Invasion History

First Non-native North American Tidal Record: 1895
First Non-native West Coast Tidal Record: 1895
First Non-native East/Gulf Coast Tidal Record:

General Invasion History:

Clava multicornis is native to the North Atlantic, ranging from Long Island to Labrador, Iceland, the White Sea, and the Bay of Biscay (Fraser 1944; Gosner 1978; Schuchert 2001; Altuna 2007). It is also found in the Mediterranean Sea (Bouillon et al. 2004). It has been introduced to San Francisco Bay, California, but has not been collected there in more than 50 years.

North American Invasion History:

Invasion History on the West Coast:

Clava multicornis was first collected on the West Coast on pilings in Oakland Harbor, San Francisco Bay in 1895 (as C. leptostyla, Torrey 1902, cited by Carlton 1979; US National Museum of Natural History 2011). According to Fraser (1937) it still occurred at that time. However, it was not found in 1993 and 2004 surveys, and there have been no records there in 50+ years (Mills et al., in Carlton 2007).

In March and July 2013, extensive, established colonies of C. multicornis were found on floating docks in Coos Bay, Oregon. This hydroid had not been found in previous surveys. The source of the Coos Bay invasion is unclear, whether from undiscovered colonies in San Francisco Bay or other Pacific sources, ships from the East Coast, or possibly seaweed used to pack baitworms from Maine or the Maritime provinces (Calder et al. 2014).

Invasion History Elsewhere in the World:

A record of this mostly cold-to-temperate water hydroid from Puerto Rico may represent an introduction (Wedler and Larson 1986; USNM 60744, US National Museum of Natural History 2011). Its establishment here is improbabale, however, and needs verification. 


Clava multicornis is a sessile hydrozoan, which lacks a planktonic medusa stage. The polyps arise from stolons, which do not coalesce, but can form a tight network. The polyps can be dispersed or clustered in tufts. The hydranths are slender and 10-20 mm high. The hypostome is large, and surrounded by 20-40 filiform tentacles, arranged in a broad band below the hypostome. The sac-like gonophores (up to 50) are arranged in clusters below the tentacles. The gonophores usually contain one egg each, occasionally two, which develop into planula larvae. The hydroids are pinkish, and frequently grow on seaweeds, especially Fucus spp., rocks, and gastropod shells, from the intertidal zone to 163 m (description from: Fraser 1944; Gosner 1978; Schuchert 2001).


Taxonomic Tree

Kingdom:   Animalia
Phylum:   Cnidaria
Class:   Hydrozoa
Subclass:   Hydroidolina
Order:   Anthoathecatae
Suborder:   Filifera
Family:   Hydractiniidae
Genus:   Clava
Species:   multicornis


Clava leptostyla (Agassiz, 1862)
Hydra multicornis (Forsskål, 1775)
Clava squamata (Hincks, 1868)

Potentially Misidentified Species



Clava multicornis is a sessile hydrozoan which lacks a planktonic medusa stage. Colonies grow on a solid substrate, and produce gonophores, which give rise to either eggs or sperm. Colonies are probably single-sexed. Female gonophores produce a single egg, which is brooded and fertilized by sperm in the water column. The egg develops into a ciliated non-feeding planula larva (Barnes 1983; Schuchert 2001). Larvae can settle immediately, but may delay settlement for up to 10 days. They settle on a variety of substrates, including seaweeds, mollusk shells, rocks, pilings, and buoys (Gosner 1978; Orlov 1996; Schuchert 2001). Once settled they develop into hydroid colonies (Barnes 1983). This hydroid is primarily associated with marine waters, but it survives in brackish water (MarLin 2011).



Trophic Status:

Suspension Feeder



General HabitatGrass BedNone
General HabitatMarinas & DocksNone
General HabitatRockyNone
Salinity RangePolyhaline18-30 PSU
Salinity RangeEuhaline30-40 PSU
Tidal RangeSubtidalNone
Tidal RangeLow IntertidalNone
Tidal RangeMid IntertidalNone
Vertical HabitatEpibenthicNone

Tolerances and Life History Parameters

Minimum Salinity (‰)18.3Field, Coos Bay, OR (Calder et al. 2014)
Maximum Salinity (‰)38Typical salinity, Mediterranean
Minimum Duration0Orlov 1996
Maximum Duration10Orlov 1996
Maximum Height (mm)20Fraser 1944; Gosner 1978; Schuchert 2001
Broad Temperature RangeNoneCold-temperate-Warm temperate
Broad Salinity RangeNoneMesohaline-Euhaline

General Impacts

The hydroid Clava multicornis has no reported impacts in its native or introduced range.

Regional Distribution Map

Bioregion Region Name Year Invasion Status Population Status
NEA-III None 0 Native Estab
NEA-II None 0 Native Estab
AR-V None 0 Native Estab
B-I None 0 Native Estab
B-II None 0 Native Estab
B-III None 0 Native Estab
B-IV None 0 Native Estab
AR-IV None 0 Native Estab
AR-III None 0 Native Estab
NA-S2 None 0 Native Estab
NA-S3 None 0 Native Estab
NA-ET1 Gulf of St. Lawrence to Bay of Fundy 0 Native Estab
NA-ET2 Bay of Fundy to Cape Cod 0 Native Estab
NA-ET3 Cape Cod to Cape Hatteras 0 Native Estab
NEP-V Northern California to Mid Channel Islands 1895 Def Unk
P090 San Francisco Bay 1895 Def Unk
NEA-V None 0 Native Estab
NEA-IV None 0 Native Estab
MED-II None 0 Native Estab
MED-I None 0 Native Estab
MED-III None 0 Native Estab
MED-VII None 0 Native Estab
CAR-IV None 1974 Crypto Unk
MED-IV None 0 Native Estab
MED-V None 0 Native Estab
MED-VI None 0 Native Estab
B-VII None 0 Native Estab
B-IV None 0 Native Estab
B-V None 0 Native Estab
NEP-IV Puget Sound to Northern California 2013 Def Estab
P170 Coos Bay 2013 Def Estab

Occurrence Map

OCC_ID Author Year Date Locality Status Latitude Longitude


Altuna, Alvaro (2007) Bathymetric distribution patterns and biodiversity of benthic Medusozoa (Cnidaria) in the Bay of Biscay(north-eastern Atlantic)., Journal of the Marine Biological Association of the United Kingdom 87: 681-694

Appeltans, W. et al. 2011-2015 World Registry of Marine Species. <missing URL>

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

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.; Carlton, James T.; Choon, Henry H. C. (2014) Clava multicornis (Forsskål, 1775): rediscovery of a North Atlantic hydroid (Cnidaria, Hydrozoa, Anthoathecata) on the Pacific coast of North America, Bioinvasions Records 3(2): 71-76

Calder, Dale R.; Hester, Betty S. (1978) Phylum Cnidaria., In: Zingmark, Richard G.(Eds.) An Annotated Checklist of the Biota of the Coastal Zone of South Carolina. , Columbia. Pp. 87-93

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. (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>

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>

Dziubinska, Anna; Janas, Urszula (2007) Submerged objects: a nice place to live and develop. Succession of fouling communities in the Gulf of Gdansk, Southern Baltic, Oceanological and Hydrobiological Studies 36(4): 65-78

Fraser, C. McLean (1937) <missing title>, The University of Toronto Press, Toronto,. Pp. <missing location>

Fraser, C. McLean (1944) Hydroids of the Atlantic Coast of North America, In: (Eds.) . , Toronto. Pp. 1-441

Gosner, Kenneth L. (1978) A field guide to the Atlantic seashore., In: (Eds.) . , Boston. Pp. <missing location>

Looby, Audrey; Ginsburg, David W. (2021) Nearshore species biodiversity of a marine protected area off Santa Catalina Island, California, Western North American Naturalist 81(1): 113-130

MarLin- Marine Life Information Network 2006-2024 MarLin- Marine Life Information Network. <missing URL>

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

Orlov, Dmitri (1996) Observations on the settling behavior of planulae of Clava multicornis., Scientia Marina 60(1): 121-128

Schuchert, Peter (2001) Hydroids of Greenland and Iceland (Cnidaria, Hydrozoa)., Meddelelser om Grønland, Bioscience 53: 1-184

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