Basic Biology  
    Ascidians are members of the subphylum Urochordata, a subset of the phylum Chordata that includes fish, birds, reptiles and us, humans. Four characters unite ascidians with chordates and these are: 1) a dorsal hollow nerve cord, 2) a notochord, 3) a post-anal tail and 4) pharyngeal gill slits. It is very difficult to see how adult ascidians are related to chordates, as the notochord, nerve cord and post-anal tail are present only in the larva stage. When the larva, or tadpole, settles it absorbs these parts and metamorphosizes into a tail-less, spine-less adult.
     While tadpoles are motile and can move short distances, nearly all adult ascidians are sessile suspension-feeders. This means that they do not move and feed by trapping small particles out of the water column. Water enters through the incurrent siphons and passes through the mucous-covered pharynx (on which the food is trapped) and then exits through the excurrent siphon. The endostyle, an organ thought to be the precursor to the thyroid gland, produces the mucous, which is rolled up and ingested along with any food that has been trapped in it. A new net is then secreted over the pharynx and the process starts all over again.
         The internal anatomy of ascidians is surrounded by a body wall made up of fibers and a carbohydrate known as tunicin. This body wall is called a tunic and hence the common name for ascidians "Tunicate". The density and composition of the tunic varies by species, ranging from extremely tough and leathery (ex. Pyrua haustor) to very soft and gelatinous (ex. Corella inflata). For an unknown reason, ascidian tunics have extremely high concentrations of the trace mineral vanadium (can be up to 1 million X that of surrounding seawater!). It is unclear why they do this. Some species also harbor endosymbiotic algae and bacteria in their tunics.
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Three Types of Ascidian
Ascidians can be divided into 3 main types based on their growth forms:

• Solitary ascidians, such as Styela montereyensis, are single, discrete individuals that can only reproduce sexually. Species may have separate sexes, but many are hermaphroditic, meaning that they produce both sperm and eggs. Most of these species are sequential hermaphrodites, meaning that they are not make and female at the same time. This is thought to help prevent self-fertilization and to increase genetic diversity.
  
• Social ascidians, such as Metandrocarpa dura, include species where genetically-identical individuals, or clones, that are vascularly connected to each other in some way. Each "zooid" or discrete unit has an incurrent and excurrent siphon. The degree that zooids are integrated can vary from those that are only connected by thin runners or "stolons" to those that share body tissue. Social ascidians can reproduce both sexually and asexually.
• In Compound or colonial ascidians, such as Botryllus schlosseri, individual zooid are integrated in a gelatinous matrix to the extent of sharing a common excurrent siphon with neighboring zooids. These species often looks like blobs, but are actually hundreds or thousands of genetically identical individuals. In some species, colonies will actually fuse with other colonies to form genetically-mixed colonies, known as chimeras. Compound ascidians can reproduce both sexually and asexually.