Invasion History

First Non-native North American Tidal Record: 1986
First Non-native West Coast Tidal Record: 1986
First Non-native East/Gulf Coast Tidal Record:

General Invasion History:

The freshwater kamptozoan (Entoprocta) Urnatella gracilis was first described from the Schuylkill River near Philadelphia, Pennsylvania by Joseph Leidy in 1851. It has been found in the eastern United States from the Connecticut River, Lake Erie, and Michigan to the Gulf Coast (Wurtz and Roback 1955; McCullough and Smith 1975; Hull et al. 1980; Wood 1991; Cohen and Carlton 1995), and is presumed to be native to this region. In 2011-2012, high densities of U. gracilis were found on shells of the Asian Freshwater Clam (Corbicula fluminea), on the south shore of the tidal fresh St. Lawrence River, near a nuclear power plant near Becancour, Quebec. This is the first record of this entoproct in the Great Lakes-St. Lawrence system, and its northernmost record (Hamelin et al. 2016).

North American Invasion History:

Invasion History on the West Coast:

In 1972, Urnatella gracilis was first collected west of the Rocky Mountains, in the Delta-Mendota Canal in California, and by 1986, it was collected in the Sacramento-San Joaquin Delta (Cohen and Carlton 1995). In 2002, it was collected in tidal fresh waters of the Columbia River estuary (Systma et al. 2004).

Invasion History Elsewhere in the World:

Urnatella gracilis is known from tidal waters entering the Black Sea and Sea of Azov (Gomiou et al. 2002; Grigorevich et al. 2002), and fresh waters of France, Germany (Gollasch and Nehring 2007), Brazil, and Japan (Oda 1982). Likely vectors include aquatic plants and freshwater canals. Urnatella produces thick-shelled resting cysts (Wood 1991), but their ability to tolerate salt or brackish water is unknown.


The freshwater kamptozoan Urnatella gracilis has a stalk up to 500 µm, which is segmented, resembling a string of beads. Young segments are transparent, but older ones are yellowish. New stolons of about 2-3 segments can form from the main stalk, but then break off and form new colonies. The body is up to 450 µm long and has 12-16 tentacles. The bodies live only about a month, but new ones are continually being formed by lateral budding from the stalk. Segments of the stalk, as well as the basal plate may serve as hibernacula, regenerating a new colony after unfavorable conditions (Nielsen 1989).


Taxonomic Tree

Kingdom:   Animalia
Phylum:   Entoprocta
Order:   Urnatellida
Family:   Urnatellidae
Genus:   Urnatella
Species:   gracilis


Potentially Misidentified Species

Loxosomatoides sirindhornae
Loxosomatoides sirindhornae is the only other known freshwater kamptozoan, so far known only from Thailand (Wood et al. 2006).



Urnatella gracilis is one of only two known freshwater entoprocts. It forms large, fuzzy colonies, reproducing asexually by budding new zooids from stolons. Embryos have not been seen in wild colonies, but a few have been seen in laboratory cultures. The basal plate and the stalk segments serve as hibernacula, providing a means for the colonies to persist through unfavorable conditions (Nielsen 1989). Urnatella gracilis is known only from fresh and slightly brackish waters, on stones, twigs, and mollusk shells (McCullough and Smith 1975; Hull et al. 1980; Nielsen 1989). Its salinity tolerance is unknown. Some of its occurrences are in estuarine regions where intrusion of brackish water is possible.


Phytoplankton, detritus

Trophic Status:

Suspension Feeder



General HabitatFresh (nontidal) MarshNone
General HabitatGrass BedNone
General HabitatCoarse Woody DebrisNone
General HabitatSwampNone
General HabitatNontidal FreshwaterNone
General HabitatTidal Fresh MarshNone
General HabitatMarinas & DocksNone
General HabitatCanalsNone
Salinity RangeLimnetic0-0.5 PSU
Salinity RangeOligohaline0.5-5 PSU
Tidal RangeSubtidalNone
Vertical HabitatEpibenthicNone

Tolerances and Life History Parameters

Minimum Salinity (‰)0Urnatella gracilis is a freshwater organism.
Maximum Salinity (‰)5Field record, approximate (Paavola et al. 2005)
Maximum Height (mm)1Nielsen 1986
Broad Temperature RangeNoneCold temperate-Warm temperate
Broad Salinity RangeNoneNontidal Limnetic-Oligohaline

General Impacts

No impacts have been reported for Urnatella gracilis in its native or introduced ranges.

Regional Distribution Map

Bioregion Region Name Year Invasion Status Population Status
P090 San Francisco Bay 1986 Def Estab
P260 Columbia River 2002 Def Estab
M130 Chesapeake Bay 0 Native Estab
G260 Galveston Bay 0 Native Estab
M090 Delaware Bay 1851 Native Estab
GL-II Lake Erie 0 Native Estab
G070 Tampa Bay 1977 Native Estab
M040 Long Island Sound 0 Native Estab
MED-X None 1962 Def Estab
MED-IX None 1954 Def Estab
G250 Sabine Lake 0 Native Estab
L095 _CDA_L095 (Cedar-Portage) 0 Native Estab
NA-S3 None 2012 Crypto Estab

Occurrence Map

OCC_ID Author Year Date Locality Status Latitude Longitude
31034 Light, Grosholz and Moyle, 2005 2005 2005-01-01 Delta General Location Def 38.0500 -121.8100
31036 Markmann 1986, cited by Cohen and Carlton 1995 1983 1983-01-01 Delta General Location Def 38.0500 -121.8100
31044 Markmann 1986, cited by Cohen and Carlton 1995 1982 1982-01-01 CA/Sacramento-San Joaquin Delta Def 38.0500 -121.8100
33577 Eng 1977, cited by Cohen and Carlton, 1995 1974 1974-01-01 Delta Mendota Canal Def 37.6930 -121.4660


Cohen, Andrew N.; Carlton, James T. (1995) Nonindigenous aquatic species in a United States estuary: a case study of the biological invasions of the San Francisco Bay and Delta, U.S. Fish and Wildlife Service and National Sea Grant College Program (Connecticut Sea Grant), Washington DC, Silver Spring MD.. Pp. <missing location>

Gollasch, Stephan; Nehring, Stefan (2006) National checklist for aquatic alien species in Germany., Aquatic Invasions 1(2): 245-269

Gomiou, Marian-Traian; Alexandrov, Boris; Shadrin, Nikolai; Zaitsev, Yuvenaly (2002) The Black Sea- a recipient, donor, and transit area for alien species., In: Leppakoski, E.; Gollasch, S.; Olenin, S.(Eds.) Invasive aquatic species of Europe: Distribution, impacts, and management.. , Dordrecht. Pp. 341-350

Grigorovich, Igor A.; MacIsaac, Hugh J.; Shadrin, Nikolai V.; Mills, Edward L. (2002) Patterns and mechanisms of aquatic invertebrate introductions in the Ponto-Caspian region., Canadian Journal of Fisheries and Aquatic Science 59: 1189-1208

Hamelin, Kayla M.; Castaneda, Rowshyra A.; Ricciardi, Anthony (2016) Cryptic invaders: nonindigenous and cryptogenic freshwater Bryozoa and Entoprocta in the St. Lawrence River, Biological Invasions Published online: <missing location>

Hull, H. C.; Bartos, F.; Martz, R. A. (1980) Occurrence of Urnatella gracilis Leidy in the Tampa Bypass Canal, FL., Florida Scientist 43(1): 12-14

LeConte, John L.; Horn, George H.; Leidy. Joseph; Hunt, J. Gibbons; Meehan, Thomas (1880) Report on plants introduced by means of the International Exhibition, 1876, Proceedings of the Academy of Natural Sciences of Philadelphia 32: 132

Leland, Harry V.; Fend, Steven V. (1998) Benthic invertebrate distributions in the San Joaquin River, California, in relation to physical and chemical factors., Canadian Journal of Fisheries and Aquatic Science 55: 1051-1067

Light, Theo; Grosholtz, Ted; Moyle, Peter (2005) Delta ecological survey (phase1): Nonindigenous aquatic species in the Sacramento-San Joaquin Delta, a literature review, In: None(Eds.) None. , Stockton, CA. Pp. <missing location>

McCullough, Jack D.; Smith, Betsy (1975) Some ecological observations on Urnatella gracilis Leidy., Southwestern Naturalist 20(2): 171-176

Nielsen, Claus (1989) <missing title>, E.J. Brill, Leiden. Pp. <missing location>

Oda, Shuzitu (1982) Urnatella gracilis, a freshwater kamptozoan, occurring in Japan., Annotationes Zoologiacae Japonenses 55(3): 151-166

Paavola, Marjo; Olenin, Sergei; Leppakoski, Erkki (2005) Are invasive species most successful in habits of low native species richness across European brackish water seas?, Estuarine Coastal and Shelf Science 64: 738-750

Rogick, Mary D. (1935) Studies on Freshwater Bryozoa: II. The Bryozoa of Lake Erie., Transactions of the American Microscopical Society 54(3): 245-263

Sytsma, Mark D.; Cordell, Jeffrey R.; Chapman, John W.; Draheim, Robyn, C. (2004) <missing title>, Center for Lakes and Reservoirs, Portland State University, Portland OR. Pp. <missing location>

Wood, Timothy S. (1991) Ecology and Classification of North American Freshwater Invertebrates, Academic Press, San Diego. Pp. 481-499

Wood, Timothy S.; Anurakpongsatorn, Patana Mahujchariyawong, Jukkrit (2006) Freshwater bryozoans of Thailand (Ectoprocta and Entoprocta), Natural History Journal of Chulalongkorn University 6(2): 83-119

Wurtz, Charles B.; Roback, Selwyn S. (1955) Invertebrate fauna of some Gulf coast rivers, Proceedings of the Academy of Natural Sciences of Philadelphia 107: 167-206