Larvaceans (Class
Appendicularia) are solitary, free-swimming
tunicateTunicates, also known as urochordates, are members of the subphylum Tunicata or Urochordata, a group of underwater saclike filter feeders with incurrent and excurrent siphons that is classified within the phylum Chordata...
s found throughout the world's oceans. Like most tunicates, appendicularians are filter feeders. Unlike other tunicates, appendicularians live in the
pelagic zoneAny water in the sea that is not close to the bottom is in the pelagic zone. The word pelagic comes from the Greek πέλαγος or pélagos, which means open sea....
, specifically in the upper sunlit portion of the ocean (
photic zoneThe photic zone or euphotic zone is the depth of the water in a lake or ocean, that is exposed to sufficient sunlight for photosynthesis to occur...
) or sometimes deeper. The appendicularian body shape (
morphologyIn biology morphology is the form, structure and configuration of an organism.This includes aspects of the outward appearance as well as the form and structure of the internal parts like bones and organs...
) resembles that of the tadpole-like larvae of most tunicates. Like a common tunicate larva, the adult appendicularian form has a discrete trunk and tail.
Larvaceans (Class
Appendicularia) are solitary, free-swimming
tunicateTunicates, also known as urochordates, are members of the subphylum Tunicata or Urochordata, a group of underwater saclike filter feeders with incurrent and excurrent siphons that is classified within the phylum Chordata...
s found throughout the world's oceans. Like most tunicates, appendicularians are filter feeders. Unlike other tunicates, appendicularians live in the
pelagic zoneAny water in the sea that is not close to the bottom is in the pelagic zone. The word pelagic comes from the Greek πέλαγος or pélagos, which means open sea....
, specifically in the upper sunlit portion of the ocean (
photic zoneThe photic zone or euphotic zone is the depth of the water in a lake or ocean, that is exposed to sufficient sunlight for photosynthesis to occur...
) or sometimes deeper. The appendicularian body shape (
morphologyIn biology morphology is the form, structure and configuration of an organism.This includes aspects of the outward appearance as well as the form and structure of the internal parts like bones and organs...
) resembles that of the tadpole-like larvae of most tunicates. Like a common tunicate larva, the adult appendicularian form has a discrete trunk and tail. Evolution by retaining juvenile traits as an adult is known as
neotenyNeoteny , also called juvenilization, is the retention, by adults in a species, of traits previously seen only in juveniles , and is a subject studied in the field of developmental biology. In neoteny, the physiological development of an animal or organism is slowed or delayed...
.
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
test (skeleton)In biology, a skeleton is a rigid framework that provides protection and structure in many types of animal, particularly those of the phylum Chordata and of the superphylum Ecdysozoa. Exoskeletons are external, as is typical of many invertebrates; they enclose the soft tissues and organs of the...
known as 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 regularly as the animal grows in size and the filters become clogged. No other Tunicate is able to abandon its test in this fashion. Discarded larvacean houses account for a significant fraction of organic material descending to the ocean deeps.
The tail of appendicularians contain a central
notochordThe notochord is a flexible, rod-shaped body found in embryos of all chordates. It is composed of cells derived from the mesoderm and defines the primitive axis of the embryo. In some chordates, it persists throughout life as the main axial support of the body, while in most vertebrates it is...
, a dorsal 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.
The recent development of techniques for expressing foreign genes in
OikopleuraOikopleura is a genus of urchordates in the class Appendicularia. It forms a mucus house every four hours at 20 degrees Celsius...
dioica has led to the advancement of this species as a model organism for the study of gene regulation, chordate evolution and development.
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