Sagul The body shape of the solitary asexual stage is symmetrical; it is asymmetrical for the aggregate sexual stage, with short outerior and posterior projections. The class is divided into 3 orders; the Pyrosomida which are colonial living and the Salpida and Htaliacea which are not colonial. The group includes species with complex life cycles with both solitary and colonial forms. A preliminary study using 28S ribosomal RNA partial sequences: At times, because of their fast generation timelarge numbers of thaliaceans can make up the largest proportion of gelatinous zooplankton. The three claass of thaliaceans are filter feeders. In the communal Pyrosomids each zooid or animal is arranged within a common test such that it draws water in from the outside and passes it into the inside or lumen of the communal form.
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Anatomy[ edit ] Sea squirts are rounded or cylindrical animals ranging from about 0. One end of the body is always firmly fixed to rock, coral, or some similar solid surface. The lower surface is pitted or ridged, and in some species has root-like extensions that help the animal grip onto the surface. The body wall is covered by a smooth thick tunic, which is often quite rigid. The tunic consists of cellulose along with proteins and calcium salts.
Unlike the shells of molluscs, the tunic is composed of living tissue, and often has its own blood supply. In some colonial species, the tunics of adjacent individuals are fused into a single structure. When removed from the water, the animal often violently expels water from these siphons, hence the common name of "sea squirt".
The body itself can be divided into up to three regions, although these are not clearly distinct in most species. The pharyngeal region contains the pharynx , while the abdomen contains most of the other bodily organs, and the postabdomen contains the heart and gonads. In many sea squirts, the postabdomen, or even the entire abdomen, are absent, with their respective organs being located more anteriorly.
The large buccal siphon opens into the pharynx, acting like a mouth. The pharynx itself is ciliated and contains numerous perforations, or stigmata, arranged in a grid-like pattern around its circumference. The beating of the cilia sucks water through the siphon, and then through the stigmata.
A long ciliated groove, or endostyle , runs along one side of the pharynx, and a projecting ridge along the other. The endostyle may be homologous with the thyroid gland of vertebrates, despite its differing function.
Cords of connective tissue cross the atrium to maintain the general shape of the body. The outer body wall consists of connective tissue, muscle fibres, and a simple epithelium directly underlying the tunic. 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. 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. 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. 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.
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. 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.
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. The gonads are located in the abdomen or postabdomen, and include one testis and one ovary, each of which opens via a duct into the cloaca. Different species of ascidians can have markedly different reproductive strategies, with colonial forms having mixed modes of reproduction. Solitary ascidians release many eggs from their atrial siphons; external fertilization in seawater takes place with the coincidental release of sperm from other individuals.
A fertilized egg spends 12 hours to a few days developing into a free-swimming tadpole -like larva , which then takes no more than 36 hours to settle and metamorphose into a juvenile. But one group, the molgulid ascidians, have evolved tailless species on at least four separate occasions, and even direct development.
The tunic develops early in embryonic life, and extends to form a fin along the tail in the larva. The larva also has a statocyst and a pigmented cup above the mouth, which opens into a pharynx lined with small clefts opening into a surrounding atrium. The mouth and anus are originally at opposite ends of the animal, with the mouth only moving to its final posterior position during metamorphosis. When its anterior end touches a surface, papillae small, finger-like nervous projections secrete an adhesive for attachment.
Adhesive secretion prompts an irreversible metamorphosis : various organs such as the larval tail and fins are lost while others rearrange to their adult positions, the pharynx enlarges, and organs called ampullae grow from the body to permanently attach the animal to the substratum. The siphons of the juvenile ascidian become orientated to optimise current flow through the feeding apparatus. Sexual maturity can be reached in as little as a few weeks. Colonies can survive for decades.
An ascidian colony consists of individual elements called zooids. Zooids within a colony are usually genetically identical and some have a shared circulation. Sexual reproduction[ edit ] 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. 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 pharyngeal 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. 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 recognize self from non-self as a mechanism to prevent unrelated colonies from fusing to them and parasitizing them. Fertilization[ edit ] 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 swims through the perivitelline space, finally reaching the egg plasma membrane and entering the egg. 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. Promotion of out-crossing[ edit ] 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. 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. Although avoided, self-fertilization is still possible in B. 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.
In the simplest forms, the members of the colony are linked only by rootlike projections from their undersides known as stolons. 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. 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.
This property has made some species sensitive indicators of pollution. 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. They 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.
Fossil record[ edit ] Ascidians are soft-bodied animals, and for this reason their fossil record is almost entirely lacking. Older Triassic records are ambiguous.
Anatomy[ edit ] Sea squirts are rounded or cylindrical animals ranging from about 0. One end of the body is always firmly fixed to rock, coral, or some similar solid surface. The lower surface is pitted or ridged, and in some species has root-like extensions that help the animal grip onto the surface. The body wall is covered by a smooth thick tunic, which is often quite rigid. The tunic consists of cellulose along with proteins and calcium salts. Unlike the shells of molluscs, the tunic is composed of living tissue, and often has its own blood supply. In some colonial species, the tunics of adjacent individuals are fused into a single structure.