitive to water conditions. Local pressures include excess nitrogen and phosphorus, blast fishing, cyanide fishing, spearfishing on scuba, coral mining, bottom trawling and the digging of canals into islands and bays. Marine pollution includes the banned anti-fouling biocide tributyltin, which persists where regulation is weak. Chemicals in sunscreens may awaken latent viral infections in zooxanthellae. Even so, concentrating tourism on offshore platforms has been shown to limit the spread of coral disease by tourists.
Greenhouse gas emissions drive the broader threat. Rising sea temperature causes widespread bleaching and loss of coral cover. Climate change also brings more frequent and severe storms and shifts ocean circulation. Ocean acidification lowers calcification rates and raises dissolution rates, although corals can adapt their calcifying fluids to changes in seawater pH. In 2011, two researchers suggested that marine invertebrates face the same combination of stressors that occurred during the end-Permian extinction, with corals especially vulnerable.
Every 4 to 7 years, an El Nino event bleaches some reefs with heat-sensitive corals, with especially widespread bleaching in 1998 and 2010. Corals hosting Clade C algae bleach easily, while tougher genera such as Porites and Montipora resist. Reefs that suffer a severe event become resistant to future heat-induced bleaching through rapid directional selection. Jarvis Island's reef, studied through 10 El Nino-coincident bleaching events from 1960 to 2016, recovered from near-total mortality after severe events.
Overgrowth by seaweed can do as much damage as heat. Sewage and fertilizer runoff carry nitrogen and phosphorus that help algae outcompete coral for space and smother it. In the Caribbean, sea urchins and herbivorous fish like parrotfish normally graze it back. Jamaica's reefs were thriving before 1980. Then Hurricane Allen struck, and an unknown disease swept the Caribbean. As little as 2% of the original sea urchin population survived. Decades of overfishing had already left few parrotfish, so macroalgae took over. Coral cover on the Jamaican coast fell from 90% to 5% by the 1990s. Corals eventually recovered in areas where sea urchin populations were increasing.
On Ahus Island in Manus Province, Papua New Guinea, inhabitants have for generations restricted fishing in six areas of their reef lagoon. Their traditions allow line fishing but no nets or spears. Both biomass and individual fish sizes there are significantly larger than where fishing goes unrestricted.
Formal marine protected areas have a mixed record. A study of a small number of them in Indonesia, the Philippines and Papua New Guinea found no significant differences from unprotected sites. Some generate local conflict when communities are left out. In the Phoenix Islands Protected Area, though, protection provides revenue to locals. The Caribbean Coral Reefs Status Report 1970-2012 argues that decline may be reduced or even reversed. Its recommendations include stopping overfishing, especially of parrotfish, and minimising sewage. The healthier Caribbean reefs are in countries that protect parrotfish and sea urchins and ban fish trapping and spearfishing. Belize's barrier reef, the Galapagos Islands and Palau are among the world heritage sites. Australia protects the Great Barrier Reef through its Marine Park Authority and has compiled a Coral Reef Resilience Action Plan.
Restoration became prominent in the 1970s-1980s, as die-offs mounted. In 1995, Baruch Rinkevich first tested coral gardening, which uses a coral's ability to fragment and keep growing when pieces anchor on new ground. Seeds are grown in nurseries and replanted, skipping the early stages when corals are most at risk of dying. Nurseries became the most widespread restoration method, though they cannot stop warming or hurricanes. Teams from the Reef Restoration and Adaptation Program have trialled a robotic camera that uses computer vision and machine learning to count young corals and track their growth in real time.
Builders also supply surfaces for corals to settle on, using discarded vehicle tires, scuttled ships, subway cars and concrete reef balls. Biorock runs low-voltage electrical currents through seawater so dissolved minerals form aragonite on steel structures, and corals colonize them rapidly. Complexity matters. In a 2013 experiment near Ticao Island in the Philippines, medium-complexity plots performed best, and larvae favored rough rocks over smooth ones. After a year, many sites supported local fisheries.
A boat channel at the Hawaii Institute of Marine Biology on Coconut Island, near Oahu, was overcrowded with coral that past dredging had damaged. The University of Hawaii relocated some coral with help from United States Army divers, with little to no damage and no mortality at the new site. Sound can help too. In damaged sections of the Great Barrier Reef, loudspeakers playing recordings of healthy reefs attracted fish twice as often as silent patches and raised species diversity by 50%.
Genetic approaches focus on the algae inside corals. Madeleine van Oppen, James Oliver, Hollie Putnam and Ruth Gates described four levels of intervention, each more intense than the last. The final one, genetically modifying the zooxanthellae themselves and introducing them to an aposymbiotic polyp, is the most labor-intensive. Researchers believe it holds the most significant promise.
Updated
Common questions
What is a coral reef?
A coral reef is an underwater ecosystem characterized by reef-building corals, which form colonies of coral polyps held together by calcium carbonate. Corals belong to the class Anthozoa in the animal phylum Cnidaria, alongside sea anemones and jellyfish.
How old are coral reefs?
Coral reefs first appeared 485 million years ago, at the dawn of the Early Ordovician. Most of today's reefs are less than 10,000 years old, because they formed after the Last Glacial Period flooded the continental shelves.
How much of the ocean do coral reefs cover and how many species live there?
Coral reefs occupy less than 0.1% of the world's ocean area, about half the area of France. They are home to at least 25% of all marine species.
How did Charles Darwin explain how coral atolls form?
In The Structure and Distribution of Coral Reefs, published in 1842, Darwin proposed that a fringing reef around an extinct volcanic island becomes a barrier reef as the island subsides, and finally an atoll. He predicted that bedrock from the original volcano lies beneath each lagoon, and later research supported this.
Why are coral reefs so productive in nutrient-poor water?
Reefs recycle nutrients efficiently, and this resolves what is called Darwin's paradox. Algae, sponges and coral tentacles capture and pass on nutrients, and internal waves and cyanobacteria bring in more.
What is coral bleaching?
Coral bleaching happens when a coral loses a significant fraction of its zooxanthellae and turns white. It can kill the coral if not corrected, and El Nino events every 4 to 7 years cause widespread bleaching on reefs with heat-sensitive corals.
How much are coral reefs worth to people?
Estimates of the annual global economic value of coral reefs range from US$30-375 billion in 1997 and 2003 estimates up to US$9.9 trillion in a 2014 estimate. About 500 million people benefit from reef services such as tourism, fisheries and shoreline protection.
How are coral reefs being restored?
Restoration methods include coral gardening in nurseries, first tested by Baruch Rinkevich in 1995, artificial substrates such as reef balls and Biorock, and relocation of coral. Researchers are also exploring genetic modification of the zooxanthellae that live inside corals.
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