Everything about coral reefs

Coral reefs have existed for 450 million years. They are probably the oldest ecosystems on our planet. Coral animals, the small organisms that build the reefs, are responsible for the largest structures ever made by living creatures. Some reefs are so large that they surpass human structures. Coral is a resilient but (very) fragile ecosystem. An ecosystem includes everything that contributes to the preservation of life within. An ecosystem is not only the society of organisms (plants, animals and micro-organisms) within a certain environment. It is above all also the exchange of matter and energy between the organisms themselves and between life and the non-living environment: soil, water and air. Within such an ecosystem, different communities have their own residential areas (biotopes). The relationships between the various organisms are determined by eating or food relationships and protection functions. Coral reefs are resilient ecosystems. Natural threats (eg storms) can be good for the biodiversity of the reef. For example, fast-growing coral is destroyed in a storm, giving slow-growing species a chance. But coral is also particularly vulnerable. It is mainly threatened by environmental factors and human actions. I will come back to this later. Coral takes years to grow. A simple touch can already undo growth for decades. About 30% of all corals in the world are damaged to such an extent that they are actually already lost. If we continue to use coral like this, another 30% will be added in the coming years.

Coral reefs: 'Rainforests' of the ocean

Position and climate

Because of their great biodiversity, their high primary production (the energy that is needed for this comes from photosynthesis), their importance for people and nature and their splendor of colors, coral reefs are also called the rain forests of the ocean.
Coral reefs are found between 35 ° North latitude and 28 ° South latitude. The large coral reefs of the world are all in the tropical zone. They develop best at a water temperature of 25 to 29 ° C. Warm wave currents extend this belt beyond the tropical latitudes. The coral triangle is located in an area in Indonesia, east of Sulawesi and west of Papua New Guinea. Here you can find the most different types of coral and the greatest biodiversity. This is because the underwater landscape here is very diverse. There are, for example, deep and shallow sections, underwater mountains and very deep canals. A lot of nutrients are also supplied by the sea currents. This combination has created an explosion of life. It is thought that all coral species originally came from this coral triangle. Coral has been transported from the coral triangle to other areas. The farther away from the coral triangle, the fewer types of coral present. That is why there are far fewer species in the Caribbean than in areas closer to the coral triangle.

Residents of the coral reefs

Only 0.25% of all oceans and seas are coral areas where 25% of all fish depend on! They are hiding places, food sources and delivery rooms for them. The ecosystem of the reefs is the most species-rich in the sea and can be compared in terms of importance and variation with that of the tropical rainforest. At least 5,000 species of fish and hundreds of thousands of other marine and marine species depend on these reefs, such as molluscs, sponges, crustaceans, etc. Many of these species have not yet been described and discovered! On 18 September 2006, the British daily newspaper The Independent reported another discovery of dozens of new fish species and corals off the coast of Indonesia. A reef can contain 200 species of coral, 300 species of fish, and 10,000 to 100,000 invertebrates. The total wealth of species of the coral reefs is probably above 2 million! After all, a lot of food is present here, while the tropical water is usually low in food. There are around 800 different types of coral. Some species are for example brain coral, stone coral, labyrinth coral, etc.

Coral polyps

Coral polyps are small animals that belong to the nettle animals together with the anemone and the jellyfish. Coral polyps are transparent and at most a few centimeters in size. The polyps contain algae, the zooxanthellae, which are necessary for the growth of these polyps. The alga converts via photosynthesis, sunlight into CO2, and water into carbohydrates and oxygen, which the polyp uses. In this way the algae provide food and the material for the skeleton of the coral polyp. The polyp in turn provides the alga with, among other things, nitrates and protection. These algae are therefore again dependent on the transparent coral polyp tissue because they need sufficient light (therefore, corals only occur in shallow, clear water). Zooxanthellae and coral polyps are therefore dependent on each other in a so-called symbiotic relationship, or a mutualistic symbiosis. A mutualistic symbiosis is actually a "for-what-should-what" strategy: both the coral animals and the zoxanthellae benefit from the fact that they live together (mutuus = borrow, exchange; sym = together; bio = life). The zooxanthellae use the waste from their host (the coral polyp) and the sunlight for their photosynthesis, the coral benefits from the lime skeleton and the oxygen produced by the zooxanthellae. This symbiotic relationship determines the productivity, color and diversity of the coral and is the reason why such strict environmental requirements are imposed on the ecosystem. The above process therefore occurs during the day when the coral ecosystem is set to relative rest so that the algae can take good advantage of the sunlight. The coral polyps then retreat to their lime skeleton (the exoskeleton). The coral reef then looks like a mountain of stone. In addition to the nutrients that coral receives from its algae, the coral itself also 'catches' food. This happens at night, then the coral only "comes alive" and the polyps feed on plankton by means of. their tentacles. They can spread them to filter plankton (very small marine animals) from the water. The beautiful colors and shapes of the polyps can only be seen at night. In groups of thousands, they form colonies, each with its own shape and color with different purposes (eg, deterrence and defense). The colors arise thanks to the algae that are present in large numbers in the transparent polyp. They release a type of pigment that determines the color of the polyp.

Reproduction of the coral polyps

Coral polyps reproduce both sexually and sexually:
  • Asexual reproduction: Coral can 'stick' itself. A parent polyp produces a daughter polyp by budding or splitting itself. Genetically this is an exact copy of the parent, with which she remains connected by living tissues. When polyps stick themselves, they stay connected. This is how true coral colonies arise. Living polyps only occur on the outside of the colony; the inner part consists of stacked, dead skeletons of former polyps. Coral colonies have a common skeleton. Food is distributed throughout the entire colony and enemies are also fought together. New colonies can arise from fragments that have been broken off from a larger colony, provided that the conditions necessary for growth are good. For example, reefs recover from storm damage. There are also coral species that live solitary. Their polyps grow up but do not form groups.

  • Sexual reproduction: Every year around the same time (depending on the species, on a certain night in the early summer or in November) all the coral in a certain area releases thousands of eggs and seeds into the sea simultaneously (this process is called coralspawning). There the eggs are fertilized by the sperm and the so-called planular larvae develop. But this suspension of sperm, eggs and larvae attracts whale sharks, mantras and other hungry fish. But due to the large amount of eggs and seeds, a large number of fertilization still takes place, enough to maintain the reefs. The planula larvae that originated from fertilization are up to 1 mm long and have all kinds of shapes, depending on the species. The parent polyp has given them zooxanthellae in their tissue cells. If they stay alive, they float in the sea for a few days or weeks. They then sink to the bottom with plankton and attach themselves to a hard, stony surface. There they develop into a coral polyp and their lime skeleton starts to form. From there they can reproduce asexually and form a new coral colony.

The growth of reefs

Coral reefs arise because limestone is deposited by the coral animals. There are both hard and soft corals. The hard coral is reef-building; it extracts calcium carbonate (lime) from seawater and then deposits it at the bottom of the polyp. If the coral polyp dies, the exoskeleton remains. These coral skeletons form the basis for new coral polyps. A coral reef can in this way become tens, even hundreds of meters high. Coral reefs are therefore actually an accumulation of the exoskeletons of dead coral polyps. That a reef gets higher and higher is necessary. The zooxanthellae that live in the polyps need sunlight for their photosynthesis (just like plants). However, the seabed tends to collapse a little under the weight of a reef every year. To stay close enough to the surface of the seawater, where the sunlight is strong enough for photosynthesis, the reef must continue to grow upwards. Just like plants and shrubs, "coral colonies" grow towards the light. The corals mutually provide a battle (by the way they do this by separating deadly chemicals) for light, food and space. These are therefore the factors that determine the growth rate of a reef.

Coral species

Hard coral or stone coral is reef-building and occurs in many species. Hard coral has a skeleton of lime. When the coral animal dies, the lime skeleton remains. A new coral animal is established on this. Every species has different characteristics. For example, with its antler-shaped protrusions, deer crop coral grows much faster than brain coral, which resembles a human's brain. The deer crop coral can sell up to one hundred kilograms of lime per square meter per year. Other examples of hard coral are: lettuce coral (really resembles a head of lettuce), finger coral, fire coral (leaves burn marks on contact). Soft coral or horn coral generally grows faster than hard coral. It has a skeleton with a fleshy structure, which is reinforced with small lime particles. The structure of soft coral is similar to that of a sponge. The soft coral in particular ensures that coral reefs sometimes look like a flower garden with different colors and shapes. An example of soft coral is the gorgon, which appears in beautiful colors such as black, red, yellow and purple. Most corals are between 5,000 and 10,000 years old. Algae, seaweed, sponges, oysters and other animal and plant species contribute to the construction of the reef. Reefs grow very slowly: between 5 and 25 millimeters per year. But over the centuries, structures of sometimes hundreds of meters high can arise. The estimated volume of lime that the coral polyps have excreted on the Great Barrier Reef is around twenty thousand square kilometers!


  1. Franche reefs or coastal reefs: Here the coral forms a border along the coast of continents and islands and borders almost directly on land. Sometimes there is a shallow lagoon in between. This type of reef is most common, especially in the Caribbean, Southeast Asia, the Red Sea and around the Indian Ocean. In turbid water, fringing reefs rarely occur at greater depths. An example of a fringe reef is the reef that has formed around the island of Bonaire.
  2. Barrier reefs: Barrier reefs occur farther off the coast. They arise on the edge of a continental shelf or around partially sunken islands. Between the reef and the coast there is a wide, fairly deep lagoon (a lagoon is a shallow stretch of sea). More vulnerable species of coral grow on the lagoon side of the barrier than on the ocean side. On the ocean side, the coral must withstand the power of higher waves. The most famous example of a barrier reef is the Great Barrier Reef. This extends over a length of almost 2000 km. along the east coast of Australia. This is the largest living organism in the world and here 3% of all reefs in the world can be found. It does not form a contiguous whole, but it consists of a series of separate pieces that are called band riffs. Other important barrier reefs are located in Belize and the Bahamas in the Caribbean, and in Papua New Guinea, New Caledonia and the Fiji Islands in the Pacific.
  3. Atolls: Atolls are ring-shaped coral reefs with low islands around a lagoon. They actually originate as a fringed reef around a volcano island. If the island sinks due to the bottom sinking or rising sea level, the fringing reef forms a circular barrier reef that is separated from the island by a lagoon. If the island disappears completely under water, the reef remains. Sometimes a circle of coral islands surrounds the lagoon. The whole is called an atoll. There are 330 atolls in total. Almost all of them are in the Pacific Ocean and around Polynesia. The Maldives are a good example of atolls. From the air you can see how beautifully the atolls have formed. In the middle you can see a small piece of land, surrounded by a light blue lagoon, then the reef edge and then the sea with a deep blue color.
  4. Plate or platform: These are also called spot reefs. They occur on the continental shelf and in shallow parts of deep lagoons, on the hard, rocky parts of the seabed. The larger ones (reef banks) are created in comparable but deeper water. They often resemble atolls but do not have the horseshoe or ring shape.

The value of coral reefs for people and society

The economic value of coral reefs

The economic value of products and services that produce coral reefs is estimated by environmental economists to be over $ 350 billion annually (!). Coral reef ecosystems therefore provide numerous functions, services and products (especially) on the coast or island population.

Source of income and food

The coral reefs are an important food source for hundreds of millions of people. The coral provides 10% of the global fish catch. In Southeast Asia, this percentage of coastal catch is around 80%. The coral is also very important for diving tourism in (in particular) Bonaire. This generates several tens of millions of euros per year. There is also a lively trade in reef products, such as coral and shells. In some countries, the reefs are even the only source of income. So you can say that the coral reefs are actually an indispensable source of life, both for humans and animals.

Coral reefs as a "medicine cabinet"

In addition to being a food source, the reef is a source of innumerable substances that are suitable for the preparation of medicines. The potential of the coral for this is even estimated to be higher than that of the tropical rainforest! The fact that the coral still hides so many of these incredibly useful substances is mainly due to sponges and tunicates that fight the reef. Marine organisms such as sponges are stuck in one place and cannot flee during an attack. During this fight, many uses are made of all kinds of chemical agents that serve as a defense. These substances can be useful as a medicine against a wide variety of diseases. The substance didemnide, found in tunicates, is effective against meningitis and herpes. There are also substances with cancer-inhibiting and HIV-inhibiting properties. The scallop shell, for example, contains a substance that delays cancer in mice. The substance ATZ, a medicine used in the treatment of AIDS, is originally from a sponge.

Natural protection of the land

Coral reefs are not only an important source of fishing and substances for the preparation of medicines, they also function as a natural barrier against ocean waves. If this barrier were missing, the country / island in question would have to invest a huge amount of money in artificial dikes / coastal works.

The Threats

Coral reefs and rain forests are among the most vulnerable ecosystems in the world and are therefore threatened in their survival. Coral reefs around the world have been doing remarkably poorly in recent years. About 30% of the original coral has already been lost. If we keep using the reef like this, another 30% will be added in the coming years.
This decline has both natural and anthropogenic (= man-made) causes. An important question that scientists are trying to answer is the extent to which anthropogenic causes play a role. Man could change something about this. After all, preserving the coral ecosystems is very important.

Natural and anthropogenic threats

Various causes play a role in the threat. We make a distinction between natural and anthropogenic causes:
Natural causes:
  • Although coral looks strong, it is extremely fragile. A big storm can also damage the coral.
  • An increase or decrease in seawater level can also cause serious damage. If the seawater level drops, the top of a reef may be above the water. However, the coral cannot stand it when it is drained. Conversely, if the seawater level rises too much, the polyps may become too far below the surface of the water. The sunlight is then no longer strong enough for the photosynthesis of the zooxanthellae.
  • In addition, coral is very sensitive to fluctuations in the temperature of seawater. Natural differences in temperature from year to year can cause the coral to go less well. Moreover, fluctuations in sea temperature, storms and differences in sea level are exacerbated by the infamous El Ninõ periods. El Niño is responsible for climate change in the world's oceans. This phenomenon takes place every 2 years, with seawater getting warmer all over the world. The El Niño of 1998 has affected coral all over the world. Especially the coral in the Great Barrier Reef and the reefs of East Africa have suffered greatly. In East Africa even more than 90% of the reefs are affected.

Anthropogenic causes:
The human influence on the health of coral reefs is very large, namely:
  • Pollution of cities and agricultural land brings poisons and pollution into the sea via rivers. The seawater and the reefs can for example be polluted with pesticides from agriculture or waste from sewers. Because of the clearing of forests or the reclamation of land, soil particles often flush into the sea. They cloud the water there and place a 'film' about the coral. This chokes the coral reefs.
  • A sharp increase in the world's population causes overfishing and therefore the coral ecosystem is disrupted. Fish eat the larger algae that grow on the reef. They ensure that the reef stays 'clean' and that enough sunlight can reach the coral. If too much fish is caught, the reef runs the risk of being overgrown by larger algae.
  • Destructive fishing methods (with anchors, trawl nets and dynamite bombs, fishermen can cause enormous damage to coral). Also fish using cyanide is disastrous for the coral. Fishermen spray this poison between the crevices of the reefs - where many reef fish hide - so that the coral can be broken off as a result. The fish are thus stunned and can be fished out of the water. Unfortunately, cyanide can kill smaller organisms at the same time and the coral dies with repeated exposure.
  • Irresponsible diving tourism. Divers damage the coral (accidentally or intentionally) and boats break the coral when, for example, they drop their anchor on it.
  • The thorn crown starfish is basically a natural enemy of the coral polyps. This starfish eats live coral animals and in that way destroys large parts of the reef. By disturbances in the ecosystem (for example, by catching the trumpet snail on a large scale, the natural enemy of the crown of thorns), this starfish gains the upper hand and thus causes a deforestation in certain coral areas.
  • The greenhouse effect causes a climate change that in turn causes a higher water temperature. As a result, algae in coral polyps produce toxic substances and this results in "coral bleaching" (bleaching of the coral).

Coral bleaching

The worldwide and most important destruction factor is coral bleaching. More than half of the dead coral (the 30% mentioned above) was caused by this. Coral bleaching is the process of coral dying, mainly due to temperature increases in the ocean water. (This is again the result of the greenhouse effect that in turn is caused by man with his industry and car traffic). If the average temperature on earth rises by 1 degrees Celsius, this can also have destructive consequences for coral. The high temperature drives the alga from the polyp, the alga also causes the breakdown of CO2. If this situation persists too long, the polyp becomes malnourished and eventually dies.

Global commitment to coral protection

Fortunately, measures are being taken to preserve the reefs. There are now around 660 protected areas with coral reefs. These protected areas become MPAs (Marine Protected Areas). Here the recovery percentage is higher than that of the coral in the unprotected areas. Because in these areas fishing is only sustainable or not permitted at all, the coral is not under stress and remains healthy. And healthy coral is much more resistant to climate change. Fish also find shelter in MPAs. Young fish can reach sexual maturity here before being caught. This increases the fish stocks in MPAs and that is also good for the coral. In collaboration with Artis there are biologists who have collected reproduction cells from coral animals and have them propagated in aquariums. They then set out in the reefs. However, it is logical that the environmental conditions in the areas of this expanded coral must be optimal for this coral to grow.
Moreover, there are various organizations active worldwide, the so-called baby coral weather for coral conservation. These are for example:
  • The International Coral Reef Action Network (ICRAN). This is a collaboration between governments, international organizations, local underwater park managers and scientists.
  • The American (non-profit) organization Coral Reef Alliance (CORAL).
  • The World Wildlife Fund. This organization works in more than 60 projects around the world on coral protection, including by means of. education and information.

Nature conservation organizations such as the WWF want to raise money for new and existing marine parks (the MPAs) and support good policies for these parks. The aim is, among other things, to make the local population aware that they must work with coral-friendly fishing techniques. Travel organizations and the world of diving must also be made aware of the consequences of water and coastal tourism and a sustainable policy must also be established in this area (the presence of tourists can also prevent dynamite and cyanide fishing). For example, the waste must be properly processed (eg no sewers from hotels that end up at sea) and divers and snorkelers must adhere to the 10 golden rules for underwater.

The 10 golden rules are:

  • Don't touch the coral.
  • Learn about coral.
  • Inform others about coral.
  • Leave all corals where they are.
  • Help keep the reef clean.
  • Avoid swirling sand.
  • Check your buoyancy and swimming technique.
  • Pay attention to loose equipment.
  • Don't buy coral products. It is forbidden by law to bring coral to the Netherlands.
  • Do not throw out anchors in coral areas.

Moreover, governments and politicians must of course get to work. Many countries have policies for protecting rainforest areas. Such a policy should actually be introduced for coral reefs. On your own you can do a lot to protect the reef, although it is not much. But, all the little bits help. By using water and energy efficiently, you limit the pollution and emissions of harmful greenhouse gases in the atmosphere. If you ever go diving yourself, you can be extra careful so that you do not damage the coral. Certainly never break pieces as a souvenir for your home. Coral is the most beautiful under water. And, even if you don't go diving yourself, you can buy pieces of coral in many places in souvenir shops. You can help the reef by not buying these souvenirs. The measures that have been taken so far, compared to the problems and the size of the coral, may seem like drops on a glowing plate, but could possibly develop into a snowball effect in the future.

Video: Coral Reefs 101. National Geographic (February 2020).

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