Didemnum vexillum Kott, 2002
This species is a compound, colonial ascidian, with small, individual zooids embedded in a common matrix or tunic. The colony can grow extensively, covering large areas of substrate. Colony shape varies from encrusting sheets to long, irregular lobes, with layers often growing more than 4 mm in thickness. Individual zooids are very small, measuring approximately 1 mm. The tunic also contains microscopic calcareous spicules; however, these spicules are not abundant and are mostly confined to the surface layer of the tunic. Each spicule measures up to 58 μm (average 30 μm) in diameter and shaped like a multi-rayed star (8-11 rays). The tunic surface is characterized by narrow, spicule-free dark lines that meander between groups of zooids. Overall color varies from creamy white, pale yellow, or beige to pinkish or pale orange. See Lambert, 2009 for more detailed description.
In the Northeast Pacific, Didemnum vexillum is most similar to other didemnid tunicates, including D. carnulentum Ritter and Forsyth, 1917, D. albidium (Verrill, 1871), Trididemnum opacum (Ritter, 1907), T. strangulatum (Ritter, 1901), and T. alexi Lambert 2003.
Didemnum vexillum has been introduced to the Northeast Pacific, and has been recorded at several sites from Alaska to California. This species also has been collected from numerous locations around the world, including the northeast United States, northern Europe and Great Britain, Japan, and New Zealand (Lambert 2009). Its native geographical range likely includes Japan (Stefaniak et al. 2012).