Ascidiacea, commonly known as the ascidians or sea squirts, is a paraphyletic class in the subphylum Tunicata of sac-like marine invertebrate filter feeders.

While members of the Thaliacea (salps, doliolids and pyrosomes) and Appendicularia (larvaceans) swim freely like plankton, sea squirts are sessile animals after their larval phase: they then remain firmly attached to their substratum, such as rocks and shells.

The outer body wall consists of connective tissue, muscle fibres, and a simple epithelium directly underlying the tunic.

An intestine runs upwards from the stomach parallel to the oesophagus and eventually opens, through a short rectum and anus, into a cloaca just below the atrial siphon.

A series of glands lie on the outer surface of the intestine, opening through collecting tubules into the stomach, although their precise function is unclear.

Nitrogenous waste, in the form of ammonia, is excreted directly from the blood through the walls of the pharynx, and expelled through the atrial siphon.

The nephrocytes collect waste material such as uric acid and accumulate it in renal vesicles close to the digestive tract.

The neural tube is composed of the sensory vesicle, the neck, the visceral or tail ganglion, and the caudal nerve cord.

[5] Sea squirts lack special sense organs, although the body wall incorporates numerous individual receptors for touch, chemoreception, and the detection of light.

[citation needed] As a general rule, the larva possesses a long tail, containing muscles, a hollow dorsal nerve tube and a notochord, both features clearly indicative of the animal's chordate affinities.

The combined effect of short sperm range and philopatric larval dispersal results in local population structures of closely related individuals/inbred colonies.

Generations of colonies which are restricted in dispersal are thought to accumulate adaptions to local conditions, thereby providing advantages over newcomers.

Sea squirt eggs are surrounded by a fibrous vitelline coat and a layer of follicle cells that produce sperm-attracting substances.

This prompts rapid modification of the vitelline coat, through processes such as the egg's release of glycosidase into the seawater, so no more sperm can bind and polyspermy is avoided.

The dramatic rearrangement of egg cytoplasm following fertilization, called ooplasmic segregation, determines the dorsoventral and anteroposterior axes of the embryo.

There are at least three types of sea squirt egg cytoplasm: ectoplasm containing vesicles and fine particles, endoderm containing yolk platelets, and myoplasm containing pigment granules, mitochondria, and endoplasmic reticulum.

[13] Self-incompatibility promotes out-crossing, and thus provides the adaptive advantage at each generation of masking deleterious recessive mutations (i.e. genetic complementation).

It was speculated that self-incompatibility evolved to avoid inbreeding depression, but that selfing ability was retained to allow reproduction at low population density.

These findings suggest that self-fertilization gives rise to inbreeding depression associated with developmental deficits that are likely caused by expression of deleterious recessive mutations.

[19] Over the last few hundred years, most of the world's harbors have been invaded by non-native sea squirts that have been introduced by accident from the shipping industry.

Several factors, including quick attainment of sexual maturity, tolerance of a wide range of environments, and a lack of predators, allow sea squirt populations to grow rapidly.

Unwanted populations on docks, ship hulls, and farmed shellfish cause significant economic problems, and sea squirt invasions have disrupted the ecosystem of several natural sub-tidal areas by smothering native animal species.

Some are also eaten by humans in many parts of the world, including Japan, Korea, Chile, and Europe (where they are sold under the name "sea violet").

As chemical defenses, many sea squirts intake and maintain an extremely high concentration of vanadium in the blood, have a very low pH of the tunic due to acid in easily ruptured bladder cells, and (or) produce secondary metabolites harmful to predators and invaders.

The earliest reliable ascidians is Shankouclava shankouense from the Lower Cambrian Maotianshan Shale (Yunnan, South China).

[22] There are also two enigmatic species from the Ediacaran period with some affinity to the ascidians – Ausia from the Nama Group of Namibia and Burykhia from the Onega Peninsula, White Sea of northern Russia.

When served raw, they have a chewy texture and peculiar flavor likened to "rubber dipped in ammonia"[34] which has been attributed to a naturally occurring chemical known as cynthiaol.

Ciona is being developed in Norway as a potential substitute meat protein, after processing to remove its 'marine taste' and to make its texture less 'squid-like'.

[36] Several factors make sea squirts good models for studying the fundamental developmental processes of chordates, such as cell-fate specification.

Its maternally-derived proteins are naturally associated with pigment (in a few species only), so cell lineages are easily labeled, allowing scientists to visualize embryogenesis from beginning to end.