Tunicate
Encyclopedia
Tunicates, also known as urochordates, are members of the subphylum
Tunicata, previously known as Urochordata, a group of underwater saclike filter feeder
s with incurrent and excurrent siphons
that is classified within the phylum Chordata. While most tunicates live on the ocean floor, others such as salp
s, doliolids
and pyrosomes live above in the pelagic zone
as adults. Various species are commonly known as sea squirts or sea pork.
Most tunicates feed by filtering sea water through pharyngeal slit
s, but some are sub-marine predators
such as the Megalodicopia hians. Like other chordate
s, tunicates have a notochord
during their early development but lack myomeric
segmentation throughout the body and tail
as adults. Tunicates lack the kidney-like metanephridial
organs, and the original coelom
body-cavity develops into a pericardial cavity
and gonads. Except for the pharynx
, heart
and gonads, the organs are enclosed in a membrane called an epicardium
, which is surrounded by the jelly-like mesenchyme
. Tunicates begin life in a mobile larva
l stage that resembles a tadpole
, later developing into a barrel-like and usually sedentary adult form.
Tunicates apparently evolved in the early Cambrian period, beginning some 540 million years ago. Despite their simple appearance, tunicates are closely related to vertebrate
s, which include fish and all land animals with bones.
in 1816. Balfour introduced the name Urochordata in 1881 in order to emphasize the affinity of the group to other chordate
s but this was unnecessary as Tunicata was a pre-existing and perfectly satisfactory name. The name Tunicata is almost invariably used to refer to this group of organisms in scientific works. It is accepted as valid by the World Register of Marine Species and by ITIS, the Integrated Taxonomic Information System.
s. The eggs are kept inside their body until they hatch, while sperm is released into the water where it fertilizes other individuals when brought in with incoming water.
Some larval forms appear very much like primitive chordate
s with a notochord
(stiffening rod). Superficially, the larva
resemble small tadpole
s. They swim with a tail, and may have a simple eye, or ocellus
, and balancing organ, or statolith. Some forms have a calcareous spicule
that may be preserved as a fossil
. They have appeared from the Jurassic
to the present, with one proposed Neoproterozoic
form, Yarnemia
.
The larval form ends when the tunicate finds a suitable rock to affix to and cements itself in place. The larval form is not capable of feeding, though it may have a digestive system, and is only a dispersal mechanism. Many physical changes occur to the tunicate's body, one of the most interesting being the digestion of the cerebral ganglion, which controls movement and is the equivalent of the human brain. From this comes the common saying that the sea squirt "eats its own brain". In some classes, the adults remain pelagic (swimming or drifting in the open sea), although their larvae undergo similar metamorphoses to a higher or lower degree.
Once grown, adults can develop a thick covering, called a tunic, to protect their barrel-shaped bodies from enemies.
During embryonic development, tunicates exhibit "determinate cleavage", where the fate of the cells is set early on with reduced cell numbers and genome
s that are rapidly evolving. In contrast, the amphioxus and vertebrates show cell determination relatively late in development and cell cleavage is indeterminate. The genome evolution of amphioxus and vertebrates is also relatively slow.
Tunicates are suspension feeders. They have two openings in their body cavity: an in-current and an ex-current siphon. The in-current siphon is used to intake food and water, and the ex-current siphon expels waste and water. The tunicate's primary food source is plankton
. Plankton gets entangled in the mucus
secreted from the endostyle
. The tunicate's pharynx
is covered by miniature hairs called ciliate cells which allow the consumed plankton to pass down through to the esophagus
. Their guts are U-shaped, and their anuses empty directly to the outside environment. Tunicates are also the only animals able to create cellulose
.
Tunicate blood
is particularly interesting. It contains high concentrations of the transition metal vanadium
and vanadium-associated proteins
as well as higher than usual levels of lithium
. Some tunicates can concentrate vanadium up to a level one million times that of the surrounding seawater. Specialized cells can concentrate heavy metals, which are then deposited in the tunic.
, lamprey
s, and jawed vertebrate
s) than to lancelet
s, echinoderm
s, hemichordates, Xenoturbella
or other invertebrate
s.
The clade
comprising Tunicates and Vertebrates is called Olfactores.
The Tunicata contains about 3,000 species, usually divided into the following classes:
Although the traditional classification is followed for now, newer evidence suggests that the Ascidiacea is an artificial group of paraphyletic status.
The new classification would be:
The species Ciona intestinalis
and Ciona savignyi have attracted interest in biology for developmental studies. Both species' mitochondrial
and nuclear
genomes have been sequenced. Moreover, the nuclear genome of the appendicularian Oikopleura dioica appears to be one of the smallest among metazoans.
Sea squirts have become a testing ground in the controversy about the extent to which cross-species gene transfer
and hybridization have influenced animal evolution. In 1990, Donald I. Williamson
of the University of Liverpool
(U.K.) fertilised sea squirt (Ascidia mentula) eggs with sea urchin
(Echinus esculentus) sperm resulting in fertile adults that resembled urchins, but Michael W. Hart of Simon Fraser University
failed to find sea-squirt DNA in tissue samples from the supposed hybrids. Williamson claims to have repeated the experiment with sea urchin eggs and sea squirt sperm, producing sea urchin larvae which developed into squirt-like juveniles. On the other hand, Syvanen and Ducore of the University of California
have suggested that sea squirts descended from a hybrid between a chordate and a likely extinct protostome ancestor at a time before the diversification of round worms and arthropods. This study also examined whether there was evidence of a sea urchin/tunicate hybridization event that could possibly explain the distribution of genes in modern sea squirts—none could be seen.
Undisputed fossils of tunicates are rare. The best known (and earliest) is Shankouclava shankouense from the Lower Cambrian
Maotianshan Shale at Shankou village, Anning, near Kunming
(South China
).
There is also a common bioimmuration of a tunicate (Catellocaula vallata) found in Upper Ordovician
bryozoa
n skeletons of the upper midwestern United States.
There are also two enigmatic species from the Ediacaran
period - Ausia fenestrata from the Nama Group of Namibia
and a second new Ausia-like genus from the Onega Peninsula, White Sea
of northern Russia
. Results of new study have shown possible affinity of these Ediacaran organisms to the ascidians. These two organisms lived in the shallow waters of a sea, slightly more than 555-548 million years ago and are likely the oldest evidence of the chordate lineage of metazoans.
A Precambrian fossil known as Yarnemia has been referred to the Urochordata, however this assignment is doubtful. Complete body fossils of tunicates are rare, but in some tunicate families, microscopic spicules are generated which may be preserved as microfossils. Such spicules have occasionally been described from Jurassic and later rocks. Few paleontologists are familiar with them; tunicate spicules may be mistaken for sponge spicules.
and Styela
) have been invading
coastal waters in many countries, and are spreading quickly. These mat-like organisms can smother other sea life, have very few natural predators, and are causing much concern. They form colonies which are yellowish cream in color, and look like thick sponge-like masses that overgrow themselves on stationary objects on the sea floor such as gravel, mollusc shells, and possibly other encrusting species. These colonies are flexible, irregular, long, flat, and often exist as branched outgrowths projected from the surface. Some of the outgrowths result from the colony encrusting worm tubes or other cylindrical objects but many are solid with a firm gelatinous core. The individuals of the colony are called zooids and many zooids with individual siphonal openings cover the surface of the colony.
Transportation of invasive tunicates is usually in the ballast water or on the hulls of ships. Current research indicates that many tunicates previously thought to be indigenous to Europe and the Americas are, in fact, invaders. Some of these invasions may have occurred centuries or even millennia ago. In some areas, tunicates are proving to be a major threat to aquaculture
operations.
The U.S. Geological Survey
, NOAA Fisheries , and the University of Rhode Island
are investigating this phenomenon as they have been spotted in 2004 in Georges Bank
. They requested that any information or sightings of these invading colonies be reported to United States Geological Survey
to aid in their investigation.
s, including:
In the May 2007 issue of The FASEB Journal, researchers from Stanford University
showed that tunicates can correct abnormalities over a series of generations, and they suggest that a similar regenerative process may be possible for humans. The mechanisms underlying the phenomenon may lead to insights about the potential of cells and tissues to be reprogrammed and regenerate compromised human organs. Gerald Weissman, editor-in-chief of the journal, said "This study is a landmark in regenerative medicine; the Stanford group has accomplished the biological equivalent of turning a sow's ear into a silk purse and back again."
species are consumed as food around the world.
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...
Tunicata, previously known as Urochordata, a group of underwater saclike filter feeder
Filter feeder
Filter feeders are animals that feed by straining suspended matter and food particles from water, typically by passing the water over a specialized filtering structure. Some animals that use this method of feeding are clams, krill, sponges, baleen whales, and many fish and some sharks. Some birds,...
s with incurrent and excurrent siphons
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...
that is classified within the phylum Chordata. While most tunicates live on the ocean floor, others such as salp
Salp
A salp or salpa is a barrel-shaped, planktonic tunicate. It moves by contracting, thus pumping water through its gelatinous body...
s, doliolids
Doliolida
Doliolida are small marine animals of the Tunicata phylum, related to salps and pyrosomas. The Doliolid body is small, typically 1–2 cm long, and barrel-shaped; it features two wide siphons, one at the front and the other at the back end, and eight or nine circular muscle strands reminiscent...
and pyrosomes live above in the pelagic zone
Pelagic zone
Any water in a sea or lake that is not close to the bottom or near to the shore can be said to be in the pelagic zone. The word pelagic comes from the Greek πέλαγος or pélagos, which means "open sea". The pelagic zone can be thought of in terms of an imaginary cylinder or water column that goes...
as adults. Various species are commonly known as sea squirts or sea pork.
Most tunicates feed by filtering sea water through pharyngeal slit
Pharyngeal slit
Pharyngeal slits are filter-feeding organs found in non-vertebrate chordates and hemichordates living in aquatic environments. These repeated segments are controlled by similar developmental mechanisms. Some hemichordate species can have as many as 200 gill slits...
s, but some are sub-marine predators
Predation
In ecology, predation describes a biological interaction where a predator feeds on its prey . Predators may or may not kill their prey prior to feeding on them, but the act of predation always results in the death of its prey and the eventual absorption of the prey's tissue through consumption...
such as the Megalodicopia hians. Like other 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...
s, tunicates have 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...
during their early development but lack myomeric
Myomere
Myomere are the blocks of skeletal muscle tissue found commonly in chordates. They are commonly zig-zag, "W" or "V"-shaped muscle fibers. The myomeres are separated from adjacent myomere by connective tissues and most easily seen in larval fishes or in the olm...
segmentation throughout the body and tail
Tail
The tail is the section at the rear end of an animal's body; in general, the term refers to a distinct, flexible appendage to the torso. It is the part of the body that corresponds roughly to the sacrum and coccyx in mammals, reptiles, and birds...
as adults. Tunicates lack the kidney-like metanephridial
Nephridium
A Nephridium is an invertebrate organ which occurs in pairs and function similar to kidneys. Nephridia remove metabolic wastes from an animal's body. They are present in many different invertebrate lines. There are two basic types, metanephridia and protonephridia, but there are other...
organs, and the original coelom
Coelom
The coelom is a fluid-filled cavity formed within the mesoderm. Coeloms developed in triploblasts but were subsequently lost in several lineages. Loss of coelom is correlated with reduction in body size...
body-cavity develops into a pericardial cavity
Pericardial cavity
The pericardial cavity is a potential space between the parietal pericardium and visceral layer. It contains a supply of serous fluid. The serous fluid that is found in this space is known as the pericardial fluid....
and gonads. Except for 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...
, heart
Heart
The heart is a myogenic muscular organ found in all animals with a circulatory system , that is responsible for pumping blood throughout the blood vessels by repeated, rhythmic contractions...
and gonads, the organs are enclosed in a membrane called an epicardium
Epicardium
Epicardium describes the outer layer of heart tissue . When considered as a part of the pericardium, it is the inner layer, or visceral pericardium, continuous with the serous layer....
, which is surrounded by the jelly-like mesenchyme
Mesenchyme
Mesenchyme, or mesenchymal connective tissue, is a type of undifferentiated loose connective tissue that is derived mostly from mesoderm, although some are derived from other germ layers; e.g. some mesenchyme is derived from neural crest cells and thus originates from the ectoderm...
. Tunicates begin life in a mobile 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...
l stage that resembles a 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 :...
, later developing into a barrel-like and usually sedentary adult form.
Tunicates apparently evolved in the early Cambrian period, beginning some 540 million years ago. Despite their simple appearance, tunicates are closely related to vertebrate
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...
s, which include fish and all land animals with bones.
Taxonomy
Urochordata is a junior synonym of the name Tunicata which was established by LamarckJean-Baptiste Lamarck
Jean-Baptiste Pierre Antoine de Monet, Chevalier de la Marck , often known simply as Lamarck, was a French naturalist...
in 1816. Balfour introduced the name Urochordata in 1881 in order to emphasize the affinity of the group to other 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...
s but this was unnecessary as Tunicata was a pre-existing and perfectly satisfactory name. The name Tunicata is almost invariably used to refer to this group of organisms in scientific works. It is accepted as valid by the World Register of Marine Species and by ITIS, the Integrated Taxonomic Information System.
Life cycle
Most tunicates are hermaphroditeHermaphrodite
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. The eggs are kept inside their body until they hatch, while sperm is released into the water where it fertilizes other individuals when brought in with incoming water.
Some larval forms appear very much like primitive 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...
s with 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...
(stiffening rod). Superficially, the 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...
resemble small 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 :...
s. They swim with a tail, and may have a simple eye, or ocellus
Ocellus
A simple eye refers to a type of eye design or optical arrangement that contains a single lens which detect light. A "simple eye" is so-called in distinction from a multi-lensed "compound eye", and is not necessarily at all simple in the usual sense of the word...
, and balancing organ, or statolith. Some forms have a calcareous spicule
Spicule
Spicules are tiny spike-like structures of diverse origin and function found in many organisms, such as the copulatory spicules of certain nematodes or the grains on the skin of some frogs.In sponges, spicules perform a structural function....
that may be preserved as a fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
. They have appeared from the Jurassic
Jurassic
The Jurassic is a geologic period and system that extends from about Mya to Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...
to the present, with one proposed Neoproterozoic
Neoproterozoic
The Neoproterozoic Era is the unit of geologic time from 1,000 to 542.0 ± 1.0 million years ago. The terminal Era of the formal Proterozoic Eon , it is further subdivided into the Tonian, Cryogenian, and Ediacaran Periods...
form, Yarnemia
Yarnemia
Yarnemia is a fossil tentatively classified as a tunicate. While tunicates and Yarnemia look similar, the oldest commonly accepted tunicate dates to the Jurassic period, while Yarnemia is Ediacaran....
.
The larval form ends when the tunicate finds a suitable rock to affix to and cements itself in place. The larval form is not capable of feeding, though it may have a digestive system, and is only a dispersal mechanism. Many physical changes occur to the tunicate's body, one of the most interesting being the digestion of the cerebral ganglion, which controls movement and is the equivalent of the human brain. From this comes the common saying that the sea squirt "eats its own brain". In some classes, the adults remain pelagic (swimming or drifting in the open sea), although their larvae undergo similar metamorphoses to a higher or lower degree.
Once grown, adults can develop a thick covering, called a tunic, to protect their barrel-shaped bodies from enemies.
During embryonic development, tunicates exhibit "determinate cleavage", where the fate of the cells is set early on with reduced cell numbers and genome
Genome
In modern molecular biology and genetics, the genome is the entirety of an organism's hereditary information. It is encoded either in DNA or, for many types of virus, in RNA. The genome includes both the genes and the non-coding sequences of the DNA/RNA....
s that are rapidly evolving. In contrast, the amphioxus and vertebrates show cell determination relatively late in development and cell cleavage is indeterminate. The genome evolution of amphioxus and vertebrates is also relatively slow.
Feeding
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...
. Plankton gets entangled in the mucus
Mucus
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...
secreted from the 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...
. The tunicate's 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...
is covered by miniature hairs called ciliate cells which allow the consumed plankton to pass down through to the esophagus
Esophagus
The esophagus is an organ in vertebrates which consists of a muscular tube through which food passes from the pharynx to the stomach. During swallowing, food passes from the mouth through the pharynx into the esophagus and travels via peristalsis to the stomach...
. Their guts are U-shaped, and their anuses empty directly to the outside environment. Tunicates are also the only animals able to create cellulose
Cellulose
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....
.
Tunicate blood
Blood
Blood is a specialized bodily fluid in animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells....
is particularly interesting. It contains high concentrations of the transition metal 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...
and vanadium-associated proteins
Vanabins
Vanabins are a specific group of vanadium-binding metalloproteins. Vanabins are found almost exclusively in the blood cells, or vanadocytes of some ascidians and tunicates . The vanabins extracted from tunicate vanadocytes are often called hemovanadins...
as well as higher than usual levels of lithium
Lithium
Lithium is a soft, silver-white metal that belongs to the alkali metal group of chemical elements. It is represented by the symbol Li, and it has the atomic number 3. Under standard conditions it is the lightest metal and the least dense solid element. Like all alkali metals, lithium is highly...
. Some tunicates can concentrate vanadium up to a level one million times that of the surrounding seawater. Specialized cells can concentrate heavy metals, which are then deposited in the tunic.
Classification
Tunicates are more closely related to craniates (including hagfishHagfish
Hagfish, the clade Myxini , are eel-shaped slime-producing marine animals . They are the only living animals that have a skull but not a vertebral column. Along with lampreys, hagfish are jawless and are living fossils whose next nearest relatives include all vertebrates...
, lamprey
Lamprey
Lampreys are a family of jawless fish, whose adults are characterized by a toothed, funnel-like sucking mouth. Translated from an admixture of Latin and Greek, lamprey means stone lickers...
s, and jawed vertebrate
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...
s) than to lancelet
Lancelet
The lancelets , also known as amphioxus, are the modern representatives of the subphylum Cephalochordata, formerly thought to be the sister group of the craniates. They are usually found buried in sand in shallow parts of temperate or tropical seas. In Asia, they are harvested commercially as food...
s, echinoderm
Echinoderm
Echinoderms are a phylum of marine animals. Echinoderms are found at every ocean depth, from the intertidal zone to the abyssal zone....
s, hemichordates, Xenoturbella
Xenoturbella
Xenoturbella is a genus of bilaterian animals; it contains two marine worm-like species. The first known species was discovered in 1915 by Sixten Bock but the first published description was only in 1949 by Einar Westblad...
or other 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...
s.
The clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
comprising Tunicates and Vertebrates is called Olfactores.
The Tunicata contains about 3,000 species, usually divided into the following classes:
- AscidiaceaAscidiaceaAscidiacea 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...
(AplousobranchiaAplousobranchiaAplousobranchia is a suborder of sea squirts in the order Enterogona. They are colonial animals, and are distinguished from other sea squirts by the presence of relatively simple pharyngeal baskets...
, PhlebobranchiaPhlebobranchiaPhlebobranchia is a suborder of sea squirts in the order Enterogona. The group includes both colonial and solitary animals. They are distinguished from other sea squirts by the presence of longitudinal vessels in the pharyngeal basket and by the gonads being surrounded by a loop of gut...
, and StolidobranchiaStolidobranchiaStolidobranchia is an order of tunicates in the class Ascidiacea. The group includes both colonial and solitary animals. They are distinguished from other tunicates by the presence of folded pharyngeal baskets and the complete absence of an abdomen...
) - ThaliaceaThaliaceaThe 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:...
(Pyrosomida, DoliolidaDoliolidaDoliolida are small marine animals of the Tunicata phylum, related to salps and pyrosomas. The Doliolid body is small, typically 1–2 cm long, and barrel-shaped; it features two wide siphons, one at the front and the other at the back end, and eight or nine circular muscle strands reminiscent...
, and SalpSalpA salp or salpa is a barrel-shaped, planktonic tunicate. It moves by contracting, thus pumping water through its gelatinous body...
ida) - Appendicularia (LarvaceaLarvaceaLarvaceans 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...
) - SorberaceaSorberaceaSorberacea are benthic animals and a subgroup of the Tunicata. They superficially resemble sea squirts but prey on invertebrates such as crustaceans...
Although the traditional classification is followed for now, newer evidence suggests that the Ascidiacea is an artificial group of paraphyletic status.
The new classification would be:
- Stolidobranchia
- Aplousobranchia, Phlebobranchia and Thaliacea
- Appendicularia
- Sorberacea would belong somewhere in Ascidiacea, or be in a taxon on its own
The species 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 have attracted interest in biology for developmental studies. Both species' mitochondrial
and nuclear
genomes have been sequenced. Moreover, the nuclear genome of the appendicularian Oikopleura dioica appears to be one of the smallest among metazoans.
Sea squirts have become a testing ground in the controversy about the extent to which cross-species gene transfer
Horizontal gene transfer
Horizontal gene transfer , also lateral gene transfer , is any process in which an organism incorporates genetic material from another organism without being the offspring of that organism...
and hybridization have influenced animal evolution. In 1990, Donald I. Williamson
Donald I. Williamson
Donald Irving Williamson is a British planktologist and carcinologist, born 8 January 1922, Alnham, Northumberland, England. He gained his first degree from the Durham University in 1942, his Ph.D. from the same university in 1948, and a D.Sc. from the Newcastle University in 1972...
of the University of Liverpool
University of Liverpool
The University of Liverpool is a teaching and research university in the city of Liverpool, England. It is a member of the Russell Group of large research-intensive universities and the N8 Group for research collaboration. Founded in 1881 , it is also one of the six original "red brick" civic...
(U.K.) fertilised sea squirt (Ascidia mentula) eggs with sea urchin
Sea urchin
Sea urchins or urchins are small, spiny, globular animals which, with their close kin, such as sand dollars, constitute the class Echinoidea of the echinoderm phylum. They inhabit all oceans. Their shell, or "test", is round and spiny, typically from across. Common colors include black and dull...
(Echinus esculentus) sperm resulting in fertile adults that resembled urchins, but Michael W. Hart of Simon Fraser University
Simon Fraser University
Simon Fraser University is a Canadian public research university in British Columbia with its main campus on Burnaby Mountain in Burnaby, and satellite campuses in Vancouver and Surrey. The main campus in Burnaby, located from downtown Vancouver, was established in 1965 and has more than 34,000...
failed to find sea-squirt DNA in tissue samples from the supposed hybrids. Williamson claims to have repeated the experiment with sea urchin eggs and sea squirt sperm, producing sea urchin larvae which developed into squirt-like juveniles. On the other hand, Syvanen and Ducore of the University of California
University of California
The University of California is a public university system in the U.S. state of California. Under the California Master Plan for Higher Education, the University of California is a part of the state's three-tier public higher education system, which also includes the California State University...
have suggested that sea squirts descended from a hybrid between a chordate and a likely extinct protostome ancestor at a time before the diversification of round worms and arthropods. This study also examined whether there was evidence of a sea urchin/tunicate hybridization event that could possibly explain the distribution of genes in modern sea squirts—none could be seen.
Fossil record
Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
Maotianshan Shale at Shankou village, Anning, near Kunming
Kunming
' is the capital and largest city of Yunnan Province in Southwest China. It was known as Yunnan-Fou until the 1920s. A prefecture-level city, it is the political, economic, communications and cultural centre of Yunnan, and is the seat of the provincial government...
(South China
Northern and southern China
Northern China and southern China are two approximate regions within China. The exact boundary between these two regions has never been precisely defined...
).
There is also a common bioimmuration of a tunicate (Catellocaula vallata) found in Upper Ordovician
Ordovician
The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...
bryozoa
Bryozoa
The Bryozoa, also known as Ectoprocta or commonly as moss animals, are a phylum of aquatic invertebrate animals. Typically about long, they are filter feeders that sieve food particles out of the water using a retractable lophophore, a "crown" of tentacles lined with cilia...
n skeletons of the upper midwestern United States.
There are also two enigmatic species from the Ediacaran
Ediacaran
The Ediacaran Period , named after the Ediacara Hills of South Australia, is the last geological period of the Neoproterozoic Era and of the Proterozoic Eon, immediately preceding the Cambrian Period, the first period of the Paleozoic Era and of the Phanerozoic Eon...
period - Ausia fenestrata from the Nama Group of Namibia
Namibia
Namibia, officially the Republic of Namibia , is a country in southern Africa whose western border is the Atlantic Ocean. It shares land borders with Angola and Zambia to the north, Botswana to the east and South Africa to the south and east. It gained independence from South Africa on 21 March...
and a second new Ausia-like genus from the Onega Peninsula, White Sea
White Sea
The White Sea is a southern inlet of the Barents Sea located on the northwest coast of Russia. It is surrounded by Karelia to the west, the Kola Peninsula to the north, and the Kanin Peninsula to the northeast. The whole of the White Sea is under Russian sovereignty and considered to be part of...
of northern Russia
Russia
Russia or , officially known as both Russia and the Russian Federation , is a country in northern Eurasia. It is a federal semi-presidential republic, comprising 83 federal subjects...
. Results of new study have shown possible affinity of these Ediacaran organisms to the ascidians. These two organisms lived in the shallow waters of a sea, slightly more than 555-548 million years ago and are likely the oldest evidence of the chordate lineage of metazoans.
A Precambrian fossil known as Yarnemia has been referred to the Urochordata, however this assignment is doubtful. Complete body fossils of tunicates are rare, but in some tunicate families, microscopic spicules are generated which may be preserved as microfossils. Such spicules have occasionally been described from Jurassic and later rocks. Few paleontologists are familiar with them; tunicate spicules may be mistaken for sponge spicules.
Invasive species
Over the past few years, urochordates (notably of the genera DidemnumDidemnum
Didemnum is a genus of tunicates in the family Didemnidae. Some species of Didemnum can be characterized as invasive species. This is particularly an issue off the east coast of the United States, where in early 2006, Didemnum were estimated to inhabit more than 175 square kilometers of this...
and Styela
Pleurogona
Pleurogona is an order of tunicates. One of the more invasive species of this order is the "stalked sea squirt" Styela clava . Another invasive genus include the "chain sea squirts or chain tunicates" of the genus Botrylloides and the "golden star tunicate" Botryllus schlosseri ....
) have been invading
Invasive species
"Invasive species", or invasive exotics, is a nomenclature term and categorization phrase used for flora and fauna, and for specific restoration-preservation processes in native habitats, with several definitions....
coastal waters in many countries, and are spreading quickly. These mat-like organisms can smother other sea life, have very few natural predators, and are causing much concern. They form colonies which are yellowish cream in color, and look like thick sponge-like masses that overgrow themselves on stationary objects on the sea floor such as gravel, mollusc shells, and possibly other encrusting species. These colonies are flexible, irregular, long, flat, and often exist as branched outgrowths projected from the surface. Some of the outgrowths result from the colony encrusting worm tubes or other cylindrical objects but many are solid with a firm gelatinous core. The individuals of the colony are called zooids and many zooids with individual siphonal openings cover the surface of the colony.
Transportation of invasive tunicates is usually in the ballast water or on the hulls of ships. Current research indicates that many tunicates previously thought to be indigenous to Europe and the Americas are, in fact, invaders. Some of these invasions may have occurred centuries or even millennia ago. In some areas, tunicates are proving to be a major threat to aquaculture
Aquaculture
Aquaculture, also known as aquafarming, is the farming of aquatic organisms such as fish, crustaceans, molluscs and aquatic plants. Aquaculture involves cultivating freshwater and saltwater populations under controlled conditions, and can be contrasted with commercial fishing, which is the...
operations.
The U.S. Geological Survey
Geological survey
The term geological survey can be used to describe both the conduct of a survey for geological purposes and an institution holding geological information....
, NOAA Fisheries , and the University of Rhode Island
University of Rhode Island
The University of Rhode Island is the principal public research university in the U.S. state of Rhode Island. Its main campus is located in Kingston. Additional campuses include the Feinstein Campus in Providence, the Narragansett Bay Campus in Narragansett, and the W. Alton Jones Campus in West...
are investigating this phenomenon as they have been spotted in 2004 in Georges Bank
Georges Bank
Georges Bank is a large elevated area of the sea floor which separates the Gulf of Maine from the Atlantic Ocean and is situated between Cape Cod, Massachusetts and Cape Sable Island, Nova Scotia ....
. They requested that any information or sightings of these invading colonies be reported to United States Geological Survey
United States Geological Survey
The United States Geological Survey is a scientific agency of the United States government. The scientists of the USGS study the landscape of the United States, its natural resources, and the natural hazards that threaten it. The organization has four major science disciplines, concerning biology,...
to aid in their investigation.
Medical uses
Tunicates contain a host of potentially useful chemical compoundChemical compound
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...
s, including:
- Didemnins, effective against various types of cancerCancerCancer , known medically as a malignant neoplasm, is a large group of different diseases, all involving unregulated cell growth. In cancer, cells divide and grow uncontrollably, forming malignant tumors, and invade nearby parts of the body. The cancer may also spread to more distant parts of the...
, as antiviralsAntiviral drugAntiviral drugs are a class of medication used specifically for treating viral infections. Like antibiotics for bacteria, specific antivirals are used for specific viruses...
and immunosuppressantImmunosuppressantAn immunosuppressant is any substance that performs immunosuppression of the immune system. They may be either exogenous, as immunosuppressive drugs, or endogenous, as ,e. g., testosterone...
s - AplidineAplidineAplidine, also known as dihydrodidemnin B, is a chemical compound extracted from the ascidian Aplidium albicans. It is currently undergoing clinical trial testing...
, effective against various types of cancer - TrabectedinTrabectedinTrabectedin is an anti-tumor drug. It is sold by Zeltia and Johnson and Johnson under the brand name Yondelis. It is approved for use in Europe, Russia and South Korea for the treatment of advanced soft tissue sarcoma. It is also undergoing clinical trials for the treatment of breast, prostate,...
, effective against various types of cancer
In the May 2007 issue of The FASEB Journal, researchers from Stanford University
Stanford University
The Leland Stanford Junior University, commonly referred to as Stanford University or Stanford, is a private research university on an campus located near Palo Alto, California. It is situated in the northwestern Santa Clara Valley on the San Francisco Peninsula, approximately northwest of San...
showed that tunicates can correct abnormalities over a series of generations, and they suggest that a similar regenerative process may be possible for humans. The mechanisms underlying the phenomenon may lead to insights about the potential of cells and tissues to be reprogrammed and regenerate compromised human organs. Gerald Weissman, editor-in-chief of the journal, said "This study is a landmark in regenerative medicine; the Stanford group has accomplished the biological equivalent of turning a sow's ear into a silk purse and back again."
As food
Various AscidiaceaAscidiacea
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...
species are consumed as food around the world.