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Colonial tunicate (Didemnum perlucidum)

Didemnum perlucidum is an introduced marine pest recorded in Western Australia, where it grows on artificial structures and native seagrasses. Its colonies can smother seagrass and other marine life, and fragments may help it spread.

Key facts

Type: Pest animal, invertebrate.

Native to: Tropical Indo-Pacific.

Established in: .

Other names: Didemnum perlucidum, Perlucid ascidian, Colonial ascidian.

Best time to act: Survey and, where authorised, manage colonies in late summer to autumn, when they are more likely to be visible, and check again after winter.

Identification

  • Look for thin, spreading colonies attached to underwater surfaces, sometimes forming mats across several seagrass leaves.
  • Colonies are often translucent or pale, with small zooids visible within the shared outer tissue.
  • The colony surface may show small white calcareous specks.
  • It can grow on moorings, jetty pylons, navigation markers, seagrass, algae and other submerged surfaces.
  • Look-alike: Other pale, encrusting colonial ascidians can look similar; expert identification may be needed to confirm the species.

Where it lives

In Australia it occurs in coastal and estuarine waters, growing on submerged artificial structures as well as seagrass and other natural surfaces.

Why it matters

  • Colonies can overgrow and smother native organisms and foul submerged infrastructure.
  • In the Swan River, colonies enveloped Halophila ovalis leaves and were associated with significantly lower seagrass biomass.
  • Seagrass-associated mud snails were less abundant in samples where colonies were present.
  • Fouling may restrict light reaching seagrass leaves and contribute to plant damage.

How it spreads

  • Colonies are commonly found near infrastructure such as moorings, jetties and navigation markers, which may facilitate spread into nearby habitats.
  • Larvae can disperse, and colonies can regenerate from fragments, so fragments may establish new colonies.
  • Fouled vessels and drifting or dislodged plant material may also transport colonies or fragments.

Control methods

  • Report suspected colonies to the relevant state or territory biosecurity agency or marine pest authority, providing clear photographs and the location.
  • Do not scrape, pressure-wash or remove colonies in the water without advice, as fragments may regenerate and spread.
  • If removal is authorised, use trained crews and contain colonies and debris so fragments do not enter the water; dispose of collected material as directed by the responsible authority.
  • Use only control methods approved by the relevant authority; herbicides are not a general treatment for marine ascidians.

Site hygiene and welfare

  • Before moving a boat or marine equipment, check hulls, moorings and gear for attached colonies; clean them only in a contained facility and collect all debris and washwater.
  • Keep equipment and fouling waste contained, and follow local marine biosecurity guidance before using gear at 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): No national listing. Managed under state and territory law.
  • Queensland: Restricted matter, categories 2, 3, 4. Report it to Biosecurity Queensland within 24 hours; do not give it away, sell or release it into the environment; do not move it. Everyone must also take all reasonable and practical steps to minimise its risks (the general biosecurity obligation).
  • New South Wales: General biosecurity duty. Not prohibited matter in NSW. 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: Noxious fish. Prescribed as a noxious fish under the Fish Resources Management Regulations 1995, so it must not be brought into WA, kept, sold or released. This applies only in Montebello Islands Marine Park. Report any sighting to FishWatch on 1800 815 507.
  • 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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