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NIWA's guide to spotting marine pest invaders

Fuseworks Media
Fuseworks Media

Every year NIWA carries out numerous marine surveillance missions; surveys at ports and harbours around the country. Their divers are looking for the pests that have hitched a ride to New Zealand waters and are capable of harming our unique ecosystems, fisheries and aquaculture.

Knowing your enemy is crucial. So swot up on six of these beasties if you’re enjoying the ocean this summer. Find out where they are by searching for them online at And if you do happen to spot something that shouldn’t be there, you can report 24/7 by calling the Ministry for Primary Industries - Pest-and-disease hotline - 0800 80 99 66

Mediterranean fanworm

The Mediterranean fanworm is a spectacularly large worm with a characteristic whirl of filaments. Its tubes can be over 40cm long and the worm looks like a graceful, golden flower in the water. Populations are increasing, particularly around Auckland. This species can be a major fouling problem on structures and boat hulls, and competes for food with native filter feeders like mussels.

The Mediterranean fanworm can form dense colonies of up to 1000 individuals per square metre that will exclude the settlement of other organisms. The presence of the Mediterranean fanworm in areas where mussels or oysters are located may affect their growth due to competition for food. Droplet tunicate

The droplet tunicate forms large colonies and is generally found in muddy bottomed tidal habitats and on structures like wharf piles and aquaculture equipment. It is generally submerged just below the waterline, but can often be seen at low tide. Its size is influenced by seawater temperatures, with it decreasing over winter, but rapidly re-growing in summer. It can smother beaches, rocks and tide-pools and displace native species and is becoming more prolific in Northland.

The droplet tunicate looks like clusters of white tubes. Each tube is actually a cylinder containing numerous small individuals. The white tunics can sometimes appear flecked with orange.

Clubbed tunicate

The clubbed tunicate grows on natural and artificial hard marine surfaces. It is most commonly seen on wharf pilings, aquaculture structures, ropes and hulls and may also be found attached to rocks, seaweed and on shellfish. The clubbed tunicate can reach densities of 500-1500 individuals per square metre enabling it to outcompete other species.

The clubbed tunicate has a long, leathery skinned cylindrical form, tapering to a stalk with a disc shaped holdfast that anchors them to hard surfaces. The sea squirt ranges in colour from yellowish to reddish to brownish and can grow up to 160mm in length. It often appears fuzzy underwater.

Asian date mussel

The Asian date mussel, also known as the Asian mussel or bag mussel, is a small saltwater mussel. It inhabits intertidal and subtidal sediments, and produces threads that form extensive mats on the sediment surface. It grows fast, is short-lived and can obtain densities of thousands per square metre.

The Asian date mussel alters seafloor habitats and can also limit the amount of plankton available as food for other species. It is relatively small and has a greenish shell which has zig-zag markings and iridescent radiating bands. The shell is easily crushed and can grow up to 3cm long.

Wakame Undaria

Asian kelp is spread mainly by fouling on vessel hulls and is present in almost all of New Zealand’s international ports and harbours. It grows from the low intertidal area to subtidal depths of around 15 metres on any hard surface and can form dense forests in sheltered reef areas, resulting in competition for light and space which may lead to the exclusion of native plants and animals.

Mature plants are a brown, green or yellow colour and grow up to one to two metres with a visible midrib up the plant. Juvenile Asian kelp have an undivided blade.

Asian paddle crab

The Asian paddle crab is found in the Waitematā, Whangarei and Kaipara harbours and Waikare Inlet in Northland. Adults can produce hundreds of thousands of offspring. It is also aggressive, can inflict a vicious nip with its claws and competes with other species for habitat and food.

It has paddle-like hind legs with adults having a shell width of up to 12cm with six distinct spikes on each side of the eyes. It ranges in colour from pale green through to a deep chestnut brown with purplish markings. Most found in the Waitemata Harbour tend to have brown-orange markings and white tips on the claws.

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