California Aquatic Invasive Species (AIS)
University of California
California Aquatic Invasive Species (AIS)

Invasive Seaweeds and Hull Fouling Species

Colonial Sea Squirt or Tunicate

Scientific Name

Diplosoma listerianum - Visit ITIS for full scientific classification. 



  • Colonial tunicate ("sea squirt") with small, cylindrical-shaped individuals immersed in a communal structure.
  • The colony forms flat, thin, mucous-covered sheets, up to 2 inches wide that grow on a variety of surfaces.  
  • Translucent appearance.
  • Can be milky yellow, or greenish, with white or grey colored spots.
  • Feed through incurrent siphons (tubular openings) that bring food particles in with the water current.
  • Subtidal marine environments.
  • Attach to a variety of surfaces including rocks, shells, other marine animals, seaweeds, buoys, lines, woody debris, vessel hulls, etc. 
Invasion Pathways and Distribution
  • Spread by attaching to vessel hulls, from which larvae swim to settle on surfaces in marinas.
  • Originally described from the English Channel.
  • However, there are many similar species and it has a very broad global distribution, therefore the exact native origin is difficult to determine.
  • It appears to be introduced along much of the North American coast, including California.
  • See NEMESIS for distribution maps. 
Life History
  • Filter-feeder. 
  • Reproduces asexually by budding or sexually by releasing sperm and eggs into the water, where they unite to form swimming larvae.
  • Colony forms a dense aggregation on boat hulls.
  • This fouling growth roughens the hull’s surface, creating friction or “drag” that slows sailboats and increases fuel consumption for powerboats.
  • Tolerant of copper in antifouling paint.
References and Useful Links

For references by category and links to other useful AIS sites see our LEARN MORE page.

Top of page

Webmaster Email: