Great Barrier Reef
The most-massive animal-made structures on earth, reefs are giant colonies, often resembling huge undersea rock outcrops, made of a layer of millions of living polyps lying in top of a supporting structure of trillions of dead coral skeletons. Reefs vary in size from a human fist to a small country. Hard stony corals, resembling brown and green colored rocks, are the major reef builders.

Coral colonies cover about 400,000 square kilometers (an area the size of British Columbia, or about 0.4 percent of the ocean floor), supply 9 million tons of fish to the annual worldwide catch of 80 million tons, and produce 100,000 to 500,000 tons of limestone per square mile. Some existing reefs are 70 million years old. Reefs themselves are estimated to be over 250 million years old. The largest reefs are made up of smaller reefs. The biggest is the Great Barrier Reef, a collection of 2,900 reefs along 2,100 km of Australia's north east coast in a marine park the size of Germany.

Coral reefs are found in 109 countries. The provide fish, mollusks and other food for an estimated 300 million people. A study done in the early 2000s found that collectively the world’s reefs cover much less land that previously though. When added together they cover only 110,000 square miles, an about the size of Nevada, and half of what scientist thought.

Reefs are an incredibly old ecosystem. Coral, sea urchins, brittlestars, sponges and mollusk found in reefs today are closely related to species found on reefs over 200 million years ago. Elizabeth Kolbert wrote in National Geographic, “The reefs that corals maintain are crucial to an incredible diversity of organisms. Somewhere between one and nine million marine species live on or around coral reefs. These include not just the fancifully colored fish and enormous turtles that people visit reefs to see, but also sea squirts and shrimps, anemones and clams, sea cucumbers and worms---the list goes on and on. The nooks and crevices on a reef provide homes for many species, which in turn provide resources for many others. [Source: Elizabeth Kolbert, National Geographic, April 2011]

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Great Barrier Reef
Reefs provide a critical source of food for millions of people, attract tourists and are potential storehouse of medicines for cancer and other diseases. They are home to 10 percent of the world's fish catch and provide natural breakwaters to protect land from storms. During extremely low tides, sometimes large expanse of reef are exposed.

"Something like 25 percent of all species in the oceans spend at least part of their life in coral reef systems," Ken Caldeira, an oceanographer at the Carnegie Institution, told National Geographic. "Corals build the architecture of the ecosystem, and it's pretty clear if they go, the whole ecosystem goes."

Websites and Resources: National Oceanic and Atmospheric Administration ; Smithsonian Oceans Portal ; Ocean World ; Woods Hole Oceanographic Institute ; Cousteau Society ; Montery Bay Aquarium

Websites and Resources on Fish and Marine Life: MarineBio ; Census of Marine Life ; Marine Life Images ; Marine Species Gallery

Websites and Resources on Coral Reefs: Coral Reef Information System (NOAA) ; International Coral Reef Initiative ; Wikipedia article Wikipedia ; Coral Reef Alliance ; Global Coral reef Alliance ; Coral Reef Pictures ; The Global Coral Reef Monitoring Network; the International Coral Reef Action Network.

Reef Growth

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coral head
Both corals and independent algae produce calcium carbonate (limestone) which they derive from sea water. Coral polyps produce it their entire lives. Each polyp build a protective chamber. When it is finished it sends out filaments from which sprout out a new polyp. Construction begins on top of the parent polyp, which dies after it is buried.

New polyps build their coral shell on top of the exoskeletons of dead polyps. On the ocean floor when enough coral polyps have been joined together a small colony called a coral head is formed. A large coral head, about three meters in height, is often 500 years old or older. When enough coral heads are all linked together the amalgamation becomes a reef, which grows slowly at a rate of 5 to 28 millimeters a year.

Boring animals drill into and honeycomb the "dead part" of the reef, while sponges, sea weed, bryozoans and algae attach themselves to it, bringing color and life. The caves, niches and crevices created in the dead coral make perfect homes and hiding places for reef fish and lobsters, who need a place to hide to survive.

Most kinds of coral need to sunlight to grow and cannot survive at depths of more than 150 meters. Damaged parts of a reef take a long time to grow back.

Types of Reefs and Parts of the Reef

Types of reef: 1) barrier reefs, coral breakwaters rising from the edge of a continental shelf: 2) platform reefs, irregular circles and crescents of corals that grow in calm waters; 3) patch reefs, smaller, scattered formations that grow in shallow water; and 4) fringing reefs, which grow outward from the shore.

Oil platforms create artificial reefs. Discarded cars, sunken ships and junk can also make good homes for tropical sea life. In May 2006,a retired U.S. aircrafts carrier, the Oriskany, was sunk off the coast of Pensacola, Florida, creating the world’s biggest artificial reef. Within two years it was covered with coral and other marine hardware and was teaming with fish and scuba divers.

Reefs have been described as cities with each part playing a roll in maintaining the whole. Sponges filter the water; sea urchins and parrotfish keep coral free of light-blocking algae; the outer living layer of the reef is like skin.

The most active part of the reef is the edge. This is where there is enough plankton floating in water to supply the coral with food. As new coral grow sand enlarges the reef edge, the reef further back, which doesn't get as much plankton, dies off. Scientist can drill into reefs and examine bands in the same way other scientists examine tree rings to study the age and health of the reef.

The blocking of heavy wave action by barrier reefs and islands allows sea grass beds and costal mangrove forest to form. These in turn "trap sediments, store nutrients and serve as nurseries for a number of reef residents."

Living Reef

Reefs are incredibly rich ecosystems, supporting thousands of other kinds of life living together in a complex web of food and resource competition. The limestone structure produced by stony corals provide a perfect home of myriads of tiny bottom dwelling animals as well as being a safe haven for a dazzling array of reef fish you see swimming above the reefs. Reefs are vital for providing shelter, breeding areas and protection from predators, especially in the early stages of development, for a number of different species. Without a reef, there would be few fish and only a sandy bottom.

Only the top quarter inch of the reef itself is alive. Seaweeds grow on the skeletons of the dead coral together with sponges and other animals. Clams and barnacles bore holes into the limestones and filter plankton. Sea lilies and brittlestars, bristle worms and shell-less mollusks climb through the network of branches and graze on the algae.

Parrotfish and triggerfish eat the coral to get the polyps inside. Small schools of damselfish hang out by antler coral feeding on plankton and organic particles and ready to dash for safety among the branches, Predators such as reef sharks, groupers, rock cod, lionfish, scorpionfish, stonefish and moray eels lurk around caves and crevasses waiting to gobble up any creature that lets down its guard or shows distress.

Coral reef locations

Predator Reef

Scientists studying Kingman Reef, a mostly submerged reef near Palmyra atoll and Kiribati, about 1,600 kilometers south of Hawaii, were surprised by what they found. In reef that was about as untouched and far from human influence as was possible, they found lost of colorful coral and sponges but instead of large schools of colorful fish they found large numbers of predators---including whitetip and grey reef sharks and masses of aggressive snappers---forcing smaller fish to seek hiding places in the reef. Large predators accounted for 85 percent of the fish biomass, the scientists said, a larger percentage than has been found at any other reef worldwide. [Source: Kennedy Warne, National Geographic, April 2008]

The finding raised questions about what is a healthy reef. Was one teeming with schools of colorful fish actually one in distress? Enric Sala, a marine biologist studying Kingman reef, told National Geographic, “Worldwide, there are maybe 50 reefs in this sort of condition.”

Kennedy Warne wrote in National Geographic, “Here the biomass pyramid is turned in its head. At first glance, an upside-down pyramid is counterintuitive. On land, we are familiar with the notion that an apex predator, such as a lion, must eat many wildebeest to survive. But imagine a world with one pound of wildebeest for every five pounds of lion, the only way an inverted pyramid can function is if there is rapid turnover of biomass at the lower levels. Prey must be fast growing and quick to replenish; predators must grow slowly and live long.”

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“In the warm tropical waters many prey species spawn several times a year, replenishing their stocks as rapidly as predators deplete them. Even so the prey barely manages to sustain the predators.” Researchers found the stomach of red snappers at nearby Palmyra Atoll, another protected reef, mostly empty. The picture that emerges of life on a healthy reef is one of abundant predators living in perpetual hunger and scarce prey living in perpetual fear.” The red snappers were so hungry they sometimes attacked diver, “They wanted to taste us” Sala said.

What happens when the large predators are removed through fishing? As has been the case at other islands which have had the predators fished out, initially there is a vibrant boom of smaller fish but as time passes the system begins to break down. As the number of harmful microbes increases, large numbers of algae flourish. As large grazing fish are taken the activity of the algae increases carbon dioxide production, boosting bacteria growth---ultimately turning a healthy coral reef into a sediment-caked wasteland. “Eliminating the top predators speeds the turnover rate of the entire reef community,” Sala said. “It’s like removing vital parts from a machine and expecting it to keep functioning.”

Reef Biodiversity

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barrel sponge
Reefs are among the worlds most diverse habitats. They are home to about 35,000 to 60,000 species (of the world's 274,000 known marine species) and a third of the 12,000 kinds of marine fishes. They are also incredible rich in phyla, with creatures as diverse as sea lilies, sponges, crabs, eels and sea anemones all living there. Some patches of reef boast over a 100 species of fish and more than a thousand other kinds of plants and animals.

A single reef wall often contains a wider variety of life forms---species from more phyla, or major groups---than an entire continent. The rich biodiversity of the reefs is due to abundance of light warmth provided by the tropical sun and an abundance of oxygen saturated into the water by crashing waves.

But the biodiversity of the world's reef system is nowhere near that of the world's rainforests. This is because the species in the ocean can spread out all over the world to some degree, while mountain ranges, deserts and water barriers keep land species from spreading. Moray eels for example are found in almost all the world's reef's while different species of snakes are usually localized to a specific region.

Reefs are also very poisonous environments. According to one study at the Great Barrier reef, 73 percent of the 429 species of common exposed invertebrates were toxic to fish.


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Ocean islands are basically divided into three types: 1) "low" coral and sand islands; 2) "high" islands (usually exposed peaks and ridge-tops of submerged mountains and volcanos); 3) parts of the continental shelf. Some continental islands, were mountains and hills along the coast during last Ice Age when ocean levels were lower.

Low islands or cays were formed on coral shoals from reef sediments. Atoll islands are low islands (See Below). Seabird dropping fertilize the soils of some of these islands, which allow scrubby forest to take root. Others are battered regularly by storms and are little more than shifting piles of sand. Some patches of sand are so low they disappear during low tide and lose their status as islands.

High islands generally have better soil and a better supply of water than low islands. Low islands support only a few species of plant because there little topsoil and this soil has a high salt content. Although they often have no visible water sources they often are positioned over water lenses that trap rain water and can provide fresh water through wells.

Mangrove swamps are found along the shores of islands in areas where the water is calm. Beaches and rocky shorelines are found in areas exposed to the open ocean. Barrier reef surround many ocean islands. The water between the reef and the shore is often relatively shallow and gets hot in the afternoon. Near some islands upwelling from deep sea trenches provide nutrients for abundant marine life.

Reef Islands, Atolls and Fringing Reefs

Atoll forming
Reef islands are built through a process of coral formation known as accretion. Rubble of reef rock broken off from the reef by heavy storms such as hurricanes and typhoons create reef-top shoals. Due to the normal action of waves and ocean currents other materials begin to gradually accumulate. Beaches develop around these shallows with wind heaping up lighter material into rises, hills and dunes. The material being almost entirely made of calcium carbonate readily dissolves in rainwater and the dissolved lime is then redeposited around the loose materials, cementing it together. After a while the islands are colonized by coconuts and others plants brought in by floating seeds or flotsam and birds and insects that have deposit seeds in their feces.

An atoll is a group of islands that encircle a central lagoon. Classic atolls develop on the top guyots, flat-topped volcanic sea mounts, that rise above the sea and then erode and sink under the weight of the coral reefs that grow on their slopes.

A classic atolls begins as a reef that forms around a high island. As the island sinks under the weight of the coral reef, the coral reef grows upwards causing the island to sink even more. This process eventually forms a barrier reef which is separated from the shore by a shallow lagoon. Eventually the original mountain becomes completely submerged and islands are created from the coral and sand deposits that accumulate on the highest points of the reef. These islands often roughly follow the contour of the original mountain top. The atoll-making process takes hundreds of thousands or millions of years.

A classic atoll is oval shaped. Most atolls have a lagoon sheltered by the barrier reef and islands with a small break or channel that allows boats to enter the lagoon. Sea grass beds are found along with reefs in the protected waters of the lagoons. The lagoons of some atolls are so large that it is difficult to see all the islands from any particular point. Their atoll shape is only visible from the air. Mountainous islands tend to be younger than developed atolls.

Atoll islands are usually narrow, long, flat and sandy. Most are uninhabited and many go unvisited for months or even years at a time. Around the sheltered sides of islands fringing reefs or mangroves often located right at the shore. The top of the reef is typically fairly shallow. Below the sea floor may be 10 meters deep or so. In some places there are walls or drop offs, where the reef suddenly gives way to deep ocean water. These walls are often teeming with life and are popular with scuba divers.

Darwin and Atolls

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Mataiva Atoll
Tuamotu Archipelago
In addition to developing a theory of evolution, Charles Darwin was the first man to suggest that atolls were built by coral reefs left on the remnants of volcanos and mountains uplifted by the forces of the earth. He based this observation partly on the fact he noticed fossils of sea shells at 14,000 feet in the Andes.

Darwin’s first scientific book was The Structure and Distribution of Coral Reefs . In it he described three types of coral reefs: barrier reefs, fringing reefs and atolls. In his time coral polyps hadn’t yet been discovered and for him the creatures that made up the reef were “coral insects.” His great insights about atolls and reefs was made by combining his observations of coral with his understanding of geology, which was partly shaped by his first hand experience with an earthquake and a volcanic eruption in Chile in 1835 and observation of oceanic volcanoes in the Galapagos Island and the reef-fringed volcanic island of Moorea in Tahiti.

Before Darwin mariners had noted the unusual structure of atolls and proposed several explanations. Among them was the idea that a huge mass of fish got together and carried sand in their mouths and deposited it to make an atoll. Some speculated there was a connection between coral and the islands but more viewed the islands as objects akin to wasps nests and beehives rather than features that were geological in origin. Darwin was clever enough to realize that volcanoes provide the initial structure for reefs to build on and that the growth of coral occurred as the land below them subsided.

Darwin wrote: “We must look at a Lagoon island as a monument raised by myriads of tiny architects, to mark the spot where a former land lies buried in the depths of the ocean...The naturalist will feel his astonishment more deeply after having examined the soft and almost gelatinous bodies of these apparently insignificant creatures, and when he knows that the solid reef increases only on the outer edge, which day and night is lashed by the breakers of an ocean never at rest.”

The only coral islands that Darwin is believed to have set foot on were the Cocos, or Keeling, Islands in the northeast Indian Ocean, where among other things he noted that delicate branch-like corals thrived in the lagoons while hardier, boulder-like brain corals thrived in the rougher seas on the open ocean side of the islands.

Day and Night Creatures at the Reef

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About half to two thirds of all reef species are most active in the day. Daytime fish species such as butterflyfish, angelfish, damselfish, parrotfish and wrasses, generally have good color vision and eyes that function well under water in bright light. Most daytime fish feed on plankton or graze on algae or aquatic plants. Some seek crabs, worms, snails and other creatures that hide in the crevasses of the reef. To avoid trouble day time fish stay near a crevasse---large enough for them but too small for predators that feed on them---where they can make a quick escape. [Source: Joseph Levine, Smithsonian magazine and the book The Coral Reef at Night ]

Daytime fish that wander far from the safety of the reef do so to feed on concentrations of plankton. They avoid threats from predators by traveling in schools. During the day huge schools of grunts and snappers---which are often most active at night--- hide inside crevasses and two or three moray eels may huddle together inside a single cave. [Ibid]

The character of the reef changes dramatically at night. After sunset the colorful polyps from hard coral emerge from their exoskeletons and soft corals expand their bodies four or five times and sway gently in the current. Crabs and invertebrates such as sea slugs and sea urchins---all of which are vulnerable to triggerfish attacks in the day---emerge to graze on algae and other things. Starfish began roaming around, giant clams open their bulbous lips, and sea lilies crawl slowly to a perch and unfurl their arms. [Ibid]

Between a quarter to a third of all reef species sleep or rest during the day and roam around at night. A few are active day and night. Some are most active during the twilight periods of dawn and dusk. At night spiny lobsters, octopuses and moray eels emerge from their burrows to feed. Spiny lobsters forage for small invertebrates. Moray eels seek bigger prey. Snapper, porgies and jacks graze in nearby grass beds. Day time fish such as parrotfish and angelfish change from bright to dull colors and seek the safety of crevasses and caves. [Ibid]

Some night-time fish have big eyes. Squirrelfish, soldierfish and cardinalfish have sensitive eyes that are blinded by strong light. They spend their day beneath coral ledges and in the mouths of caves. Sweepers that usually hang out in caves and young grunts that spend their days around coral heads meet at landmarks and then disperse to the floor of the open sea where they feed on worms, small shrimp and other sanddwellers. [Ibid]

Reef Predators at Twilight

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Twilight is often a chillingly quiet time on the reef. Most daytime fish have sought shelter for the night and the nighttime fish haven't emerged yet. Many chase-and-gulp predators, such as reef sharks, barracuda, schooling jacks and grunts, semi-solitary snappers and reclusive groupers, prefer to do their hunting at dawn and dusk, taking advantage of their acute twilight vision and the confusion that takes place as sea creatures adjust to the change in light and the changes in rest and wakefulness. Larger predators like reef shark patrol reef lagoons and channels looking for fish that are in some kind of trouble. [Source: Joseph Levine, Smithsonian magazine and the book The Coral Reef at Night ]

These predators have little success during the day because the sharp eyesight and quick reactions of their prey during the day makes them difficult to catch. In the twilight their prey lose their edge and stragglers and isolated and confused individuals are sitting ducks. [Ibid]

Schooling predators such as jacks hunt like a pack of wolves trying to bring down a caribou. Working in teams and individuals, the fish try to break up schools of prey into small groups and separate victims a few at time and corral them against the surface, where they are easy targets. Sometimes individual jacks dash into the center of a school of prey in an effort to scare individuals and distract them enough to be separated from the school so they can be picked off. The hunt continues until darkness. [Ibid]

Twilight Mating

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Surgeonfish often mate in the twilight. Around dusk at certain times of the year, surgeonfish suddenly stop eating and organize themselves into long chains, with individuals about half a meter from one another, and head to the open ocean side of the reef. Many chains of surgeonfish join together to form dome-shaped school with around 2,000 fish that begins an unusual reproduction ritual. [Source: Joseph Levine, Smithsonian magazine and the book The Coral Reef at Night ]

Describing the mating ritual in the Red Sea, Joseph Levine wrote in The Coral Reef at Night , "As the light dims noticeably, a dozen individuals gather more closely together within the pulsing mass. They separate slightly from the group as the dome reaches it peak. Finally, as though released from a tautly strung bow, first one group, then two groups, then five, sharply fling themselves toward the surface, pivot sharply and dash back down into the milling swarm. At the apex of each group's ascent, its members leave something behind---small clouds of transparent eggs and white sperm that float slowly away...After the mating dance, the surgeonfish return single file to their reef haunts and dart into their favorite hiding place.

Why do surgeon fish and other reef species spawn at dusk and loose themselves in what they are doing when they are most vulnerable to predators? They do it for their offspring---eggs and larvae---which are fed on by sharp-eyed daytime reef fish that see less well and seek cover in the evening.

Drugs and Other Benefits from Reefs

Coral reefs grow in and around about a third of world's tropical shorelines. The provide fish, mollusks and other food for an estimated 300 million people, erosion protection for beaches and an estimated $375 billion worth of storm protection a year. Economically coral reefs generate billions of dollars a year worldwide in tourism, recreation and fishing. [Source: Douglas Chadwick, National Geographic, January 1999]

Limestone produced over the eons and millennia by coral is now used in concrete and masonry. Metamorphosed limestone (marble) has been used by Michelangelo and sculptors to produce great works of art and by builders and architects to create great buildings and structures.

Protected lagoons and other areas created by wave-breaking reefs allow the formation of sea grass beds and mangrove swamps that incredible rich habitats in their own right but also are important areas where many forms of sea life breed and live out the early stages of their life.

Scientists are searching the coral reefs and the sea, the same way they are searching in the rainforests, for miracle drugs. Some drugs have already been found. More seem on the way.The study of virus-killing chemicals in a Caribbean sponge in the 1950s led to the discovery of the AIDS-fighting drug AZT as well as Acyclovir, used to treat herpes infections. These have been called the first marine drugs. Sponges have also yielded cytarabine, a treatment for a kind of leukemia.

Eleutherobin, a chemical that comes from a mottled, yellow, pickled-shaped soft coral found off the coast of Australia, and a similar chemical from a sponge and a Mediterranean coral have been shown to stop the growth of malignant tumors. Dolastatin is a drug taken from an Indian ocean sea hare that shows promise in treating skin cancer and has made it as far as the clinical trial stage. A painkiller derived from a blue-green algae found near Curacao is being studied.

Some of the toxins found in soft corals are anti-inflammatory agents that have the potential of treating cancer, AIDS, asthma, heart disease and a host of other ailments. Scientist have found that toxins used by some nudibranchs to repel fish also work on land in bug sprays. The calcium secreting mechanisms of coral are being studied as means for repairing bones. Potential non-addictive painkillers have been discovered in sea whips and cone snails.

Scientists are examining the mucous coating on clownfish that protects it from sea anemone toxins for various used. They are also checking out chemicals exuded by coral that protect them from sunburn during low tide for new waterproof sun protection cream.

See Sea Squirts, Sponges, Cone Snails.

Image Source: National Oceanic and Atmospheric Administration (NOAA) ; Wikimedia Commons

Text Sources: Mostly National Geographic articles. Also the New York Times, Washington Post, Los Angeles Times, Smithsonian magazine, Natural History magazine, Discover magazine, Times of London, The New Yorker, Time, Newsweek, Reuters, AP, AFP, Lonely Planet Guides, Compton’s Encyclopedia and various books and other publications.

Last updated March 2011

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