Release and Degradation of Environmental DNA and RNA From Eels in Aotearoa New Zealand

Environmental DNA (eDNA) has become a crucial tool for detecting rare species and monitoring biodiversity. However, the prolonged persistence of eDNA in water complicates the precise determination of an organism's location based on an eDNA signal alone. In contrast, environmental RNA (eRNA) degrades faster, potentially offering a more accurate detection proxy. To test this, we analyzed eDNA and eRNA release concentrations and decay rates from six longfin (Anguilla dieffenbachii) and six shortfin (Anguilla australis) eels under controlled conditions. Eels were placed in aquaria for 30 h and, after their removal, temporal water sampling was conducted over 7 days to assess the eels' eDNA and eRNA dynamics. Concentrations of eDNA and eRNA were estimated using validated droplet digital PCR assays for each species (cytb and 16S mitochondrial genes). Temporal eDNA and eRNA dynamics followed an exponential decay function over time, demonstrating a predictable decline in their concentrations. Moreover, higher decay rates of eRNA could represent a slightly more accurate proxy than eDNA for the location determination of rare species. Variability in the release and decay could be linked to the type of nucleic acid, marker genes, or eel species. Understanding these dynamics will help fine-tune detection models based on eDNA and eRNA.