The Appendicularia or Larvacea are a group of free-swimming pelagic urochordates found throughout the worlds oceans. Appendicularians are filter feeders that primarily occupy the euphotic zone (upper sunlit portion of the ocean) but some species can be found in deeper waters. The morphology of appendicularians superficially resembles that of the tadpole larvae of most urochordates as they possess a discrete trunk and tail throughout adult life. Classes Ascidiacea Thaliacea Appendicularia Urochordata (sometimes known as tunicata and commonly called urochordates, tunicates or sea squirts) is the subphylum of saclike filter feeders with input and output siphons. ...

Like most urochordates, appendicularians feed by drawing particulate food matter into their pharyngo-branchial region, where food particles are trapped on a mucus mesh produced by the pharynx and drawn into the digestive tract. However, appendicularians have greatly improved the efficiency of food intake by producing a "house" of protein and cellulose that surrounds the animal like a bubble and which contains a complicated arrangement of filters that allow food in the surrounding water to be brought in and concentrated prior to feeding. These houses are discarded and replaced on a regular basis as the animal grows in size and the filters become clogged. Discarded larvacean houses account for a significant fraction of organic material descending to the ocean deeps.

The tail of appendicularians contain a central notochord, a nerve cord, and a series of striated muscle bands enveloped either by epithelial tissue (Oikopleurids) or by an acellular basement membrane (Fritillarids). By regularly beating the tail, the appendicularian can generate water currents within its house that allow the concentration of food. The notochord is a flexible rod-shaped body found in embryos of all chordates. ...

Immature appendicularians resemble the tadpole larvae of Ascidians, albeit with the addition of developing viscera. Once the trunk is fully developed, the larval appendicularian undergoes "tail shift", in which the tail moves from a rearward position to a ventral orientation and twists 90 degrees relative to the trunk. Following tail shift, the appendicularian will begin secretion of the first house.

The species Oikopleura dioica has been advanced as a model organism for the study of chordate evolution and development. However, the inability to silence genes or introduce foreign genetic material into Oikopleura has limited its utility relative to other model urochordates.

References

• Bone, Q. 1998. The Biology of Pelagic Tunicates. Oxford University Press.
• Bruce H. Robison, Kim R. Reisenbichler, and Rob E. Sherlock. (2005) Giant Larvacean Houses: Rapid Carbon Transport to the Deep Sea Floor. Science 10 June 2005; 308: pp. 1609-1611 [DOI: 10.1126/science.1109104] (in Reports)