Invasion History

First Non-native North American Tidal Record: 1959
First Non-native West Coast Tidal Record: 1959
First Non-native East/Gulf Coast Tidal Record:

General Invasion History:

Watersipora arcuata was first described from San Diego Bay, California, but is apparently introduced in Southern California (Banta 1969). Banta considered that specimens of Watersipora collected in the Galapagos (Hastings 1930, cited by Banta 1969), the Gulf of California (Osburn 1952, Soule 1961, cited by Banta 1969), and Scammon's Lagoon (Soule and Soule 1964), on the Pacific side of Baja California also belonged to this species. The native region of W. arcuata is not clear, although Banta concluded that it was native to the eastern tropical Pacific. It appears to be introduced in Hawaii (in 1998, Carlton and Eldredge, 2009); New Zealand (in 1957, Gordon 1989; Gordon 1992), and Australia (in the 1940s, Allen and Wood 1950, cited by Keough and Ross 1999). Invasion history of this species is complicated by its taxonomic history- several species of Watersipora were lumped under the name W. cucullata (Banta 1969; Gordon 1967; Gordon 1992; Mackie et al. 2006). Ship fouling appears to be the primary mode of introduction, since its larval duration is very brief (Mackie et al. 2006). Observations in California and New Zealand suggest that W. arcuata may be a warm-water species which may be restricted in cooler waters, or else outcompeted by W. subtorquata and Watersipora sp. A (Gordon and Mawatari 1993; Geller et al. 2008).

North American Invasion History:

Invasion History on the West Coast:

Watersipora arcuata was collected in the Gulf of California before 1930, and in Pacific Mexico by the early 1960s (Banta 1969). It was first collected in US waters at the Southwestern Yacht Club Marina, in San Diego Harbor, its type locality (Banta 1969). In the same year, it was found in Oceanside Harbor, Alamitos Bay (Long Beach Marina), Redondo Harbor, and Marina del Rey (Banta 1969). In later sampling, it was found in additional southern California harbors, including Mission Bay (in 2003, de Rivera et al. 2005), Dana Point Harbor, Newport Bay, Huntington Harbor (in 2001, Fairey et al. 2002), Channel Islands Harbor, Port Hueneme (in 2000, Cohen et al. 2002), Ventura West Harbor (in 2003, de Rivera et al. 2005), and Santa Barbara Harbor (Geller et al. 2008). So far, it has not been reported north of Point Conception.

Invasion History in Hawaii:

Watersipora arcuata has been found in several small harbors in the Hawaiian Islands, including Kewalo Basin (in 1998, Coles et al. 1999), Alai Wai and Haleiwa Harbors, in Oahu (in 2001, Mackie et al. 2006) and Kahulului Harbor in Maui (in 2003, Coles et al. 2004; Carlton and Eldredge 2009). Mackie et al. (2006) confirmed the identity of W. arcuata by molecular means based on material from Oahu.

Invasion History Elsewhere in the World:

Watersipora arcuata was collected in New South Wales, Australia in the 1940s (Keough and Ross 1999) and is known from several sites near Sydney: Port Kemble (Moran and Grant 1993, cited by Keough and Ross 1999); Port Hacking (Vail and Wass 1981); Manly and The Spit (in 2001, Mackie et al. 2006). It was first collected in Port Phillip Bay, Victoria in 1973 (Holmes 1982, cited by Keough and Ross 1999), and subsequently found in South Australia, near Adelaide, and Western Australia, near Perth (2001, Mackie et al. 2006). In New Zealand, W. arcuata was first found in Auckland, in Waitemata Harbor in 1957 (Gordon 1967; Gordon and Mawatari 1992), and subsequently found in several other harbors on the North Island and at Nelson on the South Island (Gordon 1967). However, W. arcuata has been replaced by W. subtorquata at many locations. In 2013-2014, established populations of W. arcuata were found in a marina in Santa Margherita Ligure, in northwest Italy, the first record in European waters (Ferrario et al. 2015).

Molecular studies (Cytochrome oxidase 1 nucleotide sequences) of Watersipora arcuata showed great genetic diversity at each location, but relatively little geographical differentiation between populations in Australia, New Zealand, California, and Hawaii, suggesting multiple invasions from different source populations, or perhaps a diverse original population. Reduction in diversity due to founder effects was not seen (Mackie et al. 2006).


Watersipora arcuata is an encrusting bryozoan, but its colonies can become erect and leaf-like, with extensively overlapping calcareous crusts, often with curled edges. This bryozoan forms black crusts with red or orange growing edges. The zooids are roughly elongate or rectangular, and 680-1,110 µm X 290-600 µm in size. The frontal shield is perforated by pseudopores. The orifice is slightly wider than long, and the proximal edge is broad and slightly convex or straight. There are no oral spines, avicularia, or ovicells (description from Gordon and Mawatari 1992).


Taxonomic Tree

Kingdom:   Animalia
Phylum:   Bryozoa
Class:   Gymnolaemata
Order:   Cheilostomata
Suborder:   Ascophora
Family:   Watersiporidae
Genus:   Watersipora
Species:   arcuata


Watersipora cucullata (of authors, not Busk, 1854)
Watersipora nigra (of authors, not Canu and Bassler , 1930)

Potentially Misidentified Species

Watersipora n. sp.
This newly identified, undescribed species is morphologically nearly identical to W. subtorquata but distinct at the molecular level (Mackie 2006; Geller et al. 2008; Mackie et al. 2012). So far, it is known only from California and South Korea.

Watersipora subovoidea
(d'Orbigny, 1852), described from the Aegean Sea. Redescribed and redefined by Ryland et al. (2009). It has been characterized in molecular studies by Mackie et al. (2006) and Mackie et al. (2012).

Watersipora subtorquata
(d'Orbigny, 1852), described from Brazil, Redescribed and redefined morphologically by Ryland et al. (2009). It has been characterized in molecular studies by Mackie et al. (2006) and Mackie et al. (2012).



Life History- Watersipora arcuata is an encrusting, calcified bryozoan composed of many individual zooids. The zooids feed by extending the ciliated tentacles of the lophophore as a funnel, creating a current, and driving food particles into their mouths. The food is guided along the tentacles and through the pharynx by the cilia. Larger food particles can be moved or captured by flicking or contracting the tentacles. (Barnes 1983). Larvae of W. arcuata are lecithotrophic and have a short planktonic period (less than 1 day, Mackie et al. 2006). Larvae settle on a substrate and metamorphose into the first zooid of a colony, an ancestrula (Barnes 1983).

Ecology- Watersipora arcuata is known from pilings, rocks, floats and ships' hulls (Banta 1969; Gordon and Mawatari 1992).


Phytoplankton, detritus

Trophic Status:

Suspension Feeder



General HabitatCoarse Woody DebrisNone
General HabitatMarinas & DocksNone
Salinity RangePolyhaline18-30 PSU
Salinity RangeEuhaline30-40 PSU
Tidal RangeSubtidalNone
Tidal RangeLow IntertidalNone
Vertical HabitatEpibenthicNone

Tolerances and Life History Parameters

Minimum Duration1Larval duration (Wisley 1958, cited by Mackie et al. 2006)
Maximum Duration2Larval duration (Wisley 1958, cited by Mackie et al. 2006)
Broad Temperature RangeNoneWarm temperate-Subtropical
Broad Salinity RangeNonePolyhaline-Euhaline

General Impacts

Economic Impacts

Shipping- Watersipora arcuata is notable for its ability to settle on surfaces covered with copper-based antifouling paints, and to survive on fast moving ships (Allen 1953; Banta 1969). It has the potential to reduce the speed of ships and may provide settling surfaces for other organisms.

Ecological Impacts

Competition- Watersipora arcuata 'became one of the dominant intertidal organisms at several locations' in Auckland Harbor, and spread to other harbors, but in the 1980s was largely repressed by W. subtorquata (Gordon and Mawatari 1992).

Regional Impacts

NZ-IVNoneEconomic ImpactShipping/Boating
'An important fouling species' (Gordon 1989).
NZ-IVNoneEcological ImpactCompetition
It 'became one of the dominant intertidal organisms at several locations' in Auckland Harbor, and spread to other harbors, but in the 1980s was largely repressed by W. subtorquata.
AUS-XNoneEcological ImpactCompetition
It is 'now the most abundant shallow-water bryozoan on the New South Wales Coast' (Allen 1953).

Regional Distribution Map

Bioregion Region Name Year Invasion Status Population Status
SEP-Z None 1930 Crypto Estab
NEP-VII None 1940 Crypto Estab
NEP-VI Pt. Conception to Southern Baja California 1959 Def Estab
AUS-VIII None 1973 Def Estab
AUS-X None 1950 Def Estab
AUS-VII None 1953 Def Estab
AUS-V None 1997 Def Estab
AUS-XII None 1953 Def Estab
NZ-IV None 1957 Def Estab
SP-XXI None 1998 Def Estab
P020 San Diego Bay 1967 Def Estab
P030 Mission Bay 2003 Def Estab
P023 _CDA_P023 (San Louis Rey-Escondido) 1969 Def Estab
P027 _CDA_P027 (Aliso-San Onofre) 2001 Def Estab
P040 Newport Bay 2001 Def Estab
P050 San Pedro Bay 1969 Def Estab
P060 Santa Monica Bay 1969 Def Estab
P062 _CDA_P062 (Calleguas) 2000 Def Estab
P064 _CDA_P064 (Ventura) 2003 Def Estab
AUS-XI None 2001 Def Estab
AUS-IV None 1953 Def Estab
P065 _CDA_P065 (Santa Barbara Channel) 2007 Def Estab
MED-II None 2013 Def Estab
MED-IV None 2016 Def Estab
MED-VIII None 2015 Def Estab
MED-V None 2016 Def Estab
EAS-VI None 2010 Def Estab
NEA-V None 1990 Def Estab
MED-VII None 2019 Def Estab
MED-VI None 2015 Def Estab

Occurrence Map

OCC_ID Author Year Date Locality Status Latitude Longitude


Allen, F. E. (1953) Distribution of marine invertebrates by ships, Australian Journal of Marine and Freshwater Research 4(2): 307-316

Anderson, Dean P.; Pepper, Margaret A.; Travers, Shelby; Michaels, Trevor A.; Sullivan, Kevin · Ramsey, David S. L. (2022) Confirming the broadscale eradication success of nutria (Myocastor coypus) from the Delmarva Peninsula, USA , Biological Invasions 24: 509–3521

Armstrong, John D.; Bean, Colin W.; Wells, Alan (2018) The Scottish invasion of pink salmon in 2017, Journal of Fish Biology 93: 8-11
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Banta, W. C. (1969) The recent introduction of Watersipora arcuata Banta (Bryozoa, Cheilostomata) as a fouling pest in Southern California, Bulletin of the Southern California Academy of Sciences 68: 248-251

Barnes, Robert D. (1983) Invertebrate Zoology, Saunders, Philadelphia. Pp. 883

Carlton, J.T., Eldredge, L. (2001) <missing title>, Bernice P. Bishop Museum Press, Honolulu, Hawaii,. Pp. <missing location>

Carlton, James T. (1979) History, biogeography, and ecology of the introduced marine and estuarine invertebrates of the Pacific Coast of North America., Ph.D. dissertation, University of California, Davis. Pp. 1-904

Carlton, James T.; Eldredge, Lucius (2009) Marine bioinvasions of Hawaii: The introduced and cryptogenic marine and estuarine animals and plants of the Hawaiian archipelago., Bishop Museum Bulletin in Cultural and Environmental Studies 4: 1-202

Çinar, Melih Ertan and 7 authors (2021) Current status (as of end of 2020) of marine alien species in Turkey, PLOS ONE 16: Published online

Cohen, Andrew N. and 12 authors (2002) Project report for the Southern California exotics expedition 2000: a rapid assessment survey of exotic species in sheltered coastal waters., In: (Eds.) . , Sacramento CA. Pp. 1-23

Coles S. L., DeFelice R. C., Eldredge, L. G. (1999a) Nonindigenous marine species introductions in the harbors of the south and west shores of Oahu, Hawaii., Bishop Museum Technical Report 15: 1-212

Coles, S. L.; DeFelice, R. C.; Eldredge, L. G.; Carlton, J. T. (1999b) Historical and recent introductions of non-indigenous marine species into Pearl Harbor, Oahu, Hawaiian Islands., Marine Biology 135(1): 147-158

Coles, S. L.; Reath, P. R.; Longenecker, K.; Bolick, Holly; Eldredge, L. G. (2004) <missing title>, Hawai‘i Community Foundation and the U. S. Fish and Wildlife Service, Honolulu. Pp. 1-187

Cranfield, H.J.; Gordon, D.P.; Willan, R.C.; Marshall, B.A; Battershill, C.N.; Francis, M.P.; Nelson, W.A.; Glasby, C.J.; Read, G.B. (1998) <missing title>, The National Institute of Water and Atmospheric Research, New Zealand. Pp. <missing location>

Crivellaro, Marcelo Schuler; Candido, Davi Volney; ilveira, Thiago Cesar Lima; Fonseca, Adriana Carvalhal; Segal, Barbara ´ (2022) A tool for a race against time: Dispersal simulations to support ongoing monitoring program of the invasive coral Tubastraea coccinea, Marine Pollution Bulletin 185(114354): Published online

de Rivera, Catherine, and 27 authors (2005) Broad-scale non-indigenous species monitoring along the West Coast in National Marine Sanctuaries and National Estuarine Research Reserves report to National Fish and Wildlife Foundation, National Fish and Wildlife Foundation, Washington, D.C.. Pp. <missing location>

Fairey, Russell; Dunn, Roslyn; Sigala, Marco; Oliver, John (2002) Introduced aquatic species in California's coastal waters: Final Report, California Department of Fish and Game, Sacramento. Pp. <missing location>

Ferrario, Jasmine; D’Hondt, Jean-Loup; Marchini, Agnese; Occhipinti-Ambrogi, Anna (2015) From the Pacific Ocean to the Mediterranean Sea: Watersipora arcuata, a new non-indigenous bryozoan in Europe, Marine Biology Research 11(9): 909-919

Ferrario, Martha E.; Sar, Eugenia A.; Codina, Raul G. (1986) Diatomeas marinas de la provincia de Chubut (Republica Argentina). Centrales. I., Darwiniana 27(1-4): 89-106

Gordon, D. P. (1989) The marine fauna of New Zealand: Bryozoa: Gymnolaemata (Cheilostomida Ascophorina) from the Western South Island continental shelf and slope., New Zealand Oceanographic Institute Memoir 97: 5-95

Gordon, Dennis P.; Mawatari, S.F. (1992) Atlas of marine-fouling bryozoa of New Zealand ports and harbors., Miscellaneous Publications of the New Zealand Oceanographic Institute 107: 1-52

Hewitt, C. L. (Ed.) (2002) <missing title>, <missing publisher>, <missing place>. Pp. <missing location>

Hewitt, C.L.; Campbell, M.L.; Thresher, R.E.; Martin, R.B. (1999) Marine Biological Invasions of Port Phillip Bay, Victoria, In: (Eds.) . , Hobart, Tasmania. Pp. <missing location>

Huisman, John M.; Jones, Diana S.; Wells, Fred E.; Burton, Timothy S. (2008) Introduced marine biota in Western Australian waters., Records of the Western Australian Museum 25: 1-44

Keough, M. J. ; Ross, J. (1999) Introduced fouling species in Port Phillip Bay., In: Hewitt, C. L.; Campbell; M.;Thresher, R.; Martin,(Eds.) Marine Biological Invasions of Port Phillip Bay, Victoria. , Hobart, Tasmania. Pp. 193-225

Mackie, Joshua A.; Darling, John A.; Geller, Jonathan B. (2012) Ecology of cryptic invasions: latitudinal segregation among Watersipora (Bryozoa) species, Scientific Reports 2(871): 1-10

Mackie, Joshua A.; Keough, Michael J.; Christidis, Les (2006) Invasion patterns inferred from cytochrome oxidase I sequences in three bryozoans, Bugula neritina, Watersipora subtorquata; and Watersipora arcuata., Marine Biology 149: 285-295

Marshall, J. I.; Rowe, F. W. E.; Fisher; Smith, D. F. (1980) Alterations to the relative species-abundance of ascidians and barnacles in a fouling community due to screens, Australian Journal of Marine and Freshwater Research 31: <missing location>

Rodriguez, Laura F.; Ibarra-Obando, Silvia E. (2008) Cover and colonization of commercial oyster (Crassostrea gigas) shells by fouling organisms in San Quintin Bay, Mexico, Journal of Shellfish Research 27(2): 337-343

Ruiz, Gregory M.; Geller, Jonathan (2018) Spatial and temporal analysis of marine invasions in California, Part II: Humboldt Bay, Marina del Re, Port Hueneme, and San Francisco Bay, Smithsonian Environmental Research Center & Moss Landing Laboratories, Edgewater MD, Moss Landing CA. Pp. <missing location>

Ruiz, Gregory; Geller, Jonathan (2021) Spatial and temporal analysis of marine invasions: supplemental studies to evaluate detection through quantitative and molecular methodologies, Marine Invasive Species Program, California Department of Fish and Wildlife, Sacramento CA. Pp. 153 ppl.

Soule, Dorothy F.; Soule, John D. (1964) The Ectoprocta (Bryozoa) of Scammon's Lagoon, Baja California, Mexico., American Museum Novitates 2199: 1-56

Soule, John D. (1963) Results of the Puritan-American Museum of Natural History Expedition to Western Mexico: 18. Cyclostomata, Ctenostomata (Ectoprocta), and Entoprocta of the Gulf of California., American Museum Novitates 2144: 1-34

Tamburini, M. ; Ferrario, J.;; Marchini, A.;; Piazza, A.; Lo Vullo; M.; Occhipinti-Ambrogi, A. (2023) Are Fouling Non-indignousspecies invading Porto Venere Bay? An Assessment through image analysis , Biologia Marina Mediterranea <missing volume>(27): 213-216

Ulman, Aylin and 17 authors (2017) A massive update of non-indigenous species records in Mediterranean marinas, PeerJ 5( e3954): <missing location>

Vail, Lyle L.; Wass, Robin E. (1981) Experimental studies on the settlement and growth of Bryozoa in the natural environment., Australian Journal of Marine and Freshwater Research 32: 639-656

Vieira, Leandro M.; Jones, Mary Spencer; Taylor, Paul D. (2014) The identity of the invasive fouling bryozoan Watersipora subtorquata (d’Orbigny) and some other congeneric species, Zootaxa 3857: 151-182

Wiltshire, K.; Rowling, K.; Deveney, M. (2010) <missing title>, South Australian Research and Development Institute, Adelaide. Pp. 1-232

Zambrano, René; Ramos, John (2021) Alien crustacean species recorded in Ecuador, Nauplius 29: e2021043