The Thaliacea comprise a class of marine animals within the subphylum Tunicata. 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 ancient Greek, βένθος (bénthos), meaning " ...

relatives the ascidians, 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 (as illustrated on the right). The word ''pelagic'' is derived . The pelagic zone can be thought of as an imaginary cylinder or w ...

) 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 a sub-group of suspension feeding animals that feed by straining suspended matter and food particles from water, typically by passing the water over a specialized filtering structure. Some animals that use this method of feedin ...

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

zooid A zooid or zoöid is a single 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 zooi ...

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

cloaca In animal anatomy, a cloaca ( ), plural cloacae ( or ), is the posterior orifice that serves as the only opening for the digestive, reproductive, and urinary tracts (if present) of many vertebrate animals. All amphibians, reptiles and birds, ...

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 (plural: pharynges) is the part of the throat behind the mouth and nasal cavity, and above the oesophagus and trachea (the tubes going down to the stomach and the lungs). It is found in vertebrates and invertebrates, though its st ...

. 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 and mucous cells. It ...

slowly pulled across the slits by

cilia The cilium, plural cilia (), is a membrane-bound organelle found on most types of eukaryotic cell, and certain microorganisms known as ciliates. Cilia are absent in bacteria and archaea. The cilium has the shape of a slender threadlike proje ...

. 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 Among animals, viviparity is development of the embryo inside the body of the parent. This is opposed to oviparity which is a reproductive mode in which females lay developing eggs that complete their development and hatch externally from the ...

, 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 In anatomy, the notochord is a flexible rod which is similar in structure to the stiffer cartilage. If a species has a notochord at any stage of its life cycle (along with 4 other features), it is, by definition, a chordate. The notochord consi ...

found in

Chordata A chordate () is an animal of the phylum Chordata (). All chordates possess, at some point during their larval or adult stages, five synapomorphies, or primary physical characteristics, that distinguish them from all the other taxa. These fi ...

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 the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of the Earth. Carbon is the main component of biological compounds as well as a major compon ...

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 Pyrosomes, genus ''Pyrosoma'', are free-floating colonial tunicates that usually live in the upper layers of the open ocean in warm seas, although some may be found at greater depths. Pyrosomes are cylindrical or cone-shaped colonies up to long, ...

Jones 1848 ** Family

Pyrosomatidae Pyrosomes, genus ''Pyrosoma'', are free-floating colonial tunicates that usually live in the upper layers of the open ocean in warm seas, although some may be found at greater depths. Pyrosomes are cylindrical or cone-shaped colonies up to long, ...

Garstang 1929 ***Subfamily Pyrostremmatinae van Soest 1979 **** Genus '' Pyrostremma'' Garstang 1929 'Propyrosoma'' Ivanova-Kazas 1962***Subfamily Pyrosomatinae **** Genus ''

Pyrosoma Pyrosomes, genus ''Pyrosoma'', are free-floating colonial tunicates that usually live in the upper layers of the open ocean in warm seas, although some may be found at greater depths. Pyrosomes are cylindrical or cone-shaped colonies up to long, ...

'' 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'' 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 Thetis (; grc-gre, Θέτις ), is a figure from Greek mythology with varying mythological roles. She mainly appears as a sea nymph, a goddess of water, or one of the 50 Nereids, daughters of the ancient sea god Nereus. When described as ...

'' 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'' 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

* *

External links

earthlife.net
{{Taxonbar, from=Q610887 Thaliacea, Chordate classes