Didemnum is a genus of tunicates in the familyDidemnidae. Some Didemnum species, including Didemnum vexillum and Didemnum perlucidem can be characterized as invasive species. In early 2006, Didemnum was estimated to inhabit more than 175 square kilometers of the undersea region off the East Coast of the United States. Didemnum invasions have also been recorded in Canada, the West Coast of the United States, New Zealand, the Mediterranean, and the Netherlands.
Didemnum vexillum turned up on coast of state of Washington in 2009 and has been showing up in Coos Bay and Winchester Bay, Oregon as of 13 May 2010.