Introduced Species Summary Project
Squirt (Didemnum sp.)

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Common Name: Sea squirt     Sea Squirt

Scientific Name: Didemnum sp. (several species of sea squirt in the genus Didemnum have been introduced the U.S.; other invasive sea squirt species have yet to be conclusively identified)


Phylum or Division: Chordata (subphylum Tunicata)

Class: Ascidiacea      
Order: Enterogona (suborder: aplousobranchia)
Family: Didemnidae

Genus: Didemnum




Image 1 Sea squirt on sea bed (31784 bytes) Image 3 Sea squirt up close out of water (21604 bytes)


Photos (from top):   (1) Sea squirts taken from a colony on the seafloor off the New England coast; (2) thousands of individual sea squirts create a dense cover over the ocean floor, crowding out and smothering native species such as sea scallops and other organisms that are an important part of this North Atlantic fishery; (3) tunicate colonies can also take on a spongy, branch-like form when they become encrusted on surfaces or objects other than the sea floor.


Identification: In his book The Ancestor’s Tale, Richard Dawkins describes the typical sea squirt as “a bag filled with sea water, plus a gut and reproductive organs anchored to a rock.”  Didemnum live in large, mat-like colonies that encrust the rocky or gravelly surface of a seabed.  Such colonies may include many thousands of individuals, whose color range from pale pink to yellow and orange.  Colonies are also found similarly affixed to other surfaces, such as underwater rock outcroppings, ship hulls and docks.  Didemnum have a sponge-like appearance, often with “appendages” that poke out here and there from individuals in the colony.  More generally, sea squirts – which comprise their own subphylum – are known as “tunicates,” for the soft, flexible, tunic-like covering they develop as adults. 

Original Distribution:  The original distribution of Didemnum and other tunicate species is in dispute.  Some scientist now believe that certain species thought to be native to Europe and the Americas are actually invasive species that arrived centuries ago. 

Current Distribution:  Several species of the genus Didemnum, including Didemnum lahillei and Didemnum vexillum, have invaded coastal waters off New England, California, the Pacific Northwest and as far away as New Zealand.  The populations along the coastal U.S. from Maine to Long Island, and in the Pacific, have been growing rapidly.

Site and Date of Introduction:  Didemnum vexillum was documented in the coastal waters of California in 1998.  Two years later, the species was found in coastal New England.  More recently, a large colony – a mat covering 6.5 square miles -- was found further offshore, approximately 160 miles from Cape Cod on the Georges Bank, some 150 feet below the water’s surface.  It is presumed, based on anecdotal evidence, that the species arrived in New England waters in the mid-1990s.  A closely related tunicate species was also discovered in New Zealand waters in 2001.  Some species are thought to have arrived in North American waters in the 1970s.  

Mode(s) of Introduction:  Ballast water, ship hulls and fishing equipment are presumed to be the mode of introduction for most species.  In addition, pieces of sea squirt colonies, which may contain larva, have also been known to break away from the larger colony.  Such fragments may move through the water indefinitely and then reproduce asexually, creating another possible route for local introduction. 

Reason(s) Why it has Become Established:  According to immediately available research, the reasons why Didemnum have become established are not fully known.  Byron Daley and Don Scavia of the University of Michigan have written that “there is a critical need to identify the abiotic and biotic factors controlling the abundance and distribution of Didemnum, as well as the impacts that human activities have had on these controlling factors (e.g., human-mediated dispersal and habitat degradation).”

Ecological Role:   Sea squirts siphon in water and filter out plankton.  They have a very interesting development process.  When you look at the photos above, there is little to suggest that these creatures would be classified as chordates, the phylum that includes vertebrates.  They do possess a notochord, which is a flexible tube-like structure.  But the notochord is only present during the larvae stage of development, when the sea squirt looks much like a tadpole.  Once it attaches itself to a rock or other object, however, this economical organism begins to break down and recycle its larval tissue, including the notochord and “head.”  Thus, sea squirts are sometimes said to “eat their own brains.”

Benefit(s):  Researchers have found that sea squirts are a rich source of bioactive chemicals – which serve as a defense against other marine organisms that might want to eat them. 

Threat(s):  Invasive species of sea squirts, notably of the genus Didemnum, pose a threat to important fisheries and aquaculture.  The dense colonies formed by these organisms can overrun and alter the marine habitats of many other species, including scallops, mussels, oysters and sponges.  

Control Level Diagnosis:   Many species of sea squirts, whether native species or arrivals from past decades or centuries, are well known in North American waters, and have thrived in disturbed habitats such as port areas.  The more recent arrival of Didemnum species, and their rapid growth and abundance, suggests that this should be, at minimum, a medium priority.  Documented damage to marine habitats has thus far been minimal – but the characteristics of Didemnum suggests there is cause for concern, as they possess many of the ingredients that scientists fear in invasive species: 

Control Method:   A control method, or formal management policy, has not yet been determined.  Currently, a number of studies are underway to monitor changes in the distribution and abundance of Didemnum, better understand the physical and biological needs of this genus, and study the impact documented colonies are having on native species.  Fisherman are being encouraged to report sightings of tunicates to the U.S. Geological Survey or other research institutions.

At a scientific conference in April 2005, three researchers proposed potential strategies for controlling Didemnum, including “induced environmental stress” in the form of exposing the organisms to air or temperatures that would be enough to kill.

Author:             Jeffrey Zack  
Last Edited:      November 14, 2006


Photo credits:  Photo 1: Dann Blackwood, U.S. Geological Survey, from the USDA’s National Invasive Species Information Center (; Photos 2 & 3: The National Oceanic and Atmospheric Administration’s (NOAA) National Marine Fisheries Service (NMFS) (


 “An integrated assessment of the potential for further invasion of the colonial ascidian, Didemnum sp. in large marine ecosystems of the United States,” Bryon A. Daley and Don Scavia, University of Michigan’s School of Natural Resources & Environment, September 8, 2006 (draft),


United States Department of Agriculture, National Agriculture Laboratory, National Invasive Species Information Center,


Invasive Marine Species Found on Georges Bank (Nov 19, 2003),” National Oceanic and Atmospheric Administration.

Richard Dawkins, The Ancestor’s Tale, 2005


Stachowicz, J.J., Terwin, J.R., Whitlatch, R.B., and Osman, R.W., 2002, Linking climate change and biological invasions: ocean warming facilitates nonindigenous species invasions, Proceedings of the National Academy of Sciences, v. 99, no. 24, p. 15497-15500

Nonindigenous tunicates in North American continental waters: An overview of invasion patterns by time,” Fofonoff, Paul W. (Smithsonian
Environmental Research Center, Edgewater MD); Ruiz, G.M. and Lambert, G. (University of Washington Friday Harbor Laboratories, Friday Harbor, WA); and Carlton, J.T. (The Maritime Studies Program of Williams College and Mystic Seaport Mystic, CT) (last modified October 2006)

“Ecological observations of the colonial tunicate Didemnum sp. in a New England tide pool habitat and strategies for managing invasive colonial ascidian species,”  Valentine, Page C. (U.S. Geological Survey, Woods Hole, MA); Carman, Mary R. (Woods Hole Oceanographic Institution); Blackwood, Dann S. (U.S. Geological Survey, Woods Hole); and Heffron, Erin J. (Woods Hole Oceanographic Institution)

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