Urochordata Lankester 1877
A TUNICATE is a marine invertebrate animal, a member of the subphylum
TUNICATA, which is part of the
Chordata , a phylum which includes all
animals with dorsal nerve cords and notochords . The subphylum was at
one time called UROCHORDATA, and the term UROCHORDATES is still
sometimes used for these animals. Some tunicates live as solitary
individuals, but others replicate by budding and become colonies ,
each unit being known as a zooid . They are marine filter feeders with
a water-filled, sac-like body structure and two tubular openings,
known as siphons, through which they draw in and expel water. During
their respiration and feeding, they take in water through the
incurrent (or inhalant) siphon and expel the filtered water through
the excurrent (or exhalant) siphon. Most adult tunicates are sessile ,
and are permanently attached to rocks or other hard surfaces on the
ocean floor; others, such as salps , doliolids and pyrosomes , swim in
the pelagic zone of the sea as adults. Various species are commonly
known as sea squirts , sea pork, sea livers, or sea tulips.
The earliest probable species of tunicate appears in the fossil
record in the early
Cambrian period . Despite their simple appearance
and very different adult form, their close relationship to the
vertebrates is evidenced by the fact that during their mobile larval
stage, they possess a notochord or stiffening rod and resemble a
tadpole . Their name derives from their unique outer covering or
"tunic", which is formed from proteins and carbohydrates, and acts as
an exoskeleton . In some species, it is thin, translucent, and
gelatinous, while in others it is thick, tough, and stiff.
* 1 Taxonomy
* 1.1 Classification
* 1.2 Fossil record
* 1.3 Hybridization studies
* 2 Anatomy
* 2.1 Body form
* 2.2 Body structure
* 2.3 Physiology and internal anatomy
* 3 Feeding
* 4 Life cycle
* 4.1 Promotion of out-crossing
* 4.2 A model tunicate
* 6 Use by humans
* 6.1 Medical uses
* 6.2 As food
* 6.3 Other uses
* 7 See also
* 8 References
* 9 External links
Clavelina moluccensis , the bluebell tunicate
Botrylloides violaceus showing oral tentacles at openings of buccal
About 2,150 species of tunicate exist in the world's oceans, living
mostly in shallow water. The most numerous group is the ascidians ;
fewer than 100 species of these are found at depths greater than 200 m
(660 ft). Some are solitary animals leading a sessile existence
attached to the seabed, but others are colonial and a few are pelagic
. Some are supported by a stalk, but most are attached directly to a
substrate , which may be a rock, shell, coral, seaweed, mangrove root,
dock, piling, or ship's hull. They are found in a range of solid or
translucent colours and may resemble seeds, grapes, peaches, barrels,
or bottles. One of the largest is a stalked sea tulip, Pyura
pachydermatina , which can grow to be over 1 metre (3.3 ft) tall.
The Tunicata were established by
Jean-Baptiste Lamarck in 1816. In
Francis Maitland Balfour introduced a second name for the same
group, "Urochorda", to emphasize the affinity of the group to other
chordates. No doubt largely because of his influence, various authors
supported the term, either as such, or as "Urochordata", but this
usage is invalid because "Tunicata" has precedence, and grounds for
superseding the name never existed. Accordingly, the current (formally
correct) trend is to abandon the name Urochorda or Urochordata in
favour of the original Tunicata, and the name Tunicata is almost
invariably used in modern scientific works. It is accepted as valid by
World Register of Marine Species but not by the Integrated
Taxonomic Information System.
Various common names are used for different species. Sea tulips are
tunicates with colourful bodies supported on slender stalks. Sea
squirts are so named because of their habit of contracting their
bodies sharply and squirting out water when disturbed. Sea liver and
sea pork get their names from the resemblance of their dead colonies
to pieces of meat.
Tunicates are more closely related to craniates , (including hagfish
, lampreys , and jawed vertebrates ) than to lancelets , echinoderms ,
Xenoturbella or other invertebrates .
The clade consisting of tunicates and vertebrates is called
The Tunicata contain roughly 3,051 described species, traditionally
divided into these classes:
Phlebobranchia , and Stolidobranchia
Thaliacea (Pyrosomida ,
Doliolida , and
Members of the
Sorberacea were included in
Ascidiacea in 2011 as a
result of rDNA sequencing studies. Although the traditional
classification is provisionally accepted, newer evidence suggests the
Ascidiacea are an artificial group of paraphyletic status.
The star-shaped holes (Catellocaula vallata) in this Upper
Ordovician bryozoan may represent a tunicate preserved by
bioimmuration in the bryozoan skeleton.
Undisputed fossils of tunicates are rare. The best known and earliest
unequivocally identified species is Shankouclava shankouense from the
Maotianshan Shale at Shankou village, Anning, near
Kunming (South China ). There is also a common bioimmuration ,
(Catellocaula vallata), of a possible tunicate found in Upper
Ordovician bryozoan skeletons of the upper midwestern United States.
Three enigmatic species were also found from the
Ediacaran period –
Ausia fenestrata from the Nama Group of
Namibia , the sac-like
Yarnemia acidiformis , and one from a second new Ausia-like genus from
the Onega Peninsula of northern
Burykhia hunti . Results of a
new study have shown possible affinity of these
Ediacaran organisms to
the ascidians. Ausia and
Burykhia lived in shallow coastal waters
slightly more than 555 to 548 million years ago, and are believed to
be the oldest evidence of the chordate lineage of metazoans. The
Yarnemia is tentatively identified as a
tunicate, although, because its fossils are nowhere near as
well-preserved as those of Ausia and Burykhia, this identification is
Fossils of tunicates are rare because their bodies decay soon after
death, but in some tunicate families, microscopic spicules are
present, which may be preserved as microfossils. These spicules have
occasionally been found in
Jurassic and later rocks, but as few
palaeontologists are familiar with them, they may have been mistaken
for sponge spicules .
A multi-taxon molecular study in 2010 proposed that sea squirts are
descended from a hybrid between a chordate and a protostome ancestor.
This study was based on a quartet partitioning approach designed to
reveal horizontal gene transfer events among metazoan phyla
Colonial tunicate with multiple openings in a single tunic
Colonies of tunicates occur in a range of forms, and vary in the
degree to which individual organisms, known as zooids , integrate with
one another. In the simplest systems, the individual animals are
widely separated, but linked together by horizontal connections called
stolons , which grow along the seabed. Other species have the zooids
growing closer together in a tuft or clustered together and sharing a
common base. The most advanced colonies involve the integration of the
zooids into a common structure surrounded by the tunic. These may have
separate buccal siphons and a single central atrial siphon and may be
organized into larger systems, with hundreds of star-shaped units.
Often, the zooids in a colony are tiny but very numerous, and the
colonies can form large encrusting or mat-like patches.
A. Lancelet, B. Larval tunicate, C. Adult tunicate. 1.
Nerve chord, 3. Buccal cirri, 4. Pharynx, 5. Gill slit,
6. Gonad, 7. Gut, 8. V-shaped muscles, 9. Anus, 10. Inhalant syphon,
11. Exhalant syphon, 12. Heart, 13. Stomach, 14. Esophagus, 15.
Intestines, 16. Tail, 17. Atrium, 18. Tunic.
By far the largest class of tunicates is the
Ascidiacea . The body of
an ascidiacean is surrounded by a test or tunic, from which the
subphylum derives its name. This varies in thickness between species
but may be tough, resembling cartilage, thin and delicate, or
transparent and gelatinous. The tunic is composed of proteins and
complex carbohydrates, and includes tunicin , a variety of cellulose.
The tunic is unique among invertebrate exoskeletons in that it can
grow as the animal enlarges and does not need to be periodically shed.
Inside the tunic is the body wall or mantle composed of connective
tissue , muscle fibres, blood vessels , and nerves . Two openings are
found in the body wall: the buccal siphon at the top through which
water flows into the interior, and the atrial siphon on the ventral
side through which it is expelled. A large pharynx occupies most of
the interior of the body. It is a muscular tube linking the buccal
opening with the rest of the gut. It has a ciliated groove known as an
endostyle on its ventral surface, and this secretes a mucous net which
collects food particles and is wound up on the dorsal side of the
pharynx. The gullet, at the lower end of the pharynx, links it to a
loop of gut which terminates near the atrial siphon. The walls of the
pharynx are perforated by several bands of slits, known as stigmata,
through which water escapes into the surrounding water-filled cavity,
the atrium. This is criss-crossed by various rope-like mesenteries
which extend from the mantle and provide support for the pharynx,
preventing it from collapsing, and also hold up the other organs.
Thaliacea , the other main class of tunicates, is characterised
by free-swimming, pelagic individuals. They are all filter feeders
using a pharyngeal mucous net to catch their prey. The pyrosomes are
bioluminous colonial tunicates with a hollow cylindrical structure.
The buccal siphons are on the outside and the atrial siphons inside.
About 10 species are known, and all are found in the tropics. The 23
species of doliolids are small, mostly under 2 cm (0.79 in) long. They
are solitary, have the two siphons at opposite ends of their
barrel-shaped bodies, and swim by jet propulsion. The 40 species of
salps are also small, under 4 cm (1.6 in) long, and found in the
surface waters of both warm and cold seas. They also move by jet
propulsion, and often form long chains by budding off new individuals.
A third class, the
Larvacea (or Appendicularia), is the only group of
tunicates to retain their chordate characteristics in the adult state,
a product of extensive neoteny . The 70 species of larvaceans
superficially resemble the tadpole larvae of amphibians, although the
tail is at right angles to the body. The notochord is retained, and
the animals, mostly under 1 cm long, are propelled by undulations of
the tail. They secrete an external mucous net known as a house, which
may completely surround them and is very efficient at trapping
PHYSIOLOGY AND INTERNAL ANATOMY
Internal anatomy of a generalised tunicate Section
through the wall of a pyrosoma showing ascidiozooids; br) buccal
siphon; at) atrial siphon; tp) process of the test; br s) pharynx
Like other chordates , tunicates have a notochord during their early
development, but by the time they have completed their larval stages,
they have lost all myomeric segmentation throughout the body. As
members of the Chordata, they are true Coelomata with endoderm ,
ectoderm , and mesoderm , but they do not develop very clear coelomic
body cavities, if any at all. Whether they do or not, by the end of
their larval development, all that remain are the pericardial , renal,
and gonadal cavities of the adults. Except for the heart , gonads, and
pharynx (or branchial sac), the organs are enclosed in a membrane
called an epicardium , which is surrounded by the jelly-like
mesenchyme . Tunicates begin life in a mobile larval stage that
resembles a tadpole. A minority of species, those in the
retain the general larval form throughout life, but most Tunicata very
rapidly settle down and attach themselves to a suitable surface, later
developing into a barrel-like and usually sedentary adult form. The
Thaliacea , however, are pelagic throughout their lives and may have
Tunicates have a well-developed heart and circulatory system . The
heart is a double U-shaped tube situated just below the gut. The blood
vessels are simple connective tissue tubes, and their blood has
several types of corpuscle . The blood may appear pale green, but this
is not due to any respiratory pigments, and oxygen is transported
dissolved in the plasma . Exact details of the circulatory system are
unclear, but the gut, pharynx, gills, gonads, and nervous system seem
to be arranged in series rather than in parallel, as happens in most
other animals. Every few minutes, the heart stops beating and then
restarts, pumping fluid in the reverse direction.
Tunicate blood has some unusual features. In some species of
Perophoridae , it contains high concentrations of the
transitional metal vanadium and vanadium-associated proteins in
vacuoles in blood cells known as vanadocytes. Some tunicates can
concentrate vanadium up to a level ten million times that of the
surrounding seawater. It is stored in a +3 oxidation form that
requires a pH of less than 2 for stability, and this is achieved by
the vacuoles also containing sulphuric acid . The vanadocytes are
later deposited just below the outer surface of the tunic, where their
presence is thought to deter predation , although it is unclear
whether this is due to the presence of the heavy metal or low pH.
Other species of tunicates concentrate lithium , iron , niobium , and
tantalum , which may serve a similar function. Other tunicate species
produce distasteful organic compounds as chemical defenses against
Oikopleura cophocerca in its "house". Arrows indicate
water movement and (x) the lateral reticulated parts of the house
Tunicates lack the kidney-like metanephridial organs typical of
deuterostomes . Most have no excretory structures, but rely on the
diffusion of ammonia across their tissues to rid themselves of
nitrogenous waste, though some have a simple excretory system. The
typical renal organ is a mass of large clear-walled vesicles that
occupy the rectal loop, and the structure has no duct. Each vesicle is
a remnant of a part of the primitive coelom, and its cells extract
nitrogenous waste matter from circulating blood. They accumulate the
wastes inside the vesicles as urate crystals , and do not have any
obvious means of disposing of the material during their lifetimes.
Adult tunicates have a hollow cerebral ganglion, equivalent to a
brain, and a hollow structure known as a neural gland. Both originate
from the embryonic neural tube and are located between the two
siphons. Nerves arise from the two ends of the ganglion; those from
the anterior end innervate the buccal siphon and those from the
posterior end supply the rest of the body, the atrial siphon, organs,
gut and the musculature of the body wall. There are no sense organs
but there are sensory cells on the siphons, the buccal tentacles and
in the atrium.
Tunicates are unusual among animals in that they produce a large
fraction of their tunic and some other structures in the form of
cellulose . The production in animals of cellulose is so unusual that
at first some researchers denied its presence outside of plants, but
the tunicates were later found to possess a functional cellulose
synthesizing enzyme , encoded by a gene horizontally transferred from
a bacterium. When, in 1845, Carl Schmidt first announced the presence
in the test of some ascidians of a substance very similar to
cellulose, he called it "tunicine", but it is now recognized as
cellulose rather than any alternative substance.
Blue sea squirts from the genus
Fluorescent-colored sea squirts,
Clavelina robusta (black and white) and Pycnoclavella flava
(orange) showing siphons
Nearly all tunicates are suspension feeders , capturing planktonic
particles by filtering sea water through their bodies.
typical in their digestive processes, but other tunicates have similar
systems. Water is drawn into the body through the buccal siphon by the
action of cilia lining the gill slits. To obtain enough food, an
average ascidian needs to process one body-volume of water per second.
This is drawn through a net lining the pharynx which is being
continuously secreted by the endostyle. The net is made of sticky
mucus threads with holes about 0.5 µm in diameter which can trap
planktonic particles including bacteria . The net is rolled up on the
dorsal side of the pharynx, and it and the trapped particles are drawn
into the oesophagus . The gut is U-shaped and also ciliated to move
the contents along. The stomach is an enlarged region at the lowest
part of the U-bend. Here, digestive enzymes are secreted and a pyloric
gland adds further secretions. After digestion, the food is moved on
through the intestine , where absorption takes place, and the rectum ,
where undigested remains are formed into faecal pellets or strings.
The anus opens into the dorsal or cloacal part of the peribranchial
cavity near the atrial siphon. Here, the faeces are caught up by the
constant stream of water which carries the waste to the exterior. The
animal orientates itself to the current in such a way that the buccal
siphon is always upstream and does not draw in contaminated water.
Some ascidians that live on soft sediments are detritivores . A few
deepwater species, such as
Megalodicopia hians , are sit-and-wait
predators , trapping tiny crustacea, nematodes, and other small
invertebrates with the muscular lobes which surround their buccal
siphons. Certain tropical species in the family
symbiotic green algae or cyanobacteria in their tunics, and one of
Prochloron , is unique to tunicates. Excess
photosynthetic products are assumed to be available to the host .
Anatomy of a larval tunicate
Ascidians are almost all hermaphrodites and each has a single ovary
and testis, either near the gut or on the body wall. In some solitary
species, sperm and eggs are shed into the sea and the larvae are
planktonic . In others, especially colonial species, sperm is released
into the water and drawn into the atria of other individuals with the
incoming water current. Fertilization takes place here and the eggs
are brooded through their early developmental stages. Some larval
forms appear very much like primitive chordates with a notochord
(stiffening rod) and superficially resemble small tadpoles . These
swim by undulations of the tail and may have a simple eye, an ocellus
, and a balancing organ, a statocyst .
When sufficiently developed, the larva of the sessile species finds a
suitable rock and cements itself in place. The larval form is not
capable of feeding, though it may have a rudimentary digestive system,
and is only a dispersal mechanism. Many physical changes occur to the
tunicate's body during metamorphosis , one of the most significant
being the reduction of the cerebral ganglion, which controls movement
and is the equivalent of the vertebrate brain. From this comes the
common saying that the sea squirt "eats its own brain". However, the
adult does possess a cerebral ganglion which may even be larger than
in the embrionic stage, so the scientific validity of this joke is
questionable. 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. Colonial forms also
increase the size of the colony by budding off new individuals to
share the same tunic.
Pyrosome colonies grow by budding off new zooids near the posterior
end of the colony.
Sexual reproduction starts within a zooid with an
internally fertilized egg. This develops directly into an oozooid
without any intervening larval form. This buds precociously to form
four blastozooids which become detached in a single unit when the
oozoid disintegrates. The atrial siphon of the oozoid becomes the
exhalent siphon for the new, four-zooid colony. A 1901
comparison of frog tadpole and a tunicate larva
Doliolids have a very complex life cycle that includes various zooids
with different functions. The sexually reproducing members of the
colony are known as gonozooids. Each one is a hermaphrodite with the
eggs being fertilised by sperm from another individual. The gonozooid
is viviparous , and at first, the developing embryo feeds on its yolk
sac before being released into the sea as a free-swimming,
tadpole-like larva. This undergoes metamorphosis in the water column
into an oozooid. This is known as a "nurse" as it develops a tail of
zooids produced by budding asexually . Some of these are known as
trophozooids, have a nutritional function, and are arranged in lateral
rows. Others are PHOROZOOIDS, have a transport function, and are
arranged in a single central row. Other zooids link to the
phorozooids, which then detach themselves from the nurse. These zooids
develop into gonozooids, and when these are mature, they separate from
the phorozooids to live independently and start the cycle over again.
Meanwhile, the phorozooids have served their purpose and disintegrate.
The asexual phase in the lifecycle allows the doliolid to multiply
very rapidly when conditions are favourable.
Salps also have a complex lifecycle with an alternation of
generations. In the solitary life history phase, an oozoid reproduces
asexually , producing a chain of tens or hundreds of individual zooids
by budding along the length of a stolon . The chain of salps is the
'aggregate' portion of the lifecycle. The aggregate individuals, known
as blastozooids, remain attached together while swimming and feeding
and growing larger. The blastozooids are sequential hermaphrodites .
An egg in each is fertilized internally by a sperm from another
colony. The egg develops in a brood sac inside the blastozooid and has
a placental connection to the circulating blood of its "nurse". When
it fills the blastozooid's body, it is released to start the
independent life of an oozooid.
Larvaceans only reproduce sexually . They are protandrous
hermaphrodites , except for
Oikopleura dioica which is gonochoric ,
and a larva resembles the tadpole larva of ascidians. Once the trunk
is fully developed, the larva undergoes "tail shift", in which the
tail moves from a rearward position to a ventral orientation and
twists through 90° relative to the trunk. The larva consists of a
small, fixed number of cells, and grows by enlargement of these rather
than cell division. Development is very rapid and only takes seven
hours for a zygote to develop into a house-building juvenile starting
During embryonic development, tunicates exhibit determinate cleavage
, where the fate of the cells is set early on with reduced cell
numbers and genomes 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.
PROMOTION OF OUT-CROSSING
Ciona intestinalis (class Ascidiacea) is a hermaphrodite that
releases sperm and eggs into the surrounding seawater almost
simultaneously. It is self-sterile, and thus has been used for studies
on the mechanism of self-incompatibility. Self/non-self-recognition
molecules play a key role in the process of interaction between sperm
and the vitelline coat of the egg. It appears that self/non-self
recognition in ascidians such as C. intestinalis is mechanistically
similar to self-incompatibility systems in flowering plants.
Self-incompatibility promotes out-crossing, and thus provides the
adaptive advantage at each generation of the masking of deleterious
recessive mutations (i.e. genetic complementation) and the avoidance
of inbreeding depression .
Botryllus schlosseri (class Ascidiacea) is a colonial tunicate, a
member of the only group of chordates that are able to reproduce both
sexually and asexually. B. schlosseri is a sequential (protogynous)
hermaphrodite, and in a colony, eggs are ovulated about two days
before the peak of sperm emission. Thus self-fertilization is
avoided, and cross-fertilization is favored. Although avoided,
self-fertilization is still possible in B. schlosseri. Self-fertilized
eggs develop with a substantially higher frequency of anomalies during
cleavage than cross-fertilized eggs (23% vs. 1.6%). Also a
significantly lower percentage of larvae derived from self-fertilized
eggs metamorphose, and the growth of the colonies derived from their
metamorphosis is significantly lower. These findings suggest that
self-fertilization gives rise to inbreeding depression associated with
developmental deficits that are likely caused by expression of
deleterious recessive mutations.
A MODEL TUNICATE
Oikopleura dioica (class
Appendicularia ) is a semelparous organism,
reproducing only once in its lifetime. It employs an original
reproductive strategy in which the entire female germ-line is
contained within an ovary that is a single giant multinucleate cell
termed the "coenocyst". O. dioica can be maintained in laboratory
culture, and is of growing interest as a model organism because of its
phylogenetic position within the closest sister group to vertebrates .
Over the past few decades, tunicates (notably of the genera Didemnum
and Styela ) have been invading coastal waters in many countries. The
carpet tunicate (
Didemnum vexillum ) has taken over a 6.5 sq mi (17
km2) area of the seabed on the
Georges Bank off the northeast coast of
North America, covering stones, molluscs, and other stationary objects
in a dense mat. D. vexillum,
Styela clava and
Ciona savignyi have
appeared and are thriving in
Puget Sound and
Hood Canal in the Pacific
Invasive tunicates usually arrive as fouling organisms on the hulls
of ships, but may also be introduced as larvae in ballast water .
Another possible means of introduction is on the shells of molluscs
brought in for marine cultivation. Current research indicates 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.
USE BY HUMANS
Tunicates contain a host of potentially useful chemical compounds ,
Didemnins , effective against various types of cancer , as
antivirals and as immunosuppressants
Aplidine , a didemnin effective against various types of cancer
Trabectedin , another didemnin effective against various types of
Tunicates are able to correct their own cellular abnormalities over a
series of generations, and 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 to regenerate compromised human organs.
Sea squirts for sale at a market, Busan, South Korea Main
Ascidiacea § Culinary
Ascidiacea species are consumed as food around the world. In
Japan and Korea, the sea pineapple (Halocynthia roretzi) is the main
species eaten. It is cultivated on dangling cords made of palm fronds
. In 1994, over 42,000 tons were produced, but since then, mass
mortality events have occurred among the farmed sea squirts (the
tunics becoming soft), and only 4,500 tons were produced in 2004.
The use of tunicates as a source of biofuel is being researched. The
cellulose body wall can be broken down and converted into ethanol ,
and other parts of the animal are protein-rich and can be converted
into fish feed. Culturing tunicates on a large scale may be possible
and the economics of doing so are attractive. As tunicates have few
predators, their removal from the sea may not have profound ecological
impacts. Being sea-based, their production does not compete with food
production as does the cultivation of land-based crops for biofuel
Some tunicates are used as model organisms .
Ciona intestinalis and
Ciona savignyi have been used for developmental studies . Both
species' mitochondrial and nuclear genomes have been sequenced.
The nuclear genome of the appendicularian
Oikopleura dioica appears to
be one of the smallest among metazoans and this species has been used
to study gene regulation and the evolution and development of
Vetulicolia – crown-group chordates which are probably the
sister group of modern tunicates
Donald I. Williamson – claimed hybridization
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