Assessing distribution of Didemnum vexillum in Scotland using environmental DNA

Potential introduction and spread of D. vexillum in Firth of Clyde

Within the Firth of Clyde, the number of sites where D. vexillum eDNA was detected was also low, suggesting that limited introductions and/or spread has occurred outside those sites where the presence of this species is already known (Beveridge et al., 2011; Matejusova et al., 2021). There was a concordance between the presence of D. vexillum eDNA detected using qPCR at Largs Yacht Haven Marina, Largs ferry terminal, Fairlie Pacific oyster farm and Fairlie Quay Marina and D. vexillum colonies found during the traditional rapid assessments (Beveridge et al., 2011; Matejusova et al., 2021). With regard to D. vexillum eDNA detection at the ferry terminal on the Isle of Cumbrae, it is likely that eDNA could have been transported there via water currents from other D. vexillum positive sites in the Largs and Fairlie area. Alternatively, frequent ferry traffic between Largs and the Isle of Cumbrae might have translocated physical colonies via hull fouling and therefore, further intertidal and subtidal surveys on the Isle of Cumbrae are recommended.

A possible discrepancy between the outcomes of traditional rapid assessment and the detection of D. vexillum eDNA was observed at Portavadie Marina. The initial traditional survey was carried out at this site in 2010 (Beveridge et al., 2011), and no evidence of D. vexillum presence was found. However, in 2017, a remotely operated vehicle (ROV)-based survey identified 10 small colonies of D. vexillum present throughout the marina (Marine Scotland, unpublished), and colonies were also observed directly from the pontoons during marina visits in 2018 and the winter of 2019 (Matejusova et al., 2021). It is highly likely that the colonies were present at the marine at the time sampling in summer 2019, as they were again sighted in summer 2022 (Matejusova, personal communication), however this cannot be confirmed. Due to the high stringency of parameters used in the present study, the status of D. vexillum at Portavadie Marina was concluded as inconclusive, with two out of 10 water samples, showing amplification in one out of three technical replicates, respectively. For site, for which presence of D. vexillum cannot be accurately inferred from eDNA, we recommend sites are revisited and relevant analysis repeated in line with the recommendations in LeBlanc et al. (2020) or Abbott et al. (2021).