Invasive Seaweeds and Hull Fouling Species

Diplosoma listerianum - Visit ITIS for full scientific classification. 

Diplosoma listerianum. Photo by Carrie Culver © 2008 Regents, University of California.

• 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. 

• 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. 

• 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.

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