Invasion History

First Non-native North American Tidal Record: 1988
First Non-native West Coast Tidal Record: 1988
First Non-native East/Gulf Coast Tidal Record:

General Invasion History:

Cladonema radiatum is native to European waters where it can be found from Norway to the Mediterranean and Black Seas (Schuchert 2006). It has also been reported from Bermuda, Florida, the Bahamas, Puerto Rico, Belize, and Brazil (Mayer 1910; Migotto 1996; Calder 1998; US National Museum of Natural History 2009). We consider C. radiatum to be cryptogenic in warm Western Atlantic waters, perhaps representing a separate, cryptic species. Cladonema radiatum has been reported from Japan, Pacific Russia, and Korea (Hirohito 1988, cited by Schuchert 2006). However, these records could refer to C. pacificum (=C. uchidae), which has been treated as conspecific with C. radiatum by some workers (Rees 1982). Introduced populations have been reported from Padilla Bay, Washington; Oahu, Hawaii; and Wellington, New Zealand.

North American Invasion History:

Invasion History on the West Coast:

Cladonema radiatum has been collected in Padilla Bay, Washington, starting in 1988. The hydroids are abundant on the eelgrass Zostera marina (Mills, in Cohen et al. 1998; Mills et al., in Carlton 2007).

Invasion History in Hawaii:

Cladonema radiatum is considered introduced in Oahu, Hawaii, where it was first found in 1972 (Cook 1977a, cited by Carlton and Eldredge 2009).

Invasion History Elsewhere in the World:

Cladonema radiatum is considered to be introduced in Wellington, New Zealand, where it was first found in 1953 (Cranfield et al. 1998; Schuchert 2006).


The hydroid Cladonema radiatum consists of polyps, usually single, but rarely branched, arising from branching stolons. The club-shaped hydranth has two whorls of tentacles: one oral whorl, just below the mouth, consisting of four capitate tentacles (ending in bulbs); and a lower (aboral) whorl, near the base of the hydranth, consisting of filiform (threadlike) tentacles. The perisarc terminates below the filiform tentacles. The hypostome is rounded, with a pre-oral chamber. The medusa buds are naked, and located between the two whorls of tentacles. Hydroid colonies are about 25 mm long, and the hydranths about 1 mm high- polyps are estimated at about 2-3 mm high. Hydroids occur on the Mediterranean sea grass Posidonia (Bouillon et al. 2004; Schuchert 2006). In Padilla Bay, WA, the hydroids occur on Zostera marina (Eelgrass) (description from: Mills, in Cohen et al. 1998; Mills et al., in Carlton et al. 2007).

The umbrella of the medusa of C. radiatum is bell-shaped, slightly higher than wide, and sometimes with a slight apical projection. The velum is rather broad. The manubrium is spindle-shaped, and is as long as, or slightly shorter than the bell cavity. It has some pouch-like protuberances in the middle region, composed of gonadal tissue. Four to five spherical nematocyst clusters are located just above the mouth. The radial canals may divide near their junction with the manubrium. The number of tentacles corresponds with the number of radial canals, typically 8-10 in a mature medusa. The base of each tentacle contains an ocellus (eyespot). The tentacles have an elongated, thickened base, and are branched. The long main branch gives rise to several short tentacles which act as adhesive organs- these have a spherical swelling at the end. The main branch then gives off up to five side branches, with nematocyst clusters, including one large terminal cluster. The stomach and marginal tentacles are red, bright-red or brown; the ocelli are black or deep crimson. The medusae are about 1 mm in diameter when released, and about 3 mm when mature. Medusae are capable of swimming in the water column, but mostly remain attached to surfaces (description from: Bouillon et al. 2004; Schuchert 2006). It is possible that this wide-ranging hydrozoan is a complex of cryptic species (Schuchert 2006).


Taxonomic Tree

Kingdom:   Animalia
Phylum:   Cnidaria
Class:   Hydrozoa
Subclass:   Hydroidolina
Order:   Anthoathecatae
Suborder:   Capitata
Family:   Cladonematidae
Genus:   Cladonema
Species:   radiatum


Cladonema allmani (Haeckel, 1879)
Cladonema dujardinii (Haeckel, 1879)
Cladonema gegenbauri (Haeckel, 1879)
Cladonema krohnii (Haeckel, 1879)
Cladonema mayeri (Perkins, 1906)
Cladonema novae-zelandiae ( Ralph, 1953)
Cladonema perkinsii (Mayer, 1904)
Coryne stauridia (Gosse, 1853)
Eleutheria radiata (Lengerich, 1922)
Stauridia radiatum (Mayer, 1910)
Stauridium cladonema (Haeckel, 1879)
Syncoryne stauridium (Krohn, 1853)

Potentially Misidentified Species

Cladonema californicum

Cladonema uchidae
Cladonema radiatum has been reported from Japan, Pacific Russia, and Korea. These records could refer to C. pacificum. It is not clear whether these species co-occur in Northwest Pacific waters (Rees 1982).



The hydroid colonies of Cladonema radiatum produce medusa buds, which in turn develop into medusae that are released. The medusae can swim, but spend a lot of their time attached to surfaces. They feed on zooplankton and when mature release sperm or brood eggs. They are usually single-sexed, and rarely hermaphroditic. The eggs are fertilized and released as planula larvae, which settle on a suitable substrate and develop into new hydroids (Schuchert 2006). They commonly settle on algae or seagrasses, such as the Mediterranean Posidonia oceanica or the temperate eelgrass Zostera marina (Schuchert 2006).


Zooplankton, small epibenthos

Trophic Status:




General HabitatGrass BedNone
General HabitatUnstructured BottomNone
Salinity RangePolyhaline18-30 PSU
Salinity RangeEuhaline30-40 PSU
Tidal RangeSubtidalNone
Vertical HabitatEpibenthicNone
Vertical HabitatPlanktonicNone

Tolerances and Life History Parameters

Maximum Width (mm)3Medusae (Bouillon et al. 2004; Schuchert 2006)
Minimum Height (mm)NoneHydroids reach ~4 mm, while mature medusae are about 3 mm in diameter (Calder 1988; Bouillon et al. 2004; Schuchert 2006)
Maximum Height (mm)4Hydroids (Bouillon et al. 2004; Schuchert 2006)
Broad Temperature RangeNoneCold temperate-Tropical
Broad Salinity RangeNonePolyhaline-Euhaline

General Impacts

No impacts have been reported for Cladonema radiatum in invaded waters.

Regional Distribution Map

Bioregion Region Name Year Invasion Status Population Status
NA-ET4 Bermuda 1902 Crypto Estab
CAR-V None 0 Crypto Estab
SA-II None 0 Crypto Estab
NZ-IV None 1953 Def Estab
MED-IX None 0 Native Estab
NEA-IV None 0 Native Estab
B-IV None 0 Native Estab
B-III None 0 Native Estab
NEA-II None 0 Native Estab
B-II None 0 Native Estab
NEA-III None 0 Native Estab
NEP-III Alaskan panhandle to N. of Puget Sound 1988 Def Estab
NEA-V None 0 Native Estab
MED-I None 0 Native Estab
MED-II None 0 Native Estab
MED-III None 0 Native Estab
MED-IV None 0 Native Estab
MED-VI None 0 Native Estab
MED-V None 0 Native Estab
MED-VIII None 0 Native Estab
MED-VII None 0 Native Estab
B-I None 0 Native Estab
P293 _CDA_P293 (Strait of Georgia) 1988 Def Estab
SP-XXI None 1972 Def Estab
CAR-I Northern Yucatan, Gulf of Mexico, Florida Straits, to Middle Eastern Florida 0 Crypto Estab
CAR-IV None 1975 Crypto Estab
CAR-II None 1978 Crypto Estab

Occurrence Map

OCC_ID Author Year Date Locality Status Latitude Longitude


Bouillon, Jean; Medel, Maria Dolores; Pagès, Francesc; Gili, Josep-Maria; Boero, Ferdinando ; Gravili, Cinzia (2004) Fauna of the Mediterranean Hydrozoa., Scientia Marina 68(suppl. 2): 5-438

Calder, Dale R. (1988) Shallow water hydroids of Bermuda. The Athecatae.”, Royal Ontario Museum Life Sciences Contributions 148: 1-107

Calder, Dale R. (2010) Some anthoathecate hydroids and limnopolyps (Cnidaria, Hydrozoa) from the Hawaiian archipelago, Zootaxa 2590: 1-91

Carlton, James T. (Ed.) (2007) The Light and Smith Manual: Intertidal Invertebrates from Central California to Oregon Fourth Edition, Completely Revised and Expanded, University of California Press, Berkeley. Pp. <missing location>

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

Cohen, Andrew; and 16 authors. (1998) <missing title>, Washington State Department of Natural Resources, Olympia, Washington. Pp. 1-37

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>

Farrapeira, Cristiane Maria Rocha; Tenório, Deusinete de Oliveira ; do Amaral, Fernanda Duar (2011) Vessel biofouling as an inadvertent vector of benthic invertebrates occurring in Brazil, Marine Pollution Bulletin 62: 832-839

Mayer, A. G. (1910) Medusae of the World., In: (Eds.) . , Washington, D.C.. Pp. 231, 276-278

Migotto, A.E. (1996) Benthic shallow-water hydroids (Cnidaria, Hydrozoa) of the coast of Sao Sebastiao, Brazil, including a checklist of Brazilian hydroids, Zoologische Verhandelingen 306: 3-125

Mills, Claudia; Marques, Antonio; Migotto, Alvaro E; Calder, Dale R.; Hand, Cadet (2007) The Light and Smith Manual: Intertidal invertebrates from Central California to Oregon (4th edition), University of California Press, Berkeley CA. Pp. 118-168

Rees, John T. (1982) The hydrozoan Cladonema in California: a possible introduction from East Asia, Pacific Science 36(4): 439-444

Schuchert, Peter (1996) The marine fauna of New Zealand: Athecate hydroids oand their medusae (Cnidara: Hydrozoa), New Zealand Oceanographic Institute Memoir 106: 1-159

Schuchert, Peter (2006) The European athecate hydroids and their medusae (Hydrozoa, Cnidaria): Capitata Part 1., Revue Suisse de Zoologie 113(2): 325-410

U.S. National Museum of Natural History 2002-2021 Invertebrate Zoology Collections Database.

Wedler, Eberhard; Larson, Ronald (1986) Athecate hydroids from Puerto Rico and the Virgin Islands, Studies on Neotropical Fauna and Environment 21: 69-101