First eDNA-based monitoring of marine vertebrates in the urban marine ecosystem of Abu Dhabi: Mangrove and seagrass beds as biodiversity reservoirs?

The Arabian Gulf is a semi-enclosed sea with shallow waters. Its marine fauna must cope with extreme and recurring environmental conditions (high summer temperatures, hypersalinity, hypoxia). In addition, local human activities are very significant and have a marked impact on coastal areas, particularly in urban marine ecosystems (UME). Taken together, the Arabian Gulf's low species richness, its exposure to multiple human-induced stressors, and the proximity of many taxa to their environmental tolerance limits create a particularly concerning situation. Monitoring marine wildlife must be a priority in this context, particularly with respect to marine vertebrates, difficult to study using integrative approaches. The present study aimed to generate relevant data on marine vertebrate biodiversity in the UME of Abu Dhabi, and to provide evidence-based information to inform conservation policy. In addition, investigating marine biodiversity in the world's warmest sea in summer, the Arabian Gulf, will provide us with information of general interest in terms of the adaptation and evolution of marine fauna in the current period of global climate change. We conducted the first environmental DNA (eDNA)-based survey of marine vertebrate diversity in Abu Dhabi's UME. Using a standardized process of water sampling, eDNA metabarcoding using a  ~ 97 bp fragment of the mitochondrial 12S rRNA gene as barcode, and taxonomic assignment, we studied vertebrate biodiversity at four urban sites, including a marina, a newly developed area, a seagrass bed, and a protected urban mangrove. We identified 83 distinct taxa, including five birds, two elasmobranchs, one marine mammal, and 75 teleosts. The specific taxonomic richness did not vary significantly between sites. In contrast, phylogenetic diversity metrics revealed lower diversity than expected for the newly developed area, while the mangrove and, to a lesser extent, the seagrass bed exhibited a phylogenetic structure consistent with their species richness. This approach provides a powerful, non-invasive and integrative framework for long-term monitoring and supports evidence-based conservation planning for marine biodiversity in the Arabian Gulf.