Animal, mineral or vegetal? Corals at a crossroads

© Sacha Bollet

After years of decrypting the mysteries of plankton, tara’s new expedition is dedicated to studying corals, organisms that exhibit equally fascinating diversity, ecological role and ways of life. First dive into the astonishing world of corals.

Animal, mineral or vegetal? For a long time, naturalists were uncertain: are corals immobile animals, marine plants or living rocks? The first microscopes finally gave the answer when they revealed thousands of tiny animals living in huge colonies. These colonies, whose forms and colors show an incredible diversity depending on the species, are composed of thousands of units called “polyps”. Characterized by a certain anatomical simplicity, these polyps appear in the form of a bag with a mouth surrounded by a ring of stinging tentacles. Corals belong to the same class as jellyfish and anemones.

Polyps feed by directly absorbing organic matter dissolved in water and use their tentacles to capture small planktonic organisms. Paralyzed by the stinging cells, these organisms are drawn into the oral cavity of the polyp to be digested. However, most of the polyps’ energy comes from a surprising association: polyps shelter within their cells unicellular algae called zooxanthellae. Like all plants, these algae use solar energy via photosynthesis to produce nutrients, most of which feed the polyp. This type of association is referred to as “symbiosis”.


Pocillo meandrina ©Lauric Thiault

This perfect symbiosis is mainly present in tropical corals that develop close to the surface in warm waters and form the famous coral reefs. However, the world of corals is not limited to these organisms which, by producing calcareous skeletons, form reefs and atolls. The term “coral” also commonly refers to Mediterranean gorgonians and soft corals, such as the Mediterranean red coral, but also to the colonies of polyps forming deep sea reefs, several thousand meters under the surface.

The world of corals is therefore particularly vast and diverse, with hundreds of species in the world’s oceans. In the Pacific, Tara will focus on Scleractinaria, tropical hard corals. Not only do these reef builders have extraordinary ways of life and modes of reproduction – to be detailed in a future article – but above all, they are of considerable ecological interest: while coral reefs cover less than 0.2% of the oceans’ surface, they host a quarter of the marine biodiversity.

Yann Chavance




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