Carpet sea squirt (Didemnum vexillum)

Carpet sea squirt

Top facts

  • The carpet sea squirt is an unusual marine animal that lives in colonies made up of thousands of tiny individuals.
  • The unappealing appearance of carpet sea squirt colonies has led to the species’ alternative name of ‘marine vomit’.
  • Despite its simple appearance, the carpet sea squirt is more closely related to vertebrates than invertebrates.
  • Adult carpet sea squirts live permanently attached to hard substrates, but their larvae are free-swimming and resemble minute tadpoles.
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Carpet sea squirt fact file

Carpet sea squirt description

GenusDidemnum (1)

The carpet sea squirt (Didemnum vexillum) is an unusual marine animal that lives in immobile colonies made up of thousands of tiny, genetically identical individuals (2) (3) (4). The common name of this species comes from the way in which these colonies often grow as flat, carpet-like mats across the ocean floor (2) (3) (5).

Each individual in a carpet sea squirt colony is known as a ‘zooid’ and is about one millimetre in length (1) (3) (6). The zooids have soft, sac-like bodies (3) and are embedded in a firm, fleshy matrix or ‘tunic’, which has tiny white, spiky balls of calcium dotted on its surface (1) (2) (3) (4) (5). The tunic is composed of a material that resembles cellulose, the main component of plant cell walls (7).

In general, sea squirts possess two siphons through which water is pumped into and out of the body, allowing the animal to obtain food and oxygen and to discharge waste products (7) (8). These siphons sometimes expel water if the animal is squeezed, leading to the name ‘sea squirt’ (3) (8). In colonial species such as the carpet sea squirt, each zooid takes water in through one siphon and then discharges it into a common space that is shared with its neighbours (1) (3) (4) (6). Waste water then exits through a small pore in the colony surface (3) (4).

Like other sea squirts, the carpet sea squirt lives permanently attached to a hard substrate (7) (8). The colonies of this species vary in appearance depending on where they grow, with those in strong currents or on horizontal surfaces tending to form dense, flat mats with low, protruding lobes, while those in calm waters and on vertical surfaces form long, hanging, rope-like structures that can resemble dripping wax (1) (2) (3) (5) (6). Carpet sea squirt colonies are leathery in texture and are somewhat translucent, with the common waste channels appearing as a dark, leaf-like network of veins (3) (4) (9) (10) (11). The colonies of this species vary in colour from white to yellow, cream, tan or pale orange (2) (3) (4) (5) (9) (10).

Also known as
carpet tunicate, colonial ascidian, colonial sea squirt, didemnid, Didemnum tunicate, marine vomit, Whangamata sea squirt.
Didemnum sp., Didemnum sp. A., Didemnum vestum.
Colony diameter: up to 1 m or more (2)

Carpet sea squirt biology

The carpet sea squirt is a filter feeder, taking in water through its ‘inhalant’ siphon and filtering out small food particles and plankton using mucous-covered gill slits (2) (3) (4) (7) (8). Tiny, beating hairs on the gills move food particles into the gut for digestion, and the gills are also used to extract oxygen from the water (7) (8).

Inside its body, the carpet sea squirt has a small, simple heart from which blood vessels run to its various organs (7) (8). A small ball of nerve cells serves as the animal’s brain (8).

Each of the carpet sea squirt’s zooids is hermaphroditic, possessing both male and female reproductive organs (3) (7) (8) (9). Some sea squirts release their eggs and sperm into the water column, where fertilisation occurs. However, in the carpet sea squirt the eggs are fertilised internally from sperm taken in through the inhalant siphon (7) (8), and the larvae are brooded inside the colony before being released into the water. The carpet sea squirt larva spends only a few hours in the water before settling head-down on a firm surface and transforming into a zooid (2) (3) (4) (5) (8) (9).

Sea squirts are unusual in that they are more closely related to vertebrates than to invertebrates, despite the adult sea squirt’s quite simple appearance and immobile lifestyle (7). The relationship to vertebrates is more closely seen in the sea squirt larvae, which resemble tiny tadpoles with a long tail, a nerve cord, and more developed internal organs than the adults (7) (8). When the tadpole settles and transforms into a zooid, its tail and nerve chord are absorbed into its body (8).

Newly transformed zooids go on to found a new colony by reproducing asexually, splitting into many new, genetically identical individuals (2) (3) (4) (5) (9). The colony grows rapidly and the young zooids can reach sexual maturity in just a few weeks (2). New carpet sea squirt colonies can also form by fragmentation, when a portion of the colony breaks off and reattaches in a new location (2) (3) (5) (9). These fragments may also contain larvae, which can help to spread the species even further (5).

In the carpet sea squirt, the production of larvae generally depends on the water temperature (3) (9), often occurring in spring and autumn (2). Colonies of this species survive year-round, but tend to shrink during the cold winter months (5) (9). Very few predators are known to feed on the carpet sea squirt, although it has reportedly been eaten by certain snails, sea urchins and sea stars (5) (9). Carpet sea squirt colonies have an acidic tunic and exude a toxic substance onto their surface, discouraging most predators and preventing other species from growing over them (2) (4) (5).


Carpet sea squirt range

The carpet sea squirt was first described as a new species in 2002 (1). Although originally described from specimens in New Zealand (1), it is believed to originate from Japan and the Northwest Pacific Ocean (2) (3) (5) (9) (12).

Considered to be a highly invasive species, the carpet sea squirt has spread to many new locations around the world in recent years. It has so far been found along the east and west coasts of the United States, as well as in Canada, New Zealand and parts of northern Europe, such as the Netherlands, France, Spain and the United Kingdom (3) (5) (9) (12) (13) (14).


Carpet sea squirt habitat

A marine species, the carpet sea squirt is able to grow on a wide range of substrates in coastal waters, usually at depths of up to 65 metres and in water temperatures of -2 to 24 degrees Celsius (2) (9) (10). However, its optimum growing temperature is reported to be around 14 to 18 degrees Celsius (5). The carpet sea squirt is only found in waters with a salinity level above 26 parts per thousand (9) (10).

As well as growing over rock, pebbles, boulders, cobbles and gravel, the carpet sea squirt can also commonly be found on man-made structures such as boats, docks, moorings, ropes, chains, plastic and shellfish aquaculture gear (2) (3) (4) (9). This fast-growing animal can even grow over other living organisms, including sponges, barnacles, mussels, oysters and seaweed (2) (3) (4).

Although the carpet sea squirt has mostly been recorded in coastal waters, this may only be because deeper waters have not yet been surveyed to the same extent (3).


Carpet sea squirt status

The carpet sea squirt has yet to be classified by the IUCN.


Carpet sea squirt threats

The carpet sea squirt is not known to be facing any major threats, and is in fact considered to be a highly invasive species that is a potential problem in the areas to which it has spread. This marine animal is likely to have been transported around the world in ballast water, or attached to the hulls of ships or to aquaculture equipment (2) (3) (5) (9) (10) (11). Colony fragments are easily broken off by boats, divers and bottom dredges, and can drift in currents to new locations (2).

The carpet sea squirt grows rapidly and can smother the sea bed, reducing habitat for other species, and it can also grow over other animals, including native and farmed shellfish (2) (3) (5) (9) (10). Carpet sea squirt colonies can foul aquaculture equipment, boats and moorings and may be expensive to remove (2) (3) (5). It is possible that ocean warming as a result of climate change may facilitate the spread of this invasive species (3).


Carpet sea squirt conservation

There are no specific conservation measures in place to protect the carpet sea squirt. Where it is considered invasive, management measures are being investigated to try and control or eradicate the species and to prevent its further spread. These may include, for example, wrapping colonies in plastic, applying chemicals such as acid or bleach, or manually scraping them off or drying them out before disposing of them carefully (2) (3) (5) (14). Boats and other structures can also be treated with anti-fouling paint (11) (14) (15). Attempts have already been made or are underway to eradicate this invasive species from areas such as North Wales (9), New Zealand (11) and Alaska (3).

Other measures to prevent the carpet sea squirt from spreading include keeping equipment clean, avoiding discards of shellfish in non-infested waters, ensuring that removed sea squirt colonies do not enter the sea, and minimising the movement of infested structures between different locations (2) (11) (14) (15). Monitoring programmes will also be important in detecting any new carpet sea squirt populations, assessing the effectiveness of control measures and monitoring the species’ impacts (5) (13) (14).


Find out more

Find out more about the carpet sea squirt:



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Of asexual reproduction: reproduction that does not involve the formation of sex cells (‘gametes’). In many species, asexual reproduction can occur by existing cells splitting into two, or part of the organism breaking away and developing into a separate individual.
The primary structural component of plants.
The fusion of gametes (male and female reproductive cells) to produce an embryo, which grows into a new individual.
Possessing both male and female sex organs.
Animals with no backbone, such as insects, crustaceans, worms, molluscs, spiders, cnidarians (jellyfish, corals, sea anemones) and echinoderms.
Stage in an animal’s lifecycle after it hatches from the egg. Larvae are typically very different in appearance to adults; they are able to feed and move around but usually are unable to reproduce.
Aquatic organisms, usually tiny, that drift passively with water movements; may be phytoplankton (plants), zooplankton (animals), or other organisms such as bacteria.
Animals with a backbone, including mammals, birds, reptiles, amphibians and fish.
An individual member of a colony of animals in which individuals are physically united by living material.


  1. Kott, P. (2002) A complex didemnid ascidian from Whangamata, New Zealand. Journal of the Marine Biological Association of the United Kingdom, 82: 625-628.
  2. Woodward, S.L. and Quinn, J.A. (2011) Encyclopedia of Invasive Species: From Africanized Honey Bees to Zebra Mussels. Greenwood, Santa Barbara, California.
  3. Alaska Department of Fish and Game: Invasive Species - Didemnum tunicate (September, 2013)
  4. Washington Department of Fish and Wildlife: Aquatic Invasive Species - Colonial sea squirt (September, 2013)
  5. Invasive Species Compendium: Datasheets - Didemnum vexillum (September, 2013)
  6. The Marine Life Information Network (MarLIN): A colonial sea squirt - Didemnum vexillum (September, 2013)
  7. Campbell, A. and Dawes, J. (2004) Encyclopedia of Underwater Life. Oxford University Press, Oxford.
  8. Burton, M. and Burton, R. (2002) International Wildlife Encyclopedia. Third Edition. Marshall Cavendish, New York.
  9. GB Non-native Species Secretariat (NNSS): Factsheet - Carpet sea squirt (September, 2013)
  10. GB Non-native Species Secretariat (NNSS): Identification Sheet - Carpet sea squirt (September, 2013)
  11. Ministry for Primary Industries, New Zealand - Didemnum, aka Whangamata sea squirt (September, 2013)
  12. Stefaniak, L., Zhang, H., Gittenberger, A., Smith, K., Holsinger, K., Lin, S. and Whitlatch, R.B. (2012) Determining the native region of the putatively invasive ascidian Didemnum vexillum Kott, 2002. Journal of Experimental Marine Biology and Ecology, 422-423: 64-71.
  13. Griffith, K., Mowat, S., Holt, R.H.F., Ramsay, K., Bishop, J.D.D., Lambert, G. and Jenkins, S.R. (2009) First records in Great Britain of the invasive colonial ascidian Didemnum vexillum Kott, 2002. Aquatic Invasions, 4(4): 581-590.
  14. Laing, I., Bussell, J. and Somerwill, K. (2010) Assessment of the Impacts of Didemnum vexillum and Options for the Management of the Species in England. Project Report for Defra, UK. Available at:
  15. Scottish Natural Heritage - Carpet sea squirt (September, 2013)

Image credit

Carpet sea squirt  
Carpet sea squirt

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