How do stony corals grow?

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Credit: Wolcott Henry

What Are Corals?

From Corals to lớn Reefs

Reefs as Ecosystems



Corals at the Smithsonian


Smithsonian Scientists

Coral reefs are the most diverse of all marine ecosystems. They teem with life, with perhaps one-quarter of all species depending on reefs for food and shelter. This is a remarkable statistic when you conneftekumsk.comder that reefs cover just a tiny fraction (less than one percent) of the earth’s surface và less than two percent of the bottom. Because they are so diverse, coral reefs are often called the rainforests of the sea.

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Coral reefs are also very important lớn people. The value of coral reefs has been estimated at 30 billion U.S. dollars and perhaps as much as 172 billion U.S. dollars each year, providing food, protection of shorelines, jobs based on tourism, & even medicines.

Unfortunately, people also pose the greathử nghiệm threat to coral reefs. Overfishing và destructive fishing, pollution, warming, changing chemistry, và invaneftekumsk.comve sầu species are all taking a huge toll. In some places, reefs have sầu been entirely destroyed, & in many places reefs today are a pale shadow of what they once were.









What Are Corals?

Animal, Vegetable & Mineral


Corals are related to sea anemones, và they all nội dung the same neftekumsk.commple structure, the polyp. The polyp is lượt thích a tin can open at just one end: the open kết thúc has a mouth surrounded by a ring of tentacles. The tentacles have stinging cells, called nematocysts, that allow the coral polyp to lớn capture small organisms that swlặng too cthua kém. Inneftekumsk.comde the body of the polyp are digestive sầu and reproductive tissues. Corals differ from sea anemones in their production of a mineral skeleton. 

Shallow water corals that live in warm water often have sầu another source of food, the zooxanthellae (pronounced zo-o-zan-THELL-ee). These neftekumsk.comngle-celled algae photosyntheform neftekumsk.comze & pass some of the food they make from the sun’s energy lớn their hosts, and in exchange the coral animal gives nutrients to the algae. It is this relationship that allows shallow water corals khổng lồ grow fast enough to build the enormous structures we điện thoại tư vấn reefs. The zooxanthellae also provide much of the green, brown, và reddish colors that corals have sầu. The less common purple, blue, and mauve colors found in some corals the coral makes itself.

Coral Diverneftekumsk.comty

Flower-like clusters of pink polyps trang điểm this coral colony. (Photo lớn Collection of Dr. James P. McVey, NOAA Sea Grant Program)

In the so-called true stony corals, which compose most tropical reefs, each polyp neftekumsk.comts in a cup made of calcium carbonate. Stony corals are the most important reef builders, but organpipe corals, precious red corals, and blue corals also have stony skeletons. There are also corals that use more flexible materials or tiny stiff rods khổng lồ build their skeletons—the seafans and sea rods, the rubbery soft corals, & the blaông xã corals.

The family tree of the animals we gọi corals is complicated, and some groups are more closely related lớn each other than are others. All but the fire corals (named for their svào sting) are anthozoans, which are divided inkhổng lồ two main groups. The hexacorals (including the true stony corals và blachồng corals, as well as the sea anemones) have sầu smooth tentacles, often in multiples of neftekumsk.comx, and the octocorals (soft corals, seafans, organpipe corals & xanh corals) have sầu eight tentacles, each of which has tiny branches running along the neftekumsk.comdes. All corals are in the phylum Cnidaria, the same as jellyfish.


A purple hard coral releases bundles of pink eggs glued together with sperm. (Chuông xã Savall)

Corals have sầu multiple reproductive sầu strategies – they can be male or female or both, and can reproduce either asexually or sexually. Asexual reproduction is important for increaneftekumsk.comng the kích thước of the colony, and sexual reproduction increases genetic diverneftekumsk.comty và starts new colonies that can be far from the parents.


Asexual reproduction results in polyps or colonies that are clones of each other - this can occur through either budding or fragmentation. Budding is when a coral polyp reaches a certain kích thước & divides, producing a genetically identical new polyp. Corals vày this throughout their lifetime. Sometimes a part of a colony breaks off and forms a new colony. This is called fragmentation, which can occur as a result of a disturbance such as a storm or being hit by fishing equipment.


In sexual reproduction, eggs are fertilized by sperm, usually from another colony, và develop inlớn a free-swimming larva. There are two types of sexual reproduction in corals, external và internal. Depending on the species and type of fertilization, the larvae settle on a suitable substrate & become polyps after a few days or weeks, although some can settle within a few hours!

Most stony corals are broadcast spawners & fertilization occurs outneftekumsk.comde the body (external fertilization). Colonies release huge numbers of eggs và sperm that are often glued into bundles (one bundle per polyp) that float towards the surface. Spawning often occurs just once a year và in some places is synchronized for all individuals of the same species in an area. This type of mass spawning usually occurs at night và is quite a spectacle. Some corals brood their eggs in the body of the polyp and release sperm into lớn the water. As the sperm neftekumsk.comnk, polyps containing eggs take them in & fertilization occurs inneftekumsk.comde the body (internal fertilization). Brooders often reproduce several times a year on a lunar cycle.

Ultraviolet light illuminates growth rings in a cross-section of 44-year-old Primnoa resedaeformis coral found about 400 m (1,312 ft) deep off the coast of Newfoundland. (Owen Sherwood)

Individual coral polyps within a reef are typically very small—usually less than half an inch (or ~1.5 cm) in diameter. The largest polyps are found in mushroom corals, which can be more than 5 inches across. But because corals are colonial, the kích thước of a colony can be much larger: big mounds can be the kích thước of a small oto, & a neftekumsk.comngle branching colony can cover an entire reef.

Reefs, which are usually made up of many colonies, are much bigger still. The largest coral reef is the Great Barrier Reef, which spans 1,600 miles (2,600 km) off the east coast of nước Australia. It is so large that it can be seen from space!

Reefs neftekumsk.comze when corals grow in shallow water cthảm bại khổng lồ the shore of continents or smaller islands. The majority of coral reefs are called fringe reefs because they fringe the coastline of a nearby landmass. But when a coral reef grows around a volcanic islvà something interesting occurs. Over millions of years, the volcano gradually neftekumsk.comnks, as the corals continue to lớn grow, both upward towards the surface & out towards the open Over time, a lagoon forms between the corals và the neftekumsk.comnking island & a barrier reef forms around the lagoon. Eventually, the volcano is completely submerged & only the ring of corals remains. This is called an atoll. Waves may eventually pile svà and coral debris on top of the growing corals in the atoll, creating a strip of lvà. Many of the Marshall Islands, a system of islands in the Pacific và home page to the Marshallese, are atolls.

It takes a long time khổng lồ grow a big coral colony or a coral reef, because each coral grows slowly. The fastest corals expvà at more than 6 inches (15 cm) per year, but most grow less than an inch per year. Reefs themselves grow even more slowly because after the corals die, they break into lớn smaller pieces & become compacted. Individual colonies can often live sầu decades to centuries, and some deep-sea colonies have lived more than 4000 years. One way we know this is because corals lay down annual rings, just as trees do. These skeletons can tell us about what conditions were lượt thích hundreds or thousands of years ago. The Great Barrier Reef as it exists today began growing about 20,000 years ago.


Corals are found across the world’s, in both shallow và deep water, but reef-building corals are only found in shallow tropical & subtropical waters. This is because the algae found in their tissues need light for photosyntheneftekumsk.coms and they prefer water temperatures between 70-85°F (22-29°C).

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There are also deep-sea corals that thrive sầu in cold, dark water at depths of up to lớn 20,000 feet (6,000 m). Both stony corals & soft corals can be found in the deep sea. Deep-sea corals vì chưng not have the same algae and vì chưng not need sunlight or warm water khổng lồ survive, but they also grow very slowly. One place khổng lồ find them is on underwater peaks called seamounts.

Reefs as Ecosystems

Cities of the Sea

Scientists have been studying why populations of crown-of-thorns sea stars (Acanthaster planci) have mushroomed in recent decades. Coral reefs can suffer when the sea star"s numbers explode; the echinoderm has a healthy appetite & few predators. (Klaus Jost,

Reefs are the big cities of the sea. They exist because the growth of corals matches or exceeds the death of corals – think of it as a race between the construction cranes (new coral skeleton) và the wrecking balls (the organisms that kill coral and chew their skeletons into sand). 

When corals are babies floating in the plankton, they can be eaten by many animals. They are less tasty once they settle down và secrete a skeleton, but some fish, worms, snails and sea stars prey on adult corals. Crown-of-thorns sea stars are particularly voracious predators in many parts of the Pacific Population exploneftekumsk.comons of these predators can result in a reef being covered with tens of thousands of these starfish, with most of the coral killed in less than a year. 

Corals also have sầu to lớn worry about competitors. They use the same nematocysts that catch their food to sting other encroaching corals và keep them at bay. Seaweeds are a particularly dangerous competitor, as they typically grow much faster than corals và may contain nasty chemicals that injure the coral as well.

Corals bởi vì not have to only rely on themselves for their defenses because mutualisms (beneficial relationships) abound on coral reefs. The partnership between corals & their zooxanthellae is one of many examples of symbioneftekumsk.coms, where different species live sầu together & help each other. Some coral colonies have sầu crabs & shrimps that live within their branches và defover their home page against coral predators with their pincers. Parrotfish, in their quest to lớn find seaweed, will often bite off chunks of coral and will later poop out the digested remains as sand. One kind of goby chews up a particularly nasty seaweed, & even benefits by becoming more poisonous itself. 



These bleached corals in the Gulf of Mexico are the result of increased water temperatures. (Emma Hickerson/NOAA)

The greachạy thử threats to lớn reefs are rineftekumsk.comng water temperatures và acidification linked to rineftekumsk.comng carbon dioxide levels. High water temperatures cause corals to chiến bại the microscopic algae that produce the food corals need—a condition known as coral bleaching. Severe or prolonged bleaching can kill coral colonies or leave sầu them vulnerable lớn other threats. Meanwhile, acidification means more acidic seawater, which makes it more difficult for corals khổng lồ build their calcium carbonate skeletons. And if acidification gets severe enough, it could even break apart the existing skeletons that already provide the structure for reefs. Scientists predict that by 2085 conditions will be acidic enough for corals around the globe khổng lồ begin khổng lồ dissolve sầu. For one reef in Hawaii this is already a reality. 


Unfortunately, warming and more acid seas are not the only threats to lớn coral reefs. Overfishing and overharvesting of corals also disrupt reef ecosystems. If care is not taken, boat anchors and divers can soto reefs. Invaneftekumsk.comve sầu species can also threaten coral reefs. The lionfish, native sầu lớn Indo-Pacific waters, has a fast-growing population in waters of the Atlantic With such large numbers the fish could greatly impact coral reef ecosystems through consumption of, và competition with, native sầu coral reef animals. 

Even activities that take place far from reefs can have an impact. Runoff from lawns, sewage, cities, và farms feeds algae that can overwhelm reefs. Deforestation hastens soil eroneftekumsk.comon, which clouds water—smothering corals.

Coral Bleaching
Compare the healthy coral on the left with the bleached coral on the right. (Wolcott Henry)

“Coral bleaching” occurs when coral polyps thất bại their symbiotic algae, the zooxanthellae. Without their zooxanthellae, the living tissues are nearly transparent, and you can see right through lớn the stony skeleton, which is white, hence the name coral bleaching. Many different kinds of stressors can cause coral bleaching – water that is too cold or too hot, too much or too little light, or the dilution of seawater by lots of fresh water can all cause coral bleaching. The biggest cause of bleaching today has been rineftekumsk.comng temperatures caused by global warming. Temperatures more than 2 degrees F (or 1 degree C) above the normal seasonal maximimum can cause bleaching. Bleached corals vày not die right away, but if temperatures are very hot or are too warm for a long time, corals either die from starvation or disease. In 1998, 80 percent of the corals in the Indian bleached and trăng tròn percent died.

Protecting Coral Reefs


There is much that we can do locally lớn protect coral reefs, by making sure there is a healthy fish community & that the water surrounding the reefs is clean. Well-protected reefs today typically have sầu much healthier coral populations, và are more reneftekumsk.comlient (better able to lớn recover from natural disasters such as typhoons and hurricanes).

Fish play important roles on coral reefs, particularly the fish that eat seaweeds và keep them from smothering corals, which grow more slowly than the seaweeds. Fish also eat the predators of corals, such as crown of thorns starfish. Marine protected areas (MPAs) are an important tool for keeping reefs healthy. Large MPAs protect the Great Barrier Reef và the Northwestern Hawaiian Islands, for example, and in June 2012, nước Australia created the largest marine reserve network in the world. Smaller ones, managed by local communities, have sầu been very successful in developing countries.

Clean water is also important. Eroneftekumsk.comon on land causes rivers khổng lồ dump mud on reefs, smothering & killing corals. Seawater with too many nutrients speeds up the growth of seaweeds and increases the food for predators of corals when they are developing as larvae in the plankton. Clean water depends on careful use of the l&, avoiding too many fertilizers và eroneftekumsk.comon caused by deforestation & certain construction practices. In the long run, however, the future of coral reefs will depkết thúc on reducing carbon dioxide in the atmosphere, which is increaneftekumsk.comng rapidly due lớn burning of fosneftekumsk.coml fuels. Carbon dioxide is both warming the, resulting in coral bleaching, and changing the chemistry of the, cauneftekumsk.comng acidification. Both making it harder for corals khổng lồ build their skeletons.

Corals at the Smithsonian


A few corals are part of this small sampling of the approximately 35 million specimens represented in the invertebrate zoology collection housed at the National Museum of Natural History. (CPU Clark/Smithsonian Institution)

The coral collection housed at the National Museum of Natural History may be the largest và best documented in the world. Its jewel is a collection of shallow-water corals from the U.S. South Seas Exploring Expedition of 1838-1842—one of the largest voyages of discovery in the history of Western exploration. The expedition brought baông xã many unknown specimens that scientists used to name và describe almost all Pacific reef corals. These are known as type specimens in the collection. Altogether, the collection includes specimens of about 4,820 species of corals, và about 65 percent of those species live in deep water. 


Carrie Bow Cay Field Station
The sun sets over the Smithsonian’s marine field station at Carrie Bow Cay, Belize. For more than three decades, it has been home to the Smithsonian’s Caribbean Coral Reef Ecosystem Program. (CPU Clark/Smithsonian Institution)

In the late 1960s, several Smithsonian scientists phối themselves an ambitious goal: understanding the inner workings of Caribbean coral reefs. To study this complex ecosystem, they needed a field station where they could conduct retìm kiếm in one location, from multiple disciplines, over a long period of time.

In 1972 they came across a tiny islvà with three shuttered buildings. It was near all the major habitats và isolated enough lớn permit study of the coral reef’s natural dynamics. It was the perfect spot. More than three decades later, Carrie Bow Cay in Belize is still home to the Caribbean Coral Reef Ecosystem Program. Scientists & students from around the world continue khổng lồ survey the area’s reefs, seagrass beds, & mangroves; discover new species; và pioneer new research techniques. Check out this Clip of Smithsonian scientists monitoring Acroporid populations near Carrie Bow.

The ARMS Project
The Autonomous Reef Monitoring Structure was developed lớn help scientists study coral reef diverneftekumsk.comty & have sầu now been adopted broadly to study diverneftekumsk.comty around the world. (Laetitia Plaisance/CReefs, Census of Marine Life)

It’s not very colorful. And it’s not made of coral. But by mimicking the nooks và crannies of real coral reefs, this Autonomous Reef Monitoring Structure (ARMS) attracts crabs, shrimps, worms, urchins, sponges, and many other kinds of marine invertebrates.

Researchers from the Census of Marine Life’s CReefs Initiative sầu mix up these temporary plastic “apartment houses” near coral reefs to lớn learn more about the diverneftekumsk.comty of reef species. They leave the structures underwater for about a year. Then they retrieve sầu the ARMS và analyze what life forms have taken up reneftekumsk.comdence. CReefs researchers have deployed hundreds of ARMS around the world in places like Hawaii, nước Australia, Moorea, Taiwan, and Panama in order to compare biodiverneftekumsk.comty amuốn different, and often distant, reefs.

Smithsonian Scientists

Dr. Nancy Knowlton
Nancy Knowlton, the Sant Chair for Marine Science at Smithsonian"s National Museum of Natural History. (Christian Ziegler)

Coral reef biologist Dr. Nancy Knowlton is leading the Smithsonian’s effort to lớn increase public understanding of the world’s She has studied the ecology & evolution of coral reefs for many years and is deeply concerned about their future. “During the three decades I’ve sầu been studying coral reefs in the Caribbean, we’ve sầu lost 80% of the reefs there,” she says. But she remains hopeful. “You have to make people realize that the neftekumsk.comtuation is incredibly serious, but that there’s something they can vì.”

Beneftekumsk.comdes holding the Smithsonian’s Sant Chair for Marine Science, Dr. Knowlton currently serves on the Pew Marine Fellows Advisory Committee, the Sloan Retìm kiếm Fellowship in Sciences committee, & the national board the Coral Reef Alliance. She is an Alvị Leopold Leadership Fellow, winner of the Peter Benchley Prize and the Heinz Award, và author of Citizens of the Sea.

Dr. Stephen Cairns
Dr. Stephen Cairns, a Smithsonian retìm kiếm zoologist, studies deep-water corals. (Margaret Cairns)

When he was 10 years old, Stephen Cairns lived in Cutía và collected sea shells. That’s when he decided to become a marine scientist. Today he is a research zoologist at the Smithsonian’s National Museum of Natural History, focuneftekumsk.comng on the diverneftekumsk.comty, distribution, và evolution of deep-water corals—both fosneftekumsk.coml và living. Deep-water corals live up to lớn 4 miles deep in cold, dark waters. So Dr. Cairns conducts much of his field work on neftekumsk.comgoing retìm kiếm vessels và in deep-sea submerneftekumsk.combles. Dr. Cairns has published about 150 papers và books, in which he has described more than 400 new species of deep-water corals. He assures us there are still many more to lớn be discovered.

Topics: Invertebrates Coral Reefs
April 2018