Carpet sea squirt (Didemnum vexillum)

Current situation

The Australian Government Department of Defence (Defence) and the Department of Agriculture, Fisheries and Forestry (DAFF) are working with the Western Australian Department of Primary Industries and Regional Development (DPIRD) in response to a detection of carpet sea squirt (Didemnum vexillum) at Garden Island and at a Henderson marine facility, in Western Australia.

The carpet sea squirt (CSS) is a marine pest that is exotic to Australia and is listed on the National Priority List of Exotic Environmental Pests, Weeds and Diseases. This is the first recorded detection of CSS in Australia.

The CSS incursion at Garden Island, Western Australia is being managed as a Commonwealth response as it was found at a Commonwealth site.

The CSS incursion at the Australian Marine Complex (AMC) facility in Henderson is being managed by DPIRD as the lead agency in Western Australia.

The first confirmed detection of CSS at Garden Island was reported to the Consultative Committee on Introduced Marine Pest Emergencies (CCIMPE) in April 2021. The Australian Chief Environmental Biosecurity Officer agreed that DPIRD would work with Defence to manage the marine pest incursion.

From March to November 2021, a delimiting survey was undertaken by DPIRD at Garden Island to identify the extent of the pest colonies, followed by treatment of the colonies through underwater cleaning from November 2021.

From July 2022 onwards, Defence clean-up efforts were increased to prevent further spread of CSS outside Garden Island. Remediation and removal of CSS colonies at Garden Island is ongoing.

DPIRD has since conducted additional surveillance and testing at locations within the Port of Fremantle and Cockburn Sound. In January 2023, DPIRD confirmed new detections on wharves at AMC, east of Garden Island.

On 17 March, DPIRD issued a Quarantine Area Notice (QAN) for waters adjacent to the AMC at Henderson South, including seawalls to the north and west, extending 250 m down the coastline south of AMC.

DPIRD is working with local stakeholders to inform and assist with managing the incursion.

Following the detection at Henderson, DPIRD is extending surveillance to monitor for spread of CSS in waters surrounding Garden Island and the Henderson marine facility in Cockburn Sound.

Work is continuing to implement the necessary actions to manage the response to this marine pest incursion, including additional vessel management procedures.

About carpet sea squirt

CSS is a highly invasive marine animal that can overgrow rocks, shellfish, sea sponges and algae. It can also overgrow man-made structures such as wharves, jetty pylons, pontoons, buoys and vessels.

CSS reproduces both sexually and asexually, where it can grow new colonies from small fragments that may break off from larger colonies.

It is native to east Asia, likely Japan, and has been introduced to New Zealand, North America and Europe. The Garden Island incursion of CSS is the first confirmed detection of this marine pest in Australia.

More detailed and technical information on CSS is found at the National Introduced Marine Pest Information System (NIMPIS).

CSS growing on an underwater marine structure in New Zealand. The CSS is light cream-yellow in colour and surrounded by other marine organisms.
Carpet sea squirt in New Zealand. Photo courtesy of C. Woods, NIWA.

CSS can have potentially devasting impacts on aquaculture industries due to its invasive nature. It can alter the marine environment by competing with and growing on native marine species.

The main impacts of CSS are fouling of vessels, marine infrastructure or aquaculture equipment.

The pest is transported to new locations mainly as biofouling on vessels, or sometimes from natural dispersal through ocean currents.

What to look for

CSS is usually a yellow-grey mass but colour is variable. It has holes that are used for siphoning water to feed on small phytoplankton and organic matter from the sea water. CSS is a filter feeder and not a predatory animal.

It will grow as a covering over surfaces but may get quite thick (up to 10 cm), producing 'arms' or tendrils up to 1 metre long that hang below the main mass. It is known to grow rapidly and aggressively on substrates.

Close-up photograph of CSS showing many small colonial animals and larger pores for filter feeding.
Close-up photo of carpet sea squirt in New Zealand. Photo courtesy of C. Woods, NIWA.
CSS growing on an underwater structure in New Zealand and forming ‘dripping’ tendrils. The tendrils are light yellow in colour.
Carpet sea squirt in New Zealand. Photo courtesy of C. Woods, NIWA.

There are many native species of colonial ascidians in Australia that look similar to CSS. Accurate identification of CSS is made using a microscope by an expert or molecular techniques to confirm the species’ DNA.

Reporting marine pest sightings

If you find unusual marine species or suspected marine pests attached to vessels, submerged infrastructure or in the marine environment, report it to the Commonwealth, or relevant state or territory authority.

To assist with your report:

Take photos

  • Take a few close-up photos of the suspected marine pest from different angles.
  • Where possible, include an object in the photo for size reference (such as a coin, bank note, or ruler if available).
  • If your camera has GPS, turn it on.
  • Record relevant details including the number of suspected marine pests you saw.

Check the location

  • Record your exact location. Include as many details about where you found the marine pest as you can, such as the habitat (e.g., harbour, beach, rocky reef).
  • Screenshot your map app or use photo geotagging on your phone. You can turn geotagging on in your phone settings.
  • Note the date and time of the sighting.
  • If the organism was found underwater (i.e., while SCUBA diving), include the approximate water depth if known.

Stop the spread of marine pests

There are simple steps you can take to stop the spread of marine pests and help protect Australia’s marine environment and industries.

  • Recreational boaters, fishers and divers should keep their boats, trailers, wetsuits and equipment clean.
  • Check wheel arches on trailers, boat propellers, fishing tackle and footwear for bait, debris and seaweed.
  • Use soapy water to clean your boat and trailer, fishing rods and other equipment, and allow them to dry completely before using them at another location.

Government response

The Consultative Committee on Introduced Marine Pest Emergencies (CCIMPE) is providing technical and scientific advice on the CSS response.

CCIMPE is chaired by Australia’s Chief Environmental Biosecurity Officer and comprises marine pest managers from each state and the Northern Territory, and involves other specialists across government.

Defence has drafted a Biosecurity Response Management Plan detailing actions to manage this marine pest. It is implementing a national marine biosecurity surveillance program in partnership with state and territory governments.

Foreign warships are required to comply with Australian Government requirements when entering Australian waters.

Throughout the response to this pest incursion, Defence has followed the advice of DPIRD and CCIMPE when undertaking remediation actions at Garden Island.

More information

More detailed and technical information on carpet sea squirt (Didemnum vexillum) can be found at the National Introduced Marine Pest Information System (NIMPIS).

For further information on marine pests in Australia, please visit:

To report a suspected marine pest, contact your local biosecurity authority at: