Ascidiacea (commonly known as the ascidians or sea squirts) is a
paraphyletic class in the subphylum Tunicata of sac-like marine
invertebrate filter feeders. Ascidians are characterized by a tough
outer "tunic" made of the polysaccharide cellulose.
Ascidians are found all over the world, usually in shallow water with
salinities over 2.5%. While members of the
Thaliacea and Larvacea
(Appendicularia) swim freely like plankton, sea squirts are sessile
animals: they remain firmly attached to their 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 a tube, 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.
1.1 Digestive system
1.2 Circulatory system
1.3 Nervous system
2 Life history
3 Colonial species
3.1 Sexual reproduction
3.3 Promotion of out-crossing
3.4 Asexual reproduction
5.1 Fossil record
6.2 Model organisms for research
7.2 General references
8 External links
Sea squirts are rounded or cylindrical animals ranging from about 0.5
to 10 cm (0.2 to 4 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 cellulose 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 gonads. 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. Cords 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.
Polycarpa aurata sits in a bed of white bryozoans
The pharynx forms the first part of the digestive system. The
endostyle produces a supply of mucus 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 planktonic 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
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
ammonia, 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
There are four different types of blood cell: lymphocytes, 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.
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 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.
A tunicate group from East Timor
Almost all ascidians are hermaphrodites 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
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-like
larva, which then takes no more than 36 hours to settle and
metamorphose into a juvenile.
As a general rule, the larva possesses a long tail, containing
muscles, a hollow dorsal nerve tube and a notochord, both features
clearly indicative of the animal's chordate affinities. But one group,
the molgulid ascidians, have evolved tailless species on at least four
separate occasions, and even direct development. A notochord is
formed early in development, and always consist of a row of exactly 40
cells. 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'. This feature is a landmark for the 'theory of
retrogressive metamorphosis or ascidian larva theory'; the true
chordates are hypothesized to have evolved from sexually mature
A colony of Didemnum molle
Colonial ascidians reproduce both asexually and sexually. 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.
Sea squirt eggs are surrounded by a fibrous vitelline coat and a layer
of follicle cells that produce sperm-attracting substances. In
fertilization, the sperm passes through the follicle cells and binds
to glycosides on the vitelline coat. The sperm's mitochondria are left
behind as the sperm enters and drives through the coat; this
translocation of the mitochondria might provide the necessary force
for penetration. The sperm swims through the perivitelline space,
finally reaching the egg plasma membrane and entering the egg. This
prompts rapid modification of the vitelline coat, through processes
such as the egg's release of glycosidase into the seawater, so no more
sperm can bind and polyspermy is avoided. After fertilization, free
calcium ions are released in the egg cytoplasm in waves, mostly from
internal stores. The temporary large increase in calcium concentration
prompts the physiological and structural changes of development.
The dramatic rearrangement of egg cytoplasm following fertilization,
called ooplasmic segregation, determines the dorsoventral and
anteroposterior axes of the embryo. There are at least three types of
sea squirt egg cytoplasm: ectoplasm containing vesicles and fine
particles, endoderm containing yolk platelets, and myoplasm containing
pigment granules, mitochondria, and endoplasmic reticulum. In the
first phase of ooplasmic segregation, the myoplasmic actin-filament
network contracts to rapidly move the peripheral cytoplasm (including
the myoplasm) to the vegetal pole, which marks the dorsal side of the
embryo. In the second phase, the myoplasm moves to the subequatorial
zone and 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
Promotion of out-crossing
Ciona intestinalis 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 masking deleterious recessive
mutations (i.e. genetic complementation).
Ciona savignyi is highly self-fertile. However, non-self sperm
out-compete self-sperm in fertilization competition assays. Gamete
recognition is not absolute allowing some self-fertilization. It was
speculated that self-incompatibility evolved to avoid inbreeding
depression, but that selfing ability was retained to allow
reproduction at low population density.
Botryllus schlosseri 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
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 stolons.
Buds containing food storage cells can develop within the stolons and,
when sufficiently separated from the 'parent', may grow into a new
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.
A sea squirt (Polycarpa aurata) being used as a substrate for a
nudibranch's (Nembrotha lineolata) egg spiral
The exceptional filtering capability of adult sea squirts causes them
to accumulate pollutants that may be toxic to embryos and larvae as
well as impede enzyme function in adult tissues. This property has
made some species sensitive indicators of pollution.
Over the last few hundred years, most of the world's harbors have been
invaded by non-native sea squirts that have clung to ship hulls or to
introduced organisms such as oysters and seaweed. Several factors,
including quick attainment of sexual maturity, tolerance of a wide
range of environments, and a lack of predators, allow sea squirt
populations to grow rapidly. Unwanted populations on docks, ship
hulls, and farmed shellfish cause significant economic problems, and
sea squirt invasions have disrupted the ecosystem of several natural
sub-tidal areas by smothering native animal species.
Sea squirts are the natural prey of many animals, including
nudibranchs, flatworms, molluscs, rock crabs, sea stars, 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.
Ascidians are soft-bodied animals, and for this reason their fossil
record is almost entirely lacking. The earliest reliable ascidians is
Shankouclava shankouense from the Lower
Cambrian Maotianshan Shale
(South China). There are also two enigmatic species from the
Ediacaran period with some affinity to the ascidians - Ausia from the
Nama Group of
Burykhia from the Onega Peninsula, White Sea
of northern Russia. They are also recorded from Lower Jurassic
(Bonet and Benveniste-Velasquez, 1971; Buge and Monniot,1972) and
the Tertiary from France (Deflandre-Riguard, 1949, 1956; Durand, 1952;
Deflandre and Deflandre-Rigaud, 1956; Bouche, 1962; Lezaud, 1966;
Monniot and Buge, 1971; Varol and Houghton, 1996). Older
(Triassic) records are ambiguous. The representatives of the genus
Cystodytes (family Polycitoridae) have been described from the
Pliocene of France by Monniot (1970, 1971) and Deflandre-Rigaud
(1956), and from Eocene of France by Monniot and Buge (1971), and
lately from the Late Eocene of S
Australia by Łukowiak (2012).
Ernst Haeckel's interpretation of several ascidians from Kunstformen
der Natur, 1904
The ascidians were on morphological evidence treated as sister to the
Thaliacea and Appendicularia, but molecular evidence shows that
ascidians are paraphyletic within the Tunicata, as shown in the
Sea pineapple (hoya) served raw as sashimi in its own shell at a sushi
restaurant in Japan
Various ascidians are used as food.
Sea pineapple (Halocynthia roretzi) is cultivated in
Korea (meongge). When served raw, they have a chewy
texture and peculiar flavor likened to "rubber dipped in ammonia"
which has been attributed to (E,E)-2,4-decadien-1-ol. The tunicate
Styela clava is farmed in parts of
Korea where it's known as mideoduk
and is added to various seafood dishes such as agujjim. Sea squirt
bibimbap is a specialty of Geojae 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) and Greece (fouska, φούσκα), for
example raw with lemon, or in salads with olive oil, lemon and
The piure (Pyura chilensis) is used as food in the cuisine of Chile,
consumed both raw and used as ingredients in seafood stews like
Pyura praeputialis is known as cunjevoi in Australia. It was once used
as a food source by Aboriginal people living around Botany Bay, but is
now used mainly for fishing bait.
Model organisms for research
A number of factors make sea squirts good models for studying the
fundamental developmental processes of chordates, such as cell-fate
specification. The embryonic development of sea squirts is simple,
rapid, and easily manipulated. Because each embryo contains relatively
few cells, complex processes can be studied at the cellular level,
while remaining in the context of the whole embryo. The embryo's
transparency is ideal for fluorescent imaging and its
maternally-derived proteins are naturally pigmented, so cell lineages
are easily labeled, allowing scientists to visualize embryogenesis
from beginning to end.
Sea squirts are also valuable because of their unique evolutionary
position: as an approximation of ancestral chordates, they can provide
insight into the link between chordates and ancestral non-chordate
deuterostomes, as well as the evolution of vertebrates from simple
chordates. The sequenced genomes of the related sea squirts Ciona
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
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Wikimedia Commons has media related to Ascidiacea.
The Dutch Ascidians Homepage
Encyclopedia of Marine Life of Britain and Ireland
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Extant chordate classes
Ascidiacea (sea squirts)
Thaliacea (pyrosomes, salps, doliolids)
Vertebrates + Myxini)
(fish + Tetrapods)
Agnatha (jawless fish)
Chondrichthyes (cartilaginous fish: sharks, rays, chimaeras)
Actinopterygii (ray-finned fish)
Squamata (scaled reptiles)²
¹subclasses of Sarcopterygii
²orders of class Reptilia (reptiles)
³traditionally placed in Anapsida
italic are paraphyletic groups