Urochordates have four fundamental characteristics of chordates in the larval stage but adult urochordates possess neither a notochord nor a dorsal nerve cord. The tadpole-like larvae are planktonic and possess the fundamental chordate characteristics that help them in finding a substratum for attachment. Body of Ascidians is covered by a tunic or test made of cellulose-like polysaccharide, tunicin. Urochordata are a medium sized group of around 3,000 species of marine animals commonly referred to as Sea Squirts, Tunicates, Salps and Larvaceans. They are all filter feeders using a basically similar mechanism of drawing water through perforated pharynx which collects small food particles trapped in mucous. In some cases this matrix of the test contains living cells that have migrated from the main body of the animal and a network of blood vessels. The larvacea can change their house and secrete a new house every four hours or so. Urochordate adults have no limbs, no brain and except in the larvaceans the tail is only evident during larval development.
Tunicates are also known to contain useful chemical compounds, such as, Didemnins, Esteinascidin and Aplidine, which are effective as anticancer, antivirals and immunosuppressants.
The best known fossil urochordate is Shankouclava shankouense from the Lower Cambrian Maotianshan Shale at Shankou village near Kunming (South China). Another tunicate fossil of Catellocaula vallata was found in upper Ordovician rocks in the midwestern United States.
Class – Ascidiacea (Aplousobranchia, Phlebobranchia, and Stolidobranchia).
There are 4 orders in Ascidiacea. Examples: Ascidia, Botryllus, Chelyosoma, Ciona, Clavelina, Corella, Diazona, Diplosoma, Halocynthia, Lissoclinum, Molgula, Polycarpa, Psammascidia, Pyura, Styela.
The branchial and atrial siphons are close together on the top, away from the point of attachment. The water filters through a large pharyngeal basket where the gill slits filter out suspended organic particles and transfer them to the oesophageal opening through the dorsal lamina. Once attached, the animal does not move and covers itself with a protective tunic by secreting a thick substance similar to cellulose. Some species are colonial and reproduce by budding. They earn the name sea squirt by suddenly squirting out water by contraction of muscles when disturbed.
Class – Thaliacea. 3 orders (Pyrosomida, Doliolida, and Salpida). Examples: Dolioletta, Doliolum, Pyrosoma, Salpa, Thetys.
They are free-living planktonic tunicates that are barrel shaped, with the incurrent siphon and the excurrent siphon on the opposite ends. The adults swim about in water in search of plankton on which it feeds. Sometimes they can achieve astonishing concentrations of thousands of animals per cubic meter of water. Because the branchial and atrial siphons are located at opposite ends of the barrel-shaped, jet like exhalant water current provides propulsive force for fast movement. They show alternation of solitary asexual and sexual forms.
Class – Larvacea (Appendicularia). Examples: Fritillaria, Oikopleura, Stegasoma.
Only one order in this class.
They are called Larvacea because they retain larval stage throughout their lives and develop gonads. The adult animals have a relatively large tail that is used to draw current of water into a gelatinous house with a sophisticated set of filters and openings inside which the animal lives. The animal consumes small particles that are brought into the house and abandons the house when its filters are clogged with debris and constructs a new house. Pharynx has only 2 stigmata that open directly to the body surface and endostyle is reduced; tail with notochord and nerve cord is much larger than body.
Class – Sorberacea. Example: Octacnemus.
They are ascidian-like animals of deep ocean. Little is known of them except that they have the same life history as the ascidians. They retain a dorsal nerve chord in the adult stage and do not have gill slits in the pharynx. Brusca and Brusca (2003) state that they are carnivorous.
Although the traditional classification is generally followed, evidence suggests that Ascidiacea is an artificial group of paraphyletic status. The new classification should be as follows:
Class – Stolidobranchia
Class – Aplousobranchia
Class – Phlebobranchia
Class – Thaliacea
Class – Appendicularia (Larvacea)
Class – Sorberacea (may belong in Ascidiacea or as a taxon of its own).
The tunicates are marine, filter-feeding animals such as sea squirts (class Ascidiacea), which show affinities with other chordates. Adult sea squirts are sessile, globular animals, with prominent incurrent and excurrent siphons and many of them grow in colonies. The body of adult is occupied by a very large pharynx with numerous gill slits or stigmata that act as a sieve for filtering food. Water current entering through the incurrent siphon enters the pharynx and passes out through the gill slits, leaving food particles trapped in the mucus coated pharynx. A groove in the pharynx called the endostyle secretes mucus that traps the particles and conveys them through the dorsal lamina into the digestive tract. The movement of mucus is caused by the action of cilia. Water leaves the atrium, which is a sac surrounding the pharynx, by way of the excurrent siphon. Thus the gill slits in tunicates serve as filter feeding apparatus and do not have respiratory function as the primary job.
The sea squirt larva is a free-swimming animal resembling a tadpole. The head contains a brain hollow nerve cord and sense organs, a small pharynx and digestive tract, and a ventral heart. Incurrent and excurrent openings are located on the top of the head. The tail is a muscular appendage with a notochord that functions as a swimming organ. When the larva is ready to undergo metamorphosis it attaches to an object head downward. The tail, notochord, and nerve cord degenerate, the pharynx enlarges, and the other organs shift in position. The incurrent and excurrent openings develop siphons. This is called retrogressive metamorphosis.
Species of Herdmania
H. armata · H. claviformis · H. colona · H. coutieri · H. curvata · H. fimbriae · H. grandis · H. insolita · H. japonica · H. kiiensis · H. litoralis · H. mentula · H. momus (Red Throated Ascidian) · H. pallida · H. polyducta · H. subpallida.
Tunicates are suspension feeders that collect planktons from water. They have two openings in their body cavity: a branchial or oral siphon and the other atrial siphon. The branchial siphon is used to draw food and water into pharynx, while the ex-current or atrial siphon expels wastes and water. Planktons get trapped in mucus secreted from the endostyle of pharynx, which is covered by ciliated cells, which force the trapped planktons to pass down through the dorsal lamina to the oesophagus. Their gut is short U-shaped and the anus opens to the atrial siphon that leads to outside environment.
Tunicate blood is particularly interesting. It contains high concentrations of the metal vanadium and vanadium-associated proteins as well as higher than usual levels of lithium. Some tunicates can concentrate vanadium of up to one million times that of the seawater. Specialized cells can absorb heavy metals, which are then deposited in the tunic.
Most tunicates are hermaphrodite. There is a single male and female gonad in the loop of intestine. The eggs are kept inside their body until they hatch, while sperms are released in water from where they enter the other individuals through the incurrent water.
Tadpole like larval form appears like a primitive chordate, with a notochord, pharyngeal gill slits, dorsal hollow nervous system and a postanal tail. The larval stage ends when it finds a suitable rock to affix to. The larval form is not capable of feeding and is a mechanism for dispersal. Many physical changes occur in the larval body which is called retrogressive metamorphosis, in which the hollow nerve cord is replaced by a solid ganglion. Sense organs, notochord, tail and muscles of tail also disappear. However, pharynx enlarges and digestive system becomes functional. Once grown, adult develops a thick covering called a tunic, to protect their barrel-shaped bodies.