Home > Learning About Reefs > About Coral and Coral Reefs


Scientists divide natural objects and living things into several categories—animals, plants, fungi, bacteria, and protists. Although corals contain protists and also make large mineral structures that may be considered non-living, scientists have determined that corals should be classified as animals.

One of the most critical distinctions between plants and animals is that plants make their own food, while animals depend on outside sources for their nutritional requirements. Coral animals take advantage of both forms of nutrition by hosting plant-like algae (protists) in their tissues. The majority of the coral’s energy needs are provided by tiny algae called zooxanthellae, which live inside the coral and produce food using the sun’s energy from carbon dioxide and water. The protist algae and the coral animals live together in a symbiotic relationship. This partnership allows corals to live in nutrient-poor environments where sunlight is a plentiful source of energy and the supply of planktonic food is limited. Corals also use their tentacles to capture tiny marine animals called plankton. At night, the corals’ tentacles extend from their bodies and wave in the water, collecting plankton to eat.

Corals are invertebrates; like shellfish, they have no spinal column or internal bones. Coral colonies are composed of many tiny cup-shaped animals called polyps. Millions of polyps working together in a cooperative colony generation after generation create the limestone skeletons that form the framework of the beautiful reef.

There are two general types of corals—hard corals and soft corals. Hard corals take calcium and carbonate out of seawater and turn it into an external skeleton that forms beneath their tissues. This external skeleton is deposited on top of the existing skeleton; the corals grow upward and outward on top of this skeleton. Corals are mainly colonies, budding new animals as the thin layer of tissue on top of the external skeleton expands in area. The process by which it is created is similar to that used by clams, oysters and snails to create their hard shells. Only the outer layer of the reef is alive, as coral colonies grow and expand and new corals build their skeletons on those of dead corals from earlier generations. Millions of corals have built colonies in this way over thousands of years to form the structure of coral reefs we see in the oceans. Soft corals, which look more like plants, are also animals.

The symbiotic algae that live inside corals are one-celled protists called zooxanthellae. They are sensitive to high light, variations in the concentrations of salt, and especially, to high temperatures. The algae use sunlight for photosynthesis, the process all plants use to convert energy from the sun into food energy. Corals (and the algae that live in the corals) can only grow in a few regions of the world where the water is shallow enough for sufficient sunlight, where there is not too much silt in the water, and where the water temperature is warm enough---but not too warm. Corals live in areas where the temperature ranges between 25° and 29° Celsius (77° to 84° Fahrenheit). Because they are sensitive to changes in these conditions, corals are at risk of being damaged or destroyed. Corals also face natural threats such as hurricanes, and the impact of human activities such as over-fishing and destructive fishing practices, coastal development and global warming.
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Planet Earth is often called the “ocean planet” because more than 70 percent of the earth’s surface is covered by salt water. While there are very few places left on the surface of the earth that have not been explored, most of the ocean has not been visited by humans – mainly because of its depth and inaccessibility. The average depth of the ocean is more than 3,657 meters (12,000 feet) and the deepest spot in the world is the Marianas Trench in the Pacific Ocean, which is 10,911 meters (35,798 feet) deep. (Compare this with Mount Everest, the planet’s tallest mountain, which is 8,848 meters (29,029 feet) high. The seas are also teeming with life. There is no ocean environment with a greater diversity of species than tropical coral reefs. Ecologists classify coral reef environments as an ecosystem – a living community of plants and animals and their physical environment connected by a vast and intricate web of food chains. In the Coral Reef Adventure film, viewers can see a broad diversity of tropical reefs in the Pacific Ocean.



Coral reefs are beautiful vibrant underwater cities, home to one hundred thousand different species of sea creatures. Some, such as Australia’s 2,000-kilometer (1,243-mile) long Great Barrier Reef are so large that they can be seen from outer space. From the air, coral reefs may form large patches of blue, green and brown shapes. The deep-water areas appear dark blue in color while the shallow sandy patches appear light blue-green. The living reef structure appears brown. Up close, the thin veneer of living organisms, both plants and animals that form the coral reef, is visible in many colors.

Coral reefs are only found in tropical and sub-tropical waters around the world. However, since ocean currents can bring warm water to cooler places, reef corals grow in surprising locations such as off the coast of Texas and near Tokyo, Japan.

Coral reefs in different parts of the world vary in the number and composition of coral species. The number of coral species is highest on the Indo-Pacific reefs featured in Coral Reef Adventure. For example, the number of reef-building coral species on the Great Barrier Reef exceeds five hundred whereas a typical Caribbean reef has no more than fifty species. A subject of great debate among scientist is why no species are found in both the Caribbean and Pacific Oceans – each has its own unique fauna.
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Coral reefs provide habitat for almost all forms of life including fish, crustaceans (like crabs and lobsters), seaweeds, reptiles, bacteria and fungi. As seen in the film, coral reefs attract a diverse array of organisms, each carving out its own home, or niche. A coral reef is like a city. Little crabs dig their holes, fishes find crevices to hide in, some animals even live on top of other animals; every inch of the reef is covered with life and every form of life has a job to do. For example, fishes that eat seaweeds and other algae are called herbivores, and herbivorous fishes constantly crop the reef of fast-growing seaweeds that could smother coral polyps. Many fishes such as jacks (Trevally) even “commute” onto the reef from deeper water in search of food or even cleaning. This is shown in Coral Reef Adventure when the potato cod is being cleaned, and by the cleaner shrimp in Michele’s mouth. All of these organisms live in a delicate balance. Despite covering less than 0.2 percent of the total area of oceans, coral reefs are noted for some of the highest levels of total productivity on earth and house 25 percent of all species of marine creatures.



The famous biologist Charles Darwin was the first to describe how the three main types of coral reefs — fringing reefs, barrier reefs and atolls — are formed. Fringing reefs hug the coastline and barrier reefs grow farther away from the coast, with a lagoon in-between. Darwin theorized that fringing reefs form on the sides of islands that are sinking (called “subsidence” by geologists) over many thousands of years. As the islands sink to depths where reef-building corals cannot grow, the fringing reefs turn first into barrier reefs and finally, when the island sinks beneath the waves, only a doughnut-shaped atoll is left. On reefs with a lagoon and an open ocean side, the ocean-facing side is usually more varied in coral composition than the lagoon side. Howard and Michele Hall made their dives in Australia on a barrier reef, in Fiji and Tahiti on fringing reefs and in Rangiroa on atoll reefs.

If a fringing coral reef was sliced like a cake, looking at the side or profile, there would be a gently sloping shallow Reef Flat near the shore. There is often a lot of sand on the Reef Flat interspersed with patches of bare rock, rubble and living coral. Moving farther out to sea, the seabed rises until it reaches the area called the Reef Crest where the ocean waves break on living corals. This surf zone is an area of high physical energy and biological activity. The reef corals that live here are adapted to the high-energy environment and have high growth rates. Storms can destroy them. Thus the area just landward of the Reef Crest is often covered with broken fragments of corals and accounts for the rise of the seabed towards the crest. Seaward of the Reef Crest, the seabed slopes downward – sometimes in wide terraced steps and other times with a steady downward slope to about 9 to 18 meters (30 to 60 feet) in depth. In this depth range on many reefs the reef profile suddenly “drops off” heading almost straight down – for perhaps 30 meters (98 feet) or more. This is why scuba divers call this area the “drop off.” In Coral Reef Adventure, the deep dive Howard and Richard made was on such a steep walled reef. Live reef-building corals are not found below 100 meters (328 feet); most do not occur below 30 meters (98 feet).
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Old coral reefs may be over 30 meters (98 feet) thick, but the living part is only a thin veneer of corals and other organisms, perhaps only a meter (about 3 feet) thick on the surface. As the reef grows upwards, the older parts die and are built upon. The oldest fossil reefs in the world existed more than 150 million years ago. Some of these fossil reefs are found in surprising places where the ocean used to be – such as a farmer’s field in Vermont, USA! Most living reefs are less than 25,000 years old. When scientists want to know the history of a reef, they can drill down through it, often for over 304 meters (1000 feet) and bring up the “cores” for detailed study and carbon dating. Carbon dating is a way to tell how old the reef is. By drilling and by studying places where old fossil reefs have been exposed, such as construction sites, geologists can also study the structure of the reef and determine how the reefs were formed. Coral reefs grow like high-rise buildings are built – the skeletons of large corals form the frame of the building while the sand and coral rubble fill in between the frame
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While there are many different kinds of corals of different shapes and sizes, all corals are invertebrate animals and most live in colonies consisting of many interconnected individuals called polyps. Corals are tiny animals, called polyps, that are related to and look like sea anemones. Each coral secretes a stony cup of limestone around itself as a skeleton. The polyps divide as they grow and form coral colonies. As the coral colonies build up on top of each other, they gradually form a coral reef. There are over 4,000 different coral species. The largest reef in the world, Australia’s Great Barrier Reef, alone is home to 700 different coral species, but not all of these grow fast enough to help build the reef.



As seen in Coral Reef Adventure, there are several types of corals including soft corals, sea fans and hard corals. Soft corals look something like asparagus or broccoli and feel like soft shoe leather. Their skeleton has been reduced to tiny rods that are found throughout their tissue. Soft corals are some of the most beautiful animals, featuring brilliant red and orange colors. Sea fans (known as gorgonians by scientists) have a wiry, flexible internal skeleton and lots of tiny polyps on the branches of the flat fan-shaped structure. In Coral Reef Adventure, the divers go behind one huge red sea fan that is several meters (about six feet) across. About 600 species of hard corals are called reef builders because they grow fast enough to form the reef framework. Millions of corals have built their skeletons in this way over thousands of years to form the massive coral structures seen in the film. Some coral colonies are very old; many are hundreds of years old.
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All corals are animals, but like lichens on land, which are part fungi and part algae, they have a wonderful symbiotic relationship with algae. (In this type of symbiosis, two different organisms each benefit from living closely together.) Microscopic algae called zooxanthellae (pronounced “zo-zan-thel-ee”) live within the internal tissue of each coral polyp. The zooxanthellae are the corals’ “solar panels,” providing the reef building corals with enough energy to extract calcium from the seawater to build their skeletons rapidly. The zooxanthellae use sunlight for photosynthesis and produce oxygen and sugars (like an energy bar!) which the corals can use. The coral animal produces carbon dioxide and waste products (nutrient-rich fertilizer) that the zooxanthellae use. This symbiosis is so successful that fast-growing species of branching corals can grow up to 20 cm (8 inches) per year in shallow sunlit waters. The zooxanthellae are so numerous in the coral tissue (up to several million per square centimeter) that they give many corals their characteristic greenish-brown color. Coral species that do not form symbioses with algae grow very slowly and do not get very big; hence they do not contribute much to the framework of coral reefs. Many other reef animals in shallow sunlit waters, including anemones, soft corals and giant clams, also have symbiotic relationships with these algae. Since reef-building corals require sunlight to grow, coral growth is limited by such factors as muddy water and great depth. For example, there are no coral reefs below about 200 feet (61 meters) in Fiji – there is not enough light for the zooxanthellae to carry out photosynthesis. This zone is dominated by non-reef-building corals.



The “Coral Reef City” lives in balance with the “Suburbs” -- seagrass beds and mangrove forests that are usually located closer to shore. Seagrass and mangrove ecosystems serve as nurseries for many fish and shellfish species which later migrate to reefs; many reef creatures spent their formative years in these other habitats, protected by their calm waters and nourishing nutrients. Some creatures continue to travel between the coral reefs and seagrass beds their entire lives, transporting nutrients between these ecosystems and connecting them. Mangrove trees live at the boundary between land and sea and may grow to 15 meters (50 feet) high. Their root systems form an important underwater ecosystem for marine organisms. Mangroves require some fresh water, so are found near river mouths. Seagrass beds are strictly marine and are often found in shallow protected bays. Both mangroves and seagrasses help the reefs by trapping terrestrial sediment that could smother the corals and reduce the light needed by the zooxanthellae. Coral reefs protect these habitats from the eroding action of waves.

The tropical reef ecosystems are part of a much larger marine habitat — the open ocean. A coral reef is like an oasis in a desert. Many important open ocean or pelagic fish such as tuna spend some time of their life on or near a reef to feed on reef fish or to reproduce. For example, the largest shark in the world, the whale shark, eats small free-floating microscopic organisms called plankton and, during part of the year, corals and reef fish larvae.

Learning about the connections in coral reefs and among different marine ecosystems can help us understand and appreciate other important marine habitats. If one habitat is destroyed by pollution, for example, there are consequences for other ecosystems because of these interconnections.
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People as well as marine creatures depend on tropical marine ecosystems. In the film, Howard and Michele Hall visit Fiji where the local Melanesian people are intimately connected with their coral reefs. The Fijians have a taboo system run by the Chiefs, which determines when certain types of fish can be caught and where. This traditional reef management system has worked well for thousands of years and has allowed the Fijians to survive without over-harvesting their reefs. The reefs give them fish for food, protection from storms and rough seas, income from tourists that visit their island to witness the beauty, and are part of their culture through ceremonies to thank the reefs and waters. The Fijians are just one of hundreds of cultures worldwide that rely directly on the tropical marine system for survival. Even in spite of the Fijians’ efforts to manage their coral reefs, the reefs are still vulnerable to the impact of other human activities like industrial development and global climate change.

Coral reefs provide many benefits to humans. Throughout Southeast Asia, coral reef fisheries provide ten to 25 percent of the protein for people living along the coast. Coral reefs also have high economic value. Tourism is the largest industry in the world and tropical destinations such as Cancun and Jamaica rely on their coral sand beaches to attract tourists. In the Indian Ocean, almost eighty percent of the islands are built exclusively of reef material. For the 20 million scuba divers of the world, a tropical reef diving holiday is high on the ‘must do’ list.

Increasingly, coral reefs are largely untapped resources for future pharmaceutical breakthroughs. The best-known example of coral reef’s medicinal value is AZT, a treatment for people with HIV infections, which is based on chemicals found in a Caribbean reef sponge. Also, more than half of all new cancer drug research focuses on marine organisms.
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Natural Events

Hurricanes, typhoons, El Niño, coral-eating organisms, and diseases are natural disasters coral reefs have been facing for millions of years. However, the imbalances these events create are temporary and can actually help the tropical ecosystem to maintain diversity. For example, hurricane storm waves are particularly devastating for tall, branching corals. However, broken fragments of branching corals can start to grow where they land, creating a new colony. These corals are fast growing and quickly form new reef framework under suitable environmental conditions. Coral-eating creatures, naturally kept in low numbers by predators, appear to help the reef by opening bare space where new coral recruits can settle and have a chance to grow, maintaining the diversity of the reef.

Truly devastating threats to coral reef health come when the impacts of natural events are compounded by persistent human-induced stressors that include over-fishing and destructive fishing practices, coastal development, sewage and other pollution, and rising global temperatures.

Over-fishing and destructive fishing practices

According to the Reef Check monitoring program, over-fishing is a major problem for coral reefs — and for the ocean in general. Over-fishing works in a step-by-step fashion to cause imbalances. First, fishing selectively takes larger, predatory fish off the reef causing population explosions of smaller herbivorous fish. When the larger fish become scarce, the herbivorous fish are then targeted by fishermen. Without the herbivores, seaweeds can over-grow the corals and smother them. Destructive fishing practices include the use of cyanide to stun fish for capture for the marine aquarium and live fish trades and blast fishing to kill fish for food. These fishing methods are not usually species-specific. Many organisms are killed in the process and habitats that took thousands of years to build are destroyed.

Coastal Development

Coastal development threatens the reefs in a number of ways. Tropical forests have a thin layer of soil that is kept in place by dense forest. When land is cleared for construction, this layer erodes quickly with any rainfall and is transported by rivers to the sea. In areas with pristine mangrove forests and seagrass beds, most of this sediment will be trapped before reaching the reefs. But coastal construction often removes mangroves and seagrass beds, which takes away the sediment barrier (and also removes the reef’s nursery). Excess sediments cover corals, blocking the light necessary for their symbiotic zooxanthellae and smothering polyps. Nutrients from sewage pollution disrupt the balance between corals and faster growing seaweeds. Under natural conditions, corals are able to out-compete seaweeds because of the low nutrient content in tropical waters. When outside nutrients are added, faster growing seaweeds can take over and smother corals.


Pollution adds all sorts of unnatural and potentially harmful substances to the reef system including nutrients, pathogens and trash. Pollution has been implicated in the apparent surge in coral diseases, especially in the Caribbean.

Global Warming/Rising Sea Temperatures

One of the most global threats to coral reef ecosystems that is highlighted in Coral Reef Adventure is rising seawater temperature. If the temperature is raised above a critical level, the zooxanthellae leave, causing corals to appear brilliant white. This condition is called “bleached”. Prolonged bleaching can lead to coral death. According to a report submitted to the US Coral Reefs Task Force in 1999, “coral bleaching is most often associated with a significant rise in sea surface temperatures. Activities such as the burning of fossil fuels, changes in land use, and reduction in forest cover are increasing the atmospheric concentrations of greenhouse gases (e.g., carbon dioxide), altering radiative balances and warming the atmosphere. Global climate change poses an increasing threat to coral reefs. An increase in carbon dioxide (CO2) in the atmosphere can reduce the ability of corals to form limestone skeletons, slowing their growth and making them fragile. Global mean sea-surface temperatures are projected to increase approximately one to two degrees Celsius by the year 2100. If the frequency of high-temperature episodes increases as mean temperature gradually rises, corals will experience more frequent and widespread disturbances.”
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The human stresses on coral reefs occur at every level: locally, through over-fishing and coastal development, regionally through pollution affecting rivers, and globally, through global warming which raises the temperature of the ocean water. At each level, there are ways to reduce human impacts and help corals flourish once again. Everyone can have a hand in coral reef protection, from coastal towns choosing to develop eco-tourism-based economies to people choosing to eat sustainably caught fish.


The most important way to reverse the damage to coral reefs is through what you are doing right now: education. Learning about how our actions affect coral reefs and the marine habitats we rely on is the first step to understanding how important it is to protect these amazing ecosystems. Coral Reef Adventure is also helping to bring these messages to people everywhere. As Jean-Michel Cousteau says in the film, we need to “teach our children [and adults] well” and show them the beauty that is a coral reef.

Global Climate Change and our Life Styles

As the climate continues to warm, coral bleaching events may become more frequent and severe. Even reefs that are protected as marine sanctuaries are threatened by global climate change. Greenhouse gas emissions are a major contributor to global warming. Here are a few things that you can do to help reduce greenhouse gas emissions:

1. Conserve energy
2. Buy a fuel-efficient car and drive less often
3. Reduce, re-use and recycle
4. Plant trees
5. Contact your government representative and vote responsibly on global warming issues
6. Review the “Ways to Help Coral Reefs” section of Websites of Interest on this website for more information about everyday things that you can do to help keep coral reefs healthy

Monitoring Reefs

Coral reef management is a huge job that involves identifying the threats facing coral reefs and developing ways to monitor the reefs and human activities that affect them. Learning how to monitor reefs through Reef Check training is one example of how communities in tropical coastal areas can be involved coral reef conservation. For details, go to www.reefcheck.org. Monitoring can be an important first step in raising awareness and changing destructive local and regional practices.

Marine Protected Areas

Creation of Marine Protected Areas (MPAs) is a promising strategy that is being implemented worldwide to protect biodiversity and help local communities manage their resources. These are areas that regulate fishing and extraction methods. The Great Barrier Reef Marine Park is the largest coral reef MPA in the world, and the large size and high abundance of fishes seen in the film are due to the fact the Great Barrier Reef has been protected since the early 1970s. When allowed to recover, corals attract fish and the fish spill over outside the MPA boundaries. MPAs also include the added benefit of involving the local community in the design, implementation, and enforcement of protected areas. Groups are also working on creating better sustainable fishing options, replanting lost mangroves, and diverting sewage from being dumped into coastal marine habitats.
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St. Lucia’s Marine Reserve

St. Lucia is an island in the Caribbean that experienced increasing population pressures during the 1980s. These threatened the health of St. Lucia’s fringing reefs primarily due to pollution and over-fishing. In response, government agencies and community groups formed the Soufriere Marine Management Area (SMMA), a marine reserve, in 1994. Since the reserve was created, fish populations within the management area have tripled. For more information see http://www.smma.org.lc

Cebu’s MPA

Vast areas of Philippine reefs are destroyed due to intense blast and poison fishing. Nowhere is this situation worse than on the island of Cebu in the Central Visayas. After conducting a Reef Check survey, local residents realized the terrible state of their reefs and made a decision to take action. With help from additional service organizations, the private sector and government agencies, the community established a marine-protected area at Gilutongan Island in Cebu. In late 2001, only two years later, thousands of colorful reef fish have returned to Gilutongan Reef. These fish are reproducing and repopulating the surrounding damaged reefs. Additionally, by charging divers a small visitor’s fee, the marine protected area raised over $20,000 in 2001, a significant source of income for the small island population.



What Is A Coral?

Despite the fact that corals look more like rocks or plants, they are definitely animals. Coral colonies are composed of many tiny, cup-shaped animals called polyps, which are related to jellyfish. A single coral polyp may be as large as a saucer or smaller than the head of a pin. Millions of polyps working together in a cooperative colony generation after generation create the limestone skeletons that form the framework of the beautiful coral reef.

How Do Corals Start Out Life?

Corals begin life in tropical waters as free-floating larvae. After a relatively short period of time, the larva eventually attaches itself to a hard surface and becomes a polyp. Polyps divide asexually and form colonies. Coral colonies reproduce both sexually and asexually. In sexual reproduction, the coral polyps release both eggs and sperm into the water. (This is also known as coral spawning.) One type of asexual reproduction occurs when fragments of coral are broken off as a result of storm action. The broken pieces of corals usually survive and continue to grow and produce a new colony. This process is referred to as “fragmentation”.

What Do Corals Eat?

A coral polyp consists primarily of tentacles, a mouth and a gut (think upside down jellyfish). Many corals are passive feeders on plankton. Most corals also get nutrition from microscopic algae (zooxanthellae) living within their tissue. Coral polyps are generally nocturnal feeders and are provided sugars made by their photosynthetic zooxanthellae during the day.

Where Does The Framework Of A Coral Reef Come From?

Corals extract calcium and carbonate from seawater to build an inner skeleton that is external to the coral. This external skeleton lies underneath a thin layer of tissue. Over the years millions of coral polyps in colonies create the framework of the coral reef. Coral reefs grow very slowly. It may take up to a hundred years for a coral reef to grow one meter (around three feet).

What Is The Difference Between Hard And Soft Corals?

Hard corals, also called reef-building corals, produce a rock-like skeleton made of the same material as classroom chalk (calcium carbonate). These skeletons and the various shapes of different colonies form the familiar structure of the reef. Hard corals rely on symbiotic algae (zooxanthellae) living within their tissues for nutrition and energy to build their skeleton. They must therefore live in shallow clear water to allow sunlight to reach the algae. Soft corals look like colorful plants or graceful trees and are not reef-building since they do not produce the hard calcified skeleton of many reef-building corals. However, soft corals do produce smaller amounts of calcium carbonate that help them keep their shape. Soft corals can be distinguished from hard corals by the fact that soft coral polyps always have eight tentacles, while hard coral polyps have multiples of six tentacles.

What Is Symbiosis?

Symbiosis is defined as the close association between two or more interacting organisms, usually of different species. The relationship is usually classified as belonging to one of three types: mutualism (benefiting both partners), parasitism (one partner, the parasite, benefits, at the expense of the host), or commensalism (one partner benefits while the other is unaffected). Changes in the physical environment such as the amount of sunlight or salinity, or the temperature, and in the biological community, such as the presence or absence of other organisms and how they interact with the symbiotic pair, may change the nature of the symbiotic relationship from one type to another. Like organisms, symbiotic relationships are responsive to the environment and can change over time.

What Is The Largest Coral Reef In The World?

As the name implies, the Great Barrier Reef, located off Australia’s East Coast is the largest coral reef in the world. This enormous reef is over 2023 kilometers (1257 miles) long and covers more than 300,000 square kilometers (about 186,000 miles). Home to more than 1500 species of fish, dolphins, whales and sea turtles, the Great Barrier Reef is actually a collection of more than 3000 smaller reefs. The second largest reef lies off the coast of Belize, in Central America.
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What Are The Main Types Of Coral Reefs?

Reefs are generally classified into the following three types:

Fringing reefs, the most common type of reef, form along a coastline. They grow on the continental shelf in shallow water.
  ·Barrier reefs grow parallel to shorelines but are farther from shore and are usually separated from the land by a deep lagoon. They are so called because they form a barrier between the lagoon and the seas, protecting the coastline.
  Coral Atolls are rings of coral reef growing on top of old sunken volcanoes in the ocean. They begin as fringing reefs surrounding a volcanic island; then, as the volcano sinks, the reef continues to grow, and eventually only the reef remains. There are over 300 atolls in the South Pacific. Atolls contain islands.

Do Any Animals Eat Corals?

One of the most important predators of corals is the Pacific Ocean’s Crown of Thorns Sea Star. It is estimated that a single Crown of Thorns Sea Star can eat from 2 to 6 square meters (6 to 20 square feet) of corals per year. Many fish species such as parrotfish, butterfly fish and tangs also include corals as part of their diet. Attentive divers and snorkelers hear the crunch of hungry parrotfish as they chew up their delectable meal that includes the skeleton. Other coral predators include some types of marine snails and marine slugs, known as nudibranchs. Interestingly, these coral predators digest the animal tissue and release the symbiotic algae (zooxanthellae) in their feces. The nudibranchs may also keep the nematocysts (stinging structures) and symbiotic algae for their own nutritional use.

Why Are Coral Reefs Important To Humans?

Coral reefs are among the most biologically diverse ecosystems on earth. Second only to tropical rain forests in the number of species they harbor, they are sometimes called the “rainforests of the sea”. Although coral reefs only occupy about 0.07 percent of the ocean floor (an area roughly the size of Texas), they are home to as many as one quarter of the world’s marine species. Coral reefs offer important income sources for their human neighbors through tourism and fishing, which provide both subsistence and trade. Recently, scientists have begun to discover that coral communities may contain valuable medicines that may one day lead to treatments for cancer and HIV. For coastal communities, coral reefs also play an important role in protecting their coastlines from storms.

Why Are Coral Reefs In Danger?

Coral reefs are among the most beautiful ecosystems in the world but are also among the most susceptible to human impacts and are damaged or destroyed with alarming ease. Practices such as over-fishing, the use of dynamite or poison to capture fish and dropping boat anchors on corals have produced enormous damage. Even an accidental touch from divers and snorkelers can significantly damage the delicate coral polyps. Pollution, silting from land-based construction, and fertilizer runoff have led to damage to coral reefs worldwide by blocking the sunlight corals require for photosynthesis by their symbiotic algae. Rising sea temperatures from global warming can also destroy corals by ending the symbiotic relationship with zooxanthellae. Hurricanes and earthquakes, which can also lead to significant damage to the reefs, are nonetheless generally viewed as a natural cycle of the ecosystem. However, when a coral reef has been damaged from human effects, it may have a more difficult time recovering from natural disasters such as hurricanes and earthquakes.
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Environmental Threats

A major environmental threat to coral reefs is increased temperature of ocean water. One of the natural causes of increased ocean temperature is El Niño, a huge weather pattern that affects the trade winds in the Pacific Ocean and changes rainfall patterns around the world. The increased temperature puts stress on the highly sensitive corals, causing them to expel the algae that live inside the coral polyps. This process makes the white coral skeleton visible, and is called bleaching. Corals can survive occasional bleaching incidents, but they cannot recover from repeated or prolonged stress because they depend on the symbiotic algae for nutrition. If the algae are deprived of sunlight for too long and the coral polyps cannot capture enough plankton to survive, the corals will eventually die.

Another form of stress that the natural environment causes for corals is related to the depth of the ocean. Because the algae inside coral polyps depend on sunlight, corals cannot survive at depths where sunlight cannot reach. It is believed that if global warming causes the polar ice caps to melt, ocean depths will increase more rapidly than corals can grow preventing corals from getting the sunlight they need to live and make their skeletons.

Although these environmental risks raise concern for coral reefs, there are some indications that corals may be able to adapt to some forms of stress. In one study cited in National Geographic, July 2001, scientists found evidence that corals might possibly be able to replace their zooxanthellae with different strains of zooxanthellae better able to survive under the new environmental conditions.

The Pacific Marine Environmental Laboratory, a division of NOAA, has a very informative web page with more details about El Niño, including an animated image and maps. It can be found at http://www.pmel.noaa.gov/tao/elnino/el-nino-story.html

Full text of the National Geographic article can be found at http://news.nationalgeographic.com/news/2001/07/0725_coralbleaching.html

Impact Of Human Activities On Coral Reefs

Local Activities

There is a long list of human activities that can negatively impact the health of coral reefs, beginning with destructive fishing methods. Blasting dynamite near the reef to bring fish to the surface can destroy the entire reef, reducing it to rubble. Another destructive practice involves divers squirting cyanide onto tropical fish to stun them and take them for collectors. In addition to unnecessarily killing many fish, the cyanide poison damages live coral polyps in the reef.

Tourism also has direct and indirect effects on coral reefs. Snorkelers and divers who are not trained in proper behavior around reefs can trample delicate soft corals, damage reef structures and injure fish by attempting to feed them. Anchors from boats can drag across the surface of the reef, breaking or damaging parts of the structure. Some divers break off pieces of living hard corals or uproot soft corals to take as souvenirs of their trip. Many tourism companies and environmental organizations have developed programs to educate reef visitors to protect reefs. Divers are encouraged to treat coral reefs with care and respect.

The basic problem of over-fishing permanently changes the ratios of fish species in the area of the reef, affecting food webs and the entire ecosystem. Removal of herbivorous fishes may allow seaweeds to overgrow corals, blocking sunlight and access to planktonTop of Page.

Regional Activities

An indirect impact of tourism is caused by over-development of coastal areas near reefs. In some locations, shoreline property has been filled in to provide solid ground for building or harbors have been dredged to open waterways for large ships. Deforestation of inland areas causes runoff into waterways that lead to the ocean. All of these practices produce enormous amounts of particles in the water. The suspended particles smother corals and prevent sunlight from passing through to the zooxanthellae in corals and seaweeds that keep the reef alive.

Development along coastlines may also lead to increased amounts of sewage being dumped into the ocean. This material changes the nutrient balance in the water and can cause plankton and seaweed blooms. Fertilizer runoff and water pollution from inland waterways also contribute to the problems. Coastal power plants release large amounts of water into the ocean, often raising local water temperatures and stressing marine life.

Medical and Scientific Research

Coral reefs contain many species that make chemicals that might be used to relieve pain or even cure diseases. Doctors and scientists are studying many forms of coral life for their medical potential. From natural poisons that can be used as painkillers to natural sunscreen compounds in corals that can prevent sunburns, researchers are finding numerous benefits to humans in coral reefs.

The Australian Institute of Marine Science has patented a sunscreen product based on the natural sun protection chemicals found in corals. The product, which is still being tested, absorbs ultraviolet light efficiently without irritating skin or causing an allergic reaction. Scientists are able to create the substance synthetically so that they do not have to damage existing reefs to extract it. Another coral reef species that is being investigated for skin care properties is the soft coral sea whip. Compounds called pseudopterosins may be able to reduce swelling and relieve irritated skin.
Scientists at the U.S. Food and Drug Administration have developed a method of using a coral product called hydroxyapatite (HA) as a bone-building substance for people with broken bones. The material can be injected into the area of the break. HA has similar properties to human bone and fuses to the existing bone, forming a strong bond over time. HA was approved by the FDA in 1992.

Even some poisonous creatures whose toxins are highly dangerous in their natural environment might be adapted for medical purposes. A snail that lives near coral reefs contains a chemical poison that might be useful as a painkiller. Studies have shown that the poison might be even stronger than morphine in relieving pain. Medicines derived from the chemical structure of the poison may have great potential for patients suffering from severe pain.

Full text of sunscreen article: http://www.abc.net.au/science/news/stories/s102327.htm
Full text of FDA article:

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