Larvacean

Appendicularia
Oikopleura dioica
Houses of Bathochordaeus charon and B. stygius
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Subphylum:	Tunicata
Class:	Appendicularia Fol, 1872
Order:	Copelata Haeckel, 1866
Families
Kowalevskiidae Lahille, 1888 Kowalevskia Fol, 1872 Fritillariidae Seeliger, 1895 Appendicularia Fol, 1874 Tectillaria Lohmann & Buckman, 1926 Fritillaria Fol, 1872 Oikopleuridae Lohmann, 1896 Bathochordaeus Chun, 1900 Mesochordaeus Fenaux & Youngbluth, 1990 Althoffia Lohmann, 1892 Mesoikopleura Fenaux, 1993 Pelagopleura Lohmann, 1926 Sinisteroffia Tokioka, 1957 Chunopleura Lohmann, 1914 Folia Lohmann, 1892 Megalocercus Chun, 1887 Oikopleura Mertens, 1830 Stegosoma Chun, 1887
Synonyms
Larvacea Herdman, 1882 Perennichordata Balfour, 1881

Larvaceans, class Appendicularia, are solitary, free-swimming tunicates found throughout the world's oceans. Like most tunicates, larvaceans are filter feeders. Unlike most other tunicates, they keep their tadpole-like shape as adults, with the notochord running through the tail. They can be found in the pelagic zone, specifically in the photic zone, or sometimes deeper. They are transparent planktonic animals, generally less than 1 cm (0.39 in) in body length, excluding the tail.

Anatomy

The adult larvaceans resemble the tadpole-like larvae of most tunicates. Like a common tunicate larva, the adult Appendicularia have a discrete trunk and tail. It was originally believed that larvaceans were neotenic tunicates, giving them their common name. Recent studies hint at an earlier divergence, with ascidians having developed their sessile adult form later on.

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 ascidian larvae, the tail nerve cord in larvaceans contain some neurons.

As the larvae of ascidian tunicates don't feed at all, the larvae of doliolids goes through their metamorphosis while still inside the egg, and salps and pyrosomes have both lost the larval stage, it makes the larvaceans the only tunicates that feed and has fully functional internal organs during their tailed "tadpole stage", which in Appendicularia is permanent.

House

Larvaceans produce a "house" made of mucopolysaccharides and cellulose, secreted from specialized cells termed oikoplasts. 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, the house is always present and attached to at least one surface.

The houses possess several sets of filters, with external filters stopping food particles too big for the larvacean to eat, and internal filters redirecting edible particles to the larvacean's mouth. Including the external filters, the houses can reach over a meter, an order of magnitude larger than the larvacean itself. The house varies in shape: incomplete in Fritillaria, it is shaped like a pair of kidneys in Bathochordaeus, and toroidal in Kowalevskia.

These houses are discarded and replaced regularly as the animal grows in size and its filters become clogged; in Oikopleura, 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 descending to the ocean depths.

Feeding

Larvaceans have greatly improved the efficiency of food intake by producing a test, 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 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 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, 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° 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.

The recent development of techniques for expressing foreign genes in Oikopleura dioica, which unlike all other known larvaceans have separate sexes instead of being a protandric hermaphrodite, has led to the advancement of this species as a model organism for the study of gene regulation, chordate evolution, and development.

Classification

Appendicularia is most often recovered as the sister group of the other tunicate groups (Ascidiacea and Thaliacea).

The following cladogram is based on the 2018 phylogenomic study of Delsuc and colleagues.

Tunicata	Appendicularia Thaliacea Phlebobranchia Aplousobranchia  Stolidobranchia Ascidiacea

References

• Bone, Q. (1998). The Biology of Pelagic Tunicates. Oxford, UK: Oxford University Press.
• Clarke, T.; Bouquet, JM; Fu, X; Kallesøe, T.; Schmid, M; Thompson, E.M. (2007). "Rapidly evolving lamins in a chordate, Oikopleura dioica, with unusual nuclear architecture". Gene. 396 (1): 159–169. doi:10.1016/j.gene.2007.03.006. PMID 17449201.