This species is a simple, solitary ascidian, with a thin outer covering or tunic. Overall shape is cylindrical, similar to a vase. The body measures 4-15 cm in length, and attaches to the substrate at its base. The oral and atrial siphons are positioned close together at the anterior end of the body. Overall, the tunic is smooth and translucent, lacking any pigment flecks. A diagnostic red spot at the end of the sperm duct is visible through the tunic.

Ciona intestinalis is well distributed throughout the world, including many European oceans (Ricketts, et al 1985).

Ciona intestinalis has been introduced in the Northeast Pacific, with records ranging from Washington to Mexico. This species also has been collected from other locations around the world, including parts of South Africa, southeast Asia, Australia, New Zealand, Hawaii, and South America. In the Atlantic, the species is cryptogenic, with a geographical range that includes northeast United States, Canada, and Greenland, and stretches across northern Europe, Great Britain, and the Mediterranean. Its native geographical range likely includes northern Europe, but remains unclear (Therriault and Herborg 2008, Zhan et al. 2010).

All the animals in the family Cionidae are called tunicates because they have a "tunic" of cellulose-like substance that covers the body. In this tunic are scattered cells, nerves, and blood vessels (Larousse 1967). The animals in the genus Ciona are known for their soft tunics and flexible bodies because the upper part of their bodies can be drawn into the lower part, "like the finger of a glove" (Nichols and Cooke 1971). A branching vascular system is interlaced within the entire tunic, which makes up about 60% of the animal's weight (Grzimek 1972).

The Ciona intestinalis grows to be about 120 mm (5 in) high and is simple and elongated. It is a sessile, usually attached to a substrate such as seeweed, solitary, and non-colonial organism. It is greenish, translucent, and tubular with terminal inhalent and sub-terminal exhalent siphons (openings). The inhalent siphon is surrounded by eight distinct lobes and the exhalent siphon by six; the lobes are interposed with red or orange pigment spots. The retractor muscles, gut, gonads, and large filter-feeding and respiratory pharynx can sometimes be seen through the body wall (Coleman 1991).

The pharynx has a ring of tentacles at the beginning that prevents large objects from entering it. It then gets larger, and the walls contain many gill slits lined with cilia. The sweeping movement of these cilia sets up the current circulation of water from the pharynx to the alimentary canal and back out to the exterior. An organ known as the endostyle lies on the floor of the branchial chamber; it is believed to be the precursor to the thyroid gland. The endostyle secretes mucus that traps food particles, and then the cilia lining the endostyle pass the mucus to the dorsal midline of the pharynx "where it is rolled into a mucus rope" (Larousse 1967). The rope is then passed to the oesophagus, stomach, and intestine, and faecal pellets are discharged through the atrial opening. These animals contain few blood vessels, no capillaries, and the circulatory system is made up of haemocoelic cavities. The neural gland contains the gonatropic substances and is thought to match the pituitary gland in vertebrates (Larousse 1967).

The larva of the Ciona intestinalis is dispersive, lasting only 36 hours. It contains a notochord within its muscular tail that is lost at metamorphosis, a dorsal nerve cord, a brain and sense-organs. In its life-history, the tunicate shows retrogressive evolution because the larva contains more features similar to the vertebrates than the adult does (Larousse 1967).

In the Northeast Pacific, Ciona intestinalis is most similar to another non-native species found in the region, C. savignyi Herdman, 1882.

Ciona intestinalis are usually found in silty conditions in 0-500 meters (1640 feet) of water. Most are found near rocky shores and estuaries, where the tide of the ocean meets a river current. They are often found growing in great numbers on manmade structures such as piers, pilings, and even in marine aquaria (Coleman 1991). They, and many other species in the class Ascidiacea, are also "common fouling organisms" on buoys, pier-piles, and hulls (Larousse 1967).

Ciona intestinalis feed mainly on fine detrital particles and phytoplankton (Coleman 1991). During the cirulation of water through the large gill basket, food particles are taken from the water and the endostyle secretes mucus to trap the food. It is then passed to the dorsal midline of the pharynx where it is rolled into a mucus rope and passed to the stomach. From here it passes to the intestine, and the faecal pellets formed go from the anus to the atrial opening where they are expelled from the body (Larousse 1967).

The life span of most ascidians is about one year (Grzimek 1972). The Ciona intestinalis is hermaphroditic and releases sperm and eggs through the exhalent siphon. Fertilization occurs at sea, and a tadpole-like larva is formed about 25 hours later. The larva lasts about 36 hours, depending on the temperature of the area, after which it settles and metamorphoses into an adult (Coleman 1991).

The Ciona intestinalis are not an endangered species; therefore, there are no conservation details on them.

US Federal List: no special status

CITES: no special status

Ciona intestinalis are problematic for marine aquaria keepers because they settle within the metal plumbing pipes and filters; this does not occur when these structures are made of plastic (Nichols and Cooke 1971). The concentration of them on piers, pilings, buoys, and ship hulls can also be a nuisance to humans (Coleman 1991, Larousse 1967).

The Ciona intestinalis has no known benefit for humans.