Ascidiacea
Encyclopedia
Ascidiacea is a class
in the Tunicata
subphylum
of sac-like marine
invertebrate
filter feeders. Ascidians are characterized by a tough outer "tunic" made of the polysaccharide
tunicin, as compared to other tunicates which are less rigid.
Ascidians 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 zooid) 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.
-like substance called tunicin along with proteins and calcium salts. Unlike the shells of molluscs, the tunic is composed of living tissue, and often has its own blood supply. In some colonial species, the tunics of adjacent individuals are fused into a single structure.
The upper surface of the animal, opposite to the part gripping the substratum, has two openings, or siphons. When removed from the water, the animal often violently expels water from these siphons, hence the common name of "sea squirt". The body itself can be divided into up to three regions, although these are not clearly distinct in most species. The pharyngeal region contains the pharynx, while the abdomen contains most of the other bodily organs, and the postabdomen contains the heart and gonad
s. In many sea squirts, the postabdomen, or even the entire abdomen, are absent, with their respective organs being located more anteriorly.
As its name implies, the pharyngeal region is occupied mainly by the pharynx. The large buccal siphon opens into the pharynx, acting like a mouth. The pharynx itself is ciliated and contains numerous perforations, or stigmata, arranged in a grid-like pattern around its circumference. The beating of the cilia sucks water through the siphon, and then through the stigmata. A long ciliated groove, or endostyle
, runs along one side of the pharynx, and a projecting ridge along the other. The endostyle may be homologous
with the thyroid
gland of vertebrates, despite its differing function.
The pharynx is surrounded by an atrium, through which water is expelled through a second, usually smaller, siphon. Cord of connective tissue cross the atrium to maintain the general shape of the body. The outer body wall consists of connective tissue, muscle fibres, and a simple epithelium
directly underlying the tunic.
which is then passed into the rest of the pharynx by the beating of flagella
along its margins. The mucus then flows in a sheet across the surface of the pharynx, trapping plankton
ic food particles as they pass through the stigmata, and is collected in the ridge on the dorsal surface. The ridge bears a groove along one side, which passes the collected food downwards and into the oesophageal opening at the base of pharynx.
The oesophagus runs downwards to a stomach
in the abdomen, which secretes enzymes
that digest the food. An intestine
runs upwards from the stomach parallel to the oesophagus and eventually opens, through a short rectum
and anus
, into a cloaca
just below the atrial siphon. In some highly developed colonial species, clusters of individuals may share a single cloaca, with all the atrial siphons opening into it, although the buccal siphons all remain separate. A series of glands lie on the outer surface of the intestine, opening through collecting tubules into the stomach, although their precise function is unclear.
, is excreted directly from the blood through the walls of the pharynx, and expelled through the atrial siphon.
Unusually, the heart of sea squirts alternates the direction in which it pumps blood every three to four minutes. There are two excitatory areas, one at each end of the heart, with first one being dominant, to push the blood through the ventral vessel, and then the other, pushing it dorsally.
There are four different types of blood cell: lymphocyte
s, phagocytic amoebocytes, nephrocytes and morula cells. The nephrocytes collect waste material such as uric acid
and accumulate it in renal vesicles close to the digestive tract. The morula cells help to form the tunic, and can often be found within the tunic substance itself. In some species, the morula cells possess pigmented reducing agents containing iron
(hemoglobin
), giving the blood a red colour, or vanadium
(hemovanadin
) giving it a green colour. In that case the cells are also referred to as vanadocytes.
Although there is no true brain, the largest ganglion is located in the connective tissue between the two siphons, and sends nerves throughout the body. Beneath this ganglion lies an exocrine gland
that empties into the pharynx. The gland is formed from the nerve tube, and is therefore homologous to the spinal cord of vertebrates.
Sea squirts lack special sense organs, although the body wall has numerous individual receptors for touch, chemoreception, and the detection of light.
Almost all ascidians are hermaphrodite
s and conspicuous mature ascidians are sessile
. The gonads are located in the abdomen or postabdomen, and include one testis and one ovary, each of which opens via a duct into the cloaca. Broadly speaking, the ascidians can be divided into species which exist as independent animals (the solitary ascidians) and those which are interdependent (the colonial ascidians). Different species of ascidians can have markedly different reproductive strategies, with colonial forms having mixed modes of reproduction.
Solitary ascidians 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 no more than 36 hours to settle and metamorphose
into a juvenile.
The larva possesses a long tail, containing a hollow dorsal nerve tube and a notochord
, both features clearly indicative of the animal's chordate
affinities. The nerve tube enlarges in the main body, and will eventually become the cerebral ganglion of the adult. The tunic develops early in embryonic life, and extends to form a fin along the tail in the larva. The larva also has a statocyst
and a pigmented cup above the mouth, which opens into a pharynx lined with small clefts opening into a surrounding atrium. The mouth and anus are originally at opposite ends of the animal, with the mouth only moving to its final (posterior) position during metamorphosis.
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. The siphons of the juvenile ascidian become orientated to optimise current flow through the feeding apparatus. Sexual maturity
can be reached in as little as a few weeks. Since the larva is more advanced than its adult, this type of metamorphosis is called 'retrogressive metamorphosis' as the adult is less advanced to its larva. This feature is a landmark for the 'theory of retrogressive metamorphosis or ascidian larva theory', the true chordates are hypothesised to have evolved from sexually mature larvae.
ly and sex
ually. Colonies can survive for decades. An ascidian colony consists of individual elements called zooids. Zooids within a colony are usually genetically identical and some have a shared circulation. Different colonial ascidian species produce sexually derived offspring by one of two dispersal strategies- Colonial species are either broadcast spawners (long-range dispersal) or philopatric (very short-range dispersal). Broadcast spawners release sperm and ova into the water column and fertilization occurs near to the parent colonies. Resultant zygotes develop into microscopic larvae that may be carried great distances by oceanic currents. The larvae of sessile forms which survive eventually settle and complete maturation on the substratum- then they may bud asexually to form a colony of zooids.
The picture is more complicated for the philopatrically dispersed ascidians: sperm from a nearby colony (or from a zooid of the same colony) enter the pharyngeal siphon and fertilization takes place within the atrium. Embryos are then brooded within the atrium where embryonic development takes place: this results in macroscopic tadpole-like larvae. When mature, these larvae exit the atrial siphon of the adult and then settle close to the parent colony (often within meters). The combined effect of short sperm range and philopatric larval dispersal results in local population structures of closely related individuals/inbred colonies. Generations of colonies which are restricted in dispersal are thought to accumulate adaptions to local conditions, thereby providing advantages over newcomers.
Trauma or predation often results in fragmentation of a colony into subcolonies. Subsequent zooid replication can lead to coalescence and circulatory fusion of the subcolonies. Closely related colonies which are proximate to each other may also fuse if they coalesce and if they are histocompatible. Ascidians were among the first animals to be able to immunologically recognize self from non-self as a mechanism to prevent unrelated colonies from fusing to them and parasitizing them.
s. Buds containing food storage cells can develop within the stolons and, when sufficiently separated from the 'parent', may grow into a new adult individual.
In other species, the postabdomen can elongate and break up into a string of separate buds, which can eventually form a new colony. In some, the pharyngeal part of the animal degenerates, and the abdomen breaks up into patches of germinal tissue, each combining parts of the epidermis, peritoneum, and digestive tract, and capable of growing into new individuals.
In yet others, budding begins shortly after the larva has settled onto the substrate. In the family Didemnidae
, for instance, the individual essentially splits into two, with the pharynx growing a new digestive tract and the original digestive tract growing a new pharynx.
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 vesicle
s 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 extends 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.
e 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 nudibranch
s, flatworms, molluscs, rock crabs, sea star
s, 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.
(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.
The Korean fish stew agujjim
traditionally contains the tunicate Styela clava
. According to the LA Weekly, "they are actually farmed in parts of Korea, and sea squirt bibimbap is a specialty of Geojae-do island, not far from Masan."
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), Greece (fouska, φούσκα). In Greece
they are consumed just raw with lemon
or in salads with olive oil
, lemon
and parsley
.
The piure (Pyura chilensis) is used as food in the cuisine of Chile
, consumed both raw and used as ingredients in seafood stews like bouillabaisse
.
Pyura stolonifera is known as cunjevoi
in Australia
. This was once used as a food source by Aboriginal people
living around Botany Bay
, but is now used mainly for fishing bait. Note: the word "cunjevoi" is also used for two species of rainforest plant, at least one of which is toxic to humans.
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 chordates and ancestral non-chordate deuterostomes, as well as the origination of vertebrates from simple chordates. 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.
Class (biology)
In biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...
in the Tunicata
Tunicate
Tunicates, also known as urochordates, are members of the subphylum Tunicata, previously known as Urochordata, a group of underwater saclike filter feeders with incurrent and excurrent siphons that is classified within the phylum Chordata. While most tunicates live on the ocean floor, others such...
subphylum
Subphylum
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...
of sac-like marine
Marine (ocean)
Marine is an umbrella term. As an adjective it is usually applicable to things relating to the sea or ocean, such as marine biology, marine ecology and marine geology...
invertebrate
Invertebrate
An invertebrate is an animal without a backbone. The group includes 97% of all animal species – all animals except those in the chordate subphylum Vertebrata .Invertebrates form a paraphyletic group...
filter feeders. Ascidians are characterized by a tough outer "tunic" made of the polysaccharide
Polysaccharide
Polysaccharides are long carbohydrate molecules, of repeated monomer units joined together by glycosidic bonds. They range in structure from linear to highly branched. Polysaccharides are often quite heterogeneous, containing slight modifications of the repeating unit. Depending on the structure,...
tunicin, as compared to other tunicates which are less rigid.
Ascidians are found all over the world, usually in shallow water with salinities
Salinity
Salinity is the saltiness or dissolved salt content of a body of water. It is a general term used to describe the levels of different salts such as sodium chloride, magnesium and calcium sulfates, and bicarbonates...
over 2.5%. While members of the Thaliacea
Thaliacea
The Thaliacea comprise a class of marine animals within the subphylum Tunicata. Unlike their bottom-dwelling relatives the ascidians, thaliaceans are free-floating for their entire lifespan. The group includes both solitary and colonial species.-Anatomy:...
and Larvacea
Larvacea
Larvaceans are solitary, free-swimming tunicates found throughout the world's oceans. Like most tunicates, appendicularians are filter feeders. Unlike other tunicates, appendicularians live in the pelagic zone, specifically in the upper sunlit portion of the ocean or sometimes deeper...
swim freely like plankton
Plankton
Plankton are any drifting organisms that inhabit the pelagic zone of oceans, seas, or bodies of fresh water. That is, plankton are defined by their ecological niche rather than phylogenetic or taxonomic classification...
, sea squirts are sessile
Sessility (zoology)
In zoology, sessility is a characteristic of animals which are not able to move about. They are usually permanently attached to a solid substrate of some kind, such as a part of a plant or dead tree trunk, a rock, or the hull of a ship in the case of barnacles. Corals lay down their own...
animals: they remain firmly attached to substratum
Substratum
In linguistics, a stratum or strate is a language that influences, or is influenced by another through contact. A substratum is a language which has lower power or prestige than another, while a superstratum is the language that has higher power or prestige. Both substratum and superstratum...
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 zooid) forming colonies up to several meters in diameter.
Sea squirts feed by taking in water through the oral siphon
Siphon (biology)
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...
. The water enters the mouth and pharynx
Pharynx
The human pharynx is the part of the throat situated immediately posterior to the mouth and nasal cavity, and anterior to the esophagus and larynx. The human pharynx is conventionally divided into three sections: the nasopharynx , the oropharynx , and the laryngopharynx...
, flows through mucus-covered gill slits (also called pharyngeal
Pharyngeal
The word pharyngeal, meaning to do with the pharynx or throat, may refer to:* Pharynx, for pharyngeal anatomy* Pharyngeal muscles**Superior pharyngeal constrictor muscle** Middle pharyngeal constrictor muscle** Inferior pharyngeal constrictor muscle...
stigmata
Stigmata
Stigmata are bodily marks, sores, or sensations of pain in locations corresponding to the crucifixion wounds of Jesus, such as the hands and feet...
) into a water chamber called the atrium, then exits through the atrial siphon.
Anatomy
Sea squirts are rounded or cylindrical animals ranging from about 0.5 to 10 cm (0.196850393700787 to 3.9 in) in size. One end of the body is always firmly fixed to rock, coral, or some similar solid surface. The lower surface is pitted or ridged, and in some species has root-like extensions that help the animal grip onto the surface. The body wall is covered by a smooth thick tunic, which is often quite rigid. The tunic consists of a celluloseCellulose
Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to over ten thousand β linked D-glucose units....
-like substance called tunicin along with proteins and calcium salts. Unlike the shells of molluscs, the tunic is composed of living tissue, and often has its own blood supply. In some colonial species, the tunics of adjacent individuals are fused into a single structure.
The upper surface of the animal, opposite to the part gripping the substratum, has two openings, or siphons. When removed from the water, the animal often violently expels water from these siphons, hence the common name of "sea squirt". The body itself can be divided into up to three regions, although these are not clearly distinct in most species. The pharyngeal region contains the pharynx, while the abdomen contains most of the other bodily organs, and the postabdomen contains the heart and gonad
Gonad
The gonad is the organ that makes gametes. The gonads in males are the testes and the gonads in females are the ovaries. The product, gametes, are haploid germ cells. For example, spermatozoon and egg cells are gametes...
s. In many sea squirts, the postabdomen, or even the entire abdomen, are absent, with their respective organs being located more anteriorly.
As its name implies, the pharyngeal region is occupied mainly by the pharynx. The large buccal siphon opens into the pharynx, acting like a mouth. The pharynx itself is ciliated and contains numerous perforations, or stigmata, arranged in a grid-like pattern around its circumference. The beating of the cilia sucks water through the siphon, and then through the stigmata. A long ciliated groove, or endostyle
Endostyle
An endostyle is a longitudinal ciliated groove on the ventral wall of the pharynx which produces mucus to gather food particles. It is found in urochordates and cephalochordates, and in the larvae of lampreys. It aids in transporting food to the esophagus. It is also called the hypopharyngeal groove...
, runs along one side of the pharynx, and a projecting ridge along the other. The endostyle may be homologous
Homology (biology)
Homology forms the basis of organization for comparative biology. In 1843, Richard Owen defined homology as "the same organ in different animals under every variety of form and function". Organs as different as a bat's wing, a seal's flipper, a cat's paw and a human hand have a common underlying...
with the thyroid
Thyroid
The thyroid gland or simply, the thyroid , in vertebrate anatomy, is one of the largest endocrine glands. The thyroid gland is found in the neck, below the thyroid cartilage...
gland of vertebrates, despite its differing function.
The pharynx is surrounded by an atrium, through which water is expelled through a second, usually smaller, siphon. Cord of connective tissue cross the atrium to maintain the general shape of the body. The outer body wall consists of connective tissue, muscle fibres, and a simple epithelium
Epithelium
Epithelium is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. Epithelial tissues line the cavities and surfaces of structures throughout the body, and also form many glands. Functions of epithelial cells include secretion, selective...
directly underlying the tunic.
Digestive system
The pharynx forms the first part of the digestive system. The endostyle produces a supply of mucusMucus
In vertebrates, mucus is a slippery secretion produced by, and covering, mucous membranes. Mucous fluid is typically produced from mucous cells found in mucous glands. Mucous cells secrete products that are rich in glycoproteins and water. Mucous fluid may also originate from mixed glands, which...
which is then passed into the rest of the pharynx by the beating of flagella
Flagellum
A flagellum is a tail-like projection that protrudes from the cell body of certain prokaryotic and eukaryotic cells, and plays the dual role of locomotion and sense organ, being sensitive to chemicals and temperatures outside the cell. There are some notable differences between prokaryotic and...
along its margins. The mucus then flows in a sheet across the surface of the pharynx, trapping plankton
Plankton
Plankton are any drifting organisms that inhabit the pelagic zone of oceans, seas, or bodies of fresh water. That is, plankton are defined by their ecological niche rather than phylogenetic or taxonomic classification...
ic food particles as they pass through the stigmata, and is collected in the ridge on the dorsal surface. The ridge bears a groove along one side, which passes the collected food downwards and into the oesophageal opening at the base of pharynx.
The oesophagus runs downwards to a stomach
Stomach
The stomach is a muscular, hollow, dilated part of the alimentary canal which functions as an important organ of the digestive tract in some animals, including vertebrates, echinoderms, insects , and molluscs. It is involved in the second phase of digestion, following mastication .The stomach is...
in the abdomen, which secretes enzymes
Digestive enzyme
'Digestive enzymes' are enzymes that break down polymeric macromolecules into their smaller building blocks, in order to facilitate their absorption by the body. Digestive enzymes are found in the digestive tract of animals where they aid in the digestion of food as well as inside the cells,...
that digest the food. An intestine
Intestine
In human anatomy, the intestine is the segment of the alimentary canal extending from the pyloric sphincter of the stomach to the anus and, in humans and other mammals, consists of two segments, the small intestine and the large intestine...
runs upwards from the stomach parallel to the oesophagus and eventually opens, through a short rectum
Rectum
The rectum is the final straight portion of the large intestine in some mammals, and the gut in others, terminating in the anus. The human rectum is about 12 cm long...
and anus
Anus
The anus is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to control the expulsion of feces, unwanted semi-solid matter produced during digestion, which, depending on the type of animal, may be one or more of: matter which the animal cannot digest,...
, into a cloaca
Cloaca
In zoological anatomy, a cloaca is the posterior opening that serves as the only such opening for the intestinal, reproductive, and urinary tracts of certain animal species...
just below the atrial siphon. In some highly developed colonial species, clusters of individuals may share a single cloaca, with all the atrial siphons opening into it, although the buccal siphons all remain separate. A series of glands lie on the outer surface of the intestine, opening through collecting tubules into the stomach, although their precise function is unclear.
Circulatory system
The heart is a curved muscular tube lying in the postabdomen, or close to the stomach. Each end opens into a single vessel, one running to the endostyle, and the other to the dorsal surface of the pharynx. The vessels are connected by a series of sinuses, through which the blood flows. Additional sinuses run from that on the dorsal surface, supplying blood to the visceral organs, and smaller vessels commonly run from both sides into the tunic. Nitrogenous waste, in the form of ammoniaAmmonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
, is excreted directly from the blood through the walls of the pharynx, and expelled through the atrial siphon.
Unusually, the heart of sea squirts alternates the direction in which it pumps blood every three to four minutes. There are two excitatory areas, one at each end of the heart, with first one being dominant, to push the blood through the ventral vessel, and then the other, pushing it dorsally.
There are four different types of blood cell: lymphocyte
Lymphocyte
A lymphocyte is a type of white blood cell in the vertebrate immune system.Under the microscope, lymphocytes can be divided into large lymphocytes and small lymphocytes. Large granular lymphocytes include natural killer cells...
s, phagocytic amoebocytes, nephrocytes and morula cells. The nephrocytes collect waste material such as uric acid
Uric acid
Uric acid is a heterocyclic compound of carbon, nitrogen, oxygen, and hydrogen with the formula C5H4N4O3. It forms ions and salts known as urates and acid urates such as ammonium acid urate. Uric acid is created when the body breaks down purine nucleotides. High blood concentrations of uric acid...
and accumulate it in renal vesicles close to the digestive tract. The morula cells help to form the tunic, and can often be found within the tunic substance itself. In some species, the morula cells possess pigmented reducing agents containing iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
(hemoglobin
Hemoglobin
Hemoglobin is the iron-containing oxygen-transport metalloprotein in the red blood cells of all vertebrates, with the exception of the fish family Channichthyidae, as well as the tissues of some invertebrates...
), giving the blood a red colour, or vanadium
Vanadium
Vanadium is a chemical element with the symbol V and atomic number 23. It is a hard, silvery gray, ductile and malleable transition metal. The formation of an oxide layer stabilizes the metal against oxidation. The element is found only in chemically combined form in nature...
(hemovanadin
Hemovanadin
Hemovanadin is used to refer to the pale green vanabin proteins found in the blood cells, called vanadocytes, of ascidians and other organisms...
) giving it a green colour. In that case the cells are also referred to as vanadocytes.
Nervous system
The ascidian central nervous system is formed from a plate that rolls up to form a neural tube. The number of cells within the central nervous system is very small. The neural tube is composed of the sensory vesicle, the neck, the visceral or tail ganglion, and the caudal nerve cord. The anteroposterior regionalization of the neural tube in ascidians is comparable to that in vertebrates.Although there is no true brain, the largest ganglion is located in the connective tissue between the two siphons, and sends nerves throughout the body. Beneath this ganglion lies an exocrine gland
Exocrine gland
Exocrine glands are a type of ductal glands that secrete their products into ducts that lead directly into the external environment...
that empties into the pharynx. The gland is formed from the nerve tube, and is therefore homologous to the spinal cord of vertebrates.
Sea squirts lack special sense organs, although the body wall has numerous individual receptors for touch, chemoreception, and the detection of light.
Life history
Hermaphrodite
In biology, a hermaphrodite is an organism that has reproductive organs normally associated with both male and female sexes.Many taxonomic groups of animals do not have separate sexes. In these groups, hermaphroditism is a normal condition, enabling a form of sexual reproduction in which both...
s and conspicuous mature ascidians are sessile
Sessility (zoology)
In zoology, sessility is a characteristic of animals which are not able to move about. They are usually permanently attached to a solid substrate of some kind, such as a part of a plant or dead tree trunk, a rock, or the hull of a ship in the case of barnacles. Corals lay down their own...
. The gonads are located in the abdomen or postabdomen, and include one testis and one ovary, each of which opens via a duct into the cloaca. Broadly speaking, the ascidians can be divided into species which exist as independent animals (the solitary ascidians) and those which are interdependent (the colonial ascidians). Different species of ascidians can have markedly different reproductive strategies, with colonial forms having mixed modes of reproduction.
Solitary ascidians release many eggs from their atrial siphons; external fertilization
External fertilization
External fertilization is a form of fertilization in which a sperm cell is united with an egg cell external to the bodies of the reproducing individuals. In contrast, internal fertilization takes place inside the female after insemination through copulation....
in seawater takes place with the coincidental release of sperm
Sperm
The term sperm is derived from the Greek word sperma and refers to the male reproductive cells. In the types of sexual reproduction known as anisogamy and oogamy, there is a marked difference in the size of the gametes with the smaller one being termed the "male" or sperm cell...
from other individuals. A fertilized egg spends 12 hours to a few days developing into a free-swimming tadpole
Tadpole
A tadpole or polliwog is the wholly aquatic larval stage in the life cycle of an amphibian, particularly that of a frog or toad.- Appellation :...
larva
Larva
A larva is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle...
, which then takes no more than 36 hours to settle and metamorphose
Métamorphose
"Métamorphose" - a song by French singer Amanda Lear released in 1989 by Carrere Records.- Song information :"Métamorphose" was the first single from Amanda's French-Italian album Tant qu'il y aura des hommes. The album was actually a re-release of Uomini più uomini, and consisted mostly of down-...
into a juvenile.
The larva possesses a long tail, containing a hollow dorsal nerve tube and a notochord
Notochord
The notochord is a flexible, rod-shaped body found in embryos of all chordates. It is composed of cells derived from the mesoderm and defines the primitive axis of the embryo. In some chordates, it persists throughout life as the main axial support of the body, while in most vertebrates it becomes...
, both features clearly indicative of the animal's chordate
Chordate
Chordates are animals which are either vertebrates or one of several closely related invertebrates. They are united by having, for at least some period of their life cycle, a notochord, a hollow dorsal nerve cord, pharyngeal slits, an endostyle, and a post-anal tail...
affinities. The nerve tube enlarges in the main body, and will eventually become the cerebral ganglion of the adult. The tunic develops early in embryonic life, and extends to form a fin along the tail in the larva. The larva also has a statocyst
Statocyst
The statocyst is a balance sensory receptor present in some aquatic invertebrates, including bivalves, cnidarians, echinoderms, cephalopods, and crustaceans. A similar structure is also found in Xenoturbella. The statocyst consists of a sac-like structure containing a mineralised mass and numerous...
and a pigmented cup above the mouth, which opens into a pharynx lined with small clefts opening into a surrounding atrium. The mouth and anus are originally at opposite ends of the animal, with the mouth only moving to its final (posterior) position during metamorphosis.
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
Adhesive
An adhesive, or glue, is a mixture in a liquid or semi-liquid state that adheres or bonds items together. Adhesives may come from either natural or synthetic sources. The types of materials that can be bonded are vast but they are especially useful for bonding thin materials...
for attachment. Adhesive secretion prompts an irreversible metamorphosis
Metamorphosis
Metamorphosis is a biological process by which an animal physically develops after birth or hatching, involving a conspicuous and relatively abrupt change in the animal's body structure through cell growth and differentiation...
: various organs (such as the larval tail and fins) are lost while others rearrange to their adult positions, the pharynx
Pharynx
The human pharynx is the part of the throat situated immediately posterior to the mouth and nasal cavity, and anterior to the esophagus and larynx. The human pharynx is conventionally divided into three sections: the nasopharynx , the oropharynx , and the laryngopharynx...
enlarges, and organs called ampullae grow from the body to permanently attach the animal to the substratum. The siphons of the juvenile ascidian become orientated to optimise current flow through the feeding apparatus. Sexual maturity
Sexual maturity
Sexual maturity is the age or stage when an organism can reproduce. It is sometimes considered synonymous with adulthood, though the two are distinct...
can be reached in as little as a few weeks. Since the larva is more advanced than its adult, this type of metamorphosis is called 'retrogressive metamorphosis' as the adult is less advanced to its larva. This feature is a landmark for the 'theory of retrogressive metamorphosis or ascidian larva theory', the true chordates are hypothesised to have evolved from sexually mature larvae.
Colonial species
Colonial ascidians reproduce both asexualAsexual reproduction
Asexual reproduction is a mode of reproduction by which offspring arise from a single parent, and inherit the genes of that parent only, it is reproduction which does not involve meiosis, ploidy reduction, or fertilization. A more stringent definition is agamogenesis which is reproduction without...
ly and sex
Sex
In biology, sex is a process of combining and mixing genetic traits, often resulting in the specialization of organisms into a male or female variety . Sexual reproduction involves combining specialized cells to form offspring that inherit traits from both parents...
ually. Colonies can survive for decades. An ascidian colony consists of individual elements called zooids. Zooids within a colony are usually genetically identical and some have a shared circulation. Different colonial ascidian species produce sexually derived offspring by one of two dispersal strategies- Colonial species are either broadcast spawners (long-range dispersal) or philopatric (very short-range dispersal). Broadcast spawners release sperm and ova into the water column and fertilization occurs near to the parent colonies. Resultant zygotes develop into microscopic larvae that may be carried great distances by oceanic currents. The larvae of sessile forms which survive eventually settle and complete maturation on the substratum- then they may bud asexually to form a colony of zooids.
The picture is more complicated for the philopatrically dispersed ascidians: sperm from a nearby colony (or from a zooid of the same colony) enter the pharyngeal siphon and fertilization takes place within the atrium. Embryos are then brooded within the atrium where embryonic development takes place: this results in macroscopic tadpole-like larvae. When mature, these larvae exit the atrial siphon of the adult and then settle close to the parent colony (often within meters). The combined effect of short sperm range and philopatric larval dispersal results in local population structures of closely related individuals/inbred colonies. Generations of colonies which are restricted in dispersal are thought to accumulate adaptions to local conditions, thereby providing advantages over newcomers.
Trauma or predation often results in fragmentation of a colony into subcolonies. Subsequent zooid replication can lead to coalescence and circulatory fusion of the subcolonies. Closely related colonies which are proximate to each other may also fuse if they coalesce and if they are histocompatible. Ascidians were among the first animals to be able to immunologically recognize self from non-self as a mechanism to prevent unrelated colonies from fusing to them and parasitizing them.
Asexual reproduction
Many colonial sea squirts are also capable of asexual reproduction, although the means of doing so are highly variable between different families. In the simplest forms, the members of the colony are linked only by rootlike projections from their undersides known as stolonStolon
In biology, stolons are horizontal connections between organisms. They may be part of the organism, or of its skeleton; typically, animal stolons are external skeletons.-In botany:...
s. Buds containing food storage cells can develop within the stolons and, when sufficiently separated from the 'parent', may grow into a new adult individual.
In other species, the postabdomen can elongate and break up into a string of separate buds, which can eventually form a new colony. In some, the pharyngeal part of the animal degenerates, and the abdomen breaks up into patches of germinal tissue, each combining parts of the epidermis, peritoneum, and digestive tract, and capable of growing into new individuals.
In yet others, budding begins shortly after the larva has settled onto the substrate. In the family Didemnidae
Didemnidae
Didemnidae is a family of tunicates in the order Enterogona. It describes a group of marine animals.-Genera:*Didemnopsis Hartmeyer, 1903*Didemnum Savigny, 1816*Diplosoma Macdonald, 1859*Echinoclinum Van Name, 1902...
, for instance, the individual essentially splits into two, with the pharynx growing a new digestive tract and the original digestive tract growing a new pharynx.
Fertilization
Sea squirt eggs are surrounded by a fibrous vitellineVitelline
Vitelline can refer to:* Vitelline duct* Vitelline veins* Vitelline membrane...
coat and a layer of follicle
Ovarian follicle
Ovarian follicles are the basic units of female reproductive biology, each of which is composed of roughly spherical aggregations of cells found in the ovary. They contain a single oocyte . These structures are periodically initiated to grow and develop, culminating in ovulation of usually a single...
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
Polyspermy
In biology, polyspermy describes an egg that has been fertilized by more than one sperm. Diploid organisms normally contain two copies of each chromosome, one from each parent. The cell resulting from polyspermy, on the other hand, contains three or more copies of each chromosome -- one from the...
is avoided. After fertilization, free calcium ions are released in the egg cytoplasm
Cytoplasm
The cytoplasm is a small gel-like substance residing between the cell membrane holding all the cell's internal sub-structures , except for the nucleus. All the contents of the cells of prokaryote organisms are contained within the 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
Cytoplasm
The cytoplasm is a small gel-like substance residing between the cell membrane holding all the cell's internal sub-structures , except for the nucleus. All the contents of the cells of prokaryote organisms are contained within the cytoplasm...
: ectoplasm
Ectoplasm
Ectoplasm may refer to:* Ectoplasm , the outer part of the cytoplasm* Ectoplasm , supposed physical substance that manifests as a result of spiritual energy or psychic phenomenon...
containing vesicle
Vesicle (biology)
A vesicle is a bubble of liquid within another liquid, a supramolecular assembly made up of many different molecules. More technically, a vesicle is a small membrane-enclosed sack that can store or transport substances. Vesicles can form naturally because of the properties of lipid membranes , or...
s and fine particles, endoderm
Endoderm
Endoderm is one of the three primary germ cell layers in the very early embryo. The other two layers are the ectoderm and mesoderm , with the endoderm as the intermost layer...
containing yolk platelets, and myoplasm containing pigment granules, mitochondria, and endoplasmic reticulum
Endoplasmic reticulum
The endoplasmic reticulum is an organelle of cells in eukaryotic organisms that forms an interconnected network of tubules, vesicles, and cisternae...
. 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
Dorsum (biology)
In anatomy, the dorsum is the upper side of animals that typically run, fly, or swim in a horizontal position, and the back side of animals that walk upright. In vertebrates the dorsum contains the backbone. The term dorsal refers to anatomical structures that are either situated toward or grow...
side of the embryo. In the second phase, the myoplasm moves to the subequatorial zone and extends into a crescent, which marks the future posterior of the embryo. The ectoplasm with the zygote
Zygote
A zygote , or zygocyte, is the initial cell formed when two gamete cells are joined by means of sexual reproduction. In multicellular organisms, it is the earliest developmental stage of the embryo...
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 larvaLarva
A larva is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle...
e as well as impede enzyme
Enzyme
Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...
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
Seaweed
Seaweed is a loose, colloquial term encompassing macroscopic, multicellular, benthic marine algae. The term includes some members of the red, brown and green algae...
. 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
Dock (maritime)
A dock is a human-made structure or group of structures involved in the handling of boats or ships, usually on or close to a shore.However, the exact meaning varies among different variants of the English language...
, ship hulls, and farmed shellfish
Shellfish
Shellfish is a culinary and fisheries term for exoskeleton-bearing aquatic invertebrates used as food, including various species of molluscs, crustaceans, and echinoderms. Although most kinds of shellfish are harvested from saltwater environments, some kinds are found only in freshwater...
cause significant economic problems, and sea squirt invasions have disrupted the ecosystem
Ecosystem
An ecosystem is a biological environment consisting of all the organisms living in a particular area, as well as all the nonliving , physical components of the environment with which the organisms interact, such as air, soil, water and sunlight....
of several natural sub-tidal areas by smothering native animal species.
Nudibranch
A nudibranch is a member of what is now a taxonomic clade, and what was previously a suborder, of soft-bodied, marine gastropod mollusks which shed their shell after their larval stage. They are noted for their often extraordinary colors and striking forms...
s, flatworms, molluscs, rock crabs, sea star
Sea star
Starfish or sea stars are echinoderms belonging to the class Asteroidea. The names "starfish" and "sea star" essentially refer to members of the class Asteroidea...
s, fish, birds, and sea otters. They are also eaten by humans in many parts of the world, including Japan
Japan
Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...
, Korea
Korea
Korea ) is an East Asian geographic region that is currently divided into two separate sovereign states — North Korea and South Korea. Located on the Korean Peninsula, Korea is bordered by the People's Republic of China to the northwest, Russia to the northeast, and is separated from Japan to the...
, Chile
Chile
Chile ,officially the Republic of Chile , is a country in South America occupying a long, narrow coastal strip between the Andes mountains to the east and the Pacific Ocean to the west. It borders Peru to the north, Bolivia to the northeast, Argentina to the east, and the Drake Passage in the far...
, and Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...
(where they are sold under the name “sea violet”). As chemical defenses, many sea squirts intake and maintain an extremely high concentration of vanadium
Vanadium
Vanadium is a chemical element with the symbol V and atomic number 23. It is a hard, silvery gray, ductile and malleable transition metal. The formation of an oxide layer stabilizes the metal against oxidation. The element is found only in chemically combined form in nature...
in the blood, have a very low pH
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
of the tunic due to acid
Acid
An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...
in easily-ruptured bladder
Urinary bladder
The urinary bladder is the organ that collects urine excreted by the kidneys before disposal by urination. A hollow muscular, and distensible organ, the bladder sits on the pelvic floor...
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.
Fossil record
Despite bearing distinctive aragonite spicules, the fossil record of the sea squirts is almost entirely lacking – a dubious Silurian specimen being the only contender.Culinary
Various Ascidiacea are used as food. Sea pineappleSea pineapple
The sea pineapple is an edible ascidian consumed primarily in Korea, where it is known as meongge , and to a lesser extent in Japan, where it is known as or ....
(Halocynthia roretzi) is cultivated in Japan
Japanese cuisine
Japanese cuisine has developed over the centuries as a result of many political and social changes throughout Japan. The cuisine eventually changed with the advent of the Medieval age which ushered in a shedding of elitism with the age of shogun rule...
(hoya, maboya) and Korea (meongge) and, when eaten raw, has been described by Lonely Planet
Lonely Planet
Lonely Planet is the largest travel guide book and digital media publisher in the world. The company is owned by BBC Worldwide, which bought a 75% share from the founders Maureen and Tony Wheeler in 2007 and the final 25% in February 2011...
as tasting like "rubber dipped in ammonia". The peculiar flavor is attributed to an unsaturated alcohol called cynthiaol.
The Korean fish stew agujjim
Agujjim
Agujjim or agwijjim is a Korean jjim dish made with the fish agwi. It is usually translated as 'braised spicy angler....
traditionally contains the tunicate Styela clava
Styela clava
The Stalked Sea Squirt, Styela clava, is a solitary, hermaphroditic, ascidian tunicate that is found off Australia and New Zealand, both coasts of North America, and Europe....
. According to the LA Weekly, "they are actually farmed in parts of Korea, and sea squirt bibimbap is a specialty of Geojae-do island, not far from Masan."
Microcosmus sabatieri
Microcosmus sabatieri
Microcosmus sabatieri are a species of Tunicates. The species has a rocky-shape appearance. It is mainly found in the Mediterranean Sea.It is used as food in parts of Europe, such as France and Italy, where it is known as a sea violet or sea fig...
and several similar species from the Mediterranean Sea
Mediterranean Sea
The Mediterranean Sea is a sea connected to the Atlantic Ocean surrounded by the Mediterranean region and almost completely enclosed by land: on the north by Anatolia and Europe, on the south by North Africa, and on the east by the Levant...
are eaten in France
French cuisine
French cuisine is a style of food preparation originating from France that has developed from centuries of social change. In the Middle Ages, Guillaume Tirel , a court chef, authored Le Viandier, one of the earliest recipe collections of Medieval France...
(figue de mer, violet), Italy
Italian cuisine
Italian cuisine has developed through centuries of social and political changes, with roots as far back as the 4th century BCE. Italian cuisine in itself takes heavy influences, including Etruscan, ancient Greek, ancient Roman, Byzantine, Jewish and Arab cuisines...
(limone di mare, uova di mare), Greece (fouska, φούσκα). In Greece
Greece
Greece , officially the Hellenic Republic , and historically Hellas or the Republic of Greece in English, is a country in southeastern Europe....
they are consumed just raw with lemon
Lemon
The lemon is both a small evergreen tree native to Asia, and the tree's ellipsoidal yellow fruit. The fruit is used for culinary and non-culinary purposes throughout the world – primarily for its juice, though the pulp and rind are also used, mainly in cooking and baking...
or in salads with olive oil
Olive oil
Olive oil is an oil obtained from the olive , a traditional tree crop of the Mediterranean Basin. It is commonly used in cooking, cosmetics, pharmaceuticals, and soaps and as a fuel for traditional oil lamps...
, lemon
Lemon
The lemon is both a small evergreen tree native to Asia, and the tree's ellipsoidal yellow fruit. The fruit is used for culinary and non-culinary purposes throughout the world – primarily for its juice, though the pulp and rind are also used, mainly in cooking and baking...
and parsley
Parsley
Parsley is a species of Petroselinum in the family Apiaceae, native to the central Mediterranean region , naturalized elsewhere in Europe, and widely cultivated as an herb, a spice and a vegetable.- Description :Garden parsley is a bright green hairless biennial herbaceous plant in temperate...
.
The piure (Pyura chilensis) is used as food in the cuisine of Chile
Cuisine of Chile
Chilean cuisine stems mainly from the combination of Spanish cuisine with traditional Chilean ingredients, with later influences from other European cuisines, particularly from Germany, Italy, Croatia, France and the Middle East. The food tradition and recipes in Chile stand out due to the...
, consumed both raw and used as ingredients in seafood stews like bouillabaisse
Bouillabaisse
Bouillabaisse is a seafood soup made with various kinds of cooked fish and shellfish and vegetables, flavored with a variety of herbs and spices such as garlic, orange peel, basil, bay leaf, fennel and saffron. Bouillabaisse is a traditional Provençal fish stew originating from the port city of...
.
Pyura stolonifera is known as cunjevoi
Cunjevoi
Cunjevoi is a word in Australian English that refers to three species: two related plants of the family Araceae and genus Alocasia, and a marine animal, Pyura stolonifera.-Plants:Cunjevoi can refer to:...
in Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
. This was once used as a food source by Aboriginal people
Australian Aborigines
Australian Aborigines , also called Aboriginal Australians, from the latin ab originem , are people who are indigenous to most of the Australian continentthat is, to mainland Australia and the island of Tasmania...
living around Botany Bay
Botany Bay
Botany Bay is a bay in Sydney, New South Wales, a few kilometres south of the Sydney central business district. The Cooks River and the Georges River are the two major tributaries that flow into the bay...
, but is now used mainly for fishing bait. Note: the word "cunjevoi" is also used for two species of rainforest plant, at least one of which is toxic to humans.
Model organisms for research
Lineage (evolution)
An evolutionary lineage is a sequence of species, that form a line of descent, each new species the direct result of speciation from an immediate ancestral species. Lineages are subsets of the evolutionary tree of life. Lineages are often determined by the techniques of molecular systematics.-...
are easily labeled, allowing scientists to visualize embryogenesis
Embryogenesis
Embryogenesis is the process by which the embryo is formed and develops, until it develops into a fetus.Embryogenesis starts with the fertilization of the ovum by sperm. The fertilized ovum is referred to as a zygote...
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 chordates and ancestral non-chordate deuterostomes, as well as the origination of vertebrates from simple chordates. The sequenced genomes of the related sea squirts Ciona intestinalis
Ciona intestinalis
Ciona intestinalis is a urochordata , a tunicate widely distributed in Northern European waters. As an invasive species, it has also spread to other parts of the world....
and Ciona savignyi are small and easily manipulated; comparisons with the genomes of other organisms such as flies
Fließ
Fließ is a municipality in the Landeck district and is located5 km south of Landeck on the upper course of the Inn River. It has 9 hamlets and was already populated at the roman age; the village itself was founded around the 6th century. After a conflagration in 1933 Fließ was restored more...
, nematodes, pufferfish
Pufferfish
Tetraodontidae is a family of primarily marine and estuarine fish of the Tetraodontiformes order. The family includes many familiar species which are variously called pufferfish, balloonfish, blowfish, bubblefish, globefish, swellfish, toadfish, toadies, honey toads, sugar toads, and sea squab...
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
Gene expression
Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product. These products are often proteins, but in non-protein coding genes such as ribosomal RNA , transfer RNA or small nuclear RNA genes, the product is a functional RNA...
, 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
Morpholino
In molecular biology, a Morpholino is a molecule in a particular structural family that is used to modify gene expression. Morpholino oligomers are an antisense technology used to block access of other molecules to specific sequences within nucleic acid...
oligos.
External links
- Ascidiacea.com – Classification and images of many ascidians.
- The Dutch Ascidians Homepage
- Encyclopedia of Marine Life of Britain and Ireland
- The Organization and Cell-lineage of the Ascidian Egg
- A fate map of the ascidian egg
- Ciona savignyi Database
- ANISEED: Ascidian Network for In Situ Expression and Embryological Data