Fakarava Atoll is a true underwater paradise that stuns with its sheer diversity of marine life and particularly high populations of manta rays, grey reef sharks and sea turtles. It is one of the only places where big marine animals are found in such dense numbers. Its diversity and richness of wildlife is so unique that Fakarava is now a UNESCO Biosphere Reserve. But the survival chances of these colourful coral reefs are looking very bleak. They may be the biggest living structures on Earth, but they are dying 4 times more quickly than tropical rainforests. Three quarters of all coral reefs are under threat, and all of them could disappear within the next 50 – 100 years. Prof. Serge Plane from the French research center on Moorea “Criobe” is trying to piece together the interrelationships of coral reef dwellers in order to better understand why they are so sensitive to environmental change, and how they could be protected in the future. This ecosystem is so fragile and sensitive to change that once one link in a coral reef’s chain of life has been removed, it can have dire consequences for the entire reef. Working out how this chain works, and what makes this ecosystem so unique is instrumental to the quest of preserving it. Prof. Serge Plane has specialised in a very particular aspect of relationships amongst reef organisms: communication. He wants to find out how the animals communicate and how this serves their quest to survive. He has already accumulated a wealth of underwater recordings of sounds in a coral reef. But his normal approach requires him to place hydrophones in strategic static positions across the reef. It is an inefficient, limiting and labour-intensive approach. Frederic is able to increase the efficiency and success of Prof Serge Plane by approaching animals directly and respond to their behaviours. The scientist’s current recordings do not follow animals during different behaviours, so being able to selectively record behaviours could be an eye-opener for this type of research. As a noiseless freediver, Frederic is able to trigger certain behaviours and associated sounds by being in the water with the animals without influencing them with cumbersome diving equipment. For example, he is able to record alarm sounds by giving sudden chase to specific fish. So far, it has not been possible for Prof Serge Plane to record behaviour as specific as this because conventional diving gear makes too much noise. The team dives at every time of day and night at a specific spot on the atoll to observe different species during their own activity cycles, and to see how life on the atoll transforms at different times. But again, Christian and Frederic have to remain cautious and plan their expedition meticulously. Again, they make the most of local knowledge to prepare themselves for the task in hand. Polynesian mythology is full of marine life – it plays a very important role in local culture. Manta rays, for example, stand for freedom, while sea turtles personify wisdom and long life. Frederic and Christian meet Maitata, a pearldiver from this island. Few people could tell the team more about the local wildlife from the point of view of Polynesian peoples. Once again, the stories this experienced local man has to offer help Christian and Frederic to build a better and more accurate picture of what lurks beneath the waves in this area. Over their time working, Frederic and Christian repeat their dives on the atoll at different times of day and night. The night-dives can be a bizarre experience with an edge of the otherworldly about it. And the sharply reduced visibility does make these excursions particularly dangerous for Frederic – it would be all too easy to loose orientation and as a result risk his life. The combination of their observations of the reef at different times of day and night and the recordings of reef sounds prove invaluable to Prof. Serge Planes research. And as he builds up his database of recordings, the interrelationships of coral reef organisms becomes clearer. Frederic and Christian’ observations have allowed fascinating insights into the complex web of connections of this finely tuned ecosystem. Ultimately, it may be this kind of research that could help to preserve the world’s coral reefs well into the future.