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USGS National Geologic Studies of Benthic Habitats, Northeastern United States

Marine Nuisance Species

Image of marine nuisance species.

Species Didemnum vexillum
colonial tunicate; ascidian; sea squirt

Modified Tuesday, 13-Sep-2011 09:26:57 EDT


Didemnum vexillum in the News new

Didemnum vexillum Video Clips

New additions.

Sandwich, MA (Kim Holzer, July 26, 2011)


Click on figures for larger image to download.

Image of New Zealand Didemnum vexillum. Photo credit: Paul Barter (CI).
New Zealand
Image of California Didemnum sp. Photo credit: Gretchen Lambert (UW).
Image of Washington Didemnum sp. Photo credit: Gretchen Lambert (UW).
Image of British Columbia Didemnum sp. Photo credit: Gordon King (TSF).
British Columbia
Image of Netherlands Didemnum sp.  Photo credit: Arjan Gittenberger (NMNH).
Image of New Hampshire Didemnum vestum.  Photo credit: Becca Toppin (UNH).
New Hampshire
Image of Massachusetts Didemnum sp. Photo credit: Dann Blackwood (USGS).
Image of Georges Bank Didemnum sp. Photo credit: Page Valentine and Dann Blackwood (USGS).
Georges Bank

Taxonomic Classification

Phylum Chordata, Subphylum Tunicata, Class Ascidiacea, Order Aplousobranchia, Family Didemnidae, Genus Didemnum, Species vexillum

Summary of Origin and Occurrence

Within the past few years, the colonial ascidian species Didemnum vexillum has been reported from many parts of the world and has drawn attention as a nuisance species because it reproduces rapidly and fouls marine habitats (including shellfish aquacultures and fishing grounds), ship's hulls, and maritime structures.  It has been reported from northern Europe (Netherlands, France, Ireland, Wales, Scotland), the U.S. east coast (New Jersey, New York, Connecticut, Rhode Island, Massachusetts, New Hampshire, Maine), offshore of New England on Georges Bank, the U.S. and Canadian west coasts (California, Oregon, Washington, British Columbia, Alaska), Japan, and New Zealand.

Until recently, the taxonomy of Didemnum vexillum and forms referred to in the scientific literature and media as “Didemnum sp.” and “Didemnum sp. A” has been in dispute.  However, new morphologic and genetic studies of species of Didemnum collected worldwide (Lambert, 2009; Stefaniak et al., 2009) have shown: 1) that the valid taxonomic name for “Didemnum sp.” and “Didemnum sp. A” is Didemnum vexillum Kott (2002), originally described from New Zealand; 2) that Didemnum vestum Kott (2004), originally described from New Hampshire, is a junior synonym of D. vexillum; and 3) the region of origin of Didemnum vexillum is most likely Japan.  We follow this taxonomy here.

For reports that address the taxonomy of Didemnum vexillum see:

Kott, P., 2002, A complex didemnid ascidian from Whangamata, New Zealand: Journal of Marine Biology Association of the United Kingdom, v. 82, no. 4, p. 625-628. [first description of Didemnum vexillum] (PDF format)

Kott, P., 2004, A new species of Didemnum (Ascidiacea, Tunicata) from the Atlantic coast of North America: Zootaxa, v. 732, p. 1-10. [description of Didemnum vestum and comparison with D. vexillum and D. lahillei] (PDF format)

Lambert, G., 2009, Adventures of a sea squirt sleuth:  unraveling the identity of Didemnum vexillum, a global ascidian invader:  Aquatic Invasions, v. 4, no. 1, p. 5-28. (PDF format)

Stefaniak, L., Lambert, G., Gittenberger, A., Zhang, H., Lin, S., and Whiltlatch, R.B., 2009, Genetic conspecificity of the worldwide populations of Didemnum vexillum Kott 2002:  Aquatic Invasions, v. 4, no. 1, p. 29-44. (PDF format)

Didemnum vexillum colonies alter marine habitats and threaten to interfere with fishing, aquaculture, and other coastal and offshore activities. The colonies shown here are found on hard substrates that include dock structures and floats, wood and metal pilings, moorings and ropes, steel chain, automobile tires, polythene plastic, rock outcrops, gravel seabed (pebbles, cobbles, boulders), and ship hulls. They overgrow organisms such as tunicates, sponges, macroalgae, hydroids, anemones, bryozoans, scallops, mussels, and oysters. Where these colonies occur on the seabed, they likely cover the siphons of infaunal bivalves and also serve as a barrier between demersal fish and benthic prey. The colonies have been found at water depths ranging from intertidal to continental shelf depths of 65m (213 ft).

Gross Morphology and Growth Habits

Colonies of Didemnum vexillum exhibit a wide variety of morphological variants.  Where current velocity is low, they form long, ropey or beard-like colonies that commonly hang from hard substrates such as docks, lines, and ship hulls.  Where current velocity is high, they form low, undulating mats with short surficial appendages that encrust and drape rocky seabeds (pebbles, cobbles, boulders, and rock outcrops).

Purpose and attribution

The goal of this website is to assemble and communicate information on the distribution, ecology, and marine habitat impacts of the invasive colonial ascidian invader Didemnum vexillum.  Researchers and others are encouraged to share their observations on these topics.

All contributions are acknowledged. The information displayed on this website is in the public domain. Users are expected to give proper credit for images, data, and ideas they incorporate into their work.

Contributions to the website can be sent to Page Valentine, USGS,

Website design: Jennifer Martin and Andrea Toran, USGS

Image processing and archival: Dann Blackwood, USGS

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Page Last Modified: Tuesday, 13-Sep-2011 09:26:57 EDT