Tropical reefs: Fish diversity revised upwards, thanks to an analysis of environmental DNA

The study of environmental DNA (eDNA) offers us – again – fascinating discoveries! After detecting more than 650 different species in the waters of the Amazon in Peru – including the extremely rare pink dolphin, or even 85 species of insects in air samples in Sweden, this scientific technique – developed over the last ten years. – this time was used to analyze seawater samples.

Living organisms constantly lose cells, leaving genetic traces in their path: hair, skin, mucus, saliva and feces, which contain their DNA. By comparing the genetic sequences of samples taken from the environment with those in databases, researchers are able to identify the species present at a given location. An interesting process for detecting the presence of discrete or few animals, but also for differentiating species of similar appearance.

The “Coral Triangle” and its exceptional biodiversity

Therefore, an international team led by researchers from the University of Montpellier and the Swiss Federal Institute of Technology in Zurich (ETHZ) chose this method to study the appearance of reef fish. Between 2017 and 2019, with the support of the Monegasque government, the scientists collected about 226 water samples from 100 stations in 5 tropical marine regions. Through eDNA analysis, they identified a total of 2,650 taxa, that is, a diversity greater than 16% of the estimated one based on visual observations (more than 2,000 censuses conducted during the dives).

Among his findings published in the scientific journal Proceedings of the Royal Society (April 2022), the authors remarkably reveal that the diversity of fish is up to 5 times greater in the “Coral Triangle” – located between Borneo, Papua New Guinea and the Philippines – than in the Caribbean, for example. This observation could be explained by the strong activity of this region at the tectonic level, leading to a great diversity of habitats, as well as a more stable surface temperature than the Caribbean during glacial periods, having offered the possibility of an exceptional diversity to stay.

Much faster detection of marine species

Thanks to the eDNA method, we can detect many species and families of fish much faster than just observationsLoïc Pellissier, professor of ecosystems and landscape evolution at ETHZ and co-author of the study, was quoted as saying in a press release. the reefs to reach the high seas or the depths – like the shy black-tipped shark, but also “cryptobenthic” fish, hidden in caves and crevices formed by reefs: so many animals that are necessarily less easy to see for divers.

The authors of the study point out, however, that currently certain species or families are less well referenced in genetic databases. This is especially the case with Wrasses and Blennies. Hence the importance not only of continuing the DNA sequencing of the reference specimens of these groups, but also of continuing to make direct observations while submerging – the latter remains essential to study other ecological criteria such as fish size. , for example-.

Reefs, essential but threatened

25% of all marine life is concentrated in 0.2% of the ocean surface represented by coral reefsDenis Allemand, director of the Monaco Science Center, emphasizes the preface to Martin Colognoli’s photographic book “Coral” (ed. Hemeria, 2022). Coastal protection against erosion, importance for fishing, income linked to tourism … About 500 million people worldwide depend on these beautiful ecosystems.Unfortunately, climate change and human activities are causing coral reefs to disappear at an alarming rate.

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