What is a coral reef?

But do we know that at the origin of this idyllic landscape, there is a small animal, a polyp that patiently multiplies and builds its limestone skeleton to form colonies of individuals that will create a reef? 

This little animal is the coral, the builder of coral reefs.

The coral reef is therefore a natural structure, and in the case of Australia's Great Barrier Reef it can even be seen from space.

Not bad for a little polyp! 

Reefs are home to some of the richest marine life on the planet. Coral reefs, which represent just 0.2% of the ocean's surface, account for 30% of known marine biodiversity. 

They boast a record 1,000 species of coral, 25,000 species of molluscs and 8,000 species of fish and other vertebrates, as well as tens of thousands of other animals such as crustaceans, starfish, sea urchins and marine worms, not to mention rays and sharks.

To develop, coral reproduces by budding, i.e. by forming an outgrowth that gives rise to a new polyp, a genetic clone of the first. Little by little, the reef grows, made up of thousands or even millions of individuals. The reef is therefore made up of several colonies of polyps.

Once a year, sexual reproduction takes place through the release of male and female gametes. The planulas are carried away by the current and those that settle on the seabed form a polyp and then a genetically distinct reef.

Coral reefs can take thousands or even millions of years to form and grow. The Great Barrier Reef has been developing for 20,000 years.

Fringing reef

The fringing reef is located along the coast, close to the beach. This area of the reef is close to the surface of the water and is therefore easily accessible to swimmers equipped with masks and flippers. It provides natural protection against coastal erosion by absorbing the force of the waves.

Barrier reef

Located further from the coast, the barrier reef lies between the open sea and the lagoon, which it protects from the force of the waves.

Drop reef

It is found along a plunging reef or barrier reef and has a rapid, vertical descent towards the outside of the reef.

Reef flat

The reef flat is the generally shallow part of the barrier reef that is exposed at low tide and covered at high tide. The coral reef is home to a large population of fish and invertebrates, as well as being a breeding and nursery ground for certain marine species.

Lagoon

The lagoon is a stretch of water between the barrier reef and the coast. Shallow, the lagoon is home to a rich biodiversity of coral, fish, seagrass and sometimes mangroves.

Like the shelf, it is a breeding and nursery ground for marine species, as well as a fishing and tourist destination.

Atoll

An atoll is a ring of coral formation that develops around islands.       

Pass

The pass is an opening in the coral reef creating a passage between the lagoon and the ocean. This opening allows the movement of fish, animals and zooplankton between the two zones, as well as the input of nutrients and sediments. These movements also maintain the quality of the water in the lagoon.

• The coral reef is a natural barrier against waves. 70 to 90% of the force of waves is absorbed by reef barriers.
• Reef-building corals become carbon sinks as they grow, trapping carbon in their calcareous skeletons.
• Thanks to the wealth of marine species that live there, the coral ecosystem represents an essential food and economic resource for local populations.
• Finally, the attraction of these tropical landscapes is a source of income for the men and women who benefit from the development of tourism and leisure activities.

Caribbean corals are suffering from declining populations of sea urchins and herbivorous fish, the main targets of fishermen. Fewer algal grazers allow algae to gain ground, covering the coral and blocking the light that zooxanthellae need for photosynthesis. The reduction in the number of large predators has encouraged an increase in these populations.

Other factors are also having an impact on the health of coral reefs: pollution, urbanisation, mass tourism, over-fishing and over-exploitation of resources. Coral diseases and the proliferation of corallivorous species such as the purple starfish Acanthaster planci also add to these threats.

• Associations carry out actions in the field and restore coral reefs by using cuttings on artificial reefs that develop and attract marine life once again. These associations also work to raise awareness among local populations.
• Researchers are studying the reaction of corals to thermal stress and identifying certain species that are capable of resilience.
• Jérémy Gobé, a visual artist, is interested in coral and has developed a project to restore coral by combining art, science and industry. His Coral Resilience project is moving towards a test phase in a natural environment, in Colombia, by installing modular structures ready to receive coral cuttings.
• The World Coral Conservatory's mission is to become a bank of life, preserving the genetic heritage of coral species. Public aquariums, including Nausicaá, host samples of coral that they will grow and take cuttings from in order to preserve endangered species in the natural environment.  

Photos : Dominique Mallevoy, Coco Tamlyn - Coral Guardian, iStock, Digital Vision