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New Zealand screw shell (Maoricolpus roseus)

Introduced to south-eastern Tasmania in the 1920s, it has spread along the south-eastern Australian coast and formed dense beds across extensive marine areas. These beds alter seabed structure, and possible impacts on scallops and native screw shells are a concern.

Key facts

Type: Pest animal, invertebrate.

Native to: New Zealand.

Established in: NSW, VIC, TAS.

Other names: Twirlies, Maoricolpus roseus.

Best time to act: Larvae were detected most often in summer, and the report describes spring to summer, generally October to March, as the main spawning period, although detections also occurred in June and August.

Identification

  • It is a marine gastropod in the turritellid family, known as a screw shell.
  • The report lists its shell length as 80 mm.
  • Collected shells may be empty or occupied by hermit crabs.
  • The report notes that its larvae cannot be distinguished by appearance from many other gastropods.

Where it lives

In Australia it occurs in marine coastal and continental shelf waters from Tasmania to New South Wales, on varied substrates and at depths extending to about 80 metres off eastern Victoria and New South Wales.

Why it matters

  • Dense beds alter the benthic structure of coastal and shelf ecosystems.
  • The snails consolidate mobile sediments, creating reefs of empty shells many layers deep.
  • They may adversely affect scallops and native Gazameda gunnii, whose numbers have declined alongside the increase in Maoricolpus roseus.
  • Empty shells provide habitat for hermit crabs and may provide holdfasts for other introduced species.

How it spreads

  • The species was unintentionally introduced to south-eastern Tasmania in the 1920s and has spread as far north as Botany Bay.
  • The report detected larvae in plankton, indicating a planktonic larval phase that could allow movement in ships’ ballast water.
  • Its broad temperature, depth and substrate tolerances may enable further spread.

Control methods

  • The report does not describe an eradication or treatment method.
  • It says the risk of spread by shipping may need to be managed within the National System for Prevention and Management of Marine Pest Incursions.
  • The report recommends further study of larval presence and abundance to assess the risk posed by ships’ ballast water.

Site hygiene and welfare

  • The report identifies larvae in water available to ships’ ballast and flags ballast-water movement as a potential spread pathway.
  • 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: 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: Not declared. Not declared under the Catchment and Land Protection Act 1994. Other Victorian laws may still apply.
  • 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. 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.

  • New South Wales: Biosecurity Act 2015 (NSW), administered by NSW Department of Primary Industries and Regional Development
  • Victoria: Catchment and Land Protection Act 1994 (Vic), administered by Agriculture Victoria
  • Tasmania: Biosecurity Act 2019 (Tas), administered by Biosecurity Tasmania

Sources