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Colonial sea squirt (Didemnum vexillum)

Didemnum vexillum is an invasive marine pest that has caused significant impacts in New Zealand and the USA, although invasive exotic colonial sea squirts have not been known to occur in Australia. It can foul marine infrastructure and aquaculture and smother marine animals, making suspected sightings important to report promptly.

Key facts

Type: Pest animal, invertebrate.

Native to: North-western Pacific, including Japan.

Established in: .

Other names: Sea vomit, Carpet sea squirt, Didemnum vexillum.

Identification

  • It forms spongy colonies that are orange-yellow or cream in colour.
  • The surface can feel leathery but is not slimy.
  • Colonies can form vertical hanging tendrils that look like dripping wax.
  • Look for dark, leaf-like veins and small pores on the surface.
  • Field identification is difficult, and confirmation requires taxonomic and genetic expertise.
  • Look-alike: Native colonial sea squirts in NSW can look similar and may be confused with D. vexillum.

Where it lives

It grows on fully submerged hard surfaces, especially artificial structures such as wharves, pilings and boat hulls.

Why it matters

  • Colonies can attach to wharves, pilings, boat ramps, ropes and boats, fouling marine infrastructure.
  • They can overgrow and smother marine animals such as mussels and oysters.
  • Invasive colonial sea squirts can significantly affect natural habitats, biodiversity and aquaculture farms.

How it spreads

  • Colonies can spread on attached infrastructure, ropes and boats.
  • Most larvae settle on hard structures after 2 to 6 hours in the water, but can survive for up to 36 hours before settling.
  • Small pieces can break off, reattach to suitable surfaces and form new colonies.

Control methods

  • If you suspect D. vexillum, report it immediately to the relevant state biosecurity agency and provide the exact location.
  • If possible, take a photo and collect a sample; freeze the sample in a plastic bag.
  • Do not scrape, wash or remove colonies in a way that could release fragments or larvae; use physical removal only under an approved response plan with containment.
  • Herbicides are not appropriate for this marine animal; follow directions from biosecurity authorities on approved treatment and follow-up.

Site hygiene and welfare

  • Inspect boat hulls, ropes and marine gear for attached colonies before moving them between sites, and clean them using approved biosecurity procedures.
  • Contain removed material and wash water so fragments are not released back into the water or transported to another site.
  • Use methods set out in the national model codes of practice and standard operating procedures for the species.
  • Coordinate control with neighbours and the local government biosecurity plan, as isolated effort is quickly undone by reinvasion.

Classification by state

  • Australia (national): National priority exotic pest. On the National Priority List of Exotic Environmental Pests, Weeds and Diseases, so any new detection should be reported straight away.
  • Queensland: Prohibited matter. Report it to Biosecurity Queensland on 13 25 23 within 24 hours and do not move or deal with it. Everyone must also take all reasonable and practical steps to minimise its risks (the general biosecurity obligation).
  • New South Wales: Notifiable aquatic pest. It must not be possessed, bought, sold, moved or released in NSW, and you must notify NSW DPIRD within one working day of finding it. The general biosecurity duty applies: anyone dealing with it must prevent, eliminate or minimise the risk it poses.
  • Victoria: Noxious aquatic species. Declared a noxious aquatic species under the Fisheries Act 1995, so it must not be brought into Victoria, kept, sold, moved or released into waters. Report any sighting to the Victorian Fisheries Authority on 13 FISH (13 3474).
  • South Australia: Noxious species. Declared a noxious species under the Fisheries Management Act 2007, so it must not be brought into South Australia, kept, sold or released, and it must not be returned to the water alive. Report any sighting to PIRSA FISHWATCH on 1800 065 522.
  • Western Australia: Declared pest (C3). C3 Management: landholders must manage it to reduce its impact and stop it spreading.
  • Tasmania: Not a declared pest. Not a declared pest or prohibited matter in Tasmania. Importing animals is still controlled by Biosecurity Tasmania.
  • Northern Territory: Not a declared pest. Not on an NT declared or prohibited list. Check with the NT Government before keeping, moving or releasing it.
  • Australian Capital Territory: Not declared. Not declared under the Biosecurity Act 2023 (ACT). A general biosecurity duty still applies.

Legal obligations

Every state and territory places a duty on landholders and people working on land to prevent the spread of invasive species, such as the general biosecurity obligation in Queensland and the general biosecurity duty in New South Wales. Declarations and control requirements differ between states and local government areas, so check the current official list before work starts.

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