Larvaceans, class Appendicularia, are solitary, free-swimming
tunicate
A tunicate is a marine invertebrate animal, a member of the subphylum Tunicata (). It is part of the Chordata, a phylum which includes all animals with dorsal nerve cords and notochords (including vertebrates). The subphylum was at one time ...
s found throughout the world's oceans. Like most tunicates, larvaceans are
filter feeders
Filter feeders are a sub-group of suspension feeding animals that feed by straining suspended matter and food particles from water, typically by passing the water over a specialized filtering structure. Some animals that use this method of feedin ...
. Unlike most other tunicates, they live in the
pelagic zone, specifically in the
photic zone, or sometimes deeper. They are transparent
plankton
Plankton are the diverse collection of organisms found in water (or air) that are unable to propel themselves against a current (or wind). The individual organisms constituting plankton are called plankters. In the ocean, they provide a crucia ...
ic animals, generally less than in body length, excluding the tail.
Anatomy
The adult larvaceans resemble the
tadpole
A tadpole is the larval stage in the biological life cycle of an amphibian. Most tadpoles are fully aquatic, though some species of amphibians have tadpoles that are terrestrial. Tadpoles have some fish-like features that may not be found i ...
-like larvae of most
tunicate
A tunicate is a marine invertebrate animal, a member of the subphylum Tunicata (). It is part of the Chordata, a phylum which includes all animals with dorsal nerve cords and notochords (including vertebrates). The subphylum was at one time ...
s. Like a common tunicate larva, the adult Appendicularia have a discrete trunk and tail.
Larvaceans produce a "house" made of mucopolysaccharides and cellulose. In most species, the house surrounds the animal like a bubble. Even for species in which the house does not completely surround the body, such as ''
Fritillaria
''Fritillaria'' (fritillaries) is a genus of spring flowering herbaceous bulbous perennial plants in the lily family (Liliaceae). The type species, ''Fritillaria meleagris'', was first described in Europe in 1571, while other species from the ...
'', the house is always present and attached to at least one surface.
These houses are discarded and replaced regularly as the animal grows in size and its filters become clogged; in ''
Oikopleura
''Oikopleura'' is a genus of Tunicata (sea-squirts) in the class Appendicularia. It forms a mucus house every four hours at 20 degrees Celsius. This house has a coarse mesh to keep out big particles, and a fine mesh that collects the small par ...
'', a house is kept for 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
Organic matter, organic material, or natural organic matter refers to the large source of carbon-based compounds found within natural and engineered, terrestrial, and aquatic environments. It is matter composed of organic compounds that have c ...
descending to the ocean depths.
The tail of larvaceans 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). Unlike the Ascidiacea larvae, the tail nerve cord in larvaceans contain some
neurons
A neuron, neurone, or nerve cell is an electrically excitable cell that communicates with other cells via specialized connections called synapses. The neuron is the main component of nervous tissue in all animals except sponges and placozoa. N ...
.
Feeding
Larvaceans have greatly improved the efficiency of food intake by producing a
test
Test(s), testing, or TEST may refer to:
* Test (assessment), an educational assessment intended to measure the respondents' knowledge or other abilities
Arts and entertainment
* ''Test'' (2013 film), an American film
* ''Test'' (2014 film), ...
, which contains a complicated arrangement of filters that allow food in the surrounding water to be brought in and concentrated prior to feeding. By regularly beating the tail, the larvacean can generate water currents within its house that allow the concentration of food. The high efficiency of this method allows larvaceans to feed on much smaller
nanoplankton
Plankton are the diverse collection of organisms found in water (or air) that are unable to propel themselves against a current (or wind). The individual organisms constituting plankton are called plankters. In the ocean, they provide a cruc ...
than most other filter feeders.
Like most tunicates, larvaceans feed by drawing particulate food matter into their pharyngobranchial 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 Appendicularia retain the ancestral
Chordate
A chordate () is an animal of the phylum Chordata (). All chordates possess, at some point during their larval or adult stages, five synapomorphies, or primary physical characteristics, that distinguish them from all the other taxa. These fi ...
characteristics of having the clefts, and the
anus open directly to the outside, and by the lack of the atrium and the atrial siphon found in related classes.
Reproduction and genetics
Larvaceans reproduce
sexually. The immature animals resemble the tadpole larvae of
ascidians
Ascidiacea, commonly known as the ascidians, tunicates (in part), and sea squirts (in part), is a polyphyletic class in the subphylum Tunicata of sac-like marine invertebrate filter feeders. Ascidians are characterized by a tough outer "tunic" ...
, albeit with the addition of developing
viscera
In biology, an organ is a collection of tissues joined in a structural unit to serve a common function. In the hierarchy of life, an organ lies between tissue and an organ system. Tissues are formed from same type cells to act together in a ...
. 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° relative to the trunk. Following tail shift, the larvacean begins secretion of the first house.
Fertilisation is external. During egg release the body wall ruptures, killing the animal.
A review of the life cycles and life-history adaptations of pelagic tunicates to environmental conditions
/ref>
The recent development of techniques for expressing foreign genes in ''Oikopleura dioica
''Oikopleura dioica'' is a species of small pelagic tunicate found in the surface waters of most of the world's oceans. It is used as a model organism in research into developmental biology.
Description
''Oikopleura dioica'' is a bioluminescent ...
'', which unlike all other known larvaceans have separate sexes instead of being a protandric hermaphrodite
In reproductive biology, a hermaphrodite () is an organism that has both kinds of reproductive organs and can produce both gametes associated with male and female sexes.
Many taxonomic groups of animals (mostly invertebrates) do not have sepa ...
, has led to the advancement of this species as a model organism for the study of gene regulation, chordate
A chordate () is an animal of the phylum Chordata (). All chordates possess, at some point during their larval or adult stages, five synapomorphies, or primary physical characteristics, that distinguish them from all the other taxa. These fi ...
evolution, and development.
References
*
*
{{Taxonbar, from=Q28960
de:Copelata