Ascidiacea

The pharynx forms the first part of the digestive system. The endostyle produces a supply of mucus which is then passed into the rest of the pharynx by the beating of flagella along its margins. The mucus then flows in a sheet across the surface of the pharynx, trapping planktonic food particles as they pass through the stigmata, and is collected in the ridge on the dorsal surface. The ridge bears a groove along one side, which passes the collected food downwards and into the oesophageal opening at the base of the pharynx.[4]

The esophagus runs downwards to a stomach in the abdomen, which secretes enzymes that digest the food. 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. In some highly developed colonial species, clusters of individuals may share a single cloaca, with all the atrial siphons opening into it, although the buccal siphons all remain separate. 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.[4]

The heart is a curved muscular tube lying in the postabdomen, or close to the stomach. Each end opens into a single vessel, one running to the endostyle, and the other to the dorsal surface of the pharynx. The vessels are connected by a series of sinuses, through which the blood flows. Additional sinuses run from that on the dorsal surface, supplying blood to the visceral organs, and smaller vessels commonly run from both sides into the tunic. 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.[4]

Unusually, the heart of sea squirts alternates the direction in which it pumps blood every three to four minutes. There are two excitatory areas, one at each end of the heart, with first one being dominant, to push the blood through the ventral vessel, and then the other, pushing it dorsally.[4]

There are four different types of blood cell: lymphocytes, phagocytic amoebocytes, nephrocytes and morula cells. The nephrocytes collect waste material such as uric acid and accumulate it in renal vesicles close to the digestive tract. The morula cells help to form the tunic, and can often be found within the tunic substance itself. In some species, the morula cells possess pigmented reducing agents containing iron (hemoglobin), giving the blood a red colour, or vanadium (hemovanadin) giving it a green colour.[4] In that case the cells are also referred to as vanadocytes.[5]

The ascidian central nervous system is formed from a plate that rolls up to form a neural tube. The number of cells within the central nervous system is very small. The neural tube is composed of the sensory vesicle, the neck, the visceral or tail ganglion, and the caudal nerve cord. The anteroposterior regionalization of the neural tube in ascidians is comparable to that in vertebrates.[6]

Although there is no true brain, the largest ganglion is located in the connective tissue between the two siphons, and sends nerves throughout the body. Beneath this ganglion lies an exocrine gland that empties into the pharynx. The gland is formed from the nerve tube, and is therefore homologous to the spinal cord of vertebrates.[4]

Sea squirts lack special sense organs, although the body wall incorporates numerous individual receptors for touch, chemoreception, and the detection of light.[4]

Different colonial ascidian species produce sexually derived offspring by one of two dispersal strategies – colonial species are either broadcast spawners (long-range dispersal) or philopatric (very short-range dispersal). Broadcast spawners release sperm and ova into the water column and fertilization occurs near to the parent colonies. Some species are also viviparous.[11] Resultant zygotes develop into microscopic larvae that may be carried great distances by oceanic currents. The larvae of sessile forms which survive eventually settle and complete maturation on the substratum- then they may bud asexually to form a colony of zooids.

The picture is more complicated for the philopatrically dispersed ascidians: sperm from a nearby colony (or from a zooid of the same colony) enter the atrial siphon and fertilization takes place within the atrium. Embryos are then brooded within the atrium where embryonic development takes place: this results in macroscopic tadpole-like larvae. When mature, these larvae exit the atrial siphon of the adult and then settle close to the parent colony (often within meters). 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.

Trauma or predation often results in fragmentation of a colony into subcolonies. Subsequent zooid replication can lead to coalescence and circulatory fusion of the subcolonies. Closely related colonies which are proximate to each other may also fuse if they coalesce and if they are histocompatible. Ascidians were among the first animals to be able to immunologically distinguish self from non-self as a mechanism to prevent unrelated colonies from fusing to them and parasitizing them.

Sea squirt eggs are surrounded by a fibrous vitelline coat and a layer of follicle cells that produce sperm-attracting substances. In fertilization, the sperm passes through the follicle cells and binds to glycosides on the vitelline coat. The sperm's mitochondria are left behind as the sperm enters and drives through the coat; this translocation of the mitochondria might provide the necessary force for penetration. The sperm swims through the perivitelline space, finally reaching the egg plasma membrane and entering the egg. 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. After fertilization, free calcium ions are released in the egg cytoplasm in waves, mostly from internal stores. The temporary large increase in calcium concentration prompts the physiological and structural changes of development.

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. In the first phase of ooplasmic segregation, the myoplasmic actin-filament network contracts to rapidly move the peripheral cytoplasm (including the myoplasm) to the vegetal pole, which marks the dorsal side of the embryo. In the second phase, the myoplasm moves to the subequatorial zone and extends into a crescent, which marks the future posterior of the embryo. The ectoplasm with the zygote nucleus ends up at the animal hemisphere while the endoplasm ends up in the vegetal hemisphere.[12]

Ciona intestinalis is a hermaphrodite that releases sperm and eggs into the surrounding seawater almost simultaneously. It is self-sterile, and thus has been used for studies on the mechanism of self-incompatibility.[13] Self/non-self-recognition molecules play a key role in the process of interaction between sperm and the vitelline coat of the egg. It appears that self/non-self recognition in ascidians such as C. intestinalis is mechanistically similar to self-incompatibility systems in flowering plants.[13] Self-incompatibility promotes out-crossing, and thus provides the adaptive advantage at each generation of masking deleterious recessive mutations (i.e. genetic complementation).[14]

Ciona savignyi is highly self-fertile.[15] However, non-self sperm out-compete self-sperm in fertilization competition assays. Gamete recognition is not absolute allowing some self-fertilization. It was speculated that self-incompatibility evolved to avoid inbreeding depression, but that selfing ability was retained to allow reproduction at low population density.[15]

Botryllus schlosseri is a colonial tunicate able to reproduce both sexually and asexually. B. schlosseri is a sequential (protogynous) hermaphrodite, and in a colony, eggs are ovulated about two days before the peak of sperm emission.[16] Thus self-fertilization is avoided, and cross-fertilization is favored. Although avoided, self-fertilization is still possible in B. schlosseri. Self-fertilized eggs develop with a substantially higher frequency of anomalies during cleavage than cross-fertilized eggs (23% vs. 1.6%).[16] Also, a significantly lower percentage of larvae derived from self-fertilized eggs metamorphose, and the growth of the colonies derived from their metamorphosis is significantly lower. 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.[14]

Many colonial sea squirts are also capable of asexual reproduction, although the means of doing so are highly variable between different families. In the simplest forms, the members of the colony are linked only by rootlike projections from their undersides known as stolons. Buds containing food storage cells can develop within the stolons and, when sufficiently separated from the 'parent', may grow into a new adult individual.[4][7]

In other species, the postabdomen can elongate and break up into a string of separate buds, which can eventually form a new colony. In some, the pharyngeal part of the animal degenerates, and the abdomen breaks up into patches of germinal tissue, each combining parts of the epidermis, peritoneum, and digestive tract, and capable of growing into new individuals.[4]

In yet others, budding begins shortly after the larva has settled onto the substrate. In the family Didemnidae, for instance, the individual essentially splits into two, with the pharynx growing a new digestive tract and the original digestive tract growing a new pharynx.[4]

Apurinic/apyrimidinic (AP) sites are a common form of DNA damage that inhibit DNA replication and transcription. AP endonuclease 1 (APEX1), an enzyme produced by C. intestinalis, is employed in the repair of AP sites during early embryonic development.[17] Lack of such repair leads to abnormal development. C. intestinalis also has a set of genes that encode proteins homologous to those employed in the repair of DNA interstrand crosslinks in humans.[18]

Ascidians are soft-bodied animals, and for this reason, their fossil record is almost entirely lacking. The earliest reliable ascidians is Shankouclava shankouense from the Lower Cambrian Maotianshan Shale (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.[1] They are also recorded from Lower Jurassic (Bonet and Benveniste-Velasquez, 1971; Buge and Monniot, 1972[23]) and the Tertiary from France (Deflandre-Riguard, 1949, 1956; Durand, 1952; Deflandre and Deflandre-Rigaud, 1956; Bouche, 1962; Lezaud, 1966; Monniot and Buge, 1971; Varol and Houghton, 1996[24]). Older (Triassic) records are ambiguous.[25] The representatives of the genus Cystodytes (family Polycitoridae) have been described from the Pliocene of France by Monniot (1970, 1971) and Deflandre-Rigaud (1956), and from Eocene of France by Monniot and Buge (1971), and lately from the Late Eocene of S Australia by Łukowiak (2012).[26]

Ernst Haeckel's interpretation of several ascidians from Kunstformen der Natur, 1904

The ascidians were on morphological evidence treated as sister to the Thaliacea and Appendicularia, but molecular evidence has suggested that ascidians could be polyphyletic within the Tunicata, as shown in the following cladogram.[27][28]

Recent studies have suggested an alternate phylogeny, placing Appendicularia as sister to the rest of Tunicata, and Thaliacea nested inside Ascidiacea.[29][30] A grouping of Thaliacea and Ascidiacea to the exclusion of Appendicularia had already been suggested for a long time, under the name of Acopa.[31]

Various ascidians are used as food.

Sea pineapple (Halocynthia roretzi) is cultivated in Japan (hoya, maboya) and Korea (meongge). When served raw, they have a chewy texture and peculiar flavor likened to "rubber dipped in ammonia"[32] which has been attributed to a naturally occurring chemical known as cynthiaol. Styela clava is farmed in parts of Korea where it is known as mideoduk and is added to various seafood dishes such as agujjim. Tunicate bibimbap is a specialty of Geojae island, not far from Masan.[33]

Microcosmus species from the Mediterranean Sea are eaten in France (figue de mer, violet), Italy (limone di mare, uova di mare) and Greece (fouska, φούσκα), for example, raw with lemon, or in salads with olive oil, lemon and parsley.

The piure (Pyura chilensis) is used in the cuisine of Chile – it is consumed both raw and used in seafood stews like bouillabaisse.

Pyura praeputialis is known as cunjevoi in Australia. It was once used as a food source by Aboriginal people living around Botany Bay, but is now used mainly for fishing bait.

Several factors make sea squirts good models for studying the fundamental developmental processes of chordates, such as cell-fate specification. The embryonic development of sea squirts is simple, rapid, and easily manipulated. Because each embryo contains relatively few cells, complex processes can be studied at the cellular level, while remaining in the context of the whole embryo. The eggs of some species contain little yolk and are therefore transparent making them transparency ideal for fluorescent imaging. 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.[34]

Sea squirts are also valuable because of their unique evolutionary position: as an approximation of ancestral chordates, they can provide insight into the link between chordates and ancestral non-chordate deuterostomes, as well as the evolution of vertebrates from simple chordates.[35] The sequenced genomes of the related sea squirts Ciona intestinalis and Ciona savignyi are small and easily manipulated; comparisons with the genomes of other organisms such as flies, nematodes, pufferfish and mammals provides valuable information regarding chordate evolution. A collection of over 480,000 cDNAs have been sequenced and are available to support further analysis of gene expression, which is expected to provide information about complex developmental processes and regulation of genes in vertebrates. Gene expression in embryos of sea squirts can be conveniently inhibited using Morpholino oligos.[36]

• Tudge, Colin (2000). The Variety of Life. Oxford University Press. ISBN 0-19-860426-2.