Assessing distribution of Didemnum vexillum in Scotland using environmental DNA

Abstract

The carpet sea squirt, Didemnum vexillum, is a non-indigenous (NIS) (or invasive non-native) marine species which, when established outside its native range, can negatively impact marine habitats, outcompete native species, and lead to economic losses in the aquaculture sector. In Scotland D. vexillum is known to be present in parts of Firth of Clyde, Loch Creran and Loch Fyne, however no systematic survey to assess its distribution had been carried out since 2011. The present pilot study explores use of environmental DNA (eDNA), DNA shed into water or sediment by organisms inhabiting marine environment, to assess the distribution of D. vexillum in Scotland. In common with any other novel monitoring tools used in the regulatory context, the need for standardized protocols is essential to ensure the robustness of analyses and to allow for benchmarking and comparisons between different data sets. Our findings show the results of eDNA-based surveys for D. vexillum carried out in 2019 in the Firth of Clyde and Loch Creran and the wider Lynn of Lorn areas, with eDNA detected mainly in the vicinity of the previously established colonies. However, D. vexillum eDNA was also detected in Dunstaffnage Marina and D. vexillum colonies were confirmed there later during a rapid assessment of marina pontoons carried out in September 2022. Additionally, the presence of D. vexillum, inferred from detection of eDNA, was also suspected near Loch Creran marina (Rubha Dearg), where its physical presence has not yet been recorded, and further surveys are recommended there. This study demonstrates how an eDNA-based surveys can be used to scaled up across large geographic regions, in a short period time, using simple equipment, and guide traditional NIS surveys. The sampling techniques used in this study are also suited for future adoption of citizen science within wider sampling programmes.