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Invasive Species — Didemnum Tunicate (Didemnum vexillum)
D. vexillum appears to be undergoing a rapid worldwide expansion, with most of the new records appearing in the past 10 – 15yrs. In all cases where it has been recorded as a new occurrence, it has undergone simultaneous population increases.
Likely to have originated in Japan, D. vex has spread to new locations either via hull or sea chest (water intake area) fouling, with subsequent local spreading by fouled recreational craft, barges, commercial vessels, movement of fouled aquaculture stock and gear, and drifting and reattachment of dislodged fragments.
The colonial ascidian has been reported from many parts of the world and drawn attention as a nuisance species. It has been reported from northern Europe (Netherlands, France, Ireland, Wales, Scotland), from Maritime Canada to Maryland on the east coast of the US (New Jersey, New York, Connecticut, Rhode Island, Massachusetts, New Hampshire, Maine), off shore of New England on Georges Bank; the U.S. and Canadian Pacific coasts ( California, Oregon, Washington, British Columbia, Alaska), Japan and New Zealand.
Throughout its current range, D. vex is abundant at many nearshore and offshore sites, preferring salinities above 25ppt and temperate water conditions. It can grow at depths ranging from <1m to at least 81 m. Capable of rapid growth and dispersal and at many subtidal sites, it is a dominant space holder.
Populations of the carpet tunicate have invaded a variety of habitats and will grow on a wide variety of hard substrata. It tends to prefer substrate that has some degree of fouling present and is able to overgrow plants, invertebrates and algae. Commonly found on pontoons, docks and pilings and is common on boat hulls that have not been regularly maintained or cleaned. In aquaculture areas, it is found on suspended nets, cages and lines. Unlike some introduced species that remain restricted to artificial substrates, D. vex can quickly colonize and overgrow apparently healthy natural benthic substrates, including subtidal rock outcrops and gravel (pebbles, cobbles and boulders) in deeper water (30 – 80) as well as shallow intertidal rock pools.
D. vex may be more common in offshore open water habitats than has been documented. Surveys of deep-water habitats are logistically difficult to perform and are conducted less frequently relative to surveys conducted in shallow, near-shore habitats. Changing maximum and minimum water temperatures affect the recruitment and growth of colonial tunicates. Ocean warming is thought to facilitate establishment of nonnative species.