Tara Pacific focuses on corals : from polyp to reef

© L. Thiault/Tara Expeditions

Tara’s new expedition in the Pacific Ocean is currently being prepared. While the crew gets ready to set sail for the coral reefs, scientists are finalizing the research protocol. Before their departure, let’s summarize what is known about coral This means diving into the complex world of polyps and limestone that form the huge colonies constituting reefs.

Investigating corals by focusing on a single polyp is as limiting as studying an anthill by observing only one ant. Let’s step back to understand how these tiny animals manage to build mineral monuments, villages and underwater cathedrals. What is the relationship between a polyp – this minuscule relative of jellyfish – and the Great Barrier Reef, visible from space? In tropical scleractinian corals, each polyp creates an external skeleton, a calcareous support that protects the animal and remains intact after its death. During its lifetime, a polyp keeps multiplying, budding new polyps that will also produce calcareous skeletons and increase the colony’s population.

In all tropical corals – multi-branched, brain or encrusting – only the external surface is covered with polyps and is therefore alive. Underneath this thin layer of living beings lies the mineral accumulation of millions of skeletons. Like tree rings, large corals enable scientists to study the geological past, to retrace the life of the colony over centuries, or more. Some colonies are estimated to be 4,000 years old (n.b. the age of the colony, not of the polyp).

©L. Thiault/Tara Expeditions

As the colony grows, the limestone mass increases, fragments eventually collapse under their own weight or are torn off by currents and waves. Carried a few meters away, they can attach themselves again on the sea floor and form a new colony, a clone of the original one. The different colonies, whose crevices will gradually be filled in by sediments and other residues of organisms (shells of mollusks, sea urchins, etc.), finally form a coral reef, sometimes stretching across kilometers.

To disperse even further away, corals are also able to reproduce sexually: a sperm fertilizes an egg which becomes an egg-cell. For 3/4 of the hermaphroditic species, each polyp releases both male and female gametes. This sexual mode of reproduction has long gone unnoticed because it lasts a particularly short time: in many species, the massive discharge of gametes only takes place one night a year!

Within a few hours, the male, female and hermaphroditic polyps of dozens of different species simultaneously release their gametes into the ocean, forming an underwater “snow” that rises to the surface. Each fertilized egg then gives birth to a larva, the planula, which is carried by ocean currents before finding a sea floor or support (often referred to as substrate) to settle on. A first polyp appears, creates a calcareous skeleton, and multiplies: a new colony is born!

Yann Chavance


Other articles :

- Animal, mineral or vegetal ? Corals at a crossroads.

- Departure of the Tara PACIFIC 2016-2018 expedition

- ELSA GUILLAUME: Dreaming of coral