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Invasive Species — Didemnum Tunicate (Didemnum vexillum)

Didemnum vexillum is probably moved between locations in ballast water as colony fragments, on fouled hulls of vessels, or possibly with infested mariculture gear or infrastructure. Its discovery in Washington and British Columbia on mussel rafts owned by the same company suggests that transfers of shellfish stock or equipment may play a role in coastwise spread. Various sea squirts, including D. vexillum, along with barnacles, tubeworms, bryozoans are commonly found growing on the hulls of boats that have not received adequate cleaning and/or maintenance.

In some parts of the world, recreational boaters are considered the mostly likely vector for distributing invasive tunicates. As fouling organisms on vessel hulls and sea chests, commercial shipping is also recognized as a pathway to move species around. In some areas, floating infrastructure is moved between coastal locations, such material may include docks, mariculture superstructure, buoys, and other such floating structures. If boats and other floating material reside in an area infested by invasive species for any length of time, it is likely they will begin to harbor both invasive and native organisms. If floating structures and hulls are not cleaned before they are moved to a new site they can easily introduce communities of organisms to the new area.