Invasion History

First Non-native North American Tidal Record: 2008
First Non-native West Coast Tidal Record: 2008
First Non-native East/Gulf Coast Tidal Record:

General Invasion History:

Molgula citrina is a small, solitary tunicate, described from the British Isles and known primarily from the North Atlantic. In European waters, it is known from Roscoff, France to Svalbard and the White Sea (Dybern 1969; Monniot 1969; Gulliksen et al. 1999; Shenkar and Swalla 2010; Khalaman et al. 2011). Off the Atlantic coast of North America, it is known mostly from shallow water (less than 30 m) from Narragansett Bay, Rhode Island to the Gulf of St. Lawrence, Canada (Van Name 1945). Museum specimens are known from the continental shelf as far south as Delaware in deeper water (65-90 m, U.S. National Museum of Natural History collections). A specimen has also been collected in the Arctic Ocean, near Northumberland Island, off the west coast of Greenland (1926, USNM 14982, U.S. National Museum of Natural History 2015, 77.4°N).

In 2008, many specimens of M. citrina were collected from Seldovia, Alaska on Kachemak Bay, the first verified records of this species from the Pacific Ocean. The genetic similarity of these specimens to Atlantic populations suggests that they were introductions from the North Atlantic, rather than part of a natural circumpolar distribution, given this tunicate's limited larval dispersal (Lambert et al. 2010). Subsequently, M. citrina was found in Umpqua Bay, Oregon and Humboldt Bay, California (Gretchen Lambert, personal communication 2010 Chapman et al. 2011).

North American Invasion History:

Invasion History on the West Coast:

In 2008, many specimens of Molgula citrina were collected from floating docks and ropes in Seldovia, Alaska on Kachemak Bay, the first verified records of this species from the Pacific Ocean. The genetic similarity of these specimens to Atlantic populations suggests that they were introductions from the North Atlantic, rather than part of a natural circumpolar distribution, given this tunicate's brooded larvae and limited larval dispersal (Lambert et al. 2010). Large M. citrina populations were discovered in the Umpqua estuary, Oregon in 2010 (Chapman et al. 2012) and in Humboldt Bay, California in 2011 (Gretchen Lambert, personal communication 2011). Hull fouling is the most likely vector for the introduction of these tunicates. The fouling of sea-chests, which are heated in Arctic waters to protect the engines, is a likely vector for the transport of M. citrina through the Arctic Ocean (Lambert et al. 2010). Both Humboldt Bay and the Umpqua estuaries are also sites of oyster culture, a potential vector for future transport to other estuaries.

Collections of the U.S. National Museum of Natural History included specimens identified as M. citrina, which were collected from 1893 to 1960 in Alaska (the Chukchi Sea, Pribilof Islands and Bristol Bay), but these have been re-identified as M. pacifica and M. retortiformis (U.S. National Museum of Natural History 2015; Gretchen Lambert personal communication).


Molgula citrina is globular shaped and is sometimes attached or broadly flattened in the ventral region. Its tunic is tough and firm, but its outer surface varies from smooth to rough, with rougher tunics having attached sand and detritus. The siphons arise from the dorsal surface some distance apart, with the oral siphon near the anterior end and the atrial siphon near the middle. The oral siphon is often wider and has six narrow pointed lobes, while the atrial siphon is narrower, but longer when extended, and has a square opening. The siphons sometimes have short, spiny processes on their external surfaces (whence the synonym Molgula echinosiphonica). The larvae are brooded in the peribranchial cavities, adjacent to the gonads on the right and left side, and grow quite large, about 0.4 mm when coiled and about 1.4 mm when swimming at release. This tunicate is small, often 6-8 mm and rarely reaching 20-23 mm. The color is 'translucent, dull greenish or olive, between the tubes usually more -or less ferruginous brown or russet; on the sides the viscera show through as dull orange-colored and darker blotches' (Verrill 1871). It usually grows firmly attached to the substrate, often in groups of several individuals, sometimes with many young individuals clumped around the adults. Description based on: Verrill 1871, Van Name 1945, and Lambert et al. 2010.


Taxonomic Tree

Kingdom:   Animalia
Phylum:   Chordata
Subphylum:   Tunicata
Class:   Ascidiacea
Order:   Stolidobranchia
Family:   Molgulidae
Genus:   Molgula
Species:   citrina


Caesira citrina (Alder and Hancock, 1848)
Caesira littoralis (Huntsman, 1912)
Molgula arctica (Kiaer, 1896)
Molgula echinosiphonica (Lacze-Duthiers, 1877)
Molgula littoralis (Verrill, 1871)
Molgula nana (Kupfer, 1873)

Potentially Misidentified Species

Molgula manhattensis

Molgula pacifica
Specimens previously identified as M. citrina, re-identified by Gretchen Lambert (USNM 2345, U.S. National Museum of Natural History 2015).

Molgula retortiformis
Specimens previously identified as M. citrina from Alaska, re-identified by Gretchen Lambert (USNM 14724; USNM 6347; USNM 7440, U.S. National Museum of Natural History 2015).



Life History- A solitary tunicate is ovoid, elongate or vase-like in shape, with two openings or siphons. Most solitary tunicates attach to substrates by their side or base, but some attach with a conspicuous stalk. They are sessile filter feeders with two siphons, an oral and an atrial siphon. Water is pumped in through the oral siphon, where phytoplankton and detritus is filtered by the gills, and passed on mucus strings to the stomach and intestines. Waste is then expelled in the outgoing atrial water.

Solitary ascidians are hermaphroditic, meaning that both eggs and sperm are released to the atrial chamber. Eggs may be self-fertilized or fertilized by sperm from nearby animals, but many species have a partial block to self-fertilization. In Molgula citrina, eggs are internally fertilized, eggs are brooded and fertilized within the atrial chamber and then released into the water column upon hatching. Fertilized eggs hatch into a tadpole larva with a muscular tail, notochord, eyespots, and a set of adhesive papillae. The lecithotrophic (non-feeding, yolk-dependent) larva swims briefly (minutes to hours) before settlement (Durante 1991; Lambert et al. 2010). Once settled, the tail is absorbed, the gill basket expands, and the tunicate begins to feed by filtering (Barnes 1983).


Phytoplankton, detritus

Trophic Status:

Suspension Feeder



General HabitatRockyNone
General HabitatUnstructured BottomNone
General HabitatMarinas & DocksNone
Salinity RangePolyhaline18-30 PSU
Salinity RangeEuhaline30-40 PSU
Tidal RangeSubtidalNone
Vertical HabitatEpibenthicNone

Tolerances and Life History Parameters

Minimum Salinity (‰)17Field data, Vagsbopollen (lagoon), Bergen, Norway(Dybern 1969)
Maximum Salinity (‰)35Average Atlantic Ocean salinity
Minimum Duration0Larvae can settle immediately upon release (Durante 1991)
Maximum Width (mm)23 Van Name 1945; Lambert et al. 2010
Broad Temperature RangeNonePolar-Cold temperate
Broad Salinity RangeNonePoly-Euhaline

General Impacts

There are no ecological or economic impacts for introduced populations of M. citrina.

Regional Distribution Map

Bioregion Region Name Year Invasion Status Population Status
NA-S2 None 0 Native Estab
NEP-II Alaska south of the Aleutians to the Alaskan panhandle 2008 Def Estab
NA-ET2 Bay of Fundy to Cape Cod 0 Native Estab
NA-ET3 Cape Cod to Cape Hatteras 0 Native Estab
AR-II None 0 Native Estab
NA-S3 None 0 Native Estab
AR-III None 0 Native Estab
AR-V None 0 Native Estab
NEA-II None 0 Native Estab
B-I None 0 Native Estab
NEA-IV None 0 Native Estab
NEP-IV Puget Sound to Northern California 2010 Def Estab
P130 Humboldt Bay 2011 Def Estab
NA-ET1 Gulf of St. Lawrence to Bay of Fundy 0 Native Estab
P180 Umpqua River 2010 Def Estab
B-III None 0 None Estab
B-II None 1933 Native Estab

Occurrence Map

OCC_ID Author Year Date Locality Status Latitude Longitude


Chapman, John W.; Therriault, Thomas; Harris, Leslie; Breitenstein, Ralph (2011) The 2010 PICES Rapid Assessment Survey of shallow water nonindigenous, native and cryptogenic marine species of central Oregon, Oregon State University, Hatfield Marine Center, Newport OR. Pp. 48

Durante, Kathleen M. (1991) Larval behavior, settlement preference, and induction of metamorphosis in the temperate solitary ascidian Molgula citrina Alder and Hancock, Journal of Experimental Marine Biology and Ecology 145: 175-187

Durante, Kathleen M.; Sebens, Kenneth P. (1994) Reproductive ecology of the ascidians, Molgula citrina Alder & Hancock, 1848 and Aplidium glabrum (Verrill 1871) from the Gulf of Maine, USA, Ophelia 39(1): 1-21

Dybern, Bernt I. (1969) Distribution and ecology of ascidians in Kvirturdvikpollen and Vagsbopollen on the west coast of Norway, Sarsia 37: 21-40

Khalaman, V. V.; Mukhina, Yu. I.; Komendantov, A. Yu. (2011) The effects of the excretory secretory products of fouling organisms on settlement of larvae of the sponge Halichondria panicea (Pallas, 1766) (Porifera: Demospongiae), Russian Journal of Marine Biology 93(9): 494-500

Lambert, Gretchen; Shenkar, Noa; Swalla, Billie J. (2010) First Pacific record of the north Atlantic ascidian Molgula citrina – bioinvasion or circumpolar distribution?, Aquatic Invasions 5: corrected proof

Monniot, Claude (1969) [The Molgulidae of European Seas] (French), Memoires du Museum d Histoire Naturelle 60(4): 172-272

Robertson, D. Ross; Simoes, Nuno Rodríguez, Carla Gutiérrez Piñeros, Victor J. (2016) An Indo-Pacific damselfish well established in the southern Gulf of Mexico: prospects for a wider, adverse invasion, Journal of the Ocean Science Foundation 19: 1-17

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>

Shenkar, Noa; Swalla, Billy J. (2010) Molecular data confirm synonymy of Roscovite molgulid ascidians, Cahiers de Biologie Marine 51: 85-87

Simkanin, Christina; Fofonoff, Paul W.; Larson, Kriste; Lambert, Gretchen; Dijkstra, Jennifer A.; Ruiz, Gregory M. (2016) Spatial and temporal dynamics of ascidian invasions in the continental United States and Alaska, Marine Biology 163: Published online

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

Van Name, Willard G. (1912) Simple ascidians of the coasts of New England and neighboring British provinces., Proceedings of the Boston Society of Natural History 34: 439-619

Van Name, Willard G. (1945) The North and South American ascidians, Bulletin of the American Museum of Natural History 84: 1-462

Yale Peabody Museum of Natural History 2008-2016 YPM Invertebrate Zoology - Online Catalog. <missing URL>