Tunicate

A tunicate is a type of marine invertebrate that belongs to the subphylum Urochordata. It belongs to the Chordata phylum, which contains all creatures with notochords and dorsal nerve cords (including vertebrates). The subphylum was once known as Urochordata, and the term urochordates are still used to describe these creatures. With the probable exception of the seriation of the gill slits,' they are the only chordates that have lost their myomeric segmentation.

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This article will study tunicate meaning, tunicata and urochordata characteristics in detail.

Ecology of Tunicata/ Urochordata Characteristics

Some tunicates live alone, but others reproduce by budding and forming colonies, each of which is referred to as a zooid. They are water-filled, sac-like marine filter feeders with two tubular openings known as syphons via which they draw in and expel water. They take in water through the incurrent (or inhalant) syphon and emit filtered water through the excurrent (or exhalant) syphon during respiration and feeding. Adult tunicates, like salps, larvaceans, doliolids, and pyrosomes, are sessile, immobile, and permanently attached to rocks or other hard surfaces on the ocean floor; others, like salps, larvaceans, doliolids, and pyrosomes, swim in the pelagic zone as adults.

In the early Cambrian epoch, the oldest possible tunicate species occur in the fossil record. Despite their basic look and extremely distinct adult shape, their close kinship to vertebrates is shown by the fact that they have a notochord or stiffening rod and resemble tadpoles during their mobile larval stage. Their name comes from their exoskeleton-like outer coating, or "tunic," which is made up of proteins and carbohydrates. It can be thin, translucent, and gelatinous in certain species, yet thick, tough, and stiff in others.

Important Features of Subphy­lum Urochordata

Under the Phylum Chordata, the subphylum Urochordata is a distinct group of creatures. The members of this subphylum have a wide range of forms, habits, and habitats.

1. AII Urochordata characteristics: They are either sessile (Ascidiacea) or permanently pelagic (Larvacea or Thaliacea).

2. The majority of the adults have deteriorated and are not segmented.

3. In the larval stage, the notochord is restricted to the tail region. Urochordata is the term given to the group because of this charac­teristic feature. Other characteristics of the larval stage include a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail. In adults, they only have pharyngeal (branchial) slits.

4. A stretchy tunic or test envelops the body. As a result, they're known as tunicates. The tunic's material is unique in that it incorporates cellulose, a plant-based material.

5. The mouth and the atriopore are two openings on the free end of the body.

6. The pharynx, which is specialised for filter feeding, is substantially expanded near the proximal end of the alimentary canal.

7. There is no trace of the coelom.

8. Respiration is accomplished through gaseous exchange through the tunic or pharyngeal slits.

9. The vascular system of the blood is open. The heart is simple, tubular, and ventral, with a pericardial space surrounding it. The heart is unique in that it flips the direction in which it pumps blood on a regular basis. Cells, but not erythrocytes, are found in the blood.

Urochordata Classification

1. Tunicata is more closely linked to craniates (which include hagfish, lampreys, and jawed vertebrates) than they are to lancelets, echinoderms, hemichordates, Xenoturbella, or other invertebrates.

2. Olfactores is the clade that includes tunicates and vertebrates.

3. There are around 3,051 known species in the Tunicata, which are conventionally split into the following classes:

4. Ascidiaceae is a type of ascidiaceae (Aplousobranchia, Phlebobranchia, and Stolidobranchia)

5. Thaliaceae is a type of Thaliaceae (Pyrosomida, Doliolida, and Salpida)

6. Appendicularia is a type of appendix (Larvacea)

7. As a consequence of rDNA sequencing investigations, members of the Sorberacea were included in the Ascidiacea family in 2011. Newer data reveals that the Ascidiacea are an artificial paraphyletic group, despite the conventional classification being provisionally recognised.

8. The cladogram below is based on Delsuc and colleagues' 2018 phylogenomic analysis.

Anatomy of Tunicata 

Tunicate colonies come in a variety of shapes and sizes, and the degree to which individual organisms, known as zooids, interact with one another varies as well. Individual animals are far apart in the simplest systems, but they are joined together by horizontal connections called stolons that grow along the seafloor. The zooids in other animals grow closer together in tufts or crowded together and share a similar basis. In the most mature colonies, the zooids are integrated into a single structure that is encircled by the tunic. These can be organised into bigger systems with hundreds of star-shaped units and have distinct buccal syphons and a single central atrial syphon. A colony's zooids are often small but numerous, and the colonies can create vast encrusting or mat-like patches.

Body Structure of Tunicate 

1. The Ascidiacea are the largest group of tunicates. An ascidiacean's body is surrounded by a test or tunic, which gives the subphylum its name. This can be tough, similar to cartilage, thin and delicate, or transparent and gelatinous, depending on the species. Tunicin, a type of cellulose, is found in the tunic, which is made up of proteins and complex carbohydrates. The tunic is unique among invertebrate exoskeletons in that it may expand with the animal's size and does not need to be shed on a regular basis.

2. The body wall or mantle, which is made up of connective tissue, muscle fibres, blood arteries, and nerves, is found inside the tunic. The body wall has two openings: the buccal syphon at the top, which allows water to enter the interior, and the atrial syphon on the ventral side, which allows water to exit. The pharynx, which is quite huge, takes up the majority of the body's inner space. It's a muscular tube that connects the buccal opening to the rest of the intestine. On its ventral surface, it features a ciliated groove known as an endostyle, which secretes a mucous net that catches food particles and winds up on the dorsal side of the throat.

3. Free-swimming, pelagic individuals characterise the Thaliacea, the other important class of tunicates. They're all filter feeders who collect their prey via a pharyngeal mucous net. Bioluminous colonial tunicates with a hollow cylindrical structure are known as pyrosomes. The buccal and atrial syphons are on the outside, and the buccal and atrial syphons are on the inside. There are about ten species, all of which are located in the tropics. Doliolids are small, with most species being less than 2 cm (0.79 in) in length. They swim using jet propulsion and have two syphons at opposing ends of their barrel-shaped bodies.

Physiology and Internal Anatomy

Tunicates, like all other chordates, have a notochord during early development, but it is gone by the time they undergo transformation. They are real Coelomata having endoderm, ectoderm, and mesoderm like members of the Chordata, but they do not have particularly distinct coelomic body cavities if any at all. Whether they do or not, the adults' pericardial, renal, and gonadal cavities are all that remain at the end of their larval development. The organs are covered in a membrane termed an epicardium, which is surrounded by the jelly-like mesenchyme, with the exception of the heart, gonads, and pharynx (or branchial sac).

Most Tunicata quickly settles down and attach themselves to a suitable surface, later developing into a barrel-like and usually sedentary adult form. A minority of species, those in the Larvacea, retain the general larval form throughout their lives, but most Tunicata quickly settle down and attach themselves to a suitable surface, later developing into a barrel-like and usually sedentary adult form. The Thaliacea, on the other hand, are pelagic all of their lives and have complicated life cycles.

Life Cycle of Tunicate 

• Ascidians are almost all hermaphrodites, with a single ovary and testis on the body wall or near the gut. Sperm and eggs are shed into the water by some solitary species, and the larvae are planktonic. Other species, particularly colonial species, release sperm into the water, which is then attracted into the atria of other individuals by the incoming water current. Fertilization occurs here, and the eggs are brooded through their early stages of development. Some larval forms resemble little tadpoles and have a notochord (stiffening rod) similar to primitive chordates with a notochord (stiffening rod). These creatures have a basic eye, an ocellus, and a balanced organ, and they swim by undulating their tails.

• The sessile species' larva seeks a suitable rock and cements itself in place once fully mature. The larval phase is only a dispersal device and is incapable of feeding, but it may have a rudimentary digestive system. During metamorphosis, the tunicate's body undergoes numerous physical changes, one of the most notable of which is the shrinkage of the cerebral ganglion, which governs movement and functions similarly to the vertebrate brain. The sea squirt is said to "consume its own brain" as a result of this. The adult, on the other hand, has a cerebral ganglion that is possibly even larger than at the embryonic stage, hence the scientific veracity of this joke is debatable.

• Pyrosome colonies expand by budding off new zooids near the colony's back end. A zooid's sexual reproduction begins with an internally fertilised egg. This develops into an oozooid without going through a larval stage. When the oozoid disintegrates, it blooms prematurely to generate four blastozooids that become separated as a single entity. The oozoid's atrial syphon serves as the exhalent syphon for the new four-zooid colony.

• Doliolids have a complicated life cycle that involves a variety of zooids with various purposes. The gonozooids are the colony's sexually reproducing members. Each is a hermaphrodite, meaning their eggs are fertilised by sperm from another person. The developing embryo of a viviparous gonozooid feeds on its yolk sac before being discharged into the water as a free-swimming, tadpole-like larva. This becomes an oozooid after transformation in the water column. This is called a "nurse" because it produces a tail of zooids by budding asexually. Some of these are called trophozoites, and they are arranged in lateral rows and have a nutritive purpose.

• Salps have a complicated life cycle with multiple generations. In the solitary life cycle phase, an oozoid reproduces asexually, budding along the length of a stolon to generate a chain of tens or hundreds of individual zooids. The 'aggregate' element of the lifecycle is the chain of salps. While swimming, feeding and becoming larger, the aggregate individuals, known as blasto zooids, remain linked together. The blasto zooids are hermaphrodites who reproduce in a sequential manner. Internally, a sperm from another colony fertilises an egg in each. The egg develops in a brood sac within the blastazoid, with a placental link to its "nurse's" circulating blood.

A Model Tunicate 

Oikopleura dioica (class Appendicularia) is a semelparous creature, meaning it only reproduces once. It uses a novel reproductive method in which the whole female germ line is contained within a single gigantic multinucleate cell called a "coenocytic," which is a single giant multinucleate cell. Because of its evolutionary position within the closest sister group to vertebrates, O. dioica is becoming more popular as a model organism in laboratories.

Application of Tunicate 

Tunicates include a variety of potentially valuable chemical substances, such as didemnins, which can be used as antivirals, anticancer agents, and immunosuppressants.

Aplidine, a didemnin effective against a variety of cancers, is currently in Phase III trials as a treatment for COVID-19 Trabectedin, another didemnin effective against a variety of cancers, as of late January 2021.

Tunicates have been shown to be able to rectify their own cellular defects over generations, and a similar regeneration process in humans may be conceivable. The principles underlying the phenomena could lead to new insights into the ability of cells and tissues to be reprogrammed and restore human organs that have been damaged.

As Food 

Around the world, different Ascidiacea species are eaten as food. Chilean cuisine includes raw piure (Pyura chilensis) as well as seafood stews. The sea pineapple (Halocynthia roretzi) is the most common type of pineapple consumed in Japan and Korea. It's grown on palm frond ropes that dangle in the air. Over 42,000 tonnes were produced in 1994, but large mortality events among farmed sea squirts (the tunics becoming mushy) have happened since then, and only 4,500 tonnes were produced in 2004.

Other Uses 

Tunicates are being studied as a potential biofuel source. The cellulose body wall can be broken down and converted to ethanol, while the animal's other sections are protein-rich and can be used as fish feed. Tunicates could be cultivated on a big scale, and the economics of doing so are appealing. Tunicates have few predators, therefore their extinction may not have a significant ecological consequence. Because they are sea-based, they do not compete with food production the way land-based crops do for biofuel programmes.

Conclusion

Tunicata is a subphylum of marine invertebrates that includes tunicates. It is a member of the Chordata phylum, which includes all animals that have notochords and dorsal nerve cords (including vertebrates). Urochordata was the old name for the subphylum, and the term urochordates is still used to designate these organisms. They are the only chordates that have lost their myomeric segmentation, with the possible exception of the seriation of the gill slits.

Some tunicates live alone, while others reproduce by budding and establishing colonies, which are referred to as zooid colonies. They're water-filled, sac-like marine filter feeders with two tubular apertures called syphons that draw in and expel water. During breathing and eating, they take in water through the incurrent (or inhalant) syphon and release filtered water through the excurrent (or exhalant) syphon. Adult tunicates such as salps, larvaceans, doliolids, and pyrosomes are sessile, immobile, and permanently attached to rocks or other hard surfaces on the ocean fareloor; others, such as salps, larvaceans, doliolids, and pyrosomes, swim in the pelagic zone.