Invasive non-native species

A species is classified as an invasive non-native species when it grows at a rate that outcompetes native species without any natural predators to keep it in check. This growth can: devastate ecosystems, destroy the balance of flora and fauna, smother entire areas and in effect creating a mono-culture; threaten the livelihood of businesses in the marine sector by causing damage to infrastructure, stock and foul equipment.

We take the threat of marine invasive non-native species seriously and work with delivery partners to promote biosecurity among marine stakeholders to manage and lower the risk associated with introducing or spreading these species.

To report a suspected marine invasive non-native species, please visit the NatureScot website for more information, or email Marine Non-Native Species

Didemnum Vexillum, Carpet Sea Squirt

Didemnum Vexillum, also known as Dvex or Carpet Sea Squirt, is a colonial ascidian thought to originate from Japan. It reproduces and grows quickly and is classed as an invasive non-native species.

What it looks like

Dvex looks similar to native species but the following characterstics can help with identification. Remember, if you have any concerns that you may have found Dvex please follow the instructions for reporting. Identification posters are also available to download below.

Dvex characteristics:

  • variable growth forms, either flat mat-like sheets or pendulous beard-like growths
  • uniform colouring, either pale orange, cream or off-white
  • Dvex is not slimy. Its surface is firm and leathery with a veined or marbled appearance
  • on close inspection small open pores can be seen on the surface

Where it is found

Dvex grows on natural hard substrates and man-made underwater structures down to a depth of 80m. It has been found in many places around the world including the UK. It was first found in Scotland, in the Firth of Clyde in 2009, and was confirmed as present in Loch Creran in 2016.

How it is spread

Dvex can spread by larvae dispersal and also by fragmentation. It is an offence to spread a non-native species to a new location beyond its native range intentionally or otherwise. There are measures in place to promote biosecurity, reduce the risk of spreading Dvex and advice to marine users to support these measures. 


The Scottish Government commissioned a Community Biosecurity Plan for Carpet Sea Squirt in Loch Creran in 2017 in response to the known incidence of this invasive species in the area.  A similar approach was taken in Loch Fyne where a Community Biosecurity Plan was commissioned in 2019.  The aim of both documents is to provide local stakeholders with a Best Practice Guide of how to manage and contain D. vexillum in the marine environment, and to control or minimise its spread.

Further information:

Loch Creran Carpet Sea Squirt information sheet

Loch Creran Community Biosecurity Action Plan

Shellfish farm guidance: Control of invasive Carpet Sea Squirt

Sea squirt (Didemnum vexillum) identification poster


Wakame, also known as Japanese kelp, is an invasive non-native species. It is a fouling seaweed which grows rapidly and can outcompete native species.

What it looks like

Wakame grows to 1-3m in length. The many differing features include:

  • wavy edges on the stalk between the end which attaches to the substrate and the large blade
  • a distinct mid-rib along the blade
  • wavy edges of the blade

Where it is found

Wakame has been found on the south coast of England, the Channel Islands and also in Scotland.

How it is spread

Wakame spreads rapidly by releasing spores which quickly attach and grow on surfaces of objects in the water.

Holyhead marina debris

Incident and potential spread of the marine invasive non-native species Didemnum vexillum.

On 2 March 2018, storm Emma caused significant damage to Holyhead Marina in North Wales. Clean-up operations followed and are on-going.

As part of clean-up operations, actions are being taken to reduce any potential environmental impacts that may occur as a result of this incident.

We are advising interested parties as, despite efforts to contain debris from the damaged marina, some debris has drifted out of the harbour area into the wider marine environment.

A consequence of this incident means there is potential for carpet sea-squirt Didemnum vexillum (D. vex), a marine invasive non-native species known to inhabit the area, to spread to other areas. However, the risk of spread may be reduced due to the current sea temperature, which at this time of year is not thought to be favourable for D. vex to reproduce.

Prior to storm Emma, D. vex was being contained in the marina after being discovered living on the submerged artificial structures, commonly found on the floating pontoons, ropes and chains. The resin surface of the floating pontoons provided a suitable area for D. vex to live.

D. vex is considered to be an invasive species because it has the potential to negatively impact fisheries, aquaculture and the conservation of native marine habitats.

Areas of potential spread

Evidence suggests that debris has the potential to spread quite far from Holyhead. Remains of a floating pontoon with resin coating has been recorded approx. 37km away from the marina.

There have also been reports of polystyrene (believed to be from Holyhead pontoons) washing up in Wicklow in Ireland. There have been no further reports of debris at other locations at this time. 

Reporting invasive non-native species

Reporting the sighting of invasive non-native species is incredibly important. This allows Marine Scotland and partners to do their best to protect the natural environment.

If you suspect you have seen an invasive non-native species then please report it immediately on the Scottish Environment and Rural Services (SEARS) telephone number 08452 30 20 50 or email:

It is helpful if you can:

  • take a photo or detailed description
  • identify the location, ideally to the nearest 100m
  • note the date and roughly how many you saw
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