Marine environment

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 contact and visit Scotland's Environment webpage for more information.

Didemnum Vexillum, Carpet Sea Squirt

Didemnum Vexillum, also known as D.vex 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

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

D.vex characteristics:

  • variable growth forms, either flat mat-like sheets or pendulous beard-like growths
  • uniform colouring, either pale orange, cream or off-white
  • D.vex 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

D.vex 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

D.vex 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 D.vex 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.

American lobster

Please note the American Lobster Retain and Report campaign where further details can be found of how and where to report any suspected American lobsters.

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