Invasive Seaweeds and Hull Fouling Species

Watersipora subtorquata - Visit ITIS for full scientific classification. 

Watersipora subtorquata. Photo by Carrie Culver © 2008 Regents, University of California.

• Colonial invertebrate animals.
• This bryozoan species forms a brittle structure containing individual animals inside.
• Encrusts hard surfaces.
• Bright orange-red to black in color.
• It develops overlapping crusts, or petals, that can grow into large masses, up 24 inches long and 12 inches high.  
• Each crust or petal is comprised of small, closely packed “houses” where the individual animals in the colony live.
• The individual animals in the colony are called zooids, and appear as small, black dots to the naked eye.
• When viewed under a microscope, the soft-bodied animals (zooids) with their translucent feeding tentacles become visible.

• Common in lower intertidal, and subtidal areas.
• More abundant in areas with higher water flow.
• Found on a wide variety of natural and man-made surfaces including: rocks, shells, vegetation, woody debris, other marine animals, pier pilings, vessel hulls, oil platforms, etc. 

• Spread by attaching to vessel hulls, from which larvae swim to settle on surfaces in marinas.
• Found worldwide, including California.
• Origin unknown, but non-native to North America.
• See NEMESIS for a distribution map. 

• Filter-feeder.
• Reproduce sexually by releasing sperm and eggs into the water, where they unite to form swimming larvae.
• The larvae settle onto hard surfaces and develop into zooids.
• The mature zooids asexually bud into a colony capable of sexual reproduction, producing new larvae. 

• The encrusting leaf-like bryozoan structure can form large masses on boat hulls, piers and other structures.
• 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.
• It also provides non-toxic surfaces for other fouling species to settle and develop.
• Competes for space with native species.

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