Assessing distribution of Didemnum vexillum in Scotland using environmental DNA

Executive summary

Carpet sea squirt, Didemnum vexillum, is non-indigenous (NIS) (or invasive non-native) marine species which, when established outside its native range, can negatively impact marine habitats, outcompete natives 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.

Traditional rapid assessment surveys (RAS), applied to monitor for presence of NIS, rely on trained personnel to carry out a field surveys, collect samples and identify NIS, either directly in the field or in laboratory, using morphological features observed under a microscope.

This pilot study explores use of environmental DNA (eDNA), DNA shed into water or sediment by organisms inhabiting marine environment, to assess the spread of D. vexillum in Scotland. This report shows the results of an eDNA-based survey, carried out in 2019, targeting NIS hotspots such as ports, harbours, ferry terminals, recreational marinas, aquaculture sites, private pontoons and jetties in the Firth of Clyde, Loch Creran, and the wider Lynn of Lorn areas.

Overall, D. vexillum eDNA was mainly detected in the vicinity of previously established colonies however, presence of D. vexillum, inferred from the eDNA, was also suspected in Rubha Dearg (in a vicinity of Loch Creran Marina) and Dunstaffnage Marina. Consequently in September 2022, D. vexillum colonies were confirmed at Dunstaffnage Marina by a rapid assessment of a marina pontoon, while further surveys are recommended to be carried out in Loch Creran marina and wider Rubha Dearg area.

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. The present study sets out protocols which were able to produce good agreement between outcomes of traditional rapid assessments and the detection of D. vexillum eDNA. Moreover, the study demonstrates how an eDNA-based survey can be scaled up across large geographic regions, within a short time period, using simple equipment and potentially non-specialist personnel. It also illustrates that eDNA-based surveys can facilitate a risk-based monitoring approach by cost-effectively directing traditional surveys to areas with suspected presence of NIS.