Introduction

Invasive exotic colonial sea squirts are found in many places around the world but they have not been known to occur in Australia. These invasive pests can significantly impact marine infrastructure, natural habitats, biodiversity and aquaculture farms. One species of colonial sea squirt, Didemnum vexillum, has caused significant impacts in New Zealand and the USA. This species can be easily spread, and can attach to infrastructure such as wharves, pilings and boat ramps, as well as ropes and boats. It is known to overgrow and smother marine animals such as mussels and oysters. Didemnum spp. (exotic invasive strains only) appears on the former Consultative Committee on Marine Pest Emergencies (CCIMPE) Trigger List as a species still exotic to Australia, and are considered marine pests of national concern.  (Note: an updated Trigger List is currently being developed by the Commonwealth).

In March 2010 a suspected incursion of the invasive colonial sea squirt D. vexillum was reported in Twofold Bay. Within days of the report, NSW DPI activated its Biosecurity First Response Team (trained specifically in emergency response) to coordinate and carry out NSW’s response to the suspected marine pest emergency.

Identification

D. Vexillum in New Zealand

D. vexillum in New Zealand (Photo: US Geological Survey, http://woodshole.er.usgs.gov/project-pages/stellwagen/didemnum)

D. vexillum in Massachusetts, USA

D. vexillum in Massachusetts, USA (Photo: US Geological Survey, http://woodshole.er.usgs.gov/project-pages/stellwagen/didemnum)

Key features

D. vexillum is a spongy-textured colonial sea squirt and can be either orange/yellow or cream in colour. It can feel leathery but is not slimy. It can form ‘tendrils’ that hang vertically and look like dripping wax. Its surface has dark veins (like those on a leaf) with small pores.

This species of sea squirt is extremely difficult to identify in the field, requiring taxonomic and genetic expertise to confirm its identity. D. vexillum may easily be confused with the many native species of colonial sea squirt that occur in NSW.

Similar Native Species

These native species may be easily confused with D. vexillum.

A native Didemnum species

A native Didemnum species (Photo: Roger Laird, NSW DPI)

A native Didemnum species (Photo: Aquenal Pty Ltd)

A native Didemnum species (Photo: Aquenal Pty Ltd)

Natural distribution & biology

Habitat: D. vexillum typically grows on fully submerged hard surfaces and seems to have a preference for artificial structures such as wharfs, pilings and boat hulls. It can tolerate a wide range of temperatures (as low as -2°C and greater than 24°C), but generally requires temperatures above 13°C to reproduce.

Reproduction and dispersal: Like all sea squirts, D. vexillum is a hermaphrodite, meaning it contains male and female reproductive organs. Internally fertilised eggs are brooded and develop into larvae (sometimes called tadpoles because of their small tails) before they are released. Most larvae remain in the water for 2-6 hours before settling on hard structures, but larvae have the potential to survive for up to 36 hours before settling. This sea squirt is also capable of asexual reproduction, whereby small pieces can break off and re-attach to suitable substrates to form new colonies.

How can you help?

Report any suspected new sightings of aquatic pests on the 24 hour recorded hotline: 02 4916 3877 or email aquatic.pests@dpi.nsw.gov.au.

If you see this pest in NSW, please report it immediately

  • Note the exact location
  • If possible take a photo and/or collect a sample
  • Freeze sample in a plastic bag
  • Report your sighting