Ascidiacea

Ascidiacea
Enlarge picture
Sea Tulips, Pyura spinifera

Sea Tulips, Pyura spinifera
Scientific classification
Kingdom:Animalia
Phylum:Chordata
Subphylum:Urochordata
Class:Ascidiacea
Nielsen, 1995
Orders


Aplousobranchia
Enterogona
Phlebobranchia
Pleurogona
Stolidobranchia
Ascidiacea (commonly known as the ascidians or sea squirts) is a 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 salinities over 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 ) forming colonies up to several meters in diameter. Sea squirts feed by taking in water through the 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 hermaphrodites. 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, 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 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 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 Japan (hoya, maboya) and 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 France (figue de mer, violet), Italy (limone di mare, uova di mare) and Chile (probecho), consumed both raw and used as ingredients in seafood stews like bouillabaisse.

Model organisms for research

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 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[1].

Enlarge picture
Ernst Haeckel's interpretation of several ascidians. From Kunstformen der Natur (Artforms of Nature), 1904.

References

1. ^ Hamada, M; Wada S, Kobayashi K, Satoh N (2007). "Novel genes involved in Ciona intestinalis embryogenesis: Characterization of gene knockdown embryos." (Pubmed). Dev Dyn. 236 (7): 1820-31. 
Further general references
  • Colin Tudge (2000). The Variety of Life. Oxford University Press. ISBN 0198604262.2000&rft.pub=Oxford%20University%20Press"> 

External links

Pyura

Species: P. spinifera

Binomial name
Pyura spinifera

Sea tulips
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Scientific classification or biological classification is a method by which biologists group and categorize species of organisms. Scientific classification also can be called scientific taxonomy, but should be distinguished from folk taxonomy, which lacks scientific basis.
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Chordata
Bateson, 1885

Typical Classes

See below

Chordates (phylum Chordata) are a group of animals that includes the vertebrates, together with several closely related invertebrates.
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Urochordata
Giribet et al., 2000

Classes

Ascidiacea (2,300 species)
Thaliacea
Appendicularia
Sorberacea

Urochordata (sometimes known as tunicata and commonly called urochordates, tunicates, sea squirts
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order (Latin: ordo, plural ordines) is a rank between class and family (termed a taxon at that rank). The superorder is a rank between class and order. Exact details of formal nomenclature depend on the Nomenclature Code which applies.
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Enterogona

Suborders and families

Suborder Aplousobranchia
Family Clavelinidae
Family Didemnidae
Family Polycitoridae
Family Polyclinidae
Suborder Phlebobranchia

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Pleurogona

Suborders and families

Suborder Stolidobranchia
  • Family Styelidae
* Genus Styela
  • Family Botryllidae
* Genus Botrylloides
* Genus Botryllus

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Urochordata
Giribet et al., 2000

Classes

Ascidiacea (2,300 species)
Thaliacea
Appendicularia
Sorberacea

Urochordata (sometimes known as tunicata and commonly called urochordates, tunicates, sea squirts
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class is the rank in the scientific classification of organisms in biology below Phylum and above Order.

For example, Mammalia is the class used in the classification of dogs, whose phylum is Chordata (animals with notochords) and order is Carnivora (mammals that eat meat).
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Urochordata
Giribet et al., 2000

Classes

Ascidiacea (2,300 species)
Thaliacea
Appendicularia
Sorberacea

Urochordata (sometimes known as tunicata and commonly called urochordates, tunicates, sea squirts
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In life, a subphylum is a taxonomic rank intermediate between phylum and superclass. The rank of subdivision in plants and fungi is equivalent to subphylum.

Not all phyla are divided into subphyla.
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Filter feeders (also known as suspension feeders) are animals that feed by straining suspended matter and food particles from water, typically by passing the water over a specialized structure, such as the baleen of baleen whales.
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Polysaccharides are relatively complex carbohydrates. They are polymers made up of many monosaccharides joined together by glycosidic bonds. They are therefore very large, often branched, macromolecules. They tend to be amorphous, insoluble in water, and have no sweet taste.
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Urochordata
Giribet et al., 2000

Classes

Ascidiacea (2,300 species)
Thaliacea
Appendicularia
Sorberacea

Urochordata (sometimes known as tunicata and commonly called urochordates, tunicates, sea squirts
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Salinity is the saltiness or dissolved salt content of a body of water. Salinity in Australian English and North American English may refer to salt in soil (see soil salination).

Definition


Water salinity
Fresh water Brackish water Saline water Brine
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Thaliacea
Nielsen, 1995

Orders
  • Pyrosomida
  • Salpida
  • Doliolida
The Thalicea comprise a class of marine animals within the subphylum Urochordata.
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Appendicularia

Families

Fritillariidae
Kowalevskiidae
Oikopleuridae

The Appendicularia or Larvacea are a group of solitary, free-swimming pelagic urochordates found throughout the world's oceans.
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Plankton are any drifting organism that inhabits the pelagic zone of oceans, seas, or bodies of fresh water. It is a description of life-style rather than a genetic classification.
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Sessile is a term in biology with two distinct meanings:

In botany

In botany, sessile means "without a stalk", as in flowers (pedicel) or leaves (petiole) that grow directly from the stem or Peduncle; however, in limnology, sessile vegetation are any organisms
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In linguistics, a substratum (lat. sub: under + stratum: layerlower layer) is a language which influences another one while that second language supplants it. The term is also used of substrate interference, i.e.
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The term siphon is used for a number of biological structures, either because flowing liquids are involved or because the object is shaped like a siphon. In these structures, the physical phenomenon known as the siphon effect is not present.
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The pharynx (plural: pharynges) is the part of the neck and throat situated immediately posterior to the mouth and nasal cavity, and cranial, or superior, to the esophagus, larynx, and trachea.
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The word pharyngeal, meaning to do with the pharynx or throat, occurs in more than one context:
  • For pharyngeal anatomy, see Pharynx
  • For pharyngeal muscles, see Pharyngeal constrictor
  • For pharyngeal sounds in phonetics, see Pharyngeal consonant

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Stigmata are bodily marks, sores, or sensations of pain in locations corresponding to the crucifixion wounds of Jesus. The term originates from the line at the end of Saint Paul's Letter to the Galatians where he says, "I bear on my body the marks of Jesus," with "marks" in the
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hermaphrodite is an organism that posses both male and female genetalia.[1] In many species, hermaphroditism is a common part of the life-cycle, particularly in some asexual animals and some plants.
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External fertilization is a form of fertilization in which a sperm cell is united with an egg cell external to the body of the female. Thus, the fertilization is said to occur "externally".
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sperm is derived from the word spermos (meaning "seed") and refers to the male reproductive cells. Sperm cells are the smaller gametes involved in fertilization.
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tadpole (also known as a pollywog, pollywag or polliwog) is a larval amphibian, the first stage of a frog or toad.

Description

Not uncommonly, during the tadpole stage of an amphibian's life cycle, the tadpole breathes by means of autonomous external
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larva (Latin; plural larvae) is a juvenile form of animal with indirect development, undergoing metamorphosis (for example, insects or amphibians).

The larva can look completely different from the adult form, for example, a caterpillar differs from a butterfly.
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