A new species of aplousobranch ascidian, Trididemnum alexi n. sp., is described from the San Juan Archipelago of Washington state, USA. Colonies are smooth dark reddish brown; the largest is 8 cm in maximum width and up to 2 cm thick in some regions because of the large and complex hypozooidal cloacal canals. A thick (200-400 um) superficial bladder cell layer is present. The spicules are 20-30 um in diameter, irregularly stellate with short pointed rays (eight to 10 in optical equatorial plane) and scattered thinly through the tunic but absent from the bladder cell layer. Zooids have three rows of stigmata, with 12-13 stigmata per side in the first two rows in most zooids (occasionally 11 or 14) and usually one or two fewer in the third row. The tubular atrial siphon opens dorsal to the middle row of stigmata. The single testis is covered by a sperm duct with eight or nine coils. Larvae form in the basal portion of the colony, are 0.8-1.0 mm in trunk length with three adhesive papillae and usually seven pairs of lateral ampullae with curved tips in the fully formed tadpoles. A redescription of Aplidiopsis pannosum (Ritter, 1899) includes the morphological analysis of several larval stages. This species, though widespread in the north Pacific, was not previously known to occur south of Alaska on the west coast of North America. Range extensions are also included for several non-indigenous ascidians: Ciona savignyi Herdman, 1882, Botrylloides violaceus Oka, 1927, Styela clava Herdman, 1881 and Molgula manhattensis (De Kay, 1843).
Ascidiacea, Aplousobranchia, Stolidobranchia, Phlebobranchia, Trididemnum, Botrylloides, Aplidiopsis, Ciona, Molgula, Styela, NE Pacific, non-indigenous, new species.