Environmental DNA-enabled monitoring offers both opportunities and challenges for the conservation of Mediterranean elasmobranchs: lessons learned within the ELASMODROP collaborative initiative

Monitoring marine biodiversity is crucial for ecosystem conservation and fisheries management. Traditional methods such as visual surveys and capture techniques are invasive, costly, laborintensive, and often ineffective at detecting rare or elusive species. Advances in environmental DNA (eDNA) allow researchers to identify organisms from water samples, offering a non-invasive, sensitive alternative for assessing biodiversity. We applied eDNA-based methods to monitor elasmobranch diversity in Italian seas, with a large-scale effort across different environments, thanks to the ELASMODROP collaborative network, which unites researchers, academics, and students to advance eDNA applications in marine biodiversity monitoring. The network enabled expanded sampling, standardized methods, and a comprehensive assessment of elasmobranch diversity. We present case studies highlighting various eDNA sampling strategies: Five eDNA sampling systems (active and passive) were tested in a controlled environment. Active samplers yielded more DNA and detected all elasmobranch species; passive tools were less efficient. Passive samplers deployed with deep-sea longlines detected 78% of the species captured and uncovered additional pelagic and mesopelagic taxa missed by traditional methods. Active filtration at 25 sites in the central Mediterranean using Niskin bottles at three depths allowed for detailed depth-dependent biodiversity assessments. Over 500 samples collected along the Italian coast in 2024 using both approaches expanded spatial coverage and provided comprehensive biodiversity data. All samples were processed by eDNA metabarcoding with an elasmobranch-specific marker. Taxonomic assignment was based on a custom 12S reference database that incorporated newly generated and curated public sequences. The accuracy was improved by increasing taxonomic coverage and integrating results from 12S, COI and NADH2 markers. Over 15% of public sequences were found to be mislabelled. Our custom database now allows reliable identification of up to 90% of Mediterranean cartilaginous fish species. These findings confirm eDNA as a powerful tool for comprehensive marine biodiversity assessments. While challenges remain, especially in standardizing methods and interpreting data, addressing these will enhance eDNA’s role as a cost-effective, large-scale tool, laying the foundation for a stronger contribution to informed conservation strategies and policy development.