Thaliacea is a class of marine within the subphylum Tunicate, comprising the , and . Unlike their benthic relatives the Ascidiacea, 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.
Anatomy
The three orders of thaliaceans are
.
are colonial animals, with multiple tiny
-like
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.
Doliolida and 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 , 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 Chordate 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.
Taxonomy
The class is a relatively small one, and is divided into three orders:
Class Thaliacea[[1] World Register of Marine Species. Retrieved 2014-02-13.]
-
Order Pyrosomida Jones 1848
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Order Salpida Hemimyaria;
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Family Salpidae Franstedt 1885
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Subfamily Cyclosalpinae Yount 1954
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Subfamily Salpinae Lahille 1888
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Genus Brooksia (tunicate) Metcalf 1918
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Genus Iasis (tunicate) Savigny 1816 Weelia
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Genus Ihlea Metcalf 1919 non Metcalf 1918 Apsteinia
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Genus Metcalfina Ihle & Ihle-Landenberg 1933
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Genus Pegea Savigny 1816
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Genus Ritteriella Metcalf 1919 Ritteria
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Genus Forskål 1775 Biphora
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Genus Soestia Holothurium
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Genus Thetys Tilesius 1802 Salpa
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Genus Thalia Blumenbach 1798 Dubreuillia
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Genus Traustedtia Metcalf 1918 Salpa
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Order Doliolida Cyclomyaria
External links
