Dominique A. Cowart ^1* Thomas Chevrier ^1,2 Anne-Elise Nieblas ¹ Jérémie Chanut ¹ Serge Bernard ³ Sylvain Bonhommeau ²

• ¹ Company for Open Ocean Observations and Logging (COOOL), Saint-Leu, La Réunion, France
• ² Ifremer Délégation océan Indien, Le Port, La Réunion, France
• ³ UMR5506 Laboratoire d'Informatique, de Robotique et de Microélectronique de Montpellier (LIRMM), Montpellier, Languedoc-Roussillon, France

The back-reef depressions, or lagoons, of Reunion Island (Western Indian Ocean) host a high abundance of organisms living amongst the coral reefs, and as such, are critical sites for artisanal fishing, tourism and shoreline stability for the island. Over the years, increasing degradation of Reunionese reefs has been observed as a result of overexploitation, beach erosion and eutrophication. Efforts to mitigate the impact of these pressures on aquatic organisms typically include biodiversity surveys, and are primarily performed through visual censuses that can be logistically complex and may unintentionally overlook organisms. Surveys integrating the collection of environmental DNA (eDNA) have emerged as a rapid method for biodiversity surveys and to circumvent some limitations of visual surveys. In the present study, we describe the results of an exploratory environmental DNA survey performed to characterize metazoan communities inhabiting Reunionese lagoons. As eDNA surveys first require deliberate study design and optimization for each new context, our study also sought to establish a workflow to facilitate future work to process local water samples. The eDNA dataset was generated from seawater samples retrieved from five sites, which were processed and metabarcoded using regions of the 12S, 18S and COI genes. The eDNA survey detected high species richness that varied by site, and in a single collection period, recovered the presence of 60 teleost families and several invertebrate organisms, including members of the coral faunal community that are less studied in Reunion. Additionally, distinct biological communities were observed at each site, as well as within a single lagoon, suggesting these differences are due to site-specific factors (e.g., environmental variables, geographic distance, etc.). These results demonstrate the applicability of eDNA surveys for biodiversity assessment, to assist the overall appraisal of the state of Reunionese lagoons.