Thaliacea is a class of marine

chordate A chordate ( ) is a bilaterian animal belonging to the phylum Chordata ( ). All chordates possess, at some point during their larval or adult stages, five distinctive physical characteristics ( synapomorphies) that distinguish them from ot ...

s within the subphylum Tunicata, comprising the

salp A salp (: salps, also known colloquially as “sea grape”) or salpa (: salpae or salpas) is a barrel-shaped, Plankton, planktonic tunicate in the family Salpidae. The salp moves by contracting its gelatinous body in order to pump water thro ...

s, pyrosomes and doliolids. Unlike their

benthic The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from the Ancient Greek word (), meaning "the depths". ...

relatives the ascidians, from which they are believed to have emerged, thaliaceans are free-floating (

pelagic The pelagic zone consists of the water column of the open ocean and can be further divided into regions by depth. The word ''pelagic'' is derived . The pelagic zone can be thought of as an imaginary cylinder or water column between the sur ...

) for their entire lifespan. The group includes species with complex life cycles, with both solitary and colonial forms.

Anatomy

The three orders of thaliaceans are

filter feeder Filter feeders are aquatic animals that acquire nutrients by feeding on organic matters, food particles or smaller organisms (bacteria, microalgae and zooplanktons) suspended in water, typically by having the water pass over or through a s ...

s. Pyrosomes are colonial animals, with multiple tiny ascidian-like

zooid A zooid or zoöid is an animal that is part of a colonial animal. This lifestyle has been adopted by animals from separate unrelated taxa. Zooids are multicellular; their structure is similar to that of other solitary animals. The zooids can ...

s arranged in a cylinder closed at one end. All of the atrial siphons point inwards, emptying into a single, common

cloaca A cloaca ( ), : cloacae ( or ), or vent, is the rear orifice that serves as the only opening for the digestive (rectum), reproductive, and urinary tracts (if present) of many vertebrate animals. All amphibians, reptiles, birds, cartilagin ...

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 The pharynx (: pharynges) is the part of the throat behind the human mouth, mouth and nasal cavity, and above the esophagus and trachea (the tubes going down to the stomach and the lungs respectively). It is found in vertebrates and invertebrates ...

. 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 Mucus (, ) is a slippery aqueous secretion produced by, and covering, mucous membranes. It is typically produced from cells found in mucous glands, although it may also originate from mixed glands, which contain both Serous fluid, serous and muc ...

slowly pulled across the slits by

cilia The cilium (: cilia; ; in Medieval Latin and in anatomy, ''cilium'') is a short hair-like membrane protrusion from many types of eukaryotic cell. (Cilia are absent in bacteria and archaea.) The cilium has the shape of a slender threadlike proj ...

. Doliolids and

s 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 Tunicates are marine invertebrates belonging to the subphylum Tunicata ( ). This grouping is part of the Chordata, a phylum which includes all animals with dorsal nerve cords and notochords (including vertebrates). The subphylum was at one time ...

. Pyrosomes are

ovoviviparous Ovoviviparity, ovovivipary, ovivipary, or aplacental viviparity is a "bridging" form of reproduction between egg-laying oviparity, oviparous and live-bearing viviparity, viviparous reproduction. Ovoviviparous animals possess embryos that develo ...

, meaning the eggs develop inside the "mother" without the tadpole stage. Salps are

viviparous In animals, viviparity is development of the embryo inside the body of the mother, with the maternal circulation providing for the metabolic needs of the embryo's development, until the mother gives birth to a fully or partially developed juve ...

, 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 (see image). The individual blastozoids then reproduce sexually to produce the eggs and the next generation of oozoids. The dorsal, hollow nerve cord and

notochord The notochord is an elastic, rod-like structure found in chordates. In vertebrates the notochord is an embryonic structure that disintegrates, as the vertebrae develop, to become the nucleus pulposus in the intervertebral discs of the verteb ...

found in

Chordata A chordate ( ) is a bilaterian animal belonging to the phylum Chordata ( ). All chordates possess, at some point during their larval or adult stages, five distinctive physical characteristics (Apomorphy and synapomorphy, synapomorphies) th ...

has been lost, except for a rudimentary one in some doliolid larvae.

The jell pump and the carbon cycle

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 The carbon cycle is a part of the biogeochemical cycle where carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of Earth. Other major biogeochemical cycles include the nitrogen cycle and the water cycl ...

Taxonomy

The class is a relatively small one, and is divided into three orders: Class Thaliacea
World Register of Marine Species. Retrieved 2014-02-13. * 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 emimyaria; 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 (tunicate)'' Metcalf 1918 **** Genus '' Iasis (tunicate)'' 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 ''

Salp A salp (: salps, also known colloquially as “sea grape”) or salpa (: salpae or salpas) is a barrel-shaped, Plankton, planktonic tunicate in the family Salpidae. The salp moves by contracting its gelatinous body in order to pump water thro ...

a'' 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 yclomyaria Uljanin 1884**Suborder Doliolidina *** Family Doliolidae Bronn 1862 **** Genus '' Dolioletta'' Borgert 1894 **** Genus '' Doliolina'' Garstang 1933 **** Genus '' Dolioloides'' Garstang 1933 **** Genus ''

Doliolum ''Doliolum'' is a genus of tunicates, the members of which move via jet propulsion. A detailed description can be found in the article, "On the anatomical structure of the trophozooid of Doliolum denticulatum." Species The World Register of Mar ...

'' 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

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External links

earthlife.net
{{Authority control Thaliacea, Chordate classes