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Assessing distribution of Didemnum vexillum in Scotland using environmental DNA

This study explores use of environmental DNA (eDNA), DNA shed into water or sediment by organisms inhabiting marine environment, to assess the distribution of Didemnum vexillum in the Firth of Clyde, Loch Creran, and the wider Lynn of Lorn areas.


Non-indigenous species (NIS) represent one of the main threats to marine biodiversity and the maritime economy (Invasive Alien Species (; Williams et al., 2010; Tillin et al., 2020). In the North-East Atlantic region, the strategic monitoring of marine NIS is carried out by individual countries to fulfil obligations under the international agreements such as The Convention for the Protection of the Marine Environment of the North-East Atlantic (OSPAR Convention) and the number of new marine NIS introductions is used as a common indicator (Non-Indigenous Species Thematic Assessment ( In United Kingdom, NIS monitoring is limited to data sets originating from existing monitoring schemes that focus on the assessment of the biological and chemical status of the UK coastal waters (Stebbing et al., 2014, 2016), a range of voluntary recording schemes (such as Seasearch), and short term research projects run by academic research groups, governmental and non-governmental organizations. This approach leads to data sets which are liable to under- or over-represent certain marine NIS introductory pathways and species and are therefore not adequate to accurately describe the environmental status of the seas. In Scotland, gaps in marine NIS monitoring gaps have been outlined in the most recent Scotland’s Marine Assessment (Moffat et al., 2020).

To address the gap, a workshop was organized to bring together the main governmental and non-governmental organizations that deliver marine NIS monitoring in the United Kingdom. The aim of the workshop was to assess each organization’s capabilities to deliver NIS monitoring (Wood et al., 2020). The findings revealed that much of the INNS surveillance data in United Kingdom is derived from limited number of rapid assessment surveys (RAS) of NIS hotspots, such as recreational marinas, and consist of detailed visual inspections of man-made structures such as pontoons, ropes, fenders and similar (for example, Nall et al., 2015; Hurst, 2016). RASs continue to be invaluable for long term monitoring of NIS (Kakkonnen et al., 2019), however they are difficult to scale up spatially, or maintain over a prolonged period, and rely on the expertise of trained personnel to identify NIS using morphological features. To overcome the constraints associated with RASs, the workshop attendees discussed options for incorporation of DNA-based tools to NIS monitoring (Wood et al., 2020). Due to the ability to scale up sampling over large geographic regions, the use of environmental DNA (eDNA) has been considered by numerous countries to augment traditional NIS surveys as part of a risk-based targeted monitoring of NIS hotspots (for example Puntila et al., 2013; Andersen et al., 2014; Abbott et al., 2021). The detection of marine NIS eDNA in water samples, sediment, or hard surface scrapings has proved to be a rapid and powerful monitoring approach (for example, Holman et al., 2019; Rey et al., 2019; Wood et al. 2019; LeBlanc et al., 2020). However, well-designed pilot studies are still needed to fully understand both the feasibility of rolling out such surveys across larger geographical scales and to identify any potential constrains when using the outcome of eDNA-based monitoring in the context of government and regulatory decision making.

Didemnum vexillum is an invasive colonial tunicate and a high risk marine species which could have a “disastrous effect on biodiversity” when introduced and established outside its native range (GBNNSS, 2011). The first observation of this species in Scotland is thought to date back to 2009, when colonies were found during a routine survey of the yacht marina in Largs, Firth of Clyde. In 2010, twelve additional locations, within close proximity to the Largs site, comprising of ten other marinas and two harbours across the west coast of Scotland, were surveyed. These surveys indicated only a local spread of D. vexillum with no evidence of colonies elsewhere on the west coast (Beveridge et al., 2011). More recently, D. vexillum was reported at two Pacific oyster farms, one situated in Loch Creran (north of Oban, in 2016) (Cottier-Cook et al., 2019) and the second in Fairlie (Firth of Clyde, in 2017) and further incidences were also reported in Portavadie Marina in Loch Fyne (in 2017) (Matejusova et al., 2021).

The scale of potential ecological and economic impacts of D. vexillum when successfully established, coupled with difficulties associated with morphological identification, means that it is the ideal subject for a pilot eDNA-based survey to ground-truth this approach. There are numerous quantitative real-time PCR (qPCR) assays available designed to target and amplify D. vexillum eDNA from environmental samples, and published data suggest that these assays exhibit high sensitivities and concordance with physical observation of D. vexillum colonies (Simpson et al., 2017; LeBlanc et al., 2020; Gargan et al., 2021). The present study uses the D. vexillum qPCR assay described in Matejusova et al. (2021), validated for use in the Northern Hemisphere, and reports outcomes of eDNA-based surveys of NIS hotspots such as ports, harbours, ferry terminals, recreational marinas, aquaculture sites, private pontoons and jetties in the Firth of Clyde and Loch Creran and wider Lynn of Lorn areas.

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