Thaliacea

{{Short description|Class of tunicates}} {{Automatic taxobox | fossil_range = {{Fossil range|515|0}}<ref name=huayuan>{{cite journal |last1=Zeng |first1=Han |last2=Liu |first2=Qi |last3=Zhao |first3=Fangchen |last4=Luo |first4=Cui |last5=Wang |first5=Dezhi |last6=Zhu |first6=Yuyan |last7=Liu |first7=Yao |last8=Chen |first8=Kai |last9=Sun |first9=Zhixin |last10=Hong |first10=Yanjie |last11=Miao |first11=Lanyun |last12=Hu |first12=Chunlin |last13=Sun |first13=Haijing |last14=Pan |first14=Bing |last15=Zhao |first15=Jialin |last16=Yin |first16=Zongjun |last17=Li |first17=Guoxiang |last18=Yang |first18=Xinglian |last19=Yang |first19=Aihua |last20=Hu |first20=Shixue |last21=Zhu |first21=Maoyan |title=A Cambrian soft-bodied biota after the first Phanerozoic mass extinction |journal=Nature |date=28 January 2026 |doi=10.1038/s41586-025-10030-0|doi-access=free }}</ref> | image = Pyrosoma atlanticum.JPG | image_caption = ''Pyrosoma atlanticum'', a pyrosome | image2 = Doliolum sp.png | image2_caption = ''Doliolum sp.'', a doliolid | taxon = Thaliacea | authority = Nielsen, 1995 | subdivision_ranks = Orders | subdivision = * Pyrosomida * Salpida * Doliolida }}

'''Thaliacea''' is a class of marine chordates within the subphylum Tunicata, comprising the salps, pyrosomes and doliolids. Unlike their benthic relatives the ascidians, from which they are believed to have emerged, thaliaceans are free-floating (pelagic) for their entire lifespan. The group includes species with complex life cycles, with both solitary and colonial forms. Because of their pelagic and gelatinous nature they are extremely rare in the fossil record, with the first unambiguous thaliacean fossil only being reported in 2026 from the mid-Cambrian Huayuan biota.<ref name=huayuan/>

==Anatomy== The three orders of thaliaceans are filter feeders. Pyrosomes are colonial animals, with multiple tiny ascidian-like zooids arranged in a cylinder closed at one end. All of the atrial siphons point inwards, emptying into a single, common cloaca in the centre of the cylinder. As the water exhaled by the zooids exits through a common opening, the water movement slowly propels the pyrosome through the sea. Salps and doliolids have a transparent barrel-shaped body through which they pump water, propelling them through the sea, and from which they extract food. The bulk of the body consists of the large pharynx. Water enters the pharynx through the large buccal siphon at the front end of the animal, and is forced through a number of slits in the pharyngeal wall into an atrium lying just behind it. From here, the water is expelled through an atrial siphon at the posterior end. The pharynx is both a respiratory organ and a digestive one, filtering food from the water with the aid of a net of mucus slowly pulled across the slits by cilia.

Doliolids and salps alternate between asexual and sexual life stages. Salp colonies can be several meters in length. Doliolids and salps rely on muscular action to propel themselves through surrounding seawater.

Thaliaceans have complex lifecycles. Doliolid eggs hatch into swimming tadpole larvae, which are the common larval stage for other urochordates. Pyrosomes are ovoviviparous, meaning the eggs develop inside the "mother" without the tadpole stage. Salps are viviparous, meaning the embryos are linked to the "mother" by a placenta. This then develops into an oozoid, which reproduces asexually by budding to produce a number of blastozoids, which form long chains. The individual blastozoids then reproduce sexually to produce the eggs and the next generation of oozoids.

The dorsal, hollow nerve cord and notochord found in Chordata has been lost, except for a rudimentary one in some doliolid larvae.<ref>{{cite journal | url=https://www.sciencedirect.com/science/article/pii/S0960982215015213 | doi=10.1016/j.cub.2015.12.024 | title=Tunicates | year=2016 | last1=Holland | first1=Linda Z. | journal=Current Biology | volume=26 | issue=4 | pages=R146–R152 | pmid=26906481 | s2cid=235602431 | doi-access=free }}</ref>

=== The jelly pump and the carbon cycle === {{See also|Jelly-falls|Gelatinous zooplankton}}

Thaliaceans play an important role in the ecology of the sea. Their dense faecal pellets sink to the bottom of the oceans, and this may be a major part of the worldwide carbon cycle.<ref>{{Cite news |date=May 21, 2009 |title=The jelly cycle |work=The Economist |url=http://www.economist.com/science/displaystory.cfm?story_id=13688170}}</ref>

==Taxonomy== The class is a relatively small one, and is divided into three orders:

Class '''Thaliacea'''<ref>[http://www.marinespecies.org/aphia.php?p=taxdetails&id=22626] World Register of Marine Species. Retrieved 2014-02-13.</ref> * Order Pyrosomida Jones 1848 ** Family Pyrosomatidae Garstang 1929 ***Subfamily Pyrostremmatinae van Soest 1979 **** Genus ''Pyrostremma'' Garstang 1929 [''Propyrosoma'' Ivanova-Kazas 1962] ***Subfamily Pyrosomatinae **** Genus ''Pyrosoma'' Péron 1804 [''Dipleurosoma'' Brooks 1906] **** Genus ''Pyrosomella'' van Soest 1979 * Order Salpida [Hemimyaria; Desmomyaria Uljanin 1884] ** Family Salpidae Franstedt 1885 ***Subfamily Cyclosalpinae Yount 1954 **** Genus ''Cyclosalpa'' Blainville 1827 [''Orthocoela'' Macdonald 1864; ''Pyrosomopsis'' Macdonald 1864] **** Genus ''Helicosalpa'' Todaro 1902 ***Subfamily Salpinae Lahille 1888 **** Genus ''Brooksia'' Metcalf 1918 **** Genus ''Iasis'' Savigny 1816 [''Weelia'' Yount 1954; ''Salpa'' (''Iasis'') Savigny 1816] **** Genus ''Ihlea'' Metcalf 1919 non Metcalf 1918 [''Apsteinia'' Metcalf 1918 non Schmeil 1894] **** Genus ''Metcalfina'' Ihle & Ihle-Landenberg 1933 **** Genus ''Pegea'' Savigny 1816 **** Genus ''Ritteriella'' Metcalf 1919 [''Ritteria'' Metcalf 1918 non Kramer 1877] **** Genus ''Salpa'' Forskål 1775 [''Biphora'' Bruguière 1789; ''Bifora'' Agassiz 1846; ''Dagysa'' Banks & Solander 1773] **** Genus ''Soestia'' [''Holothurium'' sensu Pallas 1774] **** Genus ''Thetys'' Tilesius 1802 [''Salpa'' (''Thetys'') Tilesius 1802] **** Genus ''Thalia'' Blumenbach 1798 [''Dubreuillia'' Lesson 1832; ''Edusa'' Gistl 1848] **** Genus ''Traustedtia'' Metcalf 1918 [''Salpa'' (''Traustedtia'') Metcalf 1918] * Order Doliolida [Cyclomyaria Uljanin 1884] **Suborder Doliolidina *** Family Doliolidae Bronn 1862 **** Genus ''Dolioletta'' Borgert 1894 **** Genus ''Doliolina'' Garstang 1933 **** Genus ''Dolioloides'' Garstang 1933 **** Genus ''Doliolum'' Quoy & Gaimard 1834 *** Family Doliopsoididae Godeaux 1996 **** Genus ''Doliopsoides'' Krüger 1939 **Suborder Doliopsidina *** Family Doliolunidae Robison, Raskoff & Sherlock 2005 **** Genus ''Pseudusa'' Robison, Raskoff & Sherlock 2005 *** Family Doliopsidae Godeaux 1996 **** Genus ''Doliolula'' Robison, Raskoff & Sherlock 2005 **** Genus ''Doliopsis'' Vogt 1854 *** Family Paradoliopsidae Godeaux 1996 **** Genus ''Paradoliopsis'' Godeaux 1996

==References== * {{cite book |author= Barnes, Robert D. |year=1982 |title= Invertebrate Zoology |publisher= Holt-Saunders International |location= Philadelphia, PA|pages= 1042–1043|isbn= 978-0-03-056747-6}} * {{cite book |author= Bone, Quentin |year=1998 |title= The pelagic Tunicates |publisher= Oxford University Press |location= Oxford|isbn= 978-0-19-854024-3}} {{Reflist}}

==External links== *[http://www.earthlife.net/inverts/thaliacea.html earthlife.net] *[http://www.amonline.net.au/fishes/fishfacts/fish/salp.htm amonline.net.au] *[http://www.ascidians.com/families/thaliacea/thaliacea.htm ascidians.com] *[https://www.tunicate-portal.org]

Category:Thaliacea Category:Chordate classes