Ascidiacea
930016
223334208
2008-07-03T16:51:38Z
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{{Taxobox
| name = Ascidiacea
| image = Ascidias.jpg
| image_width = 250px
| image_caption = [[Sea Tulip]]s, ''Pyura spinifera''
| regnum = [[Animal]]ia
| phylum = [[Chordata]]
| subphylum = [[Urochordata]]
| classis = '''Ascidiacea'''
| classis_authority = [[Cesare Nielsen|Nielsen]], 1995
| subdivision_ranks = [[Order (biology)|Orders]]
| subdivision =
[[Aplousobranchia]]<br/>
[[Enterogona]]<br/>
[[Phlebobranchia]]<br/>
[[Pleurogona]]<br/>
[[Stolidobranchia]]
}}
'''Ascidiacea''' (commonly known as the '''ascidians''' or [[sea squirt]]s) is a [[class (biology)|class]] in the [[Tunicata]] [[subphylum]] of sac-like marine [[filter feeders]]. Ascidians are characterized by a tough outer "tunic" made of the [[polysaccharide]] [[tunicin]], as compared to other [[tunicates]] which are much less robust. They are found all over the world, usually in shallow water with [[salinity|salinities]] over 2.5%<!-- is that right, or should it be 2.5%? -->. While members of the [[Thaliacea]] and [[Larvacea]] swim freely like [[plankton]], sea squirts are [[sessile]] animals: they remain firmly attached to [[substratum]] such as rocks and shells. There are 2,300 species of ascidians and three main types: solitary ascidians, social ascidians that form clumped communities by attaching at their bases, and compound ascidians that consist of many small individuals (each individual is called a [[wiktionary:zooid|zooid]]) forming colonies up to several meters in diameter. Sea squirts feed by taking in water through the [[Siphon (biology)|oral siphon]]. The water enters the mouth and [[pharynx]], flows through mucus-covered gill slits (also called [[pharyngeal]] [[stigmata]]) into a water chamber called the atrium, then exits through the atrial siphon.
==Life History==
Almost all sea squirts are [[hermaphrodite]]s. Solitary sea squirts release many eggs from their atrial siphons; [[external fertilization]] in seawater takes place with the coincidental release of [[sperm]] from other individuals. A fertilized egg spends 12 hours to a few days developing into a free-swimming [[tadpole]] [[larva]], which then takes several more hours/days to settle and [[metamorphose]] into a juvenile. The larva selects and settles on appropriate surfaces using receptors sensitive to light, orientation to gravity, and [[tactile]] stimuli. When its [[anterior]] end touches a surface, [[wiktionary:Papillae|papillae]] (small, finger-like nervous projections) [[secrete]] an [[adhesive]] for attachment. Adhesive secretion prompts an irreversible [[metamorphosis]]: various organs (such as the larval tail and fins) are lost while others rearrange to their adult positions, the [[pharynx]] enlarges, and organs called [[ampullae]] grow from the body to permanently attach the animal to the substratum. [[Sexual maturity]] can be reached in as little as a few weeks. Most sea squirts live between 1-3 years.
Colonial sea squirts reproduce both [[asexually]] and [[sexually]]. Sexually produced individuals, those that develop from fertilized eggs, first settle and mature on substratum, then bud asexually to form a colony of many small individuals. [[Embryonic development]] takes place within the established colony: eggs are fertilized and brooded in the atrium. Colonies can survive for decades.
==Fertilization==
Sea squirt eggs are surrounded by a fibrous [[vitelline]] coat and a layer of [[ovarian follicle|follicle]] cells that produce sperm-attracting substances. In [[fertilization]], the sperm passes through the follicle cells and binds to [[glycosides]] on the vitelline coat. The sperm's [[mitochondria]] are left behind as the sperm enters and drives through the coat; this translocation of the mitochondria might provide the necessary force for penetration. The sperm swims through the perivitelline space, finally reaching the egg [[plasma membrane]] and entering the egg. This prompts rapid modification of the vitelline coat, through processes such as the egg's release of [[glycosidase]] into the seawater, so no more sperm can bind and [[polyspermy]] is avoided. After fertilization, free [[calcium ions]] are released in the egg [[cytoplasm]] in waves, mostly from internal stores. The temporary large increase in calcium concentration prompts the [[physiological]] and structural changes of development.
The dramatic rearrangement of egg cytoplasm following fertilization, called ooplasmic segregation, determines the dorsoventral and anteroposterior axes of the embryo. There are at least three types of sea squirt egg [[cytoplasm]]: [[ectoplasm]] containing [[vesicles]] and fine particles, [[endoderm]] containing yolk [[platelets]], and myoplasm containing pigment granules, [[mitochondria]], and [[endoplasmic reticulum]]. In the first phase of ooplasmic segregation, the myoplasmic actin-filament network contracts to rapidly move the peripheral cytoplasm (including the myoplasm) to the [[vegetal pole]], which marks the [[dorsal]] side of the embryo. In the second phase, the myoplasm moves to the subequatorial zone and extend into a crescent, which marks the future posterior of the embryo. The ectoplasm with the [[zygote]] nucleus ends up at the animal hemisphere while the endoplasm ends up in the vegetal hemisphere.
==Ecology==
The exceptional filtering capability of adult sea squirts causes them to accumulate [[pollutants]] that may be [[toxic]] to [[embryos]] and [[larvae]] as well as impede [[enzyme]] function in adult tissues. This property has made some species sensitive indicators of pollution.
Over the last few hundred years, most of the world's [[harbors]] have been invaded by non-native sea squirts that have clung to ship hulls or to introduced [[organisms]] such as [[oysters]] and [[seaweed]]. Several factors, including quick attainment of sexual maturity, tolerance of a wide range of environments, and a lack of [[predators]], allow sea squirt populations to grow rapidly. Unwanted populations on [[docks]], ship hulls, and farmed [[shellfish]] cause significant economic problems, and sea squirt invasions have disrupted the [[ecosystem]] of several natural sub-tidal areas by smothering native animal species.
Sea squirts are the natural prey of many animals, including [[flatworms]], [[molluscs]], rock [[crabs]], [[starfish]], fish, birds, and [[sea otters]]. They are also eaten by humans in many parts of the world, including [[Japan]], [[Korea]], [[Chile]], and [[Europe]] (where they are sold under the name “sea violet”). As chemical defenses, many sea squirts intake and maintain an extremely high concentration of [[vanadium]] in the blood, have a very low [[pH]] of the tunic due to [[acid]] in easily-ruptured [[urinary bladder|bladder]] cells, and (or) produce [[secondary metabolites]] harmful to predators and invaders. Some of these metabolites are toxic to cells and are of potential use in [[pharmaceuticals]].
==Uses==
===Culinary===
Various ''Ascidiacea'' are used as food. [[Sea pineapple]] (''Halocynthia roretzi'') is cultivated in [[Japanese cuisine|Japan]] (''hoya'', ''maboya'') and [[Korean cuisine|Korea]] (''meongge'') and, when eaten raw, has been described by ''[[Lonely Planet]]'' as tasting like "rubber dipped in ammonia". The peculiar flavor is attributed to an unsaturated alcohol called [[cynthiaol]].
''[[Microcosmus sabatieri]]'' and several similar species from the [[Mediterranean Sea]] are eaten in [[French cuisine|France]] (''figue de mer'', ''violet''), [[Italian cuisine|Italy]] (''limone di mare'', ''uova di mare'') and [[Cuisine of Chile|Chile]] (''piure''), consumed both raw and used as ingredients in seafood stews like [[bouillabaisse]].
===Model organisms for research===
[[Image:Haeckel Ascidiae.jpg|thumb|right|[[Ernst Haeckel]]'s interpretation of several ascidians. From ''[[Kunstformen der Natur]]'', 1904.]]
A number of factors make sea squirts good models for studying the fundamental developmental processes of [[chordates]], such as cell-fate specification. The [[embryonic development]] of sea squirts is simple, rapid, and easily manipulated. Because each embryo contains relatively few cells, complex processes can be studied at the cellular level, while remaining in the context of the whole embryo. The embryo's [[Transparency (optics)|transparency]] is ideal for [[fluorescent]] imaging and its maternally-derived proteins are naturally pigmented, so cell [[lineages]] are easily labeled, allowing scientists to visualize [[embryogenesis]] from beginning to end.
Sea squirts are also valuable because of their unique [[evolutionary]] position: as an approximation of ancestral chordates, they can provide insight into the link between non-chordate [[deuterostomes]] and chordates, as well as the [[origination]] of [[vertebrates]]. The sequenced [[genomes]] of the related sea squirts [[Ciona intestinalis]] and [[Ciona savignyi]] are small and easily manipulated; comparisons with the genomes of other organisms such as [[flies]], [[nematodes]], [[pufferfish]] and [[mammals]] provides valuable information regarding chordate evolution. A collection of over 480,000 cDNAs have been sequenced and are available to support further analysis of [[gene expression]], which is expected to provide information about complex developmental processes and regulation of genes in vertebrates. Gene expression in embryos of sea squirts can be conveniently inhibited using [[Morpholino]] oligos<ref name="RNaseH">{{cite journal | last = Hamada | first = M |coauthors = Wada S, Kobayashi K, Satoh N | year = 2007 | title = Novel genes involved in Ciona intestinalis embryogenesis: Characterization of gene knockdown embryos | journal = Dev Dyn. | volume = 236 | issue = 7 | pages = 1820–31 | format = [[Pubmed]] | doi = 10.1002/dvdy.21181 | unused_data = |http://www.ncbi.nlm.nih.gov/sites/entrez?Db=pubmed&Cmd=ShowDetailView&TermToSearch=17557306 }}</ref>.
==References==
{{commonscat|Ascidiacea}}
{{cetaceaportal}}
<div class="references-small">
<references />
=====Further general references=====
* {{RefTudgeVariety}}
</div>
==External links==
*[http://ascidiacea.com/ Ascidiacea.com] - Classification and images of many ascidians.
*[http://ascidians.com/ The Dutch Ascidians Homepage]
*[http://www.habitas.org.uk/marinelife/ Encyclopedia of Marine Life of Britain and Ireland]
*[http://zygote.swarthmore.edu/Conklin/Conklin.html/ The Organization and Cell-lineage of the Ascidian Egg]
*[http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=244914&rendertype=figure&id=F1/ A fate map of the ascidian egg]
*[http://www.broad.mit.edu/annotation/ciona/ Ciona savignyi Database]
{{Chordata}}
[[Category:Ascidiacea| ]]
[[Category:Tunicates]]
[[cs:Sumky]]
[[da:Søpunge]]
[[de:Seescheiden]]
[[es:Ascidiacea]]
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[[ko:해초강]]
[[it:Ascidia]]
[[ka:ასციდიები]]
[[nl:Zakpijpen]]
[[ja:ホヤ]]
[[pl:Żachwy]]
[[ru:Асцидии]]
[[sk:Ascídie]]
[[fi:Meritupet]]
[[uk:Асцидії]]
[[zh:海鞘纲]]