Editing Tunicate

{{Short description|Marine animals, subphylum of chordates}}
{{Good article}}
{{Use dmy dates|date=September 2020}}
{{Automatic taxobox
| name = Tunicates
| fossil_range = <br>[[Cambrian Stage 3]]&ndash;[[Holocene|Present]],<br>{{fossil range|518|0|earliest=557|ref=<ref>{{Cite journal |last1=Yang |first1=Chuan |last2=Li |first2=Xian-Hua |last3=Zhu |first3=Maoyan |last4=Condon |first4=Daniel J. |last5=Chen |first5=Junyuan |date=2018 |title=Geochronological constraint on the Cambrian Chengjiang biota, South China |journal=[[Journal of the Geological Society]] |language=en |volume=175 |issue=4 |pages=659–666 |doi=10.1144/jgs2017-103 |bibcode=2018JGSoc.175..659Y |s2cid=135091168 |issn=0016-7649 |url=http://nora.nerc.ac.uk/id/eprint/521412/1/2018-JGS-Chuan%20Yang%20et%20al.pdf}}</ref>}} (Possible [[Ediacaran]] record, 557 Ma<ref>{{Cite journal |last1=Fedonkin |first1=M. A. |last2=Vickers-Rich |first2=P. |last3=Swalla |first3=B. J. |last4=Trusler |first4=P. |last5=Hall |first5=M. |title=A new metazoan from the Vendian of the White Sea, Russia, with possible affinities to the ascidians |journal=[[Paleontological Journal]] |volume=46 |pages=1–11 |year=2012 |issue=1 |doi=10.1134/S0031030112010042 |bibcode=2012PalJ...46....1F |s2cid=128415270}}</ref><ref>{{Cite journal |last1=Martyshyn |first1=Andrej |last2=Uchman |first2=Alfred |date=2021-12-01 |title=New Ediacaran fossils from the Ukraine, some with a putative tunicate relationship |journal=PalZ |language=en |volume=95 |issue=4 |pages=623–639 |doi=10.1007/s12542-021-00596-1 |bibcode=2021PalZ...95..623M |s2cid=244957825 |issn=1867-6812 |doi-access=free }}</ref>)
| image = Tunicate komodo.jpg
| image_caption = Gold-mouth sea squirt (''[[Polycarpa aurata]]'')
| image_upright = 1.15
| display_parents = 2
| taxon = Tunicata
| authority = [[Jean-Baptiste Lamarck|Lamarck]], 1816<ref name=WoRMS>{{cite WoRMS |author=Sanamyan, Karen |year=2013 |title=Tunicata |id=146420 |access-date=2013-04-04 }}</ref><ref name="Nielsen2012">{{cite journal |author=Nielsen, C. |date=2012 |title=The authorship of higher chordate taxa |journal=[[Zoologica Scripta]] |volume=41 |issue=4 |pages=435–436 |doi=10.1111/j.1463-6409.2012.00536.x |s2cid=83266247}}</ref>
| subdivision_ranks = Classes and unplaced [[genus|genera]]
| subdivision_ref = <ref name=WoRMS/><ref name="Tatian"/>
| subdivision = * "[[Ascidiacea]]"<ref>{{Cite journal |last=Giribet |first=Gonzalo |date=2018-04-27 |title=Phylogenomics resolves the evolutionary chronicle of our squirting closest relatives |journal=[[BMC Biology]] |volume=16 |issue=1 |pages=49 |doi=10.1186/s12915-018-0517-4 |issn=1741-7007 |pmc=5924484 |pmid=29703197 |doi-access=free }}</ref>
* [[Thaliacea]]
* [[Larvacean|Appendicularia]]
* {{extinct}}''[[Yarnemia]]''?
* {{extinct}}''{{Ill|Megasiphon|ia}}''
* {{extinct}}''[[Shankouclava]]''?
| synonyms = Urochordata <small>Lankester, 1877</small>
}}

A '''tunicate''' is an exclusively marine [[invertebrate]] animal, a member of the [[subphylum]] '''Tunicata''' ({{IPAc-en|ˌ|tj|uː|n|ᵻ|ˈ|k|eɪ|t|ə}} {{respelling|TEW|nih|KAY|tə}}). This grouping is part of the [[Chordata]], a [[phylum]] which includes all animals with [[dorsal nerve cord]]s and [[notochord]]s (including [[vertebrates]]). The subphylum was at one time called '''Urochordata''', and the term '''urochordates''' is still sometimes used for these animals.

Despite their simple appearance and very different adult form, their close relationship to the vertebrates is certain. Both groups are chordates, as evidenced by the fact that during their mobile larval stage, tunicates possess a [[notochord]], a hollow [[dorsal nerve cord]], [[pharyngeal slit]]s, post-anal tail, and an [[endostyle]]. They resemble a [[tadpole]].

Tunicates are the only chordates that have lost their [[Myomere|myomeric]] segmentation, with the possible exception of the seriation of the gill slits.<ref>{{cite journal |pmid=29488055 |doi=10.1007/s12064-018-0260-y |volume=137 |title=The evolutionary origin of chordate segmentation: revisiting the enterocoel theory |year=2018 |journal=Theory Biosci |pages=1–16 |author=Onai T |issue=1 |s2cid=3553695 }}</ref><ref>{{Cite book |url=https://books.google.com/books?id=WXjkBwAAQBAJ&dq=Tunicates+contrast+display+metameric+segmentation+exception+gill+slits&pg=PA163 |title=Before the Backbone: Views on the origin of the vertebrates |first=H. |last=Gee |date=27 July 2007 |publisher=Springer Science & Business Media |isbn=9780585252728 |via=Google Books}}</ref> However, [[Doliolida|doliolids]] still display segmentation of the muscle bands.<ref>{{cite journal |author=Bone, Q., and K. P. Ryan |title=On the Structure and Innervation of the Muscle Bands of Doliolum (Tunicata: Cyclomyaria) |journal=Proceedings of the Royal Society of London. Series B, Biological Sciences |volume=187 |date=1974 |issue=1088 |pages=315–327 |doi=10.1098/rspb.1974.0077 |jstor=76405 |pmid=4154453 |bibcode=1974RSPSB.187..315B |s2cid=20806327 |url=http://www.jstor.org/stable/76405 |access-date=13 May 2023|url-access=subscription }}</ref>

Some tunicates live as solitary individuals, but others replicate by [[budding]] and become [[Colony (biology)|colonies]],<ref>{{Cite journal |last1=Alié |first1=Alexandre |last2=Hiebert |first2=Laurel S. |last3=Scelzo |first3=Marta |last4=Tiozzo |first4=Stefano |date=2020-03-19 |title=The eventful history of nonembryonic development in tunicates |journal=Journal of Experimental Zoology Part B: Molecular and Developmental Evolution |volume=336 |issue=3 |pages=250–266 |language=en |doi=10.1002/jez.b.22940 |pmid=32190983 |bibcode=2021JEZB..336..250A |s2cid=213181394}}</ref> each unit being known as a [[zooid]]. They are marine [[filter feeder]]s with a water-filled, sac-like body structure and two tubular openings, known as siphons, through which they draw in and expel water. During their [[Aquatic respiration|respiration]] and feeding, they take in water through the incurrent (or inhalant) siphon and expel the filtered water through the excurrent (or exhalant) siphon. Adult ascidian tunicates are [[Sessility (zoology)|sessile]], immobile and permanently attached to rocks or other hard surfaces on the ocean floor. [[Thaliacea]]ns (pyrosomes, doliolids, and salps) and [[larvacean]]s on the other hand, swim in the [[pelagic zone]] of the sea as adults.

Various species of [[Ascidiacea|ascidians]], the most well-known class of tunicates, are commonly known as [[sea squirt]]s, sea pork, sea livers, or [[sea tulip]]s.

The earliest probable species of tunicate appears in the fossil record in the early [[Cambrian period]].

==Etymology==
The term was coined in 1760 by [[Plant nursery|nurseryman]] [[Lee and Kennedy|James Lee]],<ref>{{cite web |title=Tunicate: adjective & noun |url=https://www.etymonline.com/word/tunicate |publisher=[[Oxford English Dictionary]] |accessdate=April 14, 2025}}</ref> meaning "coated or covered with [[integument]]s", fancifully [[Anthropomorphism|anthropomorphizing]] the creatures' unique outer coverings as being "enclosed in a [[tunic]]." The word derives from Latin ''tunicātus'', meaning "clothed with a tunic only (without a [[toga]])."<ref>{{cite web |title=Origin and history of tunicate (adj.) |url=https://www.etymonline.com/word/tunicate |publisher=Etymology Online |accessdate=April 14, 2025}}</ref> This "tunic", which is formed from [[proteins]] and [[carbohydrate]]s, acts as an [[exoskeleton]]. In some species, it is thin, translucent, and gelatinous, while in others it is thick, tough, and stiff.

== Taxonomy ==
[[File:Bluebell tunicates Nick Hobgood.jpg|thumb|right|''[[Clavelina moluccensis]]'', the bluebell tunicate]]
[[File:Botrylloides violaceus (cropped).jpg|thumb|right|''[[Botrylloides violaceus]]'' showing oral tentacles at openings of buccal siphons]]

About 3,000 species of tunicate exist in the world's oceans, living mostly in shallow water. The most numerous group is the [[ascidians]]; fewer than 100 species of these are found at depths greater than {{convert|200|m|abbr=on}}.<ref name=Ruppert/> Some are solitary animals leading a [[sessility (zoology)|sessile]] existence attached to the seabed, but others are [[Colony (biology)|colonial]] and a few are [[pelagic]]. Some are supported by a stalk, but most are attached directly to a [[Substrate (biology)|substrate]], which may be a rock, shell, coral, seaweed, [[mangrove]] root, dock, piling, or ship's hull. They are found in a range of solid or translucent colours and may resemble seeds, grapes, peaches, barrels, or bottles. One of the largest is a stalked sea tulip, ''[[Pyura pachydermatina]]'', which can grow to be over {{convert|1|m|ft}} tall.<ref name=Ruppert/>

The Tunicata were established by [[Jean-Baptiste Lamarck]] in 1816. In 1881, [[Francis Maitland Balfour]] introduced another name for the same group, "Urochorda", to emphasize the affinity of the group to other chordates.<ref>Foster, M. (ed.); Sedgwick, Adam (ed.); The Works of Francis Maitland Balfour. Vol. III. Memorial edition. Pub: Macmillan and co. 1885. May be downloaded from [https://archive.org/details/worksoffrancisma03balf]</ref> No doubt largely because of his influence, various authors supported the term, either as such, or as the slightly older "Urochordata", but this usage is invalid because "Tunicata" has precedence, and grounds for superseding the name never existed. Accordingly, the current (formally correct) trend is to abandon the name Urochorda or Urochordata in favour of the original Tunicata, and the name Tunicata is almost invariably used in modern scientific works. It is accepted as valid by the World Register of Marine Species<ref>[http://www.marinespecies.org/aphia.php?p=taxdetails&id=146420 Tunicata] World Register of Marine Species. Retrieved 2011-11-12.</ref> but not by the Integrated Taxonomic Information System.<ref>[https://www.itis.gov/servlet/SingleRpt/SingleRpt?search_topic=TSN&search_value=203347 Tunicata Lamarck, 1816] Integrated Taxonomic Information System. Retrieved 2017-03-30.</ref>

Various common names are used for different species. Sea tulips are tunicates with colourful bodies supported on slender stalks.<ref>{{cite web |url=https://australian.museum/learn/animals/sea-squirts/ |title=Sea squirts and sea tulips |publisher=Australian Museum |access-date=2013-09-25}}</ref> Sea squirts are so named because of their habit of contracting their bodies sharply and squirting out water when disturbed.<ref name="TwoCeans">{{cite web |url=http://dictionary.reference.com/browse/sea+squirt |title=Sea squirt |work=Dictionary.com |access-date=2013-09-25}}</ref> Sea liver and sea pork get their names from the resemblance of their dead colonies to pieces of meat.<ref>{{cite web |url=http://www.sms.si.edu/irlfieldguide/Aplidi_stella.htm |title=Sea pork, ''Aplidium stellatum'' |publisher=Smithsonian at Fort Pierce |access-date=2013-09-25}}</ref>

=== Classification ===
Tunicates are more closely related to [[craniate]]s (including [[hagfish]], [[lamprey]]s, and jawed [[vertebrate]]s) than to [[lancelet]]s, [[echinoderm]]s, [[hemichordate]]s, ''[[Xenoturbella]]'' or other [[invertebrate]]s.<ref name="pmid16495997">{{cite journal |author1=Delsuc, F. |author2=Brinkmann, H. |author3=Chourrout, D. |author4=Philippe, H. |title=Tunicates and not cephalochordates are the closest living relatives of vertebrates |journal=Nature |volume=439 |issue=7079 |pages=965–968 |year=2006 |pmid=16495997 |doi=10.1038/nature04336 |bibcode=2006Natur.439..965D |s2cid=4382758 |url=http://hal-sde.archives-ouvertes.fr/docs/00/31/54/36/PDF/Delsuc-Nature06_HAL.pdf }}</ref><ref name="pmid19003928">{{cite journal |author1=Delsuc, F. |author2=Tsagkogeorga, G. |author3=Lartillot, N. |author4=Philippe, H. |title=Additional molecular support for the new chordate phylogeny |journal=Genesis |volume=46 |issue=11 |pages=592–604 | year=2008 |pmid=19003928 |doi=10.1002/dvg.20450 |s2cid=205771088 |url=https://hal.archives-ouvertes.fr/halsde-00338411 |doi-access=free }}</ref><ref name="pmid19922605">{{cite journal |author1=Singh, T. R. |author2=Tsagkogeorga, G. |author3=Delsuc, F. |author4=Blanquart, S. |author5=Shenkar, N. |author6=Loya, Y. |author7=Douzery, E. J. |author8=Huchon, D. |title=Tunicate mitogenomics and phylogenetics: peculiarities of the ''Herdmania momus'' mitochondrial genome and support for the new chordate phylogeny |journal=BMC Genomics |volume=10 |page=534 |year=2009 |pmid=19922605 |doi=10.1186/1471-2164-10-534 |pmc=2785839 |doi-access=free }}</ref>

The [[clade]] consisting of tunicates and vertebrates is called [[Olfactores]].<ref>Jefferies, R. P. S. (1991) in Biological Asymmetry and Handedness (eds Bock, G. R.; Marsh, J.) pp. 94–127 (Wiley, Chichester).</ref>

The Tunicata contain roughly 3,051 described species,<ref name=Ruppert/> traditionally divided into these classes:
* [[Ascidiacea]] ([[Aplousobranchia]], [[Phlebobranchia]], and [[Stolidobranchia]])
* [[Thaliacea]] ([[Pyrosome|Pyrosomida]], [[Doliolida]], and [[Salp]]ida)
* [[Larvacean|Appendicularia]] ([[Copelata]])

Members of the [[Sorberacea]] were included in Ascidiacea in 2011 as a result of [[ribosomal DNA|rDNA]] sequencing studies.<ref name="Tatian">{{cite journal |author1=Tatián, Marcos |author2=Lagger, Cristian |author3=Demarchi, Milagros |author4=Mattoni, Camilo |year=2011 |title=Molecular phylogeny endorses the relationship between carnivorous and filter-feeding tunicates (Tunicata, Ascidiacea) |journal=Zoologica Scripta |volume=40 |issue=6 |pages=603–612 |doi=10.1111/j.1463-6409.2011.00493.x |s2cid=86421513 }}</ref> Although the traditional classification is provisionally accepted, newer evidence suggests the Ascidiacea are an artificial group of [[paraphyletic]] status.<ref name="Zeng2005">{{cite journal |author1=Zeng, L. |author2=Swalla, B. J. |title=Molecular phylogeny of the protochordates: chordate evolution |journal=Can. J. Zool. |volume=83 |pages=24–33 |year=2005 |doi=10.1139/z05-010 }}</ref><ref name="pmid19656395">{{cite journal |author1=Tsagkogeorga, G. |author2=Turon, X. |author3=Hopcroft, R. R. |author4=Tilak, M. K. |author5=Feldstein, T. |author6=Shenkar, N. |author7=Loya, Y. |author8=Huchon, D. |author9=Douzery, E. J. |author10=Delsuc, F. |title=An updated 18S rRNA phylogeny of tunicates based on mixture and secondary structure models|journal=BMC Evolutionary Biology |volume=9 |page=187 |year=2009 |issue=1 |pmid=19656395 |doi=10.1186/1471-2148-9-187 |pmc=2739199 |bibcode=2009BMCEE...9..187T |doi-access=free }}</ref><ref name=Delsuc2018>{{cite journal |vauthors=Delsuc F, Philippe H, Tsagkogeorga G, Simion P, Tilak MK, Turon X, López-Legentil S, Piette J, Lemaire P, Douzery EJ |date=April 2018 |title=A phylogenomic framework and timescale for comparative studies of tunicates |journal=BMC Biology |volume=16 |issue=1 |page=39 |doi=10.1186/s12915-018-0499-2 |pmc=5899321 |pmid=29653534 |doi-access=free }}</ref> A close relationship between Thaliacea and Ascidiacea, with the former possibly emerging from the latter, had already been proposed since the early 20th century under the name of Acopa.<ref>{{cite journal |url=https://core.ac.uk/download/pdf/39301635.pdf |title=Phylogenetic Speculation of the Tunicata |last1=Tokioka |first1=Takasi |date=1971-06-30 |journal=Publications of the Seto Marine Biological Laboratory |volume=19 |issue=1 |page=47 |doi=10.5134/175655 |s2cid=55491438 }}</ref>

The following cladogram is based on the 2018 phylogenomic study of Delsuc and colleagues.<ref>{{Cite journal |last1=Franchi |first1=Nicola |last2=Ballarin |first2=Loriano |year=2017 |title=Immunity in Protochordates: The Tunicate Perspective |journal=Frontiers in Immunology |volume=8 |page=674 |doi=10.3389/fimmu.2017.00674 |pmc=5465252 |pmid=28649250 |doi-access=free}}</ref><ref name=Delsuc2018/><ref>{{Cite journal |last=Giribet |first=Gonzalo |year=2018 |title=Phylogenomics resolves the evolutionary chronicle of our squirting closest relatives |journal=BMC Biology |volume=16 |issue=1 |page=49 |doi=10.1186/s12915-018-0517-4 |pmc=5924484 |pmid=29703197 |doi-access=free}}</ref>

{{clade
|label1='''Tunicata'''
|1={{clade
   |label1=[[Larvacean|Appendicularia]]
   |1={{clade
      |1=[[Oikopleuridae]] [[File:Oikopleura_dioica.gif|40 px]]
      |2={{clade
         |1=[[Kowalevskiidae]] 
         |2=[[Fritillariidae]] [[File:Appendicularia_%28YPM_IZ_096169%29.jpeg|60 px]] }} }}
   |label2=Acopa
   |2={{clade
      |1={{clade
         |label1=[[Thaliacea]]
         |1={{clade
            |1=[[Pyrosomida]] [[File:Pyrosoma_atlanticum.JPG|60 px]]
            |2=[[Salpida]] [[File:Salp_colony,_Aorangaia_PA171899.JPG|60 px]]
            |3=[[Doliolida]] [[File: Cyclomyaria.jpg|60 px]]
            }}
         |label2=Enterogona
         |2={{clade
            |1=[[Phlebobranchia]] [[File: CionaintestinalisR.jpg|60 px]] |bar1=green
            |2=[[Aplousobranchia]]&nbsp;  [[File: Sea_Squirts_Didemnum_molle.jpg|60 px]] |bar2=green
            }}
         }}
      |grouplabel2=[[Ascidiacea]] |grouplabelstyle2=vertical-align:top;|bar2=green
      |2={{clade 
         |label1=[[Stolidobranchia]] 
         |1={{clade
            |1=[[Molgulidae]] [[File: Molgula_oculata_001.png|60 px]]
            |2={{clade
               |1=[[Styelidae]] [[File:Tunicate_komodo.jpg|60 px]]
               |2=[[Pyuridae]] [[File:Microcosmus_sabatieri.jpg|60 px]]
               }}
            }}
         }}
      }}
  }}
|style=font-size:100%;line-height:100%}}

===Fossil record===
[[Image:Catellocaula.jpg|thumb|right|The star-shaped holes (''[[Catellocaula vallata]]'') in this Upper Ordovician bryozoan may represent a tunicate preserved by [[Fossil#Bioimmuration|bioimmuration]] in the [[bryozoa]]n skeleton.]]
Undisputed fossils of tunicates are rare. The best known and earliest unequivocally identified species is ''[[Shankouclava|Shankouclava shankouense]]'' from the Lower [[Cambrian]] [[Maotianshan Shale]] at Shankou village, Anning, near [[Kunming]] ([[Northern and southern China|South China]]).<ref name=Jun-Yuan2003>{{cite journal|author1=Chen, Jun-Yuan |author2=Huang, Di-Ying |author3=Peng, Qing-Qing |author4=Chi, Hui-Mei |author5=Wang,Xiu-Qiang |author6=Feng, Man |year=2003 |title=The first tunicate from the Early Cambrian of South China |journal=[[Proceedings of the National Academy of Sciences]] |volume=100 |pmid=12835415 |issue=14 |pages=8314–8318 |pmc=166226 |doi=10.1073/pnas.1431177100 |bibcode=2003PNAS..100.8314C |doi-access=free }}</ref> There is also a common [[Fossil#Bioimmuration|bioimmuration]], (''Catellocaula vallata''), of a possible tunicate found in Upper [[Ordovician]] [[bryozoa]]n skeletons of the upper midwestern United States.<ref>{{cite journal |author1=Palmer, T. J. |author2=Wilson, M. A. |year=1988 |title=Parasitism of Ordovician bryozoans and the origin of pseudoborings |journal=Palaeontology |volume=31 |pages=939–949 |url=http://palaeontology.palass-pubs.org/pdf/Vol%2031/Pages%20939-949.pdf |access-date=7 April 2013 |archive-url=https://web.archive.org/web/20130927070328/http://palaeontology.palass-pubs.org/pdf/Vol%2031/Pages%20939-949.pdf |archive-date=27 September 2013 |url-status=usurped }}</ref> A well-preserved Cambrian fossil, ''Megasiphon thylakos'', shows that the tunicate basic body design had already been established 500 million years ago.<ref>{{Cite web |url=https://www.techexplorist.com/500-million-year-old-fossil-reveals-amazing-secrets-tunicate-origins/63818/ |title=A 500 million-year-old fossil reveals the amazing secrets of tunicate origins |first=Vidya |last=Nagalwade |date=7 July 2023 |website=Tech Explorist}}</ref>

Three enigmatic species were also found from the [[Ediacaran]] period – ''[[Ausia (animal)|Ausia fenestrata]]'' from the Nama Group of [[Namibia]], the sac-like ''[[Yarnemia|Yarnemia ascidiformis]]'', and one from a second new ''Ausia''-like genus from the Onega Peninsula of northern [[Russia]], ''[[Burykhia|Burykhia hunti]]''. Results of a new study have shown possible affinity of these Ediacaran organisms to the ascidians.<ref name=Vickers-Rich>Vickers-Rich P. (2007). "Chapter 4. The Nama Fauna of Southern Africa". In: Fedonkin, M. A.; Gehling, J. G.; Grey, K.; Narbonne, G. M.; Vickers-Rich, P. "The Rise of Animals: Evolution and Diversification of the Kingdom Animalia", Johns Hopkins University Press. pp. 69–87</ref><ref name=Oslo_2008>Fedonkin, M. A.; Vickers-Rich, P.; Swalla, B.; Trusler, P.; Hall, M. (2008). "A Neoproterozoic chordate with possible affinity to the ascidians: New fossil evidence from the Vendian of the White Sea, Russia and its evolutionary and ecological implications". HPF-07 Rise and fall of the Ediacaran (Vendian) biota. International Geological Congress - Oslo 2008.</ref> ''Ausia'' and ''Burykhia'' lived in shallow coastal waters slightly more than 555 to 548&nbsp;million years ago, and are believed to be the oldest evidence of the chordate lineage of metazoans.<ref name=Oslo_2008/> The Russian Precambrian fossil ''[[Yarnemia]]'' is identified as a tunicate only tentatively, because its fossils are nowhere near as well-preserved as those of ''Ausia'' and ''Burykhia'', so this identification has been questioned.

Fossils of tunicates are rare because their bodies decay soon after death, but in some tunicate families, microscopic spicules are present, which may be preserved as microfossils. These spicules have occasionally been found in Jurassic and later rocks, but, as few palaeontologists are familiar with them, they may have been mistaken for [[sponge spicule]]s.<ref>{{cite web |url=http://www.ucmp.berkeley.edu/chordata/urochordata.html |title=Introduction to the Urochordata |publisher=University of California Museum of Paleontology |access-date=2013-04-07 |archive-url=https://web.archive.org/web/20090421215035/http://www.ucmp.berkeley.edu/chordata/urochordata.html |archive-date=21 April 2009 |url-status=dead }}</ref>

In the Permian and the Triassic, there were also forms with a calcareous exoskeleton. At first, they were mistaken for corals.<ref>[https://www.cambridge.org/core/services/aop-cambridge-core/content/view/0FE5DCCCDFDD464B92DCA4AF68F36F2B/S0022336019001094a.pdf/rare_case_of_an_evolutionary_late_and_ephemeral_biomineralization_tunicates_with_composite_calcareous_skeletons.pdf A rare case of an evolutionary late and ephemeral biomineralization: tunicates with composite calcareous skeletons]</ref><ref>{{Cite journal |url=https://onlinelibrary.wiley.com/doi/10.1111/pala.12356 |title=The first tunicate with a calcareous exoskeleton (Upper Triassic, northern Italy) |first=Jobst |last=Wendt |editor-first=Michael |editor-last=Hautmann |date=25 July 2018 |journal=Palaeontology |volume=61 |issue=4 |pages=575–595 |via=CrossRef |doi=10.1111/pala.12356|bibcode=2018Palgy..61..575W |s2cid=135456629 |url-access=subscription }}</ref>

===Hybridization studies===

A multi-taxon [[Molecular phylogenetics|molecular study]] in 2010 proposed that sea squirts are descended from a hybrid between a chordate and a [[protostome]] ancestor (before the divergence of [[panarthropod]]s and [[nematode]]s). This study was based on a quartet partitioning approach designed to reveal [[horizontal gene transfer]] events among metazoan phyla.<ref>{{cite journal |author1=Syvanen, M. |author2=Ducore, J. |year=2010 |title=Whole genome comparisons reveals a possible chimeric origin for a major metazoan assemblage |journal=Journal of Biological Systems |volume=18 |pages=261–275 |doi=10.1142/S0218339010003408 |issue=2 }}</ref>

==Anatomy==

===Body form===
[[Image:Tunicate green.jpg|thumb|Colonial tunicate with multiple openings in a single tunic.]]
Colonies of tunicates occur in a range of forms, and vary in the degree to which individual organisms, known as [[zooids]], integrate with one another. In the simplest systems, the individual animals are widely separated, but linked together by horizontal connections called [[stolon]]s, which grow along the seabed. Other species have the zooids growing closer together in a tuft or clustered together and sharing a common base. The most advanced colonies involve the integration of the zooids into a common structure surrounded by the tunic. These may have separate buccal siphons and a single central atrial siphon and may be organized into larger systems, with hundreds of star-shaped units. Often, the zooids in a colony are tiny but very numerous, and the colonies can form large encrusting or mat-like patches.<ref name=Ruppert/>

===Body structure===
By far the largest class of tunicates is the [[Ascidiacea]]. The body of an ascidiacean is surrounded by a [[Test (biology)|test]] or tunic, from which the [[subphylum]] derives its name. This varies in thickness between species but may be tough, resembling cartilage, thin and delicate, or transparent and gelatinous. The tunic is composed of proteins, crosslinked by phenoloxidase reaction,<ref>{{Cite journal |last1=Daugavet |first1=M. A. |last2=Dobrynina |first2=M. I. |last3=Shaposhnikova |first3=T. G. |last4=Solovyeva |first4=A. I. |last5=Mittenberg |first5=A. G. |last6=Shabelnikov |first6=S. V. |last7=Babkina |first7=I. Yu |last8=Grinchenko |first8=A. V. |last9=Ilyaskina |first9=D. V. |last10=Podgornaya |first10=O. I. |date=2022-08-22 |title=New putative phenol oxidase in ascidian blood cells |journal=Scientific Reports |language=en |volume=12 |issue=1 |pages=14326 |doi=10.1038/s41598-022-18283-9 |issn=2045-2322 |pmc=9395347 |pmid=35995990 |bibcode=2022NatSR..1214326D }}</ref> and complex carbohydrates, and includes [[tunicin]], a variety of cellulose. The tunic is unique among invertebrate exoskeletons in that it can grow as the animal enlarges and does not need to be periodically shed. Inside the tunic is the body wall or mantle composed of [[connective tissue]], [[muscle]] fibres, [[blood vessel]]s, and [[nerve]]s. Two openings are found in the body wall: the buccal siphon at the top through which water flows into the interior, and the atrial siphon on the ventral side through which it is expelled. A large pharynx occupies most of the interior of the body. It is a muscular tube linking the buccal opening with the rest of the gut. It has a ciliated groove known as an [[endostyle]] on its ventral surface, and this secretes a mucous net which collects food particles and is wound up on the dorsal side of the pharynx. The gullet, at the lower end of the pharynx, links it to a loop of gut which terminates near the atrial siphon. The walls of the pharynx are perforated by several bands of slits, known as stigmata, through which water escapes into the surrounding water-filled cavity, the atrium. This is criss-crossed by various rope-like [[Mesentery (zoology)|mesenteries]] which extend from the mantle and provide support for the pharynx, preventing it from collapsing, and also hold up the other organs.<ref name=Ruppert/>

The [[Thaliacea]], the other main class of tunicates, is characterised by free-swimming, pelagic individuals. They are all filter feeders using a pharyngeal mucous net to catch their prey. The [[pyrosome]]s are [[Bioluminescence|bioluminous]] colonial tunicates with a hollow cylindrical structure. The buccal siphons are on the outside and the atrial siphons inside. About ten species are known, and all are found in the tropics. The 23 species of [[Doliolida|doliolids]] are small, mostly under {{convert|2|cm|abbr=on}} long. They are solitary, have the two siphons at opposite ends of their barrel-shaped bodies, and swim by jet propulsion. The 40 species of [[Salpida|salps]] are also small, under {{convert|4|cm|abbr=on}} long, and found in the surface waters of both warm and cold seas. They also move by jet propulsion, and often form long chains by budding off new individuals.<ref name=Ruppert/>

A third class, the [[Larvacea]] (or Appendicularia), is the only group of tunicates to retain their chordate characteristics in the adult state, a product of extensive [[neoteny]]. The 70 species of larvaceans superficially resemble the tadpole larvae of amphibians, although the tail is at right angles to the body. The [[notochord]] is retained, and the animals, mostly under 1&nbsp;cm long, are propelled by undulations of the tail. They secrete an external mucous net known as a house, which may completely surround them and is very efficient at trapping planktonic particles.<ref name=Ruppert/>

===Physiology and internal anatomy===
[[File:Uroc005b.png|thumb|400px|right|{{center|Internal anatomy of a generalised tunicate}}]]
[[File:Pyrosoma 001.png|400px|thumb|Section through the wall of an ascidian pyrosoma showing several zooids; (br)&nbsp;buccal siphon; (at)&nbsp;atrial siphon; (tp)&nbsp;test process; {{nobr|(br s) pharynx.}}]]

Like all other [[chordate]]s, tunicates have a [[notochord]] during their early development, but it is lost by the time they have completed their metamorphosis. As members of the Chordata, they are true [[Body cavity|Coelomata]] with [[endoderm]], [[ectoderm]], and [[mesoderm]], but they do not develop very clear [[coelom]]ic body cavities, if any at all. Whether they do or not, by the end of their larval development, all that remain are the [[pericardial cavity|pericardial]], renal, and gonadal cavities of the adults. Except for the [[heart]], gonads, and [[pharynx]] (or branchial sac), the organs are enclosed in a membrane called an [[epicardium]], which is surrounded by the jelly-like [[mesenchyme]].

Ascidian tunicates begin life as a lecithotrophic (non-feeding) mobile [[larva]] that resembles a tadpole,<ref>{{cite journal |url=https://www.sciencedirect.com/science/article/pii/S0012160608013766 |doi=10.1016/j.ydbio.2008.11.026 |title=Delineating metamorphic pathways in the ascidian Ciona intestinalis |year=2009 |last1=Nakayama-Ishimura |first1=Akie |last2=Chambon |first2=Jean-Phillippe |last3=Horie |first3=Takeo |last4=Satoh |first4=Nori |last5=Sasakura |first5=Yasunori |journal=Developmental Biology |volume=326 |issue=2 |pages=357–367 |pmid=19100250 |url-access=subscription }}</ref> with the exception of some members of the families Styelidae and Molgulidae which has direct development.<ref>{{cite book |chapter-url=https://www.taylorfrancis.com/chapters/edit/10.1201/9781003077992-16/ascidians-renganathan |doi=10.1201/9781003077992-16 |chapter=Ascidians |title=Fouling Organisms of the Indian Ocean |year=2020 |last1=Renganathan |first1=T.K. |pages=507–534 |isbn=9781003077992 |s2cid=241318821 }}</ref> The latter also have several species with tail-less larval forms.<ref>{{cite journal |doi=10.1098/rsob.150053 |title=Tunicates: Exploring the sea shores and roaming the open ocean. A tribute to Thomas Huxley |year=2015 |last1=Lemaire |first1=Patrick |last2=Piette |first2=Jacques |journal=Open Biology |volume=5 |issue=6 |page=150053 |pmid=26085517 |pmc=4632506 }}</ref><ref>{{cite journal |pmid=33881514 |year=2021 |author1=Fodor ACA |last2=Powers |first2=M. M. |last3=Andrykovich |first3=K. |last4=Liu |first4=J. |last5=Lowe |first5=E. K. |last6=Brown |first6=C. T. |last7=Di Gregorio |first7=A. |last8=Stolfi |first8=A. |last9=Swalla |first9=B. J. |title=The Degenerate Tale of Ascidian Tails |journal=Integrative and Comparative Biology |volume=61 |issue=2 |pages=358–369 |doi=10.1093/icb/icab022 |pmc=10452958 |doi-access=free }}</ref> The ascidian larvae very rapidly settle down and attach themselves to a suitable surface, later developing into a barrel-like and usually sedentary adult form. The species in the class [[Larvacea|Appendicularia]] are [[Pelagic zone|pelagic]], and the general larval form is kept throughout life. Also the class [[Thaliacea]] is pelagic throughout their lives and may have complex lifecycles. In this class a free living larval stage is absent: Doliolids and pyrosomatids are viviparous–lecithotrophic, and salpids are viviparous–matrotrophic. Only some species of doliolids still have a rudimentary tailed tadpole stage, which is never free-living and lacks a brain.<ref>{{cite journal |pmc=5098176 |year=2015 |last1=Ostrovsky |first1=A. N. |last2=Lidgard |first2=S. |last3=Gordon |first3=D. P. |last4=Schwaha |first4=T. |last5=Genikhovich |first5=G. |last6=Ereskovsky |first6=A. V. |title=Matrotrophy and placentation in invertebrates: A new paradigm |journal=Biological Reviews of the Cambridge Philosophical Society |volume=91 |issue=3 |pages=673–711 |doi=10.1111/brv.12189 |pmid=25925633 }}</ref><ref name=Dorit>{{cite book |last1=Dorit |first1=R.L. |last2=Walker |first2=W.F. |last3=Barnes |first3=R.D. |name-list-style=amp |year=1991 |title=Zoology |publisher=Saunders College Publishing |isbn=978-0-03-030504-7 |pages=[https://archive.org/details/zoology0000dori/page/802 802–804] |url=https://archive.org/details/zoology0000dori |url-access=registration |via=archive.org}}</ref><ref>{{Cite book |last=Schlosser |first=Gerhard |url=https://books.google.com/books?id=rPMxEAAAQBAJ&dq=Doliolids+never+free+and+lacks+a+brain&pg=PP30 |title=Evolutionary Origin of Sensory and Neurosecretory Cell Types: Vertebrate Cranial Placodes, volume 2 |date=2021-06-17 |publisher=CRC Press |isbn=978-1-000-36913-7 |language=en}}</ref>

Tunicates have a well-developed [[heart]] and [[circulatory system]]. The heart is a double U-shaped tube situated just below the gut. The blood vessels are simple connective tissue tubes, and their blood has several types of [[Blood cell|corpuscle]]. The blood may appear pale green, but this is not due to any respiratory pigments, and oxygen is transported dissolved in the [[Blood plasma|plasma]]. Exact details of the circulatory system are unclear, but the gut, pharynx, gills, gonads, and nervous system seem to be arranged in series rather than in parallel, as happens in most other animals. Every few minutes, the heart stops beating and then restarts, pumping fluid in the reverse direction.<ref name=Ruppert/>

Tunicate [[blood]] has some unusual features. In some species of [[Ascidiidae]] and [[Perophoridae]], it contains high concentrations of the transitional metal [[vanadium]] and [[Vanabins|vanadium-associated proteins]] in [[vacuole]]s in blood cells known as [[vanadocyte]]s. Some tunicates can concentrate vanadium up to a level ten million times that of the surrounding seawater. It is stored in a +3 oxidation form that requires a [[pH]] of less than 2 for stability, and this is achieved by the vacuoles also containing [[sulfuric acid]]. The vanadocytes are later deposited just below the outer surface of the tunic, where their presence is thought to deter [[predation]], although it is unclear whether this is due to the presence of the metal or low pH.<ref>{{cite journal |first1=S. |last1=Odate |first2=J.R. |last2=Pawlik |name-list-style=amp |year=2007 |title=The role of vanadium in the chemical defense of the solitary tunicate, ''Phallusia nigra'' |journal=Journal of Chemical Ecology |volume=33 |issue=3 |pages=643–654 |doi=10.1007/s10886-007-9251-z |pmid=17265174 |bibcode=2007JCEco..33..643O |s2cid=116921}}</ref> Other species of tunicates concentrate [[lithium]], [[iron]], [[niobium]], and [[tantalum]], which may serve a similar function.<ref name=Ruppert>{{cite book |last1=Ruppert |first1=E.E. |last2=Fox |first2=R.S. |last3=Barnes |first3=R.D. |name-list-style=amp |year=2004 |title=Invertebrate Zoology |edition=7th |publisher=Cengage Learning |isbn=978-81-315-0104-7 |pages=940–956 }}</ref> Other tunicate species produce distasteful [[organic compound]]s as [[chemical defense]]s against predators.<ref>{{cite journal |first1=D.P. |last1=Pisut |first2=J.R. |last2 = Pawlik |name-list-style=amp |year=2002 |title=Anti-predatory chemical defenses of ascidians: Secondary metabolites or inorganic acids? |journal=Journal of Experimental Marine Biology and Ecology |volume=270 |issue=2 |pages=203–214 |doi=10.1016/S0022-0981(02)00023-0 |bibcode=2002JEMBE.270..203P |citeseerx=10.1.1.558.3639}}</ref>

Tunicates lack the kidney-like [[Nephridium|metanephridial]] organs typical of [[deuterostome]]s. Most have no excretory structures, but rely on the diffusion of [[ammonia]] across their tissues to rid themselves of nitrogenous waste, though some have a simple excretory system. The typical [[Kidney|renal]] organ is a mass of large clear-walled [[Vesicle (biology and chemistry)|vesicles]] that occupy the rectal loop, and the structure has no duct. Each vesicle is a remnant of a part of the primitive coelom, and its cells extract nitrogenous waste matter from circulating blood. They accumulate the wastes inside the vesicles as [[Uric acid|urate crystals]], and do not have any obvious means of disposing of the material during their lifetimes.<ref name=Dorit/>

Adult tunicates have a hollow cerebral ganglion, equivalent to a brain, and a hollow structure known as a neural gland. Both originate from the embryonic neural tube and are located between the two siphons. Nerves arise from the two ends of the ganglion; those from the anterior end innervate the buccal siphon and those from the posterior end supply the rest of the body, the atrial siphon, organs, gut and the musculature of the body wall. There are no sense organs but there are sensory cells on the siphons, the buccal tentacles and in the atrium.<ref name=Ruppert/>

Tunicates are unusual among animals in that they produce a large fraction of their tunic and some other structures in the form of [[cellulose]]. The production in animals of cellulose is so unusual that at first some researchers denied its presence outside of plants, but the tunicates were later found to possess a functional cellulose [[synthase|synthesizing enzyme]], encoded by a gene horizontally transferred from a bacterium.<ref>{{cite journal |author1=Matthysse, A.G. |author2=Deschet, K. |author3=Williams, M. |author4=Marry, M. |author5=White, A.R. |author6=Smith, W.C. |name-list-style=amp |year=2004 |title=A functional cellulose synthase from ascidian epidermis |journal=[[Proceedings of the National Academy of Sciences]] |volume=101 |issue=4 |pages=986–991 |doi=10.1073/pnas.0303623101 |doi-access=free |pmid=14722352 |pmc=327129 |bibcode=2004PNAS..101..986M}}</ref> When, in 1845, [[Carl Schmidt (chemist)|Carl Schmidt]] first announced the presence in the test of some ascidians of a substance very similar to cellulose, he called it "tunicine", but it is now recognized as cellulose rather than any alternative substance.<ref>{{cite journal |author1=Hirose, E. |author2=Nakashima, K. |author3=Nishino, A. |name-list-style=amp |year=2011 |title=Is there intracellular cellulose in the appendicularian tail epidermis? A tale of the adult tail of an invertebrate chordate |journal=Communicative & Integrative Biology |volume=4 |issue=6 |pages=768–771 |doi=10.4161/cib.17757|pmid=22446551 |pmc=3306355 }}</ref><ref>{{cite journal |author1=Sasakura, Y. |author2=Ogura, Y. |author3=Treen, N. |display-authors=etal |year=2016 |title=Transcriptional regulation of a horizontally transferred gene from bacterium to chordate |journal=[[Proceedings of the Royal Society B]] |volume=283 |issue=1845 |page=20161712 |doi=10.1098/rspb.2016.1712 |pmid=28003446 |pmc=5204163 }}</ref><ref>{{cite journal |author1=Sasakura, Y. |author2=Nakashima, K. |author3=Awazu, S. |author4=Matsuoka, T. |author5=Nakayama, A. |author6=Azuma, J. |author7=Satoh, N. |name-list-style=amp |year=2005 |title=Transposon-mediated insertional mutagenesis revealed the functions of animal cellulose synthase in the ascidian ''Ciona intestinalis'' |journal=[[Proceedings of the National Academy of Sciences]] |volume=102 |issue=42 |pages=15134–15139 |doi=10.1073/pnas.0503640102 |pmid=16214891 |pmc=1257696 |bibcode=2005PNAS..10215134S |doi-access=free }}</ref>

<gallery mode="packed" heights="160px" style="float:center;">
File:Oikopleura (Vexillaria) cophocerca 001.png|''[[Oikopleura cophocerca]]'' in its "house". Arrows indicate water movement and (x) the lateral reticulated parts of the house.
File:Ascidians.jpg|Blue sea squirts from the genus ''[[Rhopalaea]]''.
File:Ascidian (Rhopalaea Crassa) (4 cm).png| Fluorescent-colored sea squirts, ''[[Rhopalaea crassa]]''.
File:Sea Squirts Didemnum molle.jpg| ''[[Didemnum molle]]''.
</gallery>

==Feeding==
[[File:Tunicate black orange.jpg|thumb|left|''[[Clavelina robusta]]'' (black and white) and ''[[Pycnoclavella flava]]'' (orange) showing siphons.]]

Nearly all adult tunicates are [[suspension feeder]]s (the larval form usually does not feed), capturing [[plankton]]ic particles by filtering sea water through their bodies. Ascidians are typical in their digestive processes, but other tunicates have similar systems. Water is drawn into the body through the buccal siphon by the action of [[Cilium|cilia]] lining the gill slits. To obtain enough food, an average ascidian needs to process one body-volume of water per second.<ref name=Ruppert/> This is drawn through a net lining the pharynx which is being continuously secreted by the endostyle. The net is made of sticky mucus threads with holes about 0.5&nbsp;μm in diameter which can trap planktonic particles including [[bacteria]]. The net is rolled up on the dorsal side of the pharynx, and it and the trapped particles are drawn into the [[esophagus]]. The gut is U-shaped and also ciliated to move the contents along. The stomach is an enlarged region at the lowest part of the U-bend. Here, digestive [[enzyme]]s are secreted and a [[Pylorus|pyloric]] gland (absent in appendicularians)<ref>[https://books.google.com/books?id=qSJsKEhV3FsC&dq=Appendicularia+pyloric+gland&pg=PA164 Response of Marine Ecosystems to Global Change: Ecological Impact of Appendicularians]</ref> adds further secretions. After digestion, the food is moved on through the [[intestine]], where absorption takes place, and the [[rectum]], where undigested remains are formed into [[Feces|faecal]] pellets or strings. The [[anus]] opens into the dorsal or [[cloaca]]l part of the peribranchial cavity near the atrial siphon. Here, the faeces are caught up by the constant stream of water which carries the waste to the exterior. The animal orientates itself to the current in such a way that the buccal siphon is always upstream and does not draw in contaminated water.<ref name=Ruppert/>

Some ascidians that live on soft sediments are [[detritivore]]s. A few deepwater species, such as ''[[Megalodicopia hians]]'', are [[Ambush predator|sit-and-wait predators]], trapping tiny crustacea, nematodes, and other small invertebrates with the muscular lobes which surround their buccal siphons. Certain tropical species in the family [[Didemnidae]] have [[Symbiosis|symbiotic]] green algae or [[cyanobacteria]] in their tunics, and one of these symbionts, ''[[Prochloron]]'', is unique to tunicates. Excess [[Photosynthesis|photosynthetic]] products are assumed to be available to the [[Host (biology)|host]].<ref name=Ruppert/>

==Life cycle==
[[File:Uroc004b Jon.png|thumb|500px|right|{{center|Anatomy of a larval tunicate}}]]

Ascidians are almost all [[hermaphrodite]]s and each has a single ovary and testis, either near the gut or on the body wall. In some solitary species, sperm and eggs are shed into the sea and the [[larva]]e are [[plankton]]ic. In others, especially colonial species, sperm is released into the water and drawn into the atria of other individuals with the incoming water current. Fertilization takes place here and the eggs are brooded through their early developmental stages.<ref name=Dorit/> Some larval forms appear very much like primitive [[chordate]]s with a [[notochord]] (stiffening rod) and superficially resemble small [[tadpole]]s. These swim by undulations of the tail and may have a simple eye, an [[ocellus]], and a balancing organ, a [[statocyst]].<ref name="microscopy-uk.org.uk">{{cite web |author=Cavanihac, Jean-Marie |url=http://www.microscopy-uk.org.uk/mag/indexmag.html?http://www.microscopy-uk.org.uk/mag/artdec00/tunicp1.html |title=Tunicates extraordinaire |publisher=Microscope UK |year=2000 |access-date=2011-12-07}}</ref>

When sufficiently developed, the larva of the sessile species finds a suitable rock and cements itself in place. The larval form is not capable of feeding, though it may have a rudimentary digestive system,<ref name="microscopy-uk.org.uk"/> and is only a dispersal mechanism. Many physical changes occur to the tunicate's body during [[metamorphosis]], one of the most significant being the reduction of the cerebral ganglion, which controls movement and is the equivalent of the vertebrate brain. From this comes the common saying that the sea squirt "eats its own brain".<ref>{{cite book |title=Consciousness Explained |year=1991 |author=Dennett, Daniel C. |page=[https://archive.org/details/consciousnessexp00denn/page/177 177] |publisher=Little Brown & Co |isbn=978-0316-18065-8 |url-access=registration |url=https://archive.org/details/consciousnessexp00denn/page/177}}</ref> However, the adult does possess a cerebral ganglion adapted to lack of self-locomotion.<ref>{{cite journal |last1=Mackie |first1=G. O. |last2=Burighel |first2=P. |year=2005 |title=The nervous system in adult tunicates: current research directions |journal=Canadian Journal of Zoology |volume=83 |issue=1 |pages=151–183 |doi=10.1139/z04-177}}</ref> In the Thaliacea, the larval stage is rudimentary or suppressed, and the adults are pelagic (swimming or drifting in the open sea).<ref name=Dorit/> Colonial forms also increase the size of the colony by budding off new individuals to share the same tunic.<ref>{{cite web|author=Parmentier, Jan |url=http://www.microscopy-uk.org.uk/mag/indexmag.html?http://www.microscopy-uk.org.uk/mag/artdec00/tunicp1.html |title=Botryllus: A colonial ascidian |publisher=Microscope UK |year=1998 |access-date=2013-04-07}}</ref>

Pyrosome colonies grow by budding off new zooids near the posterior end of the colony. Sexual reproduction starts within a zooid with an internally fertilized egg. This develops directly into an oozooid without any intervening larval form. This buds precociously to form four blastozooids which become detached in a single unit when the oozoid disintegrates. The atrial siphon of the oozoid becomes the exhalent siphon for the new, four-zooid colony.<ref name=Ruppert/>

[[File:Ascidia 005.png|thumb|300px|left|A 1901 comparison of frog tadpole and a tunicate larva.]]

[[Doliolida|Doliolids]] have a very complex life cycle that includes various zooids with different functions. The sexually reproducing members of the colony are known as gonozooids. Each one is a hermaphrodite with the eggs being fertilised by sperm from another individual. The gonozooid is [[Viviparity|viviparous]], and at first, the developing embryo feeds on its [[yolk sac]] before being released into the sea as a free-swimming, tadpole-like larva. This undergoes metamorphosis in the [[water column]] into an oozooid. This is known as a "nurse" as it develops a tail of zooids produced by budding [[Asexual reproduction|asexually]]. Some of these are known as trophozooids, have a nutritional function, and are arranged in lateral rows. Others are phorozooids, have a transport function, and are arranged in a single central row. Other zooids link to the phorozooids, which then detach themselves from the nurse. These zooids develop into gonozooids, and when these are mature, they separate from the phorozooids to live independently and start the cycle over again. Meanwhile, the phorozooids have served their purpose and disintegrate. The asexual phase in the lifecycle allows the doliolid to multiply very rapidly when conditions are favourable.<ref name=Ruppert/>

Salps also have a complex lifecycle with an [[alternation of generations]]. In the [[wikt:solitary|solitary]] life history phase, an oozoid [[asexual reproduction|reproduces asexually]], producing a chain of tens or hundreds of individual zooids by budding along the length of a [[stolon]]. The chain of salps is the 'aggregate' portion of the lifecycle. The aggregate individuals, known as blastozooids, remain attached together while swimming and feeding and growing larger. The blastozooids are [[sequential hermaphrodite]]s. An egg in each is fertilized internally by a sperm from another colony. The egg develops in a brood sac inside the blastozooid and has a placental connection to the circulating blood of its "nurse". When it fills the blastozooid's body, it is released to start the independent life of an oozooid.<ref name=Ruppert/>

Larvaceans only reproduce [[sexual reproduction|sexually]]. They are [[protandrous hermaphrodite]]s, except for ''[[Oikopleura dioica]]'' which is [[gonochoric]], and a larva resembles the tadpole larva of ascidians. Once the trunk is fully developed, the larva undergoes "tail shift", in which the tail moves from a rearward position to a ventral orientation and twists through 90° relative to the trunk. The larva consists of a small, fixed number of cells, and grows by enlargement of these rather than cell division. Development is very rapid and only takes seven hours for a [[zygote]] to develop into a house-building juvenile starting to feed.<ref name=Ruppert/>

During embryonic development, tunicates exhibit [[determinate cleavage]], where the fate of the cells is set early on with reduced cell numbers and [[genome]]s that are rapidly evolving. In contrast, the [[amphioxus]] and vertebrates show [[cell determination]] relatively late in development and cell cleavage is indeterminate. The [[genome evolution]] of amphioxus and vertebrates is also relatively slow.<ref>{{cite journal |author=Holland, Linda Z. |year=2007 |title=Developmental biology: A chordate with a difference |journal=Nature |volume=447 |issue=1 |pages=153–155 |doi=10.1038/447153a |pmid=17495912|bibcode=2007Natur.447..153H |s2cid=5549210 |doi-access=free }}</ref>

===Promotion of out-crossing===

''[[Ciona intestinalis]]'' (class Ascidiacea) 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.<ref name="pmid24878524">{{cite journal |vauthors=Sawada H, Morita M, Iwano M |title=Self/non-self recognition mechanisms in sexual reproduction: new insight into the self-incompatibility system shared by flowering plants and hermaphroditic animals |journal=Biochem. Biophys. Res. Commun. |volume=450 |issue=3 |pages=1142–8 | date=August 2014 |pmid=24878524 |doi=10.1016/j.bbrc.2014.05.099 }}</ref> 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.<ref name="pmid24878524" /> Self-incompatibility promotes out-crossing, and thus provides the adaptive advantage at each generation of the masking of deleterious recessive mutations (i.e. genetic complementation)<ref name=Bernstein87>{{cite book | last1=Bernstein | first1=H | last2=Hopf | first2=FA | last3=Michod | first3=RE | chapter=The Molecular Basis of the Evolution of Sex | year=1987 | title=Molecular Genetics of Development | volume=24 | pages=323–70 | pmid=3324702 | doi=10.1016/S0065-2660(08)60012-7| series=Advances in Genetics | isbn=9780120176243 }}</ref> and the avoidance of [[inbreeding depression]].

''[[Botryllus schlosseri]]'' (class Ascidiacea) is a colonial tunicate, a member of the only group of chordates that are 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.<ref name="pmid25044771">{{cite journal |author1=Gasparini, F |author2=Manni, L |author3=Cima, F |author4=Zaniolo, G |author5=Burighel, P |author6=Caicci, F |author7=Franchi, N |author8=Schiavon, F |author9=Rigon, F |author10=Campagna, D |author11=Ballarin, L |title=Sexual and asexual reproduction in the colonial ascidian Botryllus schlosseri |journal=Genesis |volume=53 |issue=1 |pages=105–20 | date=July 2014 |pmid=25044771 |doi=10.1002/dvg.22802 |s2cid=205772576 }}</ref> 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%).<ref name="pmid25044771" /> 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.<ref name=Bernstein87 />

===A model tunicate===
''[[Oikopleura dioica]]'' (class [[Larvacean|Appendicularia]]) is a [[semelparous]] organism, reproducing only once in its lifetime.  It employs an original [[reproductive strategy]] in which the entire female [[germ-line]] is contained within an ovary that is a single giant [[multinucleate]] cell termed the "coenocyst".<ref name="pmid17126826">{{cite journal |vauthors=Ganot P, Bouquet JM, Kallesøe T, Thompson EM |title=The Oikopleura coenocyst, a unique chordate germ cell permitting rapid, extensive modulation of oocyte production |journal=Dev. Biol. |volume=302 |issue=2 |pages=591–600 |date=February 2007 |pmid=17126826 |doi=10.1016/j.ydbio.2006.10.021 |doi-access=free}}</ref> ''O. dioica'' can be maintained in laboratory culture, and is of growing interest as a [[model organism]] because of its [[phylogenetic]] position within the closest sister group to [[vertebrates]].<ref name="pmid16495997"/>

==Invasive species==
Over the past few decades, tunicates (notably of the genera ''[[Didemnum]]'' and ''[[Pleurogona|Styela]]'') have been [[invasive species|invading]] coastal waters in many countries. The carpet tunicate (''[[Didemnum vexillum]]'') has taken over a {{convert|6.5|mi2|abbr=on}} area of the seabed on the [[Georges Bank]] off the northeast coast of North America, covering stones, molluscs, and other stationary objects in a dense mat.<ref name="sh.nefsc.noaa.gov">{{cite web |url=http://sh.nefsc.noaa.gov/tunicate.htm |title=Have You Seen This Tunicate? |publisher=NOAA Fisheries Service |date=2004-11-19 |access-date=2011-12-07 |archive-url=https://web.archive.org/web/20090109034514/http://sh.nefsc.noaa.gov/tunicate.htm |archive-date=9 January 2009 |url-status=dead }}</ref> ''D. vexillum'', ''[[Styela clava]]'' and ''[[Ciona savignyi]]'' have appeared and are thriving in [[Puget Sound]] and [[Hood Canal]] in the [[Pacific Northwest]].<ref name="dornfeld">{{cite web |url=http://www.pnwscuba.com/invasives/ |title=Invasive Tunicates of Washington State |author=Dornfeld, Ann |date=2008-05-01 |publisher=NPR |access-date=2013-04-06 |archive-url=https://web.archive.org/web/20140714124309/http://www.pnwscuba.com/invasives/ |archive-date=14 July 2014 |url-status=dead}}</ref>

Invasive tunicates usually arrive as [[Fouling community|fouling organisms]] on the hulls of ships, but may also be introduced as larvae in [[ballast water]]. Another possible means of introduction is on the shells of molluscs brought in for marine cultivation.<ref name="dornfeld"/> Current research indicates many tunicates previously thought to be indigenous to Europe and the Americas are, in fact, invaders. Some of these invasions may have occurred centuries or even millennia ago. In some areas, tunicates are proving to be a major threat to [[aquaculture]] operations.<ref>{{cite web |url=https://woodshole.er.usgs.gov/project-pages/stellwagen/didemnum/ |title=Marine Nuisance Species |publisher=Woods Hole Science Center |access-date=2011-12-07}}</ref>
{{Clear}}

==Use by humans==

===Medical uses===
Tunicates contain a host of potentially useful [[chemical compound]]s, including:
* [[Plitidepsin]], a didemnin effective against various types of cancer; as of late January 2021 undergoing Phase III trials as a treatment for COVID-19<ref>{{Cite web |url=https://www.jsonline.com/story/news/2021/01/25/international-team-finds-new-more-effective-drug-treat-covid-19/6673529002/ |title=International team of scientists identifies new treatment for COVID-19 that appears to be far more effective than drugs in use now |first=Mark |last=Johnson |website=Journal Sentinel}}</ref>
* [[Trabectedin]], an FDA approved anticancer drug.
<!-- In the May 2007 issue of the ''FASEB'' journal, researchers from [[Stanford University]] showed that -->Tunicates are able to correct their own cellular abnormalities over a series of generations, and a similar [[regeneration (biology)|regenerative]] process may be possible for humans. The mechanisms underlying the phenomenon may lead to insights about the potential of cells and tissues to be reprogrammed and to regenerate compromised human organs.<ref>{{Cite book |title=Stem cells : from hydra to man |date=2008 |publisher=Springer |last=Bosch |first=Thomas C. G. |isbn=9781402082740 |location=Dordrecht |oclc=233972733}}</ref><ref>{{cite web |url=https://www.sciencedaily.com/releases/2007/04/070424093740.htm |title=Sea Squirt, Heal Thyself: Scientists Make Major Breakthrough in Regenerative Medicine |publisher=Sciencedaily.com |date=2007-04-24 |access-date=2011-12-07}}</ref><ref>{{Cite journal |last1=Kürn |first1=Ulrich |last2=Rendulic |first2=Snjezana |last3=Tiozzo |first3=Stefano |last4=Lauzon |first4=Robert J. |date=August 2011 |title=Asexual Propagation and Regeneration in Colonial Ascidians |url=https://www.journals.uchicago.edu/doi/10.1086/BBLv221n1p43 |journal=The Biological Bulletin |language=en |volume=221 |issue=1 |pages=43–61 |doi=10.1086/BBLv221n1p43 |pmid=21876110 |s2cid=37526690 |issn=0006-3185|url-access=subscription }}</ref>

===As food===
{{main|Ascidiacea#Culinary}}
[[Image:SeaSquirt.jpg|''[[Halocynthia]]'' tunicates for sale at a market, [[Busan]], [[South Korea]]|thumb]]

Various [[Ascidiacea]] species are consumed as food around the world. The ''piure'' (''[[Pyura chilensis]]'') is used in the [[cuisine of Chile]], both raw and in seafood stews. In Japan and Korea, the [[sea pineapple]] (''Halocynthia roretzi'') is the main species eaten. It is cultivated on dangling cords made of [[Arecaceae|palm fronds]]. In 1994, over 42,000 tons were produced, but since then, mass mortality events have occurred among the farmed sea squirts (the tunics becoming soft), and only 4,500 tons were produced in 2004.<ref>{{cite web |url=http://www.lib.noaa.gov/retiredsites/korea/main_species/sea_squirt.htm |title=Sea squirt |publisher=Korea-US Aquaculture |access-date=2013-04-06 |archive-url=https://web.archive.org/web/20130302122523/http://www.lib.noaa.gov/retiredsites/korea/main_species/sea_squirt.htm |archive-date=2 March 2013 |url-status=dead }}</ref>

===Other uses===
The use of tunicates as a source of [[biofuel]] is being researched. The cellulose body wall can be broken down and converted into [[ethanol]], and other parts of the animal are protein-rich and can be converted into fish feed. Culturing tunicates on a large scale may be possible and the economics of doing so are attractive. As tunicates have few predators, their removal from the sea may not have profound ecological impacts. Being sea-based, their production does not compete with food production as does the cultivation of land-based crops for biofuel projects.<ref>{{cite web |url=http://cleantechnica.com/2013/03/26/biofuel-made-from-marine-filter-feeders-tunicates-usable-as-source-of-biofuels/ |title=Biofuel made from marine filter feeders? Tunicates usable as source of biofuels |date=2013-03-26 |work=Cleantechnica |access-date=2013-04-06}}</ref>

Some tunicates are used as [[model organism]]s. ''[[Ciona intestinalis]]'' and ''[[Ciona savignyi]]'' have been used for [[developmental biology|developmental studies]]. Both species' mitochondrial<ref name="pmid17640763">{{cite journal |author1=Iannelli, F. |author2=Pesole, G. |author3=Sordino, P. |author4=Gissi, C. |title=Mitogenomics reveals two cryptic species in ''Ciona intestinalis'' |journal=Trends Genet. |volume=23 |issue=9 |pages=419–422 |year=2007 |pmid=17640763 |doi=10.1016/j.tig.2007.07.001 |url=https://air.unimi.it/retrieve/handle/2434/63110/244706/gissi_tig_supp.pdf |hdl=2434/63110 |hdl-access=free}}</ref><ref name="pmid14738316">{{cite journal |author1=Yokobori, S. |author2=Watanabe, Y. |author3=Oshima, T. |title=Mitochondrial genome of ''Ciona savignyi'' (Urochordata, Ascidiacea, Enterogona): Comparison of gene arrangement and tRNA genes with ''Halocynthia roretzi'' mitochondrial genome |journal=J. Mol. Evol. |volume=57 |issue=5 |pages=574–587 |date=2003 |pmid=14738316 |doi=10.1007/s00239-003-2511-9 |bibcode=2003JMolE..57..574Y |s2cid=19474615 }}</ref> and nuclear<ref name="pmid12481130">{{cite journal |author=Dehal, P.; Satou, Y.; Campbell, R. K.; Chapman, J., Degnan, B., De Tomaso, A.; Davidson, B.; Di Gregorio, A.; Gelpke, M.; Goodstein, D. M.; Harafuji, N.; Hastings, K. E.; Ho, I.; Hotta, K.; Huang, W.; Kawashima, T.; Lemaire, P.; Martinez, D.; Meinertzhagen, I. A.; Necula, S.; Nonaka, M.; Putnam, N.; Rash, S.; Saiga, H.; Satake, M.; Terry, A.; Yamada L.; Wang, H. G.; Awazu, S.; Azumi, K.; Boore, J.; Branno, M.; Chin-Bow, S.; DeSantis, R.; Doyle, S., Francino, P.; Keys, D. N.; Haga, S.; Hayashi, H.; Hino, K.; Imai, K. S.; Inaba, K.; Kano, S.; Kobayashi, K.; Kobayashi, M.; Lee, B. I.; Makabe, K. W.; Manohar, C.; Matassi, G.; Medina, M.; Mochizuki, Y.; Mount, S.; Morishita, T.; Miura, S.; Nakayama, A.; Nishizaka, S.; Nomoto, H.; Ohta, F.; Oishi, K.; Rigoutsos, I.; Sano, M.; Sasaki, A.; Sasakura, Y.; Shoguchi, E.; Shin-i, T.; Spagnuolo, A.; Stainier, D.; Suzuki, M. M.; Tassy, O.; Takatori, N.; Tokuoka, M.; Yagi, K.; Yoshizaki, F.; Wada, S.; Zhang C.; Hyatt, P. D.; Larimer, F.; Detter, C.; Doggett, N.; Glavina, T.; Hawkins, T.; Richardson, P.; Lucas, S.; Kohara, Y.; Levine, M.; Satoh, N.; Rokhsar, D. S. |name-list-style=amp |title=The draft genome of ''Ciona intestinalis'': insights into chordate and vertebrate origins |journal=Science |volume=298 |issue=5601 |pages=2157–2167 |year=2002 |pmid=12481130 |doi=10.1126/science.1080049 |title-link=Ciona intestinalis |bibcode=2002Sci...298.2157D |citeseerx=10.1.1.319.2643 |s2cid=15987281 }}</ref><ref name= pmid17374142>{{cite journal |author1=Small, K. S. |author2=Brudno, M. |author3=Hill, M. M. |author4=Sidow, A. |title=A haplome alignment and reference sequence of the highly polymorphic ''Ciona savignyi'' genome |journal=Genome Biol. |volume=8 |issue=3 |pages=R41 |date=2007 |pmid=17374142 |doi=10.1186/gb-2007-8-3-r41 |pmc=1868934 |doi-access=free }}</ref> genomes have been sequenced. The nuclear genome of the appendicularian ''[[Oikopleura dioica]]'' appears to be one of the smallest among metazoans<ref name=pmid11752568>{{cite journal |author1=Seo, H. C. |author2=Kube, M. |author3=Edvardsen, R. B. |author4=Jensen, M. F. |author5=Beck, A. |author6=Spriet, E. |author7=Gorsky, G. |author8=Thompson. E. M. |author9=Lehrach, H. |author10=Reinhardt, R. |author11=Chourrout, D. |title=Miniature genome in the marine chordate ''Oikopleura dioica'' |journal=Science |volume=294 |issue=5551 |pages=2506 |year=2001 |pmid=11752568 |doi=10.1126/science.294.5551.2506 }}</ref> and this species has been used to study gene regulation and the evolution and development of chordates.<ref>{{Cite journal | last1=Clarke | first1=T. | last2=Bouquet | first2=JM  | last3=Fu  | first3=X | last4=Kallesøe | first4=T. | last5=Schmid | first5=M | last6=Thompson | first6=E.M. | title=Rapidly evolving lamins in a chordate, ''Oikopleura dioica'', with unusual nuclear architecture | journal=Gene | volume=396 |issue=1 | year=2007 | pages=159–169 | doi=10.1016/j.gene.2007.03.006 | pmid=17449201 }}</ref>

==See also==
* [[Vetulicolia]] – early [[deuterostome]]s which are possibly the sister group of modern tunicates
* [[Donald I. Williamson]] – claimed hybridization

==References==
{{Reflist|25em}}

==External links==
{{Commons category|Tunicata}}
{{Wikispecies|Urochordata}}
*[https://web.archive.org/web/20130302224944/http://www.tunicate-portal.org/wordpress/ The Tunicate Web Portal]
*[http://ascidians.com/ Dutch Ascidians: Extensive database of images from around the world]
*[http://www.aniseed.cnrs.fr/ Aniseed: A model organism database for ascidians including ''Ciona intestinalis'' and ''Halocynthia roretzi'']
* [https://www.gutenberg.org/ebooks/73609 A Guide to the Shell and Starfish Galleries: (Mollusca, Polyzoa, Brachiopoda, Tunicata, Echinoderma, and Worms)] (1901), British Museum (Natural History). Department of Zoology et al.
*{{Cite EB1911|wstitle=Tunicata|short=x}}

{{Tunicata}}
{{Chordata}}
{{Animalia}}
{{Taxonbar|from=Q165118}}
{{Authority control}}

[[Category:Tunicates| ]]
[[Category:Cambrian Series 2 first appearances]]
[[Category:Extant Cambrian first appearances]]