The First eDNA-Based Assessment at the World's Most Remote Inhabited Islands: Investigating Marine Vertebrate Diversity at Tristan da Cunha

Abstract

Oceanic islands are among the most unique and vulnerable ecosystems in the world. Biodiversity monitoring is crucial for the sustainable management of resources; however, the isolation of many islands makes routine assessment challenging. Environmental DNA (eDNA) provides a promising approach to enhance traditional marine biodiversity assessments, reducing the logistical and financial challenges of monitoring. This study employed eDNA to characterize marine vertebrate biodiversity at the world's most remote inhabited islands of Tristan da Cunha. Two 12 s rRNA gene metabarcoding assays targeting marine fish and vertebrates were applied to seawater samples from 18 sites across the archipelago. This multi-assay approach detected 51 Operational Taxonomic Units (OTUs) encompassing 24 families, 28 genera, and 13 species. Comparison with existing results from traditional survey methods (SCUBA, pelagic BRUVS, and deep-water camera drops) and fisheries by-catch showed eDNA to successfully resolve the islands characteristic diversity profile. In addition, eDNA reported rare and vulnerable taxa underrepresented by the traditional surveys and detected species previously unrecorded at the islands. eDNA resolved greater species richness in kelp versus non-kelp habitats. Dominant Tristanian taxa had the highest number of reads, adding to evidence linking reads and abundance. eDNA detection was robust to sampling technique, volume filtered, time between collection, filtration, and sequencing of samples, demonstrating the effectiveness of this technique for use in challenging remote locations. Community composition varied significantly between metabarcoding assays, with unique OTUs detected by each marker, highlighting the importance of assay selection for capturing the full depth and breadth of diversity. 23%–40% of OTUs were resolved to species level, emphasizing the need for the expansion of taxonomic and sequence databases for this region. The study demonstrates the potential of eDNA as a high-resolution tool that can provide new insights into biodiversity around Tristan and can be operationalized to monitor future changes at these isolated islands.