Development and field testing of an environmental DNA assay for the European flat oyster (Ostrea edulis) in a coastal marina.

The European Flat Oyster Ostrea edulis, once widespread across European coastal waters, has experienced severe population declines due to overexploitation, habitat degradation and disease, and is now the focus of active conservation and restoration efforts. Effective monitoring of remaining and reintroduced populations is essential but remains challenging using traditional survey methods. Environmental DNA (eDNA) offers a sensitive, non-invasive alternative for species detection and monitoring in marine environments. In this study, a species-specific qPCR assay targeting the mitochondrial cytochrome oxidase I (COI) gene of O. edulis was developed, analytically validated and field tested. The assay was validated in silico, in vitro, and in situ, demonstrating high specificity and sensitivity, with a limit of detection of 1.96 copies reaction-1 and a limit of quantification of 11 copies reaction-1. Field application of the assay in a coastal marina in Northern Ireland confirmed detection of O. edulis eDNA in proximity to suspended oyster populations, with detections varying between sampling occasions. Detection differed between sampling occasions and locations, highlighting the influence that environmental conditions may have on eDNA detection in coastal systems. These findings indicate that the assay provides a sensitive and non-invasive tool for detecting O. edulis in coastal environments and support its application for monitoring native populations, evaluating restoration efforts and informing future conservation initiatives.