The ascidians, ascidians, are invertebrates of the ascidians subphylum ascidia. There are more than 1250 species in the world, including single ascidians and compound ascidians.[4]The sea squirt has a small body, and its whole body is colorful, like a bag. It has a spinal cord to support its body, and will gradually shrink and disappear after adulthood.In the adult state, the sea squirt looks like a conical capsule with two holes.One hole is filled with water and the other is drained.[5]
Sea squirts are distributed in the world's major oceans, and their traces can be found from the intertidal zone to the depths of the ocean.[4]The sea squirt is a kind of fixed living creature. It likes to parasitize on the hull, shellfish or rocks in the sea, and sometimes it is fixed on the back of large crabs.Monoascidians live alone and do not form groups;The ascidians form colonies.The ascidians are hermaphroditic, but must be fertilized in different bodies.The ascidians will discharge sperm and eggs into water respectively, or they can also put them into the surrounding gill cavity for fertilization.After fertilization, the eggs will develop into larvae within a few hours or days.[4]Life can last for decades.[5]
The sea squirt has survived for 540 million years,[5]It is the most evolved invertebrate,[4]The origin of vertebrates can be better understood through the notochord of ascidians.[5]The ascidians also have a unique characteristic among chordates. They have an open tube blood circulation system, and the flow direction of blood is reversed every few minutes.[4]Sea squirt is a specialty of Sanlu City in Northeast China. July and August are the food season, and it is fat and fleshy.[6]
Appearance of adult ascidianseggplantOr flowers, which are often attached to the bottom of ships and the reefs at the bottom of the sea, are fixed at the same place for many years. At a glance, they are not like animals at all, but like plants.The sea squirt is also called "sea water gun". There is a small mouth called "water inlet" or "breathing hole" on the top of the sea squirt, which constantly absorbs water into it. There is also a mouth called "water outlet" or "discharge hole" on the side, which constantly drains water out.If you poke it with your finger, the sea squirt will be stimulated, and the small hole can shoot quite powerful water, which is shaped like a squirt gun, so it is called "sea squirt gun".This plant like animal neither swims nor feeds, but constantly absorbs water and drains water through its outlet and inlet holes. The gills absorb oxygen in the water, and the intestines absorb plankton and organics to eat, so as to maintain life.Sea squirts vary in size. Some species are very small, only one thousandth of an inch in size, and even invisible to the naked eye. Others are very large, and their body diameters can reach 2 inches or more.[1]
Life habits
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Ascidian
The larva of sea squirt looks like a tadpole. The larva is about 0.5mm long. There is a developed notochord in its tail. There is a hollow dorsal nerve tube at the back of the notochord. The front end of the nerve tube even expands into a brain bubble, containing eyespots and balance organs. The front section of the digestive tract differentiates into a pharynx, with a small number of paired branchial slits, and the ventral side of the body has a heart.[1]
After a few hours of free life, the larvae use the attachment protuberances at the front of the body to stick to other objects and begin their metamorphosis.In the process of metamorphosis, the tail of the larval sea squirt, together with its internal notochord and caudate muscle, gradually atrophied, and was absorbed and disappeared. The neural tube and sensory organs also degenerated and remained as a ganglion.On the contrary, the pharynx expanded greatly, and the number of gill slits increased sharply. At the same time, a peribranchial cavity around the pharynx was formed, and the attachment process was replaced by the stem of the ascidian.Due to the rapid growth of the back side of the attachment process, the position of the orifice was moved to the other end (back), thus causing the position of the internal organs to rotate 90 °~180 °.Finally, the body wall secretes the capsulin to form a protective capsule, which changes it from a free living larva to a camp living ascidian.After metamorphosis, the ascidians lost some important structures and became simpler. This metamorphosis is called retrogressive metamorphosis.[1]
The morphology and structure of adult ascidians are very different from those of typical chordates, and they have changed their lifestyle. They begin to stick to rocks and filter seawater for a living, instead of roaming around.The front part of the body has bulged, the body has changed, the notochord has disappeared, and some other important organs have also subsided or shrunk, resulting in a capsule, etc., which has also exhausted the brain.In terms of evolution, it has not progressed but has regressed. It has changed from having a notochord to having no notochord. This "retrograde metamorphosis" phenomenon is rare in the animal kingdom. Therefore, sea squirts have very important value in studying and solving the origin of chordate animals.[1]
Reproductive habits of ascidians
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Ascidian
Most ascidians areHermaphrodite(but allogeneic insemination), a few hermaphrodites, with 1-2 pairs of testes, most ascidians have one or two obvious reproductive peaks in a year, but some species (including tropical and subtropical species) can reproduce all year round.There are three modes of reproduction of ascidians: oviparous, oviparous and viviparous.Most monosomic ascidians are oviparous, and their fertilization and subsequent development are carried out in vitro. The type of oviparous birth is mostly seen in colony ascidians, and there is no nutritional transmission relationship between mother and larva after ovulation.So far, only one kind of ascidians (Hypsistozoa fasmeriana) has been found to be the true type of viviparity, that is, nutrition exchange between the maternal and embryonic disc materials.[2]
Ascidianembryonic developmentThere are two forms, tail development and tailless development.Most ascidians, especially single ascidians, develop tails, and the whole life history is as follows: oviposition → in vitro fertilization → embryonic development → hatching → swimming tadpole larvae with tails → attachment → metamorphosis → juvenile ascidians → adult.However, some species of Ascididae and Oneophilidae have no larval stage, which is tailless development. Their life history is: ovulation → in vitro fertilization → secretion of mucus → attachment → embryonic development → hatching → young ascidians → adult.[2]
Reproductive mode
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The ascidians are hermaphroditic,Allogamy, sexual reproduction. The gonad is located between the intestinal rings and on the inner wall of the mantle. The testis is large, branching, and the ovary is long tubular, containing many round egg cells.The two closely overlap, and the mature sex cells are respectively injected into the peribranchial cavity with a single reproductive duct, and then discharged into the body through the outlet pipe hole, or meet and fertilize with the germ cells of another sea squirt in the peribranchial cavity.The ascidians are hermaphroditic, but cannot self fertilize because their own sperm and eggs do not mature at the same time. Only two ascidians, one providing sperm and the other providing eggs, can form a fertilized egg.But sea squirts can alsoAsexual reproductionOnce the ascidians are mature, they can perform budding reproduction. Like branch branching, the ascidians grow a small bud, which gradually matures and grows into a large ascidian, and then leaves the mother body to become an independent ascidians.However, the second generation of ascidians grown through "germination" must be mated to produce the next generation, and the next generation of ascidians will "germinate".[1]
This method of intergenerational asexual reproduction enables the sea squirt to spread all over the world, but at the same time keeps itself at a very low level of evolution. The sea squirt has high research value in biology. The tail of the larva of the sea squirt has a notochord, which is one of the important characteristics of chordate animals.At the same time, it can not only coordinate movement, but also have primitive vibration receptors (equivalent to ears) and primitive photoreceptors (similar to simple eyes).[1]
Embryonic development - mosaic pattern
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After fertilization, the egg is completely cleavage.The left and right symmetry of the first cleavage surface of the ascidians makes the subsequent development of the cleavage balls on both sides identical.At the 8-cell stage, the developmental fate of schizoid has been determined.At the 8-cell stage, four on the back are smaller, and four on the ventral side are larger. After forming a blastocyst, the gastrula is formed from the big lobed side inner fold, forming a neural plate on the back, and mesodermal coelom, notochord and other structures occur inside.[1]
The larva looks like a tadpole. The orifice is on the front back, and there is a coeliac orifice on the back. When the larva is metamorphosed, the tail notochord and nerve tube all degenerate, the front part is attached by the fixed nipple, and the orifice and coeliac orifice move to the other end, gradually becoming an adult.Because the number of blastomeres in the cleavage and blastocyst stages of the ascidian embryo is relatively small and easy to identify, the blastomeres can be easily distinguished from each other according to the size, shape and position of the blastomeres.Therefore, it is easy to track and determine the origin and development fate of each cell in the embryo of ascidians by using this feature.[1]
Laurent Chabry, a French scholar, studied the deformed embryos by puncturing the blastomere of the encapsulated animal embryos.Chabry's experiment on ascidian embryos in 1887 showed that if a specific blastomere was separated from the whole embryo at the early stage of embryonic development of ascidians, it would form the same tissue as the structure it would form in the whole embryo, while the tissue formed in the rest of the embryo would lack the structure that could be generated by separating the blastomere, and the two just complement each other.Chabry concluded that each split ball is responsible for generating a special part of the body.It was also found that each mitosphere of the sea squirt can develop independently, and the embryo of the sea squirt seems to be a mosaic composed of various parts that can differentiate themselves.[1]
Egg isolation
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When the embryo of ascidian is cleavage, different cells receive different parts of the egg cytoplasm, and different parts of the egg cytoplasm contain differentMorphogenetic determinant(Morphogenic Determinant) can make cells differentiate in a certain direction.Morphogenetic determinants are also calledCytoplasmic determinant(Cytoplasmic Determinant) or morphogenetic element, different areas of the cytoplasm of the ascidian egg have different colors, for example, the cytoplasm of the fertilized egg of Styela stipitata can be divided into four areas according to the pigment content, the extreme part of the animal contains transparent cytoplasm, and the plant is very close to the red path with two crescent areas arranged opposite each other,One grayish crescent area and one yellow crescent area are grayish, and the other parts of the plant pole contain grayish yolk, which is a grayish yolk area.When the egg is fertilized and divided, the cytoplasm of different regions in the egg will be naturally wrapped in the division ball of the region, and the fate of the development of the division ball can be easily identified.Through tracking research, it is found that the egg cytoplasm in different regions is respectively related to the specific development fate of future embryos. The yellow crescent region contains yellow cytoplasm, called sarcoplasm. After entering the blastomere, the blastomere progeny cells will form muscle cell tissue in the future. The gray crescent region contains gray cytoplasm, which will form the notochord and neural tube in the future. The animal pole contains transparent cytoplasm,In the future, the larval epidermis will be formed, and the gray yolk area will contain a large amount of gray yolk, which will form the digestive tract of larvae and worms.[1]
These different cytoplasmic components contained in the eggs of ascidians were almost evenly distributed in the eggs at the beginning. After fertilization and before cleavage of the eggs of ascidians, after a classification process of Ooplasmic Segregation, they began to be arranged in a unique spatial pattern.Ooplasmic isolation refers to the phenomenon that the morphogenetic determinants distributed in a certain form in the cytoplasm of the egg move during fertilization, redistribute and isolate into a certain area, and are directionally allocated to a specific blastomere during cleavage, which determines the developmental fate of the blastomere.This phenomenon is also called Cytoplasmic Localization, Cytoplasmic Segregation, Cytoplasmic Regionalization or Cytoplasmic Rearrangement.[1]
Sheath processing
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Peel off the sheath of the sea squirt, remove the viscera (including digestive tube and gonad) in the pocket gill sac, and leave only the gill sac.Then add 30g of salt, 15g of sodium glutamate, 100g of sorbitol, 0.7g of potassium sorbateantioxidant1g, seasoning.Then dry it with hot air at 70 ℃ for about 4 hours to shrink the gill sac of ascidians, reduce the water content, and retain the unique pigment, which is conducive to long-term storage.Finally, finely cut the dried product into proper shape and usesynthetic resinThe bag is sealed and can be stored for a long time for food.[3]
Compared with other fish and shellfish, the content of glycogen in ascidians is higher.The sea squirt food processed by the above method does not lose its natural flavor, tastes sweet and has high nutritional value.[3]
Nutritional value of ascidians
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Some species of ascidians (true ascidians, stem ascidians, pot ascidians, etc.) can be eaten in Japan and South Korea and become economic seafood.In recent years, Dalian, Rongcheng, Long Island and other places in China have also begun to breed true ascidians, and some people have proposed that ascidians can be developed as food resources.[2]
The protein content of Styela stipitata was 6.15% by wet weight and 28.49% by dry weight, of which 19 amino acids were detected, including 8Human essential amino acidsCompared with freshwater fish and some marine organisms, the content of protein and amino acid is not high, but the content of taurine in Styela stipitata is very rich 20.20mg/100g.[2]
Although the content of trace elements in the visceral mass of Styela stipitata is not high, it contains a wide range of trace elements, including some rare trace elements, such as Zn, Co, Fe, Mg, Cr, etc.Therefore, Styela stipitata can be processed into health food to balance human nutrition. The content of fatty acids in Styela stipitata is 3.77% of wet weight and 17.12% of dry weight. It contains at least six fatty acids, five of which are unsaturated fatty acids. EPA+DHA is higher than freshwater fish, marine fish and other marine organisms.[2]
