Invasive Species Compendium

Detailed coverage of invasive species threatening livelihoods and the environment worldwide


Didemnum vexillum



Didemnum vexillum


  • Last modified
  • 14 July 2018
  • Datasheet Type(s)
  • Invasive Species
  • Preferred Scientific Name
  • Didemnum vexillum
  • Taxonomic Tree
  • Domain: Eukaryota
  •   Kingdom: Metazoa
  •     Phylum: Chordata
  •       Subphylum: Tunicata
  •         Class: Ascidiacea
  • Summary of Invasiveness
  • D. vexillum, a compound ascidian (tunicate or sea squirt), belongs to the family Didemnidae (Kott, 2002; ...

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Didemnum vexillum (colonial sea squirt sp); overgrowing solitary tunicates (Ciona intestin).
CaptionDidemnum vexillum (colonial sea squirt sp); overgrowing solitary tunicates (Ciona intestin).
Copyright©Crown Copyright 2009/CCW
Didemnum vexillum (colonial sea squirt sp); overgrowing solitary tunicates (Ciona intestin).
HabitDidemnum vexillum (colonial sea squirt sp); overgrowing solitary tunicates (Ciona intestin).©Crown Copyright 2009/CCW
Didemnum vexillum (colonial sea squirt); habit.
CaptionDidemnum vexillum (colonial sea squirt); habit.
Copyright©Crown Copyright 2009/Rohan Holt - CCW
Didemnum vexillum (colonial sea squirt); habit.
HabitDidemnum vexillum (colonial sea squirt); habit.©Crown Copyright 2009/Rohan Holt - CCW
Didemnum vexillum (colonial sea squirt); habit, on a boat hull.
CaptionDidemnum vexillum (colonial sea squirt); habit, on a boat hull.
Copyright©Crown Copyright 2009/CCW
Didemnum vexillum (colonial sea squirt); habit, on a boat hull.
HabitDidemnum vexillum (colonial sea squirt); habit, on a boat hull.©Crown Copyright 2009/CCW
Didemnum vexillum (colonial sea squirt); habit, taken from a boat hull.
CaptionDidemnum vexillum (colonial sea squirt); habit, taken from a boat hull.
Copyright©Crown Copyright 2009/CCW
Didemnum vexillum (colonial sea squirt); habit, taken from a boat hull.
HabitDidemnum vexillum (colonial sea squirt); habit, taken from a boat hull.©Crown Copyright 2009/CCW


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Preferred Scientific Name

  • Didemnum vexillum Kott, 2002

Local Common Names

  • Netherlands: druipzakpijp

Summary of Invasiveness

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D. vexillum, a compound ascidian (tunicate or sea squirt), belongs to the family Didemnidae (Kott, 2002; Lambert, 2009; Stefaniak et al., 2009). Since its identification in 1988 (Kott, 2002), sightings of this species have dramatically increased across the globe. The specific vectors for introduction are largely unknown (see Coutts and Forrest, 2007), though international shipping, local boat traffic and transport of aquaculture species are likely sources (Carlton, 1989; Dijkstra et al., 2007a). Didemnids possess chemical defences (Pisut and Pawlik, 2002) and an acidic tunic (Bullard et al., 2007a), and tolerate a wide range of temperatures (Bullard et al., 2007a; Dijkstra et al., 2007a; Valentine et al., 2007b, 2009), salinities (Dijkstra et al., 2007a; Bullard and Whitlatch, 2009) and nutrients (Carman et al., 2007). Like all ascidians, they produce lecithotrophic larvae that spend less than 24 hours in the water column before settling on suitable substrate and metamorphosing into adult colonies that allow them to build up local populations. In addition, didemnids can disperse through larvae or through fragmentation (Bullard et al., 2007b). They have few known predators (Lambert, 2009) and undergo fast rates of growth (Valentine et al., 2007b). All of these characteristics allow D. vexillum to successfully occupy new habitats and become a dominant spatial competitor.

Taxonomic Tree

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  • Domain: Eukaryota
  •     Kingdom: Metazoa
  •         Phylum: Chordata
  •             Subphylum: Tunicata
  •                 Class: Ascidiacea
  •                     Order: Enterogona
  •                         Suborder: Aplousobranchia
  •                             Family: Didemnidae
  •                                 Genus: Didemnum
  •                                     Species: Didemnum vexillum

Notes on Taxonomy and Nomenclature

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Didemnum vexillum is a compound ascidian (tunicate or sea squirt) and belongs to the family Didemnidae (Kott, 2002).


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D. vexillum is likely to originate from Japan (Lambert, 2009) and is now found in the northeast Pacific (British Columbia to Southern California (Cohen et al., 2001; Lambert, 2009), northeast of the USA (Bullard et al., 2007a; Dijkstra et al., 2007a), New Zealand (Coutts, 2002; Coutts and Forrest, 2007), the Netherlands (Lambert, 2009), northwestern France (Lambert, 2009), Ireland (Minchin and Sides, 2006; Minchin, 2007), England (Arenas et al., 2006) and North Wales (USGS, 2009).

D. vexillumn is found around the world in temperate waters (A Gittenberger, Gimaris, The Netherlands, personal communication, 2009).

Distribution Table

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The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.

Continent/Country/RegionDistributionLast ReportedOriginFirst ReportedInvasiveReferenceNotes

Sea Areas

Atlantic, NortheastWidespreadArenas et al., 2006; Minchin and Sides, 2006; Minchin, 2007; Lambert, 2009The Netherlands, England, Ireland, France
Atlantic, NorthwestWidespread Invasive Dijkstra et al., 2007a; Valentine et al., 2007b; Osman and Whitlatch, 2007New York (NY) to Eastport (ME), Georges Bank, introduced to the Gulf of Maine in the 1970s
Indian Ocean, EasternPresentCoutts, 2002; Coutts and Forrest, 2007North Island
Pacific, NortheastWidespread Invasive Cohen et al., 2001; Lambert, 2009Found in coast off Alaska, British Columbia to Ensenada, Baja California.


NetherlandsPresent1974 Invasive Gittenberger, 2007Threat to native ecosystems, especially in the province of Zeeland, as Didemnum sp.

Habitat List

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Estuaries Secondary/tolerated habitat Natural
Inland saline areas Present, no further details Natural
Coastal areas Secondary/tolerated habitat Natural
Intertidal zone Present, no further details Natural
Benthic zone Present, no further details Natural
Inshore marine Present, no further details Natural

Biology and Ecology

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Both global and local population genetics studies are being carried out by a number of institutes in the USA and New Zealand (A Gittengerber, Gimaris, The Netherlands, personal communication, 2009). For further details see Stefaniak et al. (2009)

Reproductive Biology

Like all colonial ascidians, D. vexillum has two types of reproduction: asexual and sexual. Asexual reproduction occurs through budding which produces colonies of genetically identical zooids. It is made up of individual genetically identical zooids that are not connected by an internal vascular system. Zooids appear randomly embedded in a transparent, spiculated tunic (see Kott (2002) and Lambert (2009) for a full description).

Physiology and Phenology

Colonies exhibit a wide range of morphological variation and within their tunic have calcium carbonate spicules (Bullard et al., 2007a). They range in colour from pinkish, tan or pale orange and can form undulating encrusting mats or rope-like tendrils on vertical surfaces (Bullard et al., 2007a). In its reported range, this species has an extremely rapid growth, quickly covering large areas (100 km2). In coastal zones, D. vexillum has seasonal cycles, undergoing regression during the coldest winter months (Dijkstra et al., 2007a). It grows on both horizontal and vertical surfaces on a wide variety of substrata including intertidal, subtidal rocky surfaces and man-made substrates (Bullard et al., 2007a; Dijkstra et al., 2007a; Valentine et al., 2007b). Colour and growth form are likely related to environmental parameters and substrate orientation. For example, on vertical surfaces D. vexillum tends to form rope-like tendrils whereas colonies on horizontal surfaces or in areas of high current velocities tend to be mat-like (Bullard et al., 2007a; Valentine et al., 2007b). Rope-like tendrils can break off, float away and are capable of reattachment and growth (Bullard et al., 2007a; Coutts and Forrest, 2007; Valentine et al., 2007b). These fragments may increase local recruitment as they may contain brooded larvae that are capable of being released (Lambert, 2009). Although some introduced species remain restricted to artificial substrates in harbours, D.vexillum colonizes and overgrows healthy natural subtidal surfaces (Dijkstra et al., 2007a; Osman and Whitlatch, 2007; Valentine et al., 2007a, 2007b).


Once established in an area, D. vexillum overgrows shellfish (e.g. mussels, scallops) and other sessile invertebrate species (Valentine et al., 2007b) and inhibits settlement of invertebrate species (Dijkstra et al., 2007b; Morris et al., 2009). Very few species settle on the tunic of living colonies as it can be acidic (Dijkstra et al., 2007b). Its rapid growth and acidic tunic reduces the availability of space for settlement and their short larval dispersal allows them to build up local populations. It is a pest to mussel farmers and must be removed either manually or by innovative engineering (Coutts and Forrest, 2007).

Environmental Requirements

D. vexillum is currently observed in areas that experience temperature ranges between 0-28°C (Bullard et al., 2007a; Valentine et al., 2007a). However, Gittenberger (2007) reports that the optimal growing temperature for Didemnum sp. appears to be 14-18°C with virtually all colonies dying when water temperatures are lower than 4°C. D. vexillum is common in marine waters (33 psu), though it can be observed in estuaries (Dijkstra et al., 2007a). In areas with fluctuating salinity regimes, its growth is significantly reduced (Bullard and Whitlach, 2009; J Dijkstra, University of New Hampshire, USA, personal communication, 2009). In addition, its colour can be dependent on temperature and salinity (Lambert, 2009). During periods of heavy rainfall or in estuaries, the colour of D. vexillum can change from white to tan and looks very similar to a sponge. Change in colour from white to tan results from a reduction in size of spicules and in some cases colonies lose their spicules (Lambert, 2009).

Water Tolerances

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ParameterMinimum ValueMaximum ValueTypical ValueStatusLife StageNotes
Salinity (part per thousand) Optimum ~20-45 tolerated
Water temperature (ºC temperature) 14 18 Optimum 2-28 tolerated, only surviving at extremes of temperature for a short amount of time

Notes on Natural Enemies

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Few predators have been reported to prey on D. vexillum. USGS (2009) has accumulated photographic evidence of predation by a sea star, a sea urchin (Notechinus albocinctus), a chiton (Cryptoconchus porosus) and an intertidal snail (Littorinalittorea) (Valentine et al., 2007a; Lambert, 2009). In the Netherlands, Gittenberger (2007) describes the subtidal snail, Trivia arctica, as a generalist predator of colonial ascidians (including D. vexillum) and another predatory snail of the genus Lamellaria which appears to feed solely on D. vexillum.

Means of Movement and Dispersal

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At least three potential mechanisms account for the introduction of colonial ascidians to the Gulf of Maine: 1) hull fouling, 2) ballast water, and 3) aquaculture. Because colonial ascidians produce lecithotrophic larvae that have an abbreviated planktonic stage (Lambert, 1968; Olson, 1985; Svane and Young, 1989), the likelihood of larvae surviving in ballast water is very low (Carlton and Geller, 1993). Thus, the most likely vectors for transport are hull fouling, aquaculture or rafting. Rafting on broken leaves and other debris to which they are attached can transport colonies both long and short distances. Some studies show evidence that rafting events occur frequently, and thus may have a substantial effect on population dynamics. Dijkstra et al. (2007a) found bivalve aquaculture to be the likely mechanism of long-distance (trans-oceanic) spread for this species. Other vectors include sea chests. Once transported to a site, further introduction to sites in the region (kms apart) can be from hull fouling of recreational or commercial vessels.

Economic Impact

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In New Zealand, aquaculture industries have spent $807,000 attempting to eradicate the species from economically important habitats such as mussel lines. The sea scallop, Placopecten magellanicus, is regularly overgrown by D. vexillum in Georges Bank and in Eastport, Maine (Valentine et al., 2007b). The impact of this species on biology of sea scallops and its ecological and economical impact on fishing is yet unknown, though studies suggest the presence of the species inhibits recruitment (Morris et al., 2009). Ecological knowledge gained from studies on this and other similar species suggest that D. vexillum may become a permanent member of the community.

Risk and Impact Factors

Top of page Invasiveness
  • Proved invasive outside its native range
  • Is a habitat generalist
  • Pioneering in disturbed areas
  • Tolerant of shade
  • Fast growing
  • Has high reproductive potential
  • Reproduces asexually
Impact outcomes
  • Ecosystem change/ habitat alteration
  • Modification of natural benthic communities
  • Monoculture formation
  • Negatively impacts aquaculture/fisheries
  • Threat to/ loss of native species
Impact mechanisms
  • Competition - monopolizing resources
  • Competition - smothering
  • Filtration
  • Fouling
  • Interaction with other invasive species
  • Rapid growth
Likelihood of entry/control
  • Highly likely to be transported internationally accidentally
  • Difficult to identify/detect as a commodity contaminant
  • Difficult to identify/detect in the field
  • Difficult/costly to control

Prevention and Control

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Management of D. vexillum has been restricted to attempts in New England (J Dijkstra and L Harris, University of New Hampshire, USA, personal communication, 2009) and New Zealand (Coutts and Forrest, 2007). During their study, Coutts and Forrest (2007) examined a variety of eradication techniques and determined that regional eradication is unlikely, but eradication at small-scales may be possible. Denny (2008) tested several chemical techniques and determined a 2% bleach solution worked best. Establishing monitoring programs is likely the most effective management strategy.


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Arenas F; Bishop J; Carlton J; Dyrynda P; Farnham W; Gonzales D; Jacobs M; Lambert C; Lambert G; Nielsen S; Pederson J; Porter J; Ward S; Wood C, 2006. Alien species and other notable records from arapid assessment survey of marinas on the south coast of England. Journal of the Marine Biological Association UK, 86:1329-1337.

Bullard SG; Lambert G; Carman MR; Byrnes J; Whitlatch RB; Ruiz GM; Miller RJ; Harris LG; Valentine PC; Collie JS; Pederson J; McNaught DC; Cohen AN; Asch RG; Dijkstra J; Heinonen K, 2007. The Invasive colonial ascidian Didemnum sp.: current distribution, basic biology, and potential threat to marine communities of the northeast and west coasts of the United States. J. Exp. Mar. Biol. Ecol, 342:99-108.

Bullard SG; Sedlack B; Reinhardt JF; Litty C; Gareau K; Whitlatch RB, 2007. Fragmentation of colonial ascidians: differences in reattachment capability among species. Journal of Experimental Marine Biology and Ecology, 342(1):166-168.

Bullard SG; Whitlatch RB, 2009. In situ growth of the colonial ascidian Didemnum vexillum under different environmental conditions. Aquatic Invasions, 4(1):275-278.

Carlton J; Geller J, 1993. Ecological roulette: The global transport of nonindigenous marine organisms. Science, 261:78-82.

Carlton JT, 1989. Man's role in changing the face of the ocean: Biological invasions and implications for conservation of near-shore environments. Conservation Biology, 3:265-273.

Carman MR; Bullard SG; Donnelly JP, 2007. Water quality, nitrogen pollution, and ascidian diversity in coastal waters of southern Massachusetts, USA. Journal of Experimental Marine Biology and Ecology, 342(1):175-178.

Cohen A; Berry H; Mills C; Milne D; Britton-Simmons K; Wonham M; Secord D; Barkas J; Bingham B; Bookheim B; Byers J; Chapman J; Cordell J; Dumbauld B; Fukuyama A; Harris L; Kohn A; Li K; Mumford T; Radashevsky V; Sewell A; Welch K, 2001. Washington state exotics expedition 2000: a rapid survey of exotic species in the shallow waters of Elliott Bay, Totten and Eld Inlets, and Willapa Bay. Olympia, : Washington State Department of Natural Resources.

Coutts A, 2002. A biosecurity investigation of a barge in the Marlborough Sounds. Cawthron Report No. 744.

Coutts ADM; Forrest BM, 2007. Development and application of tools for incursion response: lessons learned from the management of the fouling pest Didemnum vexillum. Journal of Experimental Marine Biology and Ecology, 342(1):154-162.

Denny CM, 2008. Development of a method to reduce the spread of the ascidian Didemnum vexillum with aquaculture transfers. ICES Journal of Marine Science, 65(5):805-810.

Dijkstra J; Harris LG; Westerman E, 2007. Distribution and long-term temporal patterns of four invasive colonial ascidians in the Gulf of Maine. Journal of Experimental Marine Biology and Ecology, 342(1):61-68.

Dijkstra J; Sherman H; Harris LG, 2007. The role of colonial ascidians in altering biodiversity in marine fouling communities. Journal of Experimental Marine Biology and Ecology, 342(1):169-171.

Gittenberger A, 2007. Recent population expansions of non-native ascidians in The Netherlands. Journal of Experimental Marine Biology and Ecology [Proceedings of the 1st International Invasive Sea Squirt Conference, Woods Hole, Massachusetts, USA, April 21-22, 2005.], 342(1):122-126.

Kott P, 2002. A complex didemnid ascidian from Whangamata, New Zealand. Journal Marine Biological Association UK, 82:625-628.

Lambert G, 1968. The general ecology and growth of a solitary ascidian, Corella willmeriana. Biological Bulletin Woods Hole, 135:296-307.

Lambert G, 2007. Invasive sea squirts: A growing global problem. Journal of Experimental Marine Biology and Ecology, 342:3-4.

Lambert L, 2009. Adventures of a sea squirt sleuth: unraveling the identity of Didemnum vexillum, a global ascidian invader. Aquatic Invasions, 4:5-28.

Minchin D, 2007. Rapid coastal survey for targeted alien species associated with floating pontoons in Ireland. Aquatic Invasions, 2(1):63-70.

Minchin D; Sides E, 2006. Appearance of a cryptogenic tunicate, a Didemnum sp. fouling marina pontoons and leisure craft in Ireland. Aquatic Invasions, 1(3):143-147.

Morris JA Jr; Carman MR; Hoagland KE; Green-Beach ERM; Karney RC, 2009. Impact of the invasive colonial tunicate Didemnum vexillum on the recruitment of the bay scallop (Argopecten irradians irradians) and implications for recruitment of the sea scallop (Placopecten magellanicus) on Georges Bank. Aquatic Invasions, 4(1):207-211.

Olson RR, 1985. The consequences of short-distance larval dispersal in a sessile marine invertebrate. Ecology, 66:30-39.

Osman RW; Whitlatch RB, 2007. Variation in the ability of Didemnum sp. to invade established communities. Journal of Experimental Marine Biology and Ecology, 342(1):40-53.

Pederson J; Bullock R; Carlton JT; Dijkstra J; Dobroski N; Dyrynda P; Fisher R; Harris L; Hobbs N; Lambert G; Lazo-Wasem E; Mathieson AC; Miglietta MP; Smith J; Smith IIIJ; Tyrrell M, 2005. Marine invaders of the Northeast: Rapid assessment survey of non-native and native marine species of floating dock communities, August 2003. Massachusetts Institute of Technology Sea Grant College Program Publication No. 05-3.

Pisut DP; Pawlik JR, 2002. Anti-predatory chemical defenses of ascidians: secondary metabolites or inorganic acids? Journal of Experimental Marine Biology and Ecology, 270:203-214.

Ruiz GM; Fofonoff PW; Carlton JT; Wonham MJ; Hines AH, 2000. Invasion of coastal marine communities in North America: apparent patterns, processes and biases. Annual Review of Ecology and Systematics, 31:481-531.

Stefaniak L; Lambert G; Gittenberger A; Zhang H; Lin SJ; Whitlatch RB, 2009. Genetic conspecificity of the worldwide populations of Didemnum vexillum Kott, 2002. Aquatic Invasions, 4(1):29-44.

Svane I; Young CM, 1989. The ecology and behaviour of ascidian larvae. Oceanography of Marine Biology Annual Review, 27:45-90.

USGS, 2009. Marine Nuisance Species. Massachusetts, USA: USGS Woods Hole Science Center.

Valentine PC; Carman MR; Blackwood DS; Heffron EJ, 2007. Ecological observations on the colonial ascidian Didemnum sp. in a New England tide pool habitat. Journal of Experimental Marine Biology and Ecology, 342(1):109-121.

Valentine PC; Carman MR; Dijkstra J; Blackwood DS, 2009. Larval recruitment of the invasive colonial ascidian Didemnum vexillum, seasonal water temperatures in New England coastal and offshore waters, and implications for spread of the species. Aquatic Invasions, 4(1):153-168.

Valentine PC; Collie JS; Reid RN; Asch RG; Guida VG; Blackwood DS, 2007. The occurrence of the colonial ascidian Didemnum sp. on Georges Bank gravel habitat: Ecological observations and potential effects on groundfish and scallop fisheries. J. Exp. Mar. Biol. Ecol, 342:179-181.

Links to Websites

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Pacific NW Critter Watchers-Invasive tunicates in Washington State
The Dutch Ascidians Homepage
USDA-National Invasive Species Information Center-Species profile-seasquirt
USGS-Woods Hole Science Center-Marine Nuisance Species-Didemnum


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30/03/09 Original text by:

Jenn Dijkstra, University of New Hampshire, USA

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