The Loch Creran SAC was established to protect the Priority Marine Features (PMFs) such as marine biogenic reef consisting of the calcareous tube-worm Serpula vermicularis, and the horse mussel Modiolus modiolus. Loch Creran is also designated as a Marine Protected Area (MPA), conferred to protect the flame shell (Limaria hians) beds and other geodiversity features. Loch Creran contains the only example of well-developed serpulid reefs in the MPA network in Europe. The invasive colonial sea squirt Didemnum vexillum (Dvex) was first recorded on an oyster farm in Loch Creran in 2016. Since 2016, Marine Scotland Science (MSS) has undertaken annual monitoring surveys to assess the distribution of Dvex at the oyster farm and in the wider Loch Creran area. In view of Dvex possible impact on the PMFs present in the loch, Scottish Natural Heritage (SNH) has carried out surveys of the subtidal environment, by establishing a diving programme to monitor spread. The surveys described in this report complement ongoing annual surveys of the impacted oyster farm and cover the wider Loch Creran area, incorporating intertidal sections of shore not previously surveyed. All suspicious samples collected during the shore intertidal survey were tested and only one sample collected was confirmed as Dvex by real time PCR and sequencing. The subtidal survey did not reported any suspicious colonies.