Didemnum
Didemnidae - Didemnum vexillum
BioSecurity

What does it look like?

Carpet sea squirts grow attached to hard surfaces.  The carpet sea squirt is leathery or spongey and light mustard in colour.  It can look like a yellowish wax dripping over a structure such as a rope or mussel line.  It can be distinguished from native species by its colour and non-slimy feel.

Colonies of Carpet sea squirt can be found from the intertidal zone in salinities greater than about 25 PSU, down to depths of at least 65 meters.  It is more common on artificial structures but has been found growing on rocks, seaweed, and seagrass in tide pools, estuaries, lagoons and open coastal areas.  In New Zealand, it is generally found on structures such as wharves, mooring lines and vessel hulls and appears to have only a limited ability to establish in natural habitats.

Why is it a problem?

Carpet sea squirt can tolerate a wide range of environmental conditions, has a high reproductive ability, and high population growth rates.  The absence of natural predators and/or diseases outside its native range also gives it a competitive advantage.  Carpet sea squirt has the ability to grow over other organisms and can hinder the settlement of larvae through the production of chemical compounds.

Carpet sea squirt can reproduce sexually and releases larvae that are carried in water currents. It can also reproduce from fragments that break off the “parent” and grow into new colonies.  The reproductive season of the carpet sea squirt in New Zealand is considerably longer than comparable northern-hemisphere populations (at least 9 months of the year compared with 3 to 5 months in the USA).

Larvae can be dispersed relatively short distances  by water currents and in ballast water.  Carpet sea squirt may also be transported by marine farming activities (e.g. transfer of aquaculture stock between harbours) and hull fouling.

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