Larvacea
Encyclopedia
Larvaceans are solitary, free-swimming tunicate
s found throughout the world's oceans. Like most tunicates, appendicularians are filter feeders. Unlike other tunicates, appendicularians live in the pelagic zone
, specifically in the upper sunlit portion of the ocean (photic zone
) or sometimes deeper. They are transparent plankton
ic animals, generally less than 1 centimetre (0.393700787401575 in) in body length (excluding the tail).
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 neoteny
.
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. The mucus mesh lies over two clefts in the pharynx, one on either side, rather than the much larger number of clefts found in most other tunicates. Furthermore, the clefts, and the anus
, open directly to the outside, and the animal therefore lacks the atrium and atrial siphon found in related classes.
Appendicularians have greatly improved the efficiency of food intake by producing a test (skeleton)
known as a "house" of protein and cellulose that, in most cases, 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. Even in those species in which the house does not completely surround the body, such as Fritillaria, it is always present and attached to at least one surface. The high efficiency of this method allows larvaceans to feed on much smaller nannoplankton than most other filter feeders.
These houses are discarded and replaced regularly as the animal grows in size and the filters become clogged; in Oikopleura
, a house is kept no more than four hours before being replaced. 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 notochord
, 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.
Appendicularians reproduce sexually
. The immature animals resemble the tadpole larvae of Ascidians, albeit with the addition of developing viscera. Once the trunk is fully developed, the larva 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 Oikopleura dioica has led to the advancement of this species as a model organism for the study of gene regulation, chordate evolution and development.
Tunicate
Tunicates, also known as urochordates, are members of the subphylum Tunicata, previously known as Urochordata, a group of underwater saclike filter feeders with incurrent and excurrent siphons that is classified within the phylum Chordata. While most tunicates live on the ocean floor, others such...
s found throughout the world's oceans. Like most tunicates, appendicularians are filter feeders. Unlike other tunicates, appendicularians live in the pelagic zone
Pelagic zone
Any water in a sea or lake that is not close to the bottom or near to the shore can be said to be in the pelagic zone. The word pelagic comes from the Greek πέλαγος or pélagos, which means "open sea". The pelagic zone can be thought of in terms of an imaginary cylinder or water column that goes...
, specifically in the upper sunlit portion of the ocean (photic zone
Photic zone
The 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. They are transparent plankton
Plankton
Plankton are any drifting organisms that inhabit the pelagic zone of oceans, seas, or bodies of fresh water. That is, plankton are defined by their ecological niche rather than phylogenetic or taxonomic classification...
ic animals, generally less than 1 centimetre (0.393700787401575 in) in body length (excluding the tail).
Biology
The appendicularian body shapeMorphology (biology)
In biology, morphology is a branch of bioscience dealing with the study of the form and structure of organisms and their specific structural features....
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 neoteny
Neoteny
Neoteny , also called juvenilization , is one of the two ways by which paedomorphism can arise. Paedomorphism is the retention by adults 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...
.
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. The mucus mesh lies over two clefts in the pharynx, one on either side, rather than the much larger number of clefts found in most other tunicates. Furthermore, the clefts, and the anus
Anus
The anus is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to control the expulsion of feces, unwanted semi-solid matter produced during digestion, which, depending on the type of animal, may be one or more of: matter which the animal cannot digest,...
, open directly to the outside, and the animal therefore lacks the atrium and atrial siphon found in related classes.
Appendicularians have greatly improved the efficiency of food intake by producing a test (skeleton)
Skeleton
The skeleton is the body part that forms the supporting structure of an organism. There are two different skeletal types: the exoskeleton, which is the stable outer shell of an organism, and the endoskeleton, which forms the support structure inside the body.In a figurative sense, skeleton can...
known as a "house" of protein and cellulose that, in most cases, 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. Even in those species in which the house does not completely surround the body, such as Fritillaria, it is always present and attached to at least one surface. The high efficiency of this method allows larvaceans to feed on much smaller nannoplankton than most other filter feeders.
These houses are discarded and replaced regularly as the animal grows in size and the filters become clogged; in Oikopleura
Oikopleura
Oikopleura is a genus of Tunicata in the class Appendicularia. It forms a mucus house every four hours at 20 degrees Celsius...
, a house is kept no more than four hours before being replaced. 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 notochord
Notochord
The 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 becomes...
, 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.
Appendicularians reproduce sexually
Sexual reproduction
Sexual reproduction is the creation of a new organism by combining the genetic material of two organisms. There are two main processes during sexual reproduction; they are: meiosis, involving the halving of the number of chromosomes; and fertilization, involving the fusion of two gametes and the...
. The immature animals resemble the tadpole larvae of Ascidians, albeit with the addition of developing viscera. Once the trunk is fully developed, the larva 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 Oikopleura dioica has led to the advancement of this species as a model organism for the study of gene regulation, chordate evolution and development.