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

Invasive Seaweeds and Hull Fouling Species

Encrusting Bryozoan

Scientific Name

Watersipora subtorquata - Visit ITIS for full scientific classification. 



  • 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. 
Invasion Pathways and Distribution
  • 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. 
Life History
  • 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.
References and Useful Links

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

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