July 27, 2021

Dives to deep sea corals

A new species of coral discovered, unprecedented scientific observations at 172 meters deep: notable results, but the collaboration of Under the Pole divers with researchers from the Insular Research Center and Environmental Observatory (Criobe / CNRS) above all looks like a nice meeting. “The complementarity between us was really obvious”, summarizes Laetitia Hédouin, researcher at Criobe, Moorea, Polynesia.

She and her colleagues, who work on mesophotic corals – that is, living between 30 m and 170 m below the surface – wanted to see below 50 m, but the legislation did not allow them. It is therefore with enthusiasm that Laetitia Hédouin responded to an ad from Ghislain and Emmanuelle Bardout, the Under the Pole team who was looking for a scientific program to integrate aboard their sailboat. Their joint adventure lasted thirteen months, from summer 2018 to summer 2019, led them to the five archipelagos of Polynesia, accumulating, in each of the 22 selected sites, more than 1,000 systematic dives at different depths, for report more than 6,000 samples of mesophotic corals.

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One of the questions that drives Criobe researchers is that of the adaptation of corals to the strong reduction in light, whereas this is essential for zooxanthella, the photosynthetic unicellular algae that lodges in their tissues and provides them with his energy. They were able to observe to what extent these modify the morphology of their skeleton to achieve this. “The pocillopora, for example, spreads its arms in a way to avoid them the slightest shadow, reports Laetitia Hédouin. We knew that mesophotic corals could take the form of flat plateaus, like large roses, but we did not know that they could extend over 2 km long and 20 to 30 m wide! And that there were 172 m. »

Is the community of fish that live near the surface around coral reefs threatened with bleaching ready to migrate deeper? “As soon as there is a habitat, even atypical, life develops, she replies. It’s good news. In the 6 to 20 meter band, the landscapes are fairly homogeneous, while the further down you go, the more each island is unique. That’s why we have to protect them all. “

A collaboration over time

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