Invasion History

First Non-native North American Tidal Record: 1904
First Non-native West Coast Tidal Record: 1970
First Non-native East/Gulf Coast Tidal Record: 1904

General Invasion History:

The medusa of the hydrozoan Blackfordia virginica was first collected in Norfolk Harbor, Virginia, in 1904, and described by Mayer (1910). Mayer also collected this species off Sandy Hook, New Jersey (as B. manhattensis, Mayer 1910). Blackfordia virginica's native distribution includes the Black Sea and the Sea of Azov (Thiel 1935; Naumov 1969; Mills and Sommer 1995), but canals enabled it to invade the Caspian Sea in 1956 (Logvinenko 1959). Based on the abundance of polyps and medusae in the Black Sea, Thiel (1935) considered that it was part of an endemic 'Sarmatic' cnidarian fauna, and only recently introduced to the United States. This was based on B. virginica's absence in early American collections (of Agassiz, Fewkes, Hargitt, etc.) and the lack of records of polyps from North America. It should be noted that Russian and Ukrainian workers consider B. virginica to be of Northwest Atlantic origin and introduced to the Black Sea (Mordukhai-Boltovskoi 1964; Goumiou et al. 2002). Provisionally, we are following Mills and Sommer (1995) and Ruiz et al. (2000) in treating this species as having a Black Sea origin. A genetic analysis by Harrison et al. (2013) found that North American populations had a very low genetic diversity, compared to that of 30 other hydrozoan species. There were no differing haplotypes among populations in four US estuaries on the East, Gulf, and West coasts, supporting the possibility of a single introduction to North America, and subsequent spread (Harrison et al. 2013).

Blackfordia virginica has been reported from European Atlantic estuaries, including the Loire, France; the Mira, Portugal (in 1984, Moore 1987); and the Guadiana estuary, Spain (Chícharo et al. 2009). It has also been found in the Caspian Sea (Logvinenko 1959), South America (Brazil in 1963, Argentina in 1983), India (in 1958, 1968, 1970; Moore 1987) and China (Chen-tsu & Chin 1962, cited by Bardi and Marques 2009). More recently, records have been reported from tropical Pacific Mexico (Alvarez 1998) and South Africa (Buecher et al. 2005). In the United States, B. virginica medusa were collected from tributaries of San Francisco Bay in 1993 and found in preserved samples from 1970 and 1974. Medusa have also been collected from Coos Bay, Oregon (in 1997; James T. Carlton, personal communication). Polyps were found in San Francisco Bay (Mills and Sommer 1995; Mills and Rees 2000), and in 1999 were reported from Baltimore Harbor in Chesapeake Bay (Ruiz et al., unpublished data). It is likely to have been widely transported in ballast water and fouling.

A genetic analysis by Harrison et al. (2013) found that North American populations had a very low genetic diversity, compared to that of 30 other hydrozoans. There were no differing haplotypes among populations in four US estuaries, on the East, Gulf, and West coasts, supporting the possibility of a single introduction to North America, and subsequent spread (Harrison et al. 2013).

North American Invasion History:

Invasion History on the West Coast:

Blackfordia virginica medusae were found in preserved samples collected in 1970-1974 from the Napa and Petaluma Rivers, brackish tributaries of San Francisco Bay. In 1992-1994, live medusae were studied (Mills and Sommer 1995), and in 1997 living polyps were found in the Napa River (Mills and Rees 2000). In fouling plate surveys in 2007 and 2008, polyps were collected in largest numbers in the Napa River, but were also found in the Petaluma River, Boynton Slough, and Montezuma Slough on the seaward edge of the Sacramento-San Joaquin Delta. Medusae occurred in the more saline portions of Suisun Marsh (6-16 PSU), but usually at low densities (0.1-.5 m-3) (Schroeter 2008). Settlement of polyps occurred in June-October, and was optimum at a salinity of 14-22 PSU (Wintzer et al. 2011a). The only other West Coast system where B. virginica has been reported is Coos Bay, Oregon. In 1997, a single medusa was collected in Isthmus Slough, Coos Bay (James T. Carlton, personal communication). In 1999, hundreds of medusae were seen there (Mills and Rees 2000).

Invasion History on the East Coast:

The medusa of B. virginica was first collected and described in Hampton Roads, Virginia in 1904, and was reported as occurring northwards from Delaware Bay to Sandy Hook, New Jersey (as B. manhattensis; Mayer 1910). It was also collected in Norfolk Sound, Virginia in 1920-21 (Cowles 1930), but was not found in later extensive surveys by Calder (Calder 1971; Calder 1972). However, medusae were collected in the Mattaponi River, a York River tributary, in 2010 (Harrison et al. 2013). In South Carolina estuaries, B. virginica medusae were reported as occurring in 'numerous areas', and as abundant and widespread (Calder and Hester 1978). In the Delaware Bay, they were described as sporadic throughout the estuary (Cronin et al. 1962). Ferrante (1970) found the medusae to be abundant around 'Artificial Island', near the mouth of the Chesapeake and Delaware Canal. Harrison et al. (2013) collected specimens at Slaughter Beach, in lower Delaware Bay in 2010. It was found in the Mullica River, in the New Jersey coastal bay system (Richards 1938), and has been reported from mesohaline waters of the Hudson River estuary (Patrick 1994). The scattered records of this medusa suggest a patchy spatial distribution and/or an eruptive population cycle on the East Coast. The only collection of polyps is from fouling plates in Baltimore Harbor, identified by Dale Calder (Royal Ontario Museum, Toronto, Canada).

Invasion History on the Gulf Coast:

In 2009, medusae of B. virginica were collected in Lake Pontchartrain, Louisiana (Harrison et al. 2013). This medusa was collected much earlier on the Gulf in Laguna Tamiahua, in Veracruz, Mexico (Signoret-Poillón 1969, cited by Alvarez et al. 2003).

Invasion History Elsewhere in the World:

In Europe, B. virginica is known from one specimen collected in the Loire estuary, France (Denayer 1973), and from established populations in the Mira estuary, Portugal (Moore 1987) and the Gaudiana estuary, Spain (Chicaro et al. 2008), where it was discovered in 1984 and 2008, respectively. In 2013, medusae and polyps were discovered in the North Sea Canal (Noordzeekanaal), connecting Amsterdam to the North Sea (Faasse and Melchers 2014). It is abundant in lagoons adjacent to the Black Sea, and in the Sea of Azov (Thiel 1935; Naumov 1969), where we consider it native. It was introduced to the Caspian Sea in 1956, probably by ships entering through canals from the Black Sea (Logvinenko 1959; Aladin et al. 2002). While many records are estuarine, and occasionally neritic (near-shore), its 1991 record off the east coast of South Africa is unusual in that it occurs in a strong ocean current (Agulhas Current) off an open coast (Buecher et al. 2005). Records in Asia are scattered, and include records from Bombay, Cochin, the Ganges estuary, and the Fujian Coast of China, from 1956 to the present (Moore 1987; Bardi and Marques 2009). In South America, B. virginica was first reported in Muribeca, near Recife, in the River Jaboatao (8⁰S) (in 1963, Moore 1987). It is established in northeastern Brazil, in several estuaries in Pernambuco and Sergipe states (Freire et al. 2013). In 1983, this medusa was found in the Rio de la Plata estuary, Argentina (36⁰S) (Genzano et al. 2006), and it has subsequently been found in several locations in southern Brazil (Nogueira and De Oliveira 2006; Bardi and Marques 2009). It is also known from Laguna Tamiahua, in Veracruz, Mexico (Signoret-Poillón, 1969, cited by Alvarez et al. 2003). So far, we know of only one record from the tropical Eastern Pacific, from a lagoon in Chiapas, Mexico in 1997 (Alvarez 1999).


The hydroid colonies of Blackfordia virginica are creeping, and arise from a stolon.The stems are occasionally branched, and can bear 2-3 hydrothecae on each stem. The stem and lower stalk sometimes have weak annular constrictions (rings). The hydrothecae are cylindrical, rising vertically above the stalk. The hydrotheca abruptly widens from the stalk, and is separated from it by a diaphragm. The mouth of the hydrotheca can be closed by an operculum, consisting of numerous triangular flaps meeting centrally and having no clear demarcation from the hydrothecal margin. The hydranth ends in a conical hypostome with a whorl of 12-16 filiform tentacles, connected at their base by an intertentacular membranous web. Gonophores develop on the stolon or on the stalk of the hydranths. They are pear-shaped sacs, containing one medusa at a time. The height of the hydrotheca without the stalk is 0.4-0.5 mm, while the gonothecae are about 0.45 mm. The total height of the hydroid is approximately 1-2 mm (description from: Naumov 1969; Mills and Sommer 1995; Bouillon et al. 2004; Bardi and Marques 2009). Polyps tend to grow on aquatic vegetation, barnacle shells, and the undersides of hard substrates (Naumov 1969; Wintzer et al. 2011a).

The medusa of B. virginica is dome-shaped, a little taller than a hemisphere, with a rounded apex. The jelly in the bell is thick, and composes nearly half the height of the bell. There are usually four radial canals, and a prominent ring canal around the base of the bell. The margin of the bell has a thin velum, which covers about 10-25% of the bell opening. The gonads extend from the manubrium along the radial canals, and cover up to about 2/3 of their length. In females, the gonads are granular and contain visible eggs, while in males they are linear and homogeneous.The medusa has a narrow, short manubrium, whose mouth has four long, fluted lips, with ruffled edges. There are 50-142 marginal tentacles, and between the roots of the tentacles are marginal statocysts. These frequently, but not always, contain black pigment and have a finger-like projection pointing towards the bell roof. Mature medusae are 4-22 mm (rarely exceeding 14-15 mm) in diameter and about 3-7 mm in height (description from: Mayer 1910; Naumov 1969; Moore 1987; Mills and Sommer 1995; Bouillon et al. 2004; Johnson and Allen 2005; Genzano et al. 2006; Bardi and Marques 2009).


Taxonomic Tree

Kingdom:   Animalia
Phylum:   Cnidaria
Class:   Hydrozoa
Subclass:   Hydroidolina
Order:   Leptothecata
Family:   Blackfordiidae
Genus:   Blackfordia
Species:   virginica


Blackfordia manhattensis (Mayer, 1910)
Blackfordia virginiana (None, None)

Potentially Misidentified Species



Blackfordia virginica has a conspicuous planktonic medusa stage, and a small obscure hydroid stage. The hydroid grows in a creeping fashion, with short stalks and polyps about 1-2 mm high, attached to vegetation or other surfaces (including fouling plates and barnacle shells). Gonophores bud off the stems, and each one releases a single planktonic medusa, which drifts in the plankton. The medusa feeds on zooplankton, and grows, developing either ovaries or testes (Naumov 1969; Mills and Sommer 1995; Bardi and Marques 2009; Wintzer et al. 2011b). In the Napa and Petaluma Rivers, the most common foods were copepod nauplii, cyclopoid copepods, and mysids (Wintzer et al. 2013). Eggs hatch into non-feeding, ciliated planula larvae, which settle about four days after fertilization and grow into polyps.

In the San Francisco estuary, polyps showed a preference for settling on the undersides of surfaces, probably as a way of avoiding being covered by sediment. Recruitment of B. virginica polyps was highest during periods of higher water clarity (Wintzer et al. 2011a). Polyps have only been reported from estuarine habitats, over a salinity range of 3-22 PSU (Naumov 1969; Wintzer et al. 2011a), but the medusae have been found in open waters off the coast of New Jersey (Mayer 1910, as B. manhattensis) and in the Agulhas Current, off South Africa (Buecher et al. 2005), as well as in brackish waters down to 2 PSU (Nogueira and De Oliveira 2006).



Trophic Status:




General HabitatCoarse Woody DebrisNone
General HabitatVessel HullNone
General HabitatGrass BedNone
General HabitatMarinas & DocksNone
Salinity RangeOligohaline0.5-5 PSU
Salinity RangeMesohaline5-18 PSU
Salinity RangePolyhaline18-30 PSU
Salinity RangeEuhaline30-40 PSU
Tidal RangeSubtidalNone
Vertical HabitatEpibenthicNone
Vertical HabitatPlanktonicNone

Tolerances and Life History Parameters

Minimum Temperature (ºC)3Field, Cronin et al. 1962, winter temperature in zone where B. virginica appeared in summer.
Maximum Temperature (ºC)29.9Field, Feire et al. 2013
Minimum Salinity (‰)2Medusae (Nogueira and De Oliveira 2006; Moore 1987). Polyps have been reported at 3-22 PSU (Naumov 1969; Schroeter 2008; Wintzer et al. 2011). In Suisun Marsh CA, medusae were not found at salinities below 5.6 PSU, and were most abundant at 6-16 PSU (Schroeter 2008).
Maximum Salinity (‰)35Medusa (Moore 1987)
Minimum Dissolved Oxygen (mg/l)4.3Field, Suisun Marsh (Schroeter 2008)
Maximum Length (mm)7Medusa height. Hydroids are 1-2 mm in height (Mayer 1910; Naumov 1969; Moore 1987; Mills and Sommer 1995).
Maximum Width (mm)22Medusa diameter- rarely exceeding 14-15 mm. (Naumov 1969; Moore 1987; Mills and Sommer 1995; Bouillon et al. 2004; Genzano et al. 2006; Bardi and Marques 2009; Wintzer et al. 2013)
Broad Temperature RangeNoneCold temperate-Tropical
Broad Salinity RangeNoneOligohaline-Euhaline

General Impacts

Blackfordia virginica, together with other introduced hydrozoans in brackish estuaries (e.g. Moerisia spp., Cordylophora caspia, Maeotias marginata), has the potential to alter planktonic food webs through predation on zooplankton. Most estuaries have few or no native brackish-water hydrozoans (Calder 1971; Calder 1972; Rees and Gershwin 2000; Wintzer et al. 2013). Sharp reductions in zooplankton abundance were observed during a Blackfordia bloom in the Guadiano estuary, Spain (Chícharo et al. 2009). However, measurements of feeding rate were not made, and the role of predation was not established. The extent and importance of hydrozoan predation in zooplankton populations has rarely been determined.

Regional Impacts

NEA-VNoneEcological ImpactPredation
Sharp reductions in zooplankton abundance were observed during a Blackfordia bloom in the Guadiana estuary, Spain-Portugal (Chícharo et al. 2009). However, measurements of feeding rate were not made, and the role of predation was not established. In the Mira Estuary, Portugal, predation rates on copepods were estimated to reduce the copepod concentration by up to 50% in 0.30 days, at the highest level of abundance (Marques et al. 2015). Modeling indicates that the predatory impact of B. virginica is strongly affected by hydrological conditions, such as river discharge (Muha et al. 2017).
NWP-2NoneEcological ImpactPredation
In brackish Nanhu Lake, China, predation by B. virginica resulted in decreaed densiyt of microzooplankton, and increased phytoplankton biomass, and NH4+ concentraions (Huang et al. 2021).

Regional Distribution Map

Bioregion Region Name Year Invasion Status Population Status
MED-IX None 0 Native Estab
MED-X None 0 Native Estab
NA-ET3 Cape Cod to Cape Hatteras 1904 Def Estab
CAR-VII Cape Hatteras to Mid-East Florida 1978 Def Estab
NEP-V Northern California to Mid Channel Islands 1970 Def Estab
NEP-IV Puget Sound to Northern California 1997 Def Estab
SA-III None 1963 Def Estab
CIO-III None 1958 Def Estab
CIO-I None 1968 Def Estab
NEA-V None 1984 Def Estab
CASP Caspian Sea 1956 Def Estab
CAR-I Northern Yucatan, Gulf of Mexico, Florida Straits, to Middle Eastern Florida 1969 Def Estab
NEP-IX None 1997 Def Estab
NEA-IV None 1967 Def Estab
P170 Coos Bay 1997 Def Estab
M060 Hudson River/Raritan Bay 1904 Def Estab
M130 Chesapeake Bay 1904 Def Estab
M090 Delaware Bay 1951 Def Estab
M080 New Jersey Inland Bays 1938 Def Unk
SA-II None 1983 Def Estab
P090 San Francisco Bay 1970 Def Estab
WA-V None 1991 Def Estab
S056 _CDA_S056 (Northeast Cape Fear) 1978 Def Estab
S060 Winyah Bay 1978 Def Estab
S070 North/South Santee Rivers 1978 Def Estab
S076 _CDA_S076 (South Carolina Coastal) 1978 Def Estab
S080 Charleston Harbor 1978 Def Estab
S090 Stono/North Edisto Rivers 1978 Def Estab
S100 St. Helena Sound 1978 Def Estab
S110 Broad River 1978 Def Estab
S120 Savannah River 1978 Def Estab
P093 _CDA_P093 (San Pablo Bay) 1970 Def Estab
NWP-3a None 1962 Def Estab
G170 West Mississippi Sound 2010 Def Estab
NEA-II None 2013 Def Estab
G330 Lower Laguna Madre 2008 Def Unk
B-IV None 2014 Def Estab
G260 Galveston Bay 2016 Def Estab
NWP-2 None 2012 Def Estab

Occurrence Map

OCC_ID Author Year Date Locality Status Latitude Longitude
29826 Mills and Sommer 1995; Cohen and Carlton, 1995 1993 1970-01-01 San Pablo Bay - Napa River Def 38.1402 -122.2764
31155 Schroeter 2008) 2005 2005-01-01 Suisun Marsh Def 38.0540 -121.8682
33871 Cohen and Carlton, 1995 1993 1993-01-01 San Pablo Bay - Petaluma River Def 38.1092 -122.4858


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