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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.
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.
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.
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.
References
- Bone, Q. 1998. The Biology of Pelagic Tunicates. Oxford University Press.
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