Publié le August 27, 2020

Coral, the planet’s largest builder, is a fragile and endangered living being


At Nausicaá, the various tanks and aquariums are home to more than 70 species of coral.

This unusual animal is made up of tiny individuals that together form the reef and live in symbiosis with microscopic algae.

What is coral?

Corals are very old animals. Their appearance dates back to the Precambrian, 900 million years ago, just like their cousins the jellyfish.

Together with gorgonians and anemones, they form the Cnidaria group. Cnidarians are characterized by a primitive body with a single orifice and stinging tentacles. In fact, the word cnidaria comes from the Greek word knidê, which means nettle.

Despite its plant-like appearance, coral is actually an animal. Each individual of coral is called a polyp. Coral is a colony of polyps, which lives fixed on the ocean floor.

Extraordinary diversity!

Different types of coral

It is estimated that there are currently 800 species of coral and about 200 genera in the world.

They are mostly found in warm waters close to the surface, but there are also cold-water or deep-water corals. They thrive along continental margins at depths ranging from a few hundred metres to more than a thousand metres. They even exist in the depths of the Mediterranean Sea.

All corals synthesize limestone throughout their lives.

They are very diverse and can be:

  • flexible, with a horny skeleton (Gorgonians and Antipathaires)
  • soft without any skeleton (Alcyonaires and Dendronephthya). In soft corals, the limestone is disseminated in the tissues.
  • hard with calcareous skeleton (Acropora, Porites, Euphyllia, etc.)

Hard corals produce an outer calcareous skeleton and then fuse firmly together to build up coral reefs. They are also called reef-building corals. They have the ability to build the reef as the colonies expand and multiply. This is the case with Scleractinians.

Biology and behaviour

In the dark, coral polyps deploy their tentacles to capture micro-organisms. However, to obtain the nutritional supplements required for their growth, hard corals (Scleractiniae) live in symbiosis with microscopic algae, zooxanthellae, which live in their tissues.

Zooxanthellae encourage the precipitation of calcium dissolved in water when they absorb carbon dioxide from the water during the photosynthesis process. This helps the coral build its skeleton.

This amazing association with zooxanthellae is essential to their survival. This is the reason why corals settle in clear, luminous waters: they expose their algae to the sun so that they can grow!

Fluorescence in corals

Most corals are fluorescent. However, it is actually the animals that have these fluorescent properties, not the coral skeleton. They have molecules called “GFP” (Green fluorescent protein) that can absorb light at a given wavelength (invisible) and radiate it at another wavelength (visible).

To date, very little is understood about the exact role of coral fluorescence.

However, among the most convincing hypotheses, it would seem that in poorly lit areas, fluorescence changes blue radiation, the only radiation that remains at depth, into useful radiation for their symbiotic algae that will carry out photosynthesis.

Therefore, fluorescence would allow corals to thrive in less favourable environments where light is scarcer. Other hypotheses have been suggested, such as a role of photo-protection and/or intensification of certain light wavelengths or, more recently, protection of cells against free radicals.

The coral reef

All corals and the creatures that live near it form the coral reef ecosystem.

“The coral reef is a biological construct of warm marine waters, formed by layers of skeletons secreted by organisms living in colonies and dominated by corals. It is both a cemetery and a nursery”.

Source: Encyclopédie Larousse de la Nature. La planète de la vie. Ed. Larousse 1993, P 73

An abundant ecosystem

It is one of the most biologically diverse ecosystems on our planet and also one of the oldest. Coral reefs exist in more than 100 countries, in tropical seas, covering an area the size of Italy (about 300,000 km²). The vast majority of coral formations develop between the surface and a depth of about thirty metres.

The conditions required for these formations are:

– a temperature of at least 20°C

– constant salt content

– sufficient luminous intensity

The growth of a coral reef can be interrupted by natural variations in these three factors.

Coral reefs cover less than 0.2% of the ocean surface, but are home to more than a quarter of all global marine biodiversity. A whole host of species at the base of marine food chains reproduce here and find shelter and food.

A coral reef can accumulate hundreds of different species. Nearly 5,000 species of fish and molluscs have been identified in them, 400 species of coral, and more than 2,500 different reefs, more than a thousand of which contribute to reef building.

The Coral Triangle, located between the Solomon Islands, Indonesia and the Philippines, includes the Indonesian islands of Raja Ampat where the most abundant marine biodiversity on the planet is believed to be located. There are more than 1,000 species of fish and 537 species of coral!

The coral reef, a world heritage for mankind

Grande barrière de corail, vue aérienneSome coral reefs are included on the World Heritage list drawn up by UNESCO.

The Great Barrier Reef is the largest coral reef in the world.

It is located to the north-east of the Australian coast, with 400 species of coral, 1,500 species of fish and 4,000 species of molluscs. It offers a magnificent display of extraordinary variety and is of immense scientific interest.

It is also home to endangered species such as the dugong and the large green turtle.

France has about 10% of the world’s coral reefs in its overseas territories. The lagoons of New Caledonia, which cover an area larger than the Hauts-de-France region, are some of the French coral reefs classified as World Heritage sites.

What is the purpose of a coral reef?

Coral reefs and their associated ecosystems are the undersea equivalent of tropical rainforests on earth in terms of biodiversity. Their importance in global ecological functioning, as well as their socio-economic role (fishing and tourism), and their contribution to the development of high medical technologies are crucial. Almost 500 million people around the world rely on coral reefs for their livelihood!

Coral reefs are extremely valuable because:

  • they form protective barriers against waves around coasts and thereby limit marine erosion by absorbing 70 to 90% of the force of the waves
  • they form part of the marine ecosystems that produce the most living biomass
  • they are a major source of food for many coastal populations
  • they support more species per unit area than any other marine habitat. Scientists estimate that there are several million species that are still unknown living on or near coral reefs.
  • this reservoir of biodiversity could provide 21st century medicine with essential molecules: many reef organisms produce biochemically potent substances, the effects of which are being studied against arthritis, cancer and other diseases.
  • through their beauty alone, the reefs contribute to the development of local economies by attracting tourists and diving enthusiasts

However, the importance of reefs extends far beyond this. It acts as a nursery for offshore species. Corals fix carbon and are involved in the CO² cycle, which is the main greenhouse gas. Finally, reefs contribute to our leisure activities, our health, our culture, etc.

Corals also serve as markers of the past. By studying the coral skeletons that form the foundations of reefs, environmental factors can be studied over the ages.

Coral reefs provide between US$1,000 and US$6,000 per hectare per year in “services” to humanity, for example, by contributing to the economy and coastal protection. They also play a cultural, ecological and tourist role.

Coral reefs are among the most complex and abundant ecosystems, but they are also the most economically valuable and the most exploited.

Endangered corals

Human actions can damage coral reefs: intensive fishing, excessive sampling, spillage of polluting substances, backfilling practices, increasing demographic pressure, etc.

Since the Second World War, the world has lost 19% of its coral reefs, especially in densely urbanized coastal areas.

Corals are now suffering from global warming. This can be seen by:


Coral bleaching

Corals turn white when their symbiotic algae, zooxanthellae, are expelled from the polyps or lose their pigments. This loss of zooxanthellae can cause the coral to die. Bleaching can be caused by a variety of stresses including pollution and rising ocean temperatures. Several massive bleaching episodes affecting large areas have been observed over the last few decades.

  • Acidification of the oceans

The rise of CO2 in the ocean increases its acidity and probably threatens the long-term survival of many marine species and more specifically corals, because it affects the building of their calcareous skeleton.

Progress in terms of protection

Reefs can be preserved by creating reserves and protected areas in which human activities are monitored and controlled. For example, in the Pacific, two huge marine protected areas (MPAs) dedicated to coral reefs were established by the governments of the United States and Kiribati in 2006.

Similarly, after a bleaching episode that devastated coral reefs in the Indian Ocean, Seychelles, Chagos and Maldives in 1998, there is now a marked improvement in the condition of these reefs linked to coral regeneration.

Some coral reefs, such as those in New Caledonia, have even been classified as World Heritage Sites to ensure that they are protected.

Other international initiatives have emerged to raise funds and develop awareness campaigns to protect corals, such as the “Coral Triangle Initiative” in South-east Asia and the International Coral Reef Initiative (ICRI) created in 1994.

What are the solutions to preserve coral reefs?

  • Limit global warming: by reducing the current level of greenhouse gases and developing sustainable energy production
  • Optimise the resilience of coral reefs: by monitoring and controlling human activities that damage the state of the reefs to allow corals to regenerate, by fighting pollution or by implementing a management method for coastal areas that does not impact the reef (coastal development)
  • Increase the number of marine protected areas
  • Protect isolated coral reefs: some coral reefs are located far from any human activity; by protecting them, they could serve as a reservoir of biodiversity
  • Restore damaged reefs The Reefscapers association, backed by Nausicaá, uses the cutting technique in the natural environment to repopulate reefs in the Maldives after bleaching episodes related to climate change.


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