Ciona intestinalis

Ciona intestinalis (sometimes known by the common name of vase tunicate) is an ascidian (sea squirt), a tunicate with very soft tunic. Its Latin name literally means "pillar of intestines", referring to the fact that its body is a soft, translucent column-like structure, resembling a mass of intestines sprouting from a rock. It is a globally distributed cosmopolitan species: this species has been introduced to various parts of the world and is considered as an invasive species.

Since Linnaeus described the species, Ciona intestinalis has been used as a model species of an invertebrate chordate in developmental biology and genomics.

Although Linnaeus first categorised Ciona intestinalis as a kind of mollusk, Alexander Kovalevsky found a tadpole-like larval stage during development that shows similarity to vertebrates. Recent molecular phylogenetic studies as well as phylogenomic studies support that sea squirts are the closest invertebrate relatives of vertebrates.

Studies conducted between 2005 and 2010 have shown that this species may in fact be a species complex, with at least two, possibly four, sister species that should be split out from C. intestinalis.^{[needs update]} One of these cryptic species were proven to be an already described species; Ciona robusta.

Ciona intestinalis is a solitary tunicate with a cylindrical, soft, gelatinous body, up to 20 centimetres (8 in) long. The body colour and colour at the distal end of siphons are major external characters distinguishing sister species within the species complex.

The body of Ciona is bag-like and covered by a tunic, which is a secretion of the epidermal cells. The body is attached by a permanent base located at the posterior end, while the opposite extremity has two openings, the buccal and atrial siphons. Water is drawn into the ascidian through the buccal (oral) siphon and leaves the atrium through the atrial siphon (cloacal).

In the sea squirt C. intestinalis a CB1 and CB2-type cannabinoid receptors is found to be targeted to axons, indicative of an ancient role for cannabinoid receptors as axonal regulators of neuronal signalling.

Ciona intestinalis was one of the first animals to have its full genome sequenced in 2002, using a specimen from Half Moon Bay in California, US, It possesses a very small genome size, about 160 Mbp, or less than 1/20 of the human genome, which consists of 14 pairs of chromosomes with about 16,000 genes. A number of these genes correspond to almost every family of genes in vertebrates.