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Thought to originate from the Northwest Pacific, this invasive sea squirt (Styela clava) has spread to many parts of the world, largely on the hulls of ships. It has the potential to alter the marine environment, impede marine farming and foul boats and fishing gear. Now it has been found in Pāuatahanui Inlet and it is important that we all look out for this little organism and do what we can to lessen the threat it poses.
 Styela clava is a non-colonial (i.e. solitary) sea squirt with a long club-shaped body. It attaches itself to rigid submerged surfaces (wharfs, boat hulls, etc) by means of a fixed, or sessile, stalk. The surface is tough, leathery, rumpled and knobbly and has a brownish-white, yellowish-brown or reddish-brown colour. It can grow up to 16cm long. A filter feeder, it has two siphons, one inhalant and the other exhalant, through which it draws water in and out of its body, straining planktonic organisms as it does so.
Like all sea squirts Styela is hermaphroditic, but the gonads mature at different times, avoiding self-fertilization. Given the right conditions it spawns during spring and autumn shedding eggs or sperm into the water every 24 hours. Larvae remain planktonic for only a short time before settling down and metamorphosing. Maturity is reached in around 6 months.
Styela clava has the potential to spread rapidly and can form densities of up to 1500 individuals per square metre. An adult can filter approximately 150 millilitres of water per minute for each gram of body weight. With such statistics it can and does compete severely for the planktonic food supplies of native invertebrate species and those that are commercially and recreationally important. It also consumes the larvae of fish and molluscs and as such it is a major problem for shellfish aquaculture. While predated upon in larval and juvenile form there are no known predators for the adult and wherever it has occurred it has proved impossible to eradicate.
Fortunately for Pāuatahanui Inlet Styela doesn't breed at temperatures below 15°C, a level exceeded only during the summer months, and while salinities are generally high enough for Styela to establish itself the general lack of firm surfaces will provide limited areas for the larvae to settle. Given these factors the threat to local filtering fauna and fish larvae is likely to be relatively small compared to many places in northerly NZ.
Despite these limitations to its spread Greater Wellington Regional Council is asking all boaties to be vigilant and reduce the chances of this organism becoming a region-wide nuisance by keeping the hulls of moored boats clean and well antifouled. If you have a boat on the Inlet, we urge you to follow this advice.
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As a point of interest sea squirts, also known as Tunicates, are an offshoot of the phylum Chordata and as such a sister group to the vertebrates. Evolutionarily therefore, believe it or not, they are very distantly related to humans.
This article first appeared in 'The Inlet', the newsletter of the Guardians of Pāuatahanui Inlet, in August 2013.
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