Didemnum, aka Whangamata sea squirt

Didemnum vexillum

Legal Status: No Status

Status in New Zealand: Established

Organism: Water snails, crabs, shellfish, starfish, and other aquatic

This pest is established in New Zealand in the Marlborough Sounds and in Whangamata and Tauranga. Efforts are being made to manage it in Marlborough.

If you suspect you have found this in the Marlborough area, call 03 578 5044. This is the contact number for the local Didemnum Working Group, which is managing this organism in the area. If you suspect you have found it anywhere else in New Zealand, call 0800 99 66.

Description

Factsheet (1177 KB)

Didemnum vexillum is a leathery or spongey textured, light mustard coloured sea squirt which often presents like a yellowish wax dripping over a structure such as a rope or mussel line.

Its surface has darkish leaf-like veins with pores.

There are a number of other marine organisms that look similar. The things that set Didemnum apart are:

It is yellow-orange in colour
It has dark leaf-like veins with pores

Didemnum vexillum is similar to another New Zealand native species, Didemnum incanum. However, Didemnum incanum is cream in colour and has raised leaf like veins without pores.

Didemnum vexillum is generally found on artificial structures such as wharf pylons, mooring lines and vessel hulls.

Colonies of Didemnum vexillum can reproduce sexually by releasing tailed larvae that are carried in water currents. It can also reproduce asexually by budding, hence fragments can break off and grow into new colonies

Impact

Didemnum vexillum poses a threat to the marine farming industry because of its ability to smother man made structures including mussel lines.

Spread

As well as the natural spread detailed above, this pest spreads easily on the hulls of vessels, aquaculture and other marine equipment as fragments in ballast water.

Its spread can be prevented by:

ensuring vessel hulls and marine equipment are free of fouling, and regularly treated with anti-fouling paint
regularly cleaning hulls in a facility with collection and land-based disposal of fouling material
minimising the movement of excessively fouled structures from one location to another.

light coloured and porous Didemnum Vexillum

Management

In Marlborough, a management strategy has been developed by a local group including representatives from the New Zealand Marine Farming Association, the New Zealand Mussel Industry Council, Marlborough District Council, Port Marlborough, the New Zealand King Salmon Company, the Cawthron Institute, Biosecurity New Zealand, the Department of Conservation and a number of individual mussel farming companies.

This group, known as the Didemnum Working Group, has undertaken a deliminating survey in the Sounds, and has undertaken localised clean up measures in a bid to control the organism in the area.

Ongoing efforts will involve public communication on preventing the organism’s spread; applied research into surveillance and response tools; developing further control strategies for badly affected areas; developing Codes of Practice for the wider marine farming industry; and undertaking impact analysis on the downstream effects of Didemnum on the marine farming industry.

Progress report on control measures