Meeting the Corals

Updated: Aug 25, 2021

We are interested in corals. What about you?

Venus and her friends in the coral reef of Tikehau, Tuamotus.


Corals are superstructures with a thousand shapes and colors built by a multitude of small animals called polyps that synthesize a calcium carbonate skeleton. They constitute reef ecosystems that :

  • are home to thousands of marine species

  • participate in the storage of carbon

  • protect the coastline by breaking ocean waves

They are able to feed themselves by capturing plankton but most of their food comes from photosynthesis. Really? But polyps are animals and only plants are capable of photosynthesis, right? Quite simply, they work as a team. They live in symbiosis with microalgae that take care of photosynthesis in exchange for shelter. Pink, purple, blue, orange, yellow... these microalgae are responsible for the color of corals.

One image worth more than a thousand words, and more even when talking about polyps!


The reproduction of corals is also worthy of interest. They reproduce asexually by cuttings but not only. Their reproduction is also sexual with occasional release of male and female gametes into the environment. Fertilization is then external or internal producing a ciliated larva called planula. Attracted by the light, these larvae will rise to the surface and join the plankton. This stage of free life will allow the usually fixed corals to conquer new horizons and to ensure the dissemination of the species. After a journey of a few days, the plankton settle on a hard substrate and become colonies of polyps which begin the construction of the reef which grows this time thanks to asexual reproduction. The growth of the colony will depend on the morphology, the environmental conditions and the constructor species. For example, a branching coral Acropora sp. will grow about 10-15 cm per year while a massive coral such as Porites sp. will grow 1-2 cm per year. Fascinating, right?

Diferent types of coal and colours, in the South Pass of Fakarava.


Corals are real markers of changing environmental conditions. They are very sensitive to multiple parameters such as water temperature, chemistry, turbidity. The increase in temperature leads to an increase in the solubility of CO2 in the oceans, contributing to its acidification. This phenomenon makes it more difficult for corals to synthesize their calcium carbonate skeleton. Also with the increase in temperature but also with an excess of zinc in the water, such as that contained in sunscreens, symbiotic microalgae begin to synthesize toxic compounds for the polyps. The symbiosis is over and the polyps seek to get rid of these troublesome invaders. This is the phenomenon of whitening of the corals. But of course, during this expulsion, the corals lose their photosynthetic faculty which represents a good part of their food. So, if the conditions are rebalanced, they will try to re-establish the symbiosis with the microalgae. Conversely, if conditions persist, the corals will eventually die out leaving behind a vast graveyard of carbonate skeletons on which the algae will begin to grow. This is obviously not what we want for the future of our oceans. Let's preserve this beauty so that future generations can have the chance to contemplate it.

It is sad to see the whitening of coral, here in Bora Bora.


There are many ways to reduce our impact on coral reefs. Reducing ocean pollution at the source and controlling the discharge of waste into the ocean, raising public awareness of the issues and stresses on these cathedrals of the sea (palm strikes, plastic waste, zinc sunscreens, etc...) are a way to protect these important ecosystems.

Some entities, at the same time, are setting up structures to do gardening and replanting of corals.

ArcticStern went to meet one of these organizations, Tamarii Pointe des Pêcheurs, in order to learn more about them, to understand the complexity of these ecosystems and to lend a hand in the gardening of corals.

Coral Gardening, Tahiti


Let's preserve this beauty so that future generations can have the chance to contemplate it.


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