Everything about Ascidian totally explained
Ascidiacea (commonly known as the
ascidians or
sea squirts) is a
class in the
Tunicata subphylum of sac-like marine
filter feeders. Ascidians are characterized by a tough outer "tunic" made of the
polysaccharide tunicin, as compared to other
tunicates which are much less robust. They are found all over the world, usually in shallow water with
salinities over 2.5%. While members of the
Thaliacea and
Larvacea swim freely like
plankton, sea squirts are
sessile animals: they remain firmly attached to
substratum such as rocks and shells. There are 2,300 species of ascidians and three main types: solitary ascidians, social ascidians that form clumped communities by attaching at their bases, and compound ascidians that consist of many small individuals (each individual is called a ) forming colonies up to several meters in diameter. Sea squirts feed by taking in water through the
oral siphon. The water enters the mouth and
pharynx, flows through mucus-covered gill slits (also called
pharyngeal stigmata) into a water chamber called the atrium, then exits through the atrial siphon.
Life History
Almost all sea squirts are
hermaphrodites. Solitary sea squirts release many eggs from their atrial siphons;
external fertilization in seawater takes place with the coincidental release of
sperm from other individuals. A fertilized egg spends 12 hours to a few days developing into a free-swimming
tadpole larva, which then takes several more hours/days to settle and
metamorphose into a juvenile. The larva selects and settles on appropriate surfaces using receptors sensitive to light, orientation to gravity, and
tactile stimuli. When its
anterior end touches a surface, (small, finger-like nervous projections)
secrete an
adhesive for attachment. Adhesive secretion prompts an irreversible
metamorphosis: various organs (such as the larval tail and fins) are lost while others rearrange to their adult positions, the
pharynx enlarges, and organs called
ampullae grow from the body to permanently attach the animal to the substratum.
Sexual maturity can be reached in as little as a few weeks. Most sea squirts live between 1-3 years.
Colonial sea squirts reproduce both
asexually and
sexually. Sexually produced individuals, those that develop from fertilized eggs, first settle and mature on substratum, then bud asexually to form a colony of many small individuals.
Embryonic development takes place within the established colony: eggs are fertilized and brooded in the atrium. Colonies can survive for decades.
Fertilization
Sea squirt eggs are surrounded by a fibrous
vitelline coat and a layer of
follicle cells that produce sperm-attracting substances. In
fertilization, the sperm passes through the follicle cells and binds to
glycosides on the vitelline coat. The sperm's
mitochondria are left behind as the sperm enters and drives through the coat; this translocation of the mitochondria might provide the necessary force for penetration. The sperm swims through the perivitelline space, finally reaching the egg
plasma membrane and entering the egg. This prompts rapid modification of the vitelline coat, through processes such as the egg's release of
glycosidase into the seawater, so no more sperm can bind and
polyspermy is avoided. After fertilization, free
calcium ions are released in the egg
cytoplasm in waves, mostly from internal stores. The temporary large increase in calcium concentration prompts the
physiological and structural changes of development.
The dramatic rearrangement of egg cytoplasm following fertilization, called ooplasmic segregation, determines the dorsoventral and anteroposterior axes of the embryo. There are at least three types of sea squirt egg
cytoplasm:
ectoplasm containing
vesicles and fine particles,
endoderm containing yolk
platelets, and myoplasm containing pigment granules,
mitochondria, and
endoplasmic reticulum. In the first phase of ooplasmic segregation, the myoplasmic actin-filament network contracts to rapidly move the peripheral cytoplasm (including the myoplasm) to the
vegetal pole, which marks the
dorsal side of the embryo. In the second phase, the myoplasm moves to the subequatorial zone and extend into a crescent, which marks the future posterior of the embryo. The ectoplasm with the
zygote nucleus ends up at the animal hemisphere while the endoplasm ends up in the vegetal hemisphere.
Ecology
The exceptional filtering capability of adult sea squirts causes them to accumulate
pollutants that may be
toxic to
embryos and
larvae as well as impede
enzyme function in adult tissues. This property has made some species sensitive indicators of pollution.
Over the last few hundred years, most of the world's
harbors have been invaded by non-native sea squirts that have clung to ship hulls or to introduced
organisms such as
oysters and
seaweed. Several factors, including quick attainment of sexual maturity, tolerance of a wide range of environments, and a lack of
predators, allow sea squirt populations to grow rapidly. Unwanted populations on
docks, ship hulls, and farmed
shellfish cause significant economic problems, and sea squirt invasions have disrupted the
ecosystem of several natural sub-tidal areas by smothering native animal species.
Sea squirts are the natural prey of many animals, including
flatworms,
molluscs, rock
crabs,
starfish, fish, birds, and
sea otters. They are also eaten by humans in many parts of the world, including
Japan,
Korea,
Chile, and
Europe (where they're sold under the name “sea violet”). As chemical defenses, many sea squirts intake and maintain an extremely high concentration of
vanadium in the blood, have a very low
pH of the tunic due to
acid in easily-ruptured
bladder cells, and (or) produce
secondary metabolites harmful to predators and invaders. Some of these metabolites are toxic to cells and are of potential use in
pharmaceuticals.
Uses
Culinary
Various
Ascidiacea are used as food.
Sea pineapple (
Halocynthia roretzi) is cultivated in
Japan (
hoya,
maboya) and
Korea (
meongge) and, when eaten raw, has been described by
Lonely Planet as tasting like "rubber dipped in ammonia". The peculiar flavor is attributed to an unsaturated alcohol called
cynthiaol.
Microcosmus sabatieri and several similar species from the
Mediterranean Sea are eaten in
France (
figue de mer,
violet),
Italy (
limone di mare,
uova di mare) and
Chile (
probecho), consumed both raw and used as ingredients in seafood stews like
bouillabaisse.
Model organisms for research
A number of factors make sea squirts good models for studying the fundamental developmental processes of
chordates, such as cell-fate specification. The
embryonic development of sea squirts is simple, rapid, and easily manipulated. Because each embryo contains relatively few cells, complex processes can be studied at the cellular level, while remaining in the context of the whole embryo. The embryo's
transparency is ideal for
fluorescent imaging and its maternally-derived proteins are naturally pigmented, so cell
lineages are easily labeled, allowing scientists to visualize
embryogenesis from beginning to end.
Sea squirts are also valuable because of their unique
evolutionary position: as an approximation of ancestral chordates, they can provide insight into the link between non-chordate
deuterostomes and chordates, as well as the
origination of
vertebrates. The sequenced
genomes of the related sea squirts
Ciona intestinalis and
Ciona savignyi are small and easily manipulated; comparisons with the genomes of other organisms such as
flies,
nematodes,
pufferfish and
mammals provides valuable information regarding chordate evolution. A collection of over 480,000 cDNAs have been sequenced and are available to support further analysis of
gene expression, which is expected to provide information about complex developmental processes and regulation of genes in vertebrates. Gene expression in embryos of sea squirts can be conveniently inhibited using
Morpholino oligos.
Further Information
Get more info on 'Ascidian'.
|
External Link Exchanges
Do you know how hard it is to get a link from a large encyclopaedia? Well we're different and will prove it. To get a link from us just add the following HTML to your site on a relevant page:
<a href="http://ascidiacea.totallyexplained.com">Ascidiacea Totally Explained</a>
Then simply click through this link from your web page. Our crawlers will verify your link, extract the title of your web page and instantly add a link back to it. If you like you can remove the words Totally Explained and embed the link in article text.
As long as your link remains in place, we'll keep our link to you right here. Please play fair - our crawlers are watching. Your site must be closely related to this one's topic. Any kind of spamming, dubious practises or removing the link will result in your link from us being dropped and, potentially, your whole site being banned. |
