Botryllus schlosseri (Pallas, 1766)
This species is a compound, colonial ascidian, with small, bean-shaped zooids embedded in a common matrix or tunic. The colony grows to form encrusting patches that typically reach 3-4 mm in thickness and 2-10 cm in diameter. Individual zooids lie horizontally relative to the substrate, and are organized into radial, star-shaped clusters or "systems", with each system measuring 0.5-1 cm in diameter. Each zooid grows up to 5 mm in length. Zooids are characterized by a pharynx with 9-10 rows of stigmata, and a hood that covers the opening of the atrial siphon. Color varies among colonies, but often includes a combination (=bi-colored) of yellow, orange, red, brown, grey, or black.
In the Northeast Pacific, Botryllus schlosseri is most similar to B. tuberatus Ritter and Forsyth, 1917 and two other non-native botryllid species found in the region, Botrylloides perspicuus (Herdman, 1886) and Botrylloides violaceus Oka, 1927.
Botryllus schlosseri has been introduced to the Northeast Pacific, with confirmed records ranging from Alaska to Baja California, Mexico. In addition, this species has been reported from several other locations around the world, including parts of northeast United States and Canada, southeast Asia, Australia, and New Zealand. Its native geographical range includes Great Britain, northern Europe, and the Mediterranean.