An atoll with a unique geomorphology

In French Polynesia's Tuamotu archipelago, the atoll of L'espoir is an exception: unlike the vast majority of its counterparts, it has no passes, which considerably reduces the mixing of oceanic and lagoon waters. At high tide, only a few shallow channels - the hoas - allow seawater to feed the lagoon. 

This unique geomorphology has an impact not only on the proliferation of species - larvae remain "imprisoned" within the lagoon - but also on temperature. Due tothe lack of mixing, the water temperature is higher inside the lagoon," explains Laetitia Hédouin, research manager at CNRS-CRIOBE. It also becomes much more variable, with variations of 3 to 4 degrees in a single day, compared with around 0.5 degrees outside. That's a huge difference!

In the heart of the lagoon, a biological safe

These "extreme" thermal conditions are normally incompatible with the presence of corals. On this atoll, however, the rules of life seem very different: despite the high temperatures, the lagoon has become a biological safe in which many coral species seem to have lived for millennia. This is all the more surprising given that the clams - also dependent on their symbiosis with zooxanthellae - did not survive the global warming that hit the region in 2016. 

If the atoll's corals are so resistant to the lagoon's temperatures, it seems that this is above all the result of a long evolutionary process. The most plausible hypothesis is that being subjected to such variability has enabled corals to develop resistance mechanisms," explains Laetitia Hédouin. Then, it's a whole process of natural selection that ensures that the best win.

Hope for coral reefs

On site, Laetitia and her team continue to study the functioning of these unusual corals. A new mission, planned for the very near future, will provide further answers, testing the ability of these corals to survive in a different environment. During this mission, scientists will attempt to carry out the first large-scale assisted colonization of coral by moving several samples to another archipelago.

"In this context of crisis for coral reefs, finding thermotolerant populations is extremely important," says Laetitia Hédouin. Lost in the middle of the Pacific, this small Tuamotu atoll represents a real source of hope: within the confines of this lagoon, which is almost identical to a thousand others, live perhaps the corals of the future, and with them, the prospect of one day ensuring the resilience of coral reefs.