Biology:Larvacean

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Short description: Class of marine animals in the subphylum Tunicata

Appendicularia
Appendicularia (YPM IZ 096169).jpeg
Appendicularia sp., a genus of fritillariid larvacean
Bathochordaeus charon compared to B. stygius.gif
Houses of Bathochordaeus charon (top) and B. stygius (bottom), two species of giant larvacean
Scientific classification e
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Subphylum: Tunicata
Class: Appendicularia
Fol, 1872[1]
Order: Copelata
Haeckel, 1866
Families and genera
Synonyms
  • Larvacea Herdman, 1882
  • Perennichordata Balfour, 1881

Larvaceans or appendicularians, 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, usually ranging from 2 mm (0.079 in) to 8 mm (0.31 in) in body length including the tail, although giant larvaceans can reach up to 10 cm (3.9 in) in length.[4]

Larvaceans are known for the large houses they build around their bodies to assist in filter-feeding. Secreted from mucus and cellulose, these structures often comprise several layers of filters and can reach up to ten times their body length. In some genera like Oikopleura, houses are built and discarded every few hours, with sinking houses playing a key role in the oceanic carbon cycle.

History

The study of larvaceans began with the description of Appendicularia flagellum by Chamisso and Eysenhardt in 1821.[1][5][n 1] More species were quickly discovered, with Oikopleura in 1830 providing the first evidence of the larvacean house, although its role in feeding wouldn't be understood until Eisen's discoveries in 1874.[5]

Larvaceans as tunicates

Huxley was the first to suggest the identity of larvaceans as tunicates in 1851. Their relationship with other tunicates remained unclear, with larvaceans being argued to be ascidian larvae or a free-swimming generation of ascidians.

An attempt at establishing the internal phylogeny of the class was realized by Fol following the discovery of the aberrant Kowalevskia. Fol grouped together the families Oikopleuridae and Fritillariidae in the putative Endostyla, based on the presence of an endostyle, absent in Kowalevskia which he placed in the sister group Anendostyla.[6]

In situ observations

Another jump in the study of larvaceans was the beginning of in situ observations, which allowed to study the creatures inside their fragile houses without damage. Researchers such as Kakani Katija Young from the Monterey Bay Aquarium Research Institute pioneered imaging techniques such as the particle image velocimetry instrument DeepPIV, revealing the complexity and inner structure of larvacean houses and leading to the first 3D simulations of their internal currents.[7]

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.

As the larvae of ascidian tunicates don't feed at all,[8] the larvae of doliolids goes through their metamorphosis while still inside the egg,[9] and salps and pyrosomes have both lost the larval stage,[10] it makes the larvaceans the only tunicates that feed and have fully functional internal organs during their tailed "tadpole stage", which in Appendicularia is permanent.

The full development of Oikopleura dioica and the fate of its cell lineages have been well-documented, providing insight into larvacean anatomy.[11] Being a model organism, most of our knowledge on larvaceans comes from this specific taxon. Variations in body shape and anatomy exist between families,[12] although the general body plan stays similar.

Trunk

The trunk can roughly be divided into three regions — pharyngeo-brachial, digestive and genital — which are more or less distinct depending on the genus.[13] Like in vertebrates, the digestive system comprises in order a mouth, pharynx, oesophagus, stomach, intestine and rectum.

The pharynx is equipped with an endostyle on its lower side, a specialized organ helping direct food particles inside. It also possesses two spiracles, each surrounded by a ring of cilia,[1] which direct food particles from the inner filter's junction to the mouth.[14]

In some genera like Oikopleura, the tract is U-shaped, with the anus located in a forwards position compared to the stomach and intestine.[15] Others like Fritillaria present a more segmented appearance, with a straighter digestive tract and well-separated pharyngeal and digestive sections. The species Appendicularia sicula doesn't have any anus at all, leading to accumulation of undigested material.[16]

Appendicularia retains the ancestral chordate characteristics of having the pharyngeal spiracles and the anus open directly to the outside, and by the lack of the atrium and the atrial siphon found in related classes.

The gonads are located in the posterior section of the trunk, beyond the digestive tract. They are the only section of the body not to be well-distinguished in the juvenile post-tail shift, instead only growing in size in the days leading to spawning.

Tail

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 contains some neurons.[17]

The tail twists during development, with its dorsal and ventral sides becoming left and right sides respectively. In this way, the dorsal nerve cord actually runs through the tail to the left of the notochord, connecting to the rest of the nervous system at the caudal ganglion at the base of the tail.[18]

The muscle bands surrounding the notochord and nerve cord consist of rows of paired muscle cells, or myocytes, running along the length of the tail.

House

To assist in their filter-feeding, larvaceans produce a "house" made of mucopolysaccharides and cellulose,[19] secreted from specialized cells termed oikoplasts.[20][21] 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 house is secreted from oikoplasts, a specialized family of cells constituting the oikoplastic epithelium. Derived from the ectoderm, it covers part (in Fritillaria) or all (in Oikopleura) of the trunk.[12] In larvae, surface fibrils are secreted by the epithelium prior to the differentiation of the oikoplasts, and have been suggested to play a part in the development of the first house, as well as the formation of the cuticular layer.

The houses possesses 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 one meter in giant larvaceans, 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.

The arrangement of filters allows food in the surrounding water to be brought in and concentrated prior to feeding, with some species able to concentrate food up to 1000 times compared to the surrounding water.[4] By regularly beating the tail, the larvacean can generate water currents within its house that allow the concentration of food. For this purpose, the tail fits into a specialized tail sheath, a funnel of the house connected to the exhalent aperture.[18] The high efficiency of this method allows larvaceans to feed on much smaller nanoplankton than most other filter feeders.

This specific niche of "mucous-mesh grazers" or "mammoth grazers" has been argued to be shared with thaliaceans (salps, pyrosomes and doliolids) — all using internal mucous structures —, as well as with sea butterflies, a clade of pelagic sea snails similarly using an external mucous web to catch prey, although through passive "flux feeding" rather than active filter-feeding.[22]

Larvaceans have been found to be able to select food particles based on factors such as nutrient availability and toxin presence, although both laboratory feeding experiments and in situ observations show no difference in feeding rate between their usual food sources and microplastics.[23] They can eat a wide range of particles sizes, down to one ten-thousandth of their own body size, far smaller than other filter-feeders of comparable size.[22] On the other side of the spectrum, Okiopleura dioica can eat prey up to 20% of its body size. The upper limit on prey size is set by the mouth size, which in the largest genus Bathochordaeus is around 1–2 mm wide for a trunk length of 1–3 cm.[24]

In some species, 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. In other genera such as Fritillaria, houses can be regularly deflated and inflated, cleaning off particles clogging the filters. Houses being reused in this manner leads to a smaller contribution in marine snow from these genera.[12]

Larvacean houses share key homologies with tunicate tunics, including the use of cellulose as a material, confirming that the ancestral tunicate already had the capability to synthesize cellulose.[25] This has been confirmed through genetic studies on Oikopleura dioica and the ascidian Ciona, pinpointing their common cellulose synthase genes as originating with a horizontal gene transfer from a prokaryote.[26] However, houses and tunics share key differences — while houses are gelatinous and can be deflated or even discarded at will, tunics are rigid structures definitively incorporated into the animal's filter-feeding apparatus.

Ecology

Habitat

Larvaceans are widespread, motile planktonic creatures, living through the water column. As their habitats are mostly defined by ocean currents,[1] many species have a cosmopolitan distribution, with some like Oikopleura dioica being found in all of the world's oceans.[27] Larvaceans have been reported as far as the Southern Ocean, where they are estimated to comprise 10.5 million tonnes of wet biomass.[5]

Most species live in the photic zone at less than 100 meters in depth,[27] although giant larvaceans such as Bathochordaeus mcnutti can be found up to 1,400 meters deep,[28] and undescribed oikopleurid and fritillariid species have been reported through the bathypelagic zone, down to the 3,500 meters deep seafloor in Monterey Bay where they constitute the dominant particle feeders in most of the water column.[29]

Reproduction and life cycle

Larvaceans reproduce sexually, with all but one species being protandric hermaphrodites. Unlike all other known larvaceans, Oikopleura dioica shows separate sexes, which are distinguished on the last day of their life cycle through differing gonad shapes.[11]

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.

The life cycle is short. The tadpole-shaped larva usually performs the tail shift less than one day after fecundation, becoming fully functional juveniles. Adults usually reproduce after 5 to 7 days depending on the species.[11]

Fertilisation is external. The body wall ruptures during egg release, killing the animal.[30]

Ecological impact

Through their discarded, nutrient-rich houses — termed sinkers — and fecal pellets falling towards the deep seafloor, larvaceans transport large amounts of organic matter towards that region, constituting a significant component of marine snow.[5] In that way, they massively contribute to the oceanic carbon cycle, being responsible for up to one-third of the carbon transfer to the deep seafloor in Monterey Bay.[31] Still in Monterey Bay, giant larvaceans have been found to have the highest filtration rate of any invertebrate,[4] and discarded larvacean houses have been observed as a consistent food source for both pelagic and benthic organisms in that same region.[29]

Both larvacean houses and fecal pellets were also found to trap microplastics, before sinking towards the seafloor. In this way, larvaceans are believed to play a part in the missing plastic paradox, transporting microplastics through the water column and to the seafloor. Experiments performed on the giant larvacean Bathochordaeus stygius confirm their ability to filter and discard microplastics.[23]

Taxonomy

Appendicularia is most often recovered as the sister group of the other tunicate groups (Ascidiacea and Thaliacea). Already in the late 19th to early 20th century, it was hypothesized by Seeliger and later by Lohmann that Appendicularia diverged first from a free-swimming ancestral tunicate, with sessile forms evolving later in the sister lineage (often termed Acopa).[32]

The following cladogram is based on the 2018 phylogenomic study of Delsuc and colleagues.[33]

Tunicata
 

Appendicularia

 
 
 
 

Thaliacea

 
 
 

Phlebobranchia 

 
 

Aplousobranchia 

 
 
 
 

Stolidobranchia 

 
Script error: No such module "Clade/labels".
 
 

Fossil record

Being delicate and soft-bodied, Appendicularia has no definitive fossil record, although the Cambrian form Oesia disjuncta has historically been suggested to belong to the class.[32] More recently, microfossils covered in an organic coat found in vanadium-rich Cambrian black shales in South China have been suggested to be traces of early larvaceans in their houses, putatively termed "paleoappendicularians".[34][35]

Vetulicolians have also been argued to represent stem-group larvaceans by Dominguez and Jefferies, on the basis of synapomorphies comprising the reduction of the atria and of the gill slits, the position of the anus, and a 90° counter-clockwise torsion of the tail (as seen from behind) around the anterior-posterior axis.[36]

Internal classification

The extant species of the class are divided into three families based on both morphological and genomic criteria: Kowalevskiidae, Fritillariidae and Oikopleuridae.[12][13] The first two are believed to be closer to each other, sharing more derived characteristics compared to the primitive Oikopleuridae.[37] Fritillariidae itself is subdivided into Fritillariinae and the monotypic Appendiculariinae, while Oikopleuridae is split into Bathochordaeinae and Oikopleurinae. Deeper phylogeny is unclear, with genera such as Oikopleura possibly being paraphyletic.

Appendicularia
 
 

Kowalevskiidae

 
Fritillariidae
 

Fritillariinae

 
 

Appendiculariinae

 
 
 
 
Oikopleuridae
 

Bathochordaeinae

 
 

Oikopleurinae

 
 
 
 

Several key morphological differences distinguish the families. Fritillariidae presents a more tapered, compressed trunk, as compared to the rounder one of the other two families. Meanwhile, Kowalevskiidae is notable for lacking the heart and endostyle present in other families, the latter replaced by a ciliated groove without glandular cells. The shape of the spiracles also differs: they appear as simple holes in Fritillariidae, long narrow slits in Kowalevskiidae, and tubular passages in Oikopleuridae.[1]

While the number of described species is comparatively low, the class is believed to harbour massive diversity in the form of cryptic species. For instance, Oikopleura dioica comprises at least three distinct, reproductively incompatible clades despite a similar morphological appearance.[38]

Not all species are equally well-studied. The popularity of Oikopleura dioica as a model organism and its ease of cultivation have led to studies disproportionately focusing on this species' anatomy, and in situ observations on Bathochordaeus charon have been performed by the Monterey Bay Aquarium Research Institute.[7] Meanwhile, studies of Kowalevskiidae and Fritillariidae are comparatively rarer and more limited.[12]

Use as a model species

The dioecious Oikopleura dioica is the only larvacean species that has successfully been cultured in laboratory.[11] The ease of cultivation, combined with extremely small genome size and recent development of techniques for expressing foreign genes in O. dioica, has led to the advancement of this species as a model organism for the study of gene regulation, chordate evolution, developmental biology, and ecology.[38]

Notes

  1. ↑ This first description would later be considered insufficient, leading to Appendicularia becoming a nomen nudum until its reuse by Fol in 1874 under its modern definition.

References

  1. ↑ 1.0 1.1 1.2 1.3 1.4 "Appendicularia". https://www.dcceew.gov.au/sites/default/files/env/pages/81f491f3-4fc8-42d0-a92e-5dce256fd340/files/05-appendicularia.pdf. 
  2. ↑ Archives de zoologie expérimentale et générale. 3. 1874. https://www.biodiversitylibrary.org/page/45507170. Retrieved 10 April 2023. 
  3. ↑ Fenaux, R.; Bone, Q.; Deibel, D. (1998). "Appendicularian distribution and zoogeography". The biology of pelagic tunicates. Oxford University Press. pp. 251–264. 
  4. ↑ 4.0 4.1 4.2 Katija, Kakani; Sherlock, Rob E.; Sherman, Alana D.; Robison, Bruce H. (16 August 2017). "New technology reveals the role of giant larvaceans in oceanic carbon cycling". Science Advances 3 (8): e1602374. doi:10.1126/sciadv.1602374. PMID 28508058. Bibcode: 2017SciA....3E2374K. 
  5. ↑ 5.0 5.1 5.2 5.3 Lindsay, Margaret Caroline Murray (June 2012). Distribution and abundance of Larvaceans in the Southern Ocean (PDF) (Thesis). University of Tasmania. Archived (PDF) from the original on 18 May 2023. Retrieved 18 May 2023.
  6. ↑ "Les Appendiculaires.". https://www.cosmovisions.com/appendiculaires.htm. 
  7. ↑ 7.0 7.1 K. Katija; G. Troni; J. Daniels; K. Lance; R. Sherlock; A.D. Sherman; B.H. Robison (3 June 2020). "Revealing enigmatic mucus structures in the deep sea using DeepPIV". Nature 583 (7814): 78–82. doi:10.1038/s41586-020-2345-2. PMID 32494011. Bibcode: 2020Natur.583...78K. https://www.nature.com/articles/s41586-020-2345-2. Retrieved 14 May 2023. 
  8. ↑ "20.pdf – Scholars' Bank Urochordata: Ascidiacea". https://scholarsbank.uoregon.edu/xmlui/bitstream/handle/1794/6123/20.pdf?sequence. 
  9. ↑ Holland, Linda Z. (2016). "Tunicates". Current Biology 26 (4): R146–R152. doi:10.1016/j.cub.2015.12.024. PMID 26906481. 
  10. ↑ Stolfi, Alberto; Brown, Federico D. (2015). "Tunicata". Evolutionary Developmental Biology of Invertebrates 6. pp. 178–179. doi:10.1007/978-3-7091-1856-6_4. ISBN 978-3-7091-1855-9. https://edisciplinas.usp.br/pluginfile.php/4361144/mod_resource/content/2/StolfiBrown2015_red.pdf. Retrieved 26 February 2023. 
  11. ↑ 11.0 11.1 11.2 11.3 Nishida, Hiroki (July 2008). "Development of the appendicularian Oikopleura dioica: Culture, genome, and cell lineages". Development Growth and Regeneration 50 Suppl 1 (50): S239-56. doi:10.1111/j.1440-169X.2008.01035.x. PMID 18494706. https://www.researchgate.net/publication/5355017. 
  12. ↑ 12.0 12.1 12.2 12.3 12.4 Henriet, Simon; Aasjord, Anne; Chourrout, Daniel (28 October 2022). "Laboratory study of Fritillaria lifecycle reveals key morphogenetic events leading to genus-specific anatomy". Frontiers in Zoology 19 (19): 26. doi:10.1186/s12983-022-00471-y. PMID 36307829. 
  13. ↑ 13.0 13.1 Aravena, Guillermo; Palma, Sergio (June 2002). "Taxonomic identification of appendicularians collected in the epipelagic waters off northern Chile (Tunicata, Appendicularia)". Revista Chilena de Historia Natural 75 (2). doi:10.4067/S0716-078X2002000200005. https://www.researchgate.net/publication/250373343. 
  14. ↑ Deibel D. Feeding and metabolism of Appendicularia. In: Bone Q, editor. The biology of pelagic tunicates. Oxford: Oxford University Press; 1998. p. 139–49.
  15. ↑ Cima, Francesca; Brena, Carlo; Burighel, P. (September 2002). "Multifarious activities of gut epithelium in an appendicularian (Oikopleura dioica: Tunicata)". Marine Biology 141 (3): 479–490. doi:10.1007/s00227-002-0850-5. Bibcode: 2002MarBi.141..479F. https://www.researchgate.net/publication/225711955. 
  16. ↑ "The Exceptional "Blind" Gut of Appendicularia sicula (Appendicularia, Tunicata) – Infona.pl". https://www.infona.pl/resource/bwmeta1.element.elsevier-4b7334eb-83f7-3054-a2aa-5dce912a1dac. 
  17. ↑ Kaas, Jon H. (2016). Evolution of nervous systems. Elsevier Science. pp. 14ff. ISBN 978-0-12-804096-6. https://books.google.com/books?id=XTUYCwAAQBAJ&pg=PA14. 
  18. ↑ 18.0 18.1 "Larvaceans". https://cronodon.com/BioTech/larvaceans.html. 
  19. ↑ Bouquet, J. M.; Troedsson, C.; Novac, A.; Reeve, M.; Lechtenbörger, A. K.; Massart, W.; Skaar, K. S.; Aasjord, A. et al. (2018). "Increased fitness of a key appendicularian zooplankton species under warmer, acidified seawater conditions". PLOS ONE 13 (1): e0190625. doi:10.1371/journal.pone.0190625. PMID 29298334. Bibcode: 2018PLoSO..1390625B. 
  20. ↑ "Appendiculaire ellipsoïdal" (in French). https://doris.ffessm.fr/Especes/Kowalevskia-tenuis-Appendiculaire-ellipsoidal-2129. 
  21. ↑ "Classification des appendiculaires" (in French). http://www.cotebleue.org/classappen.html. 
  22. ↑ 22.0 22.1 Keats R. Conley1, Fabien Lombard2 and Kelly R. Sutherland1 (2018). "Mammoth grazers on the ocean's minuteness: a review of selective feeding using mucous meshes". Proc. R. Soc. B 285 (1878). doi:10.1098/rspb.2018.0056. PMID 29720410. 
  23. ↑ 23.0 23.1 Katija, Kakani; Choy, C. Anela; Sherlock, Rob E.; Sherman, Alana D.; Robison, Bruce H. (16 August 2017). "From the surface to the seafloor: How giant larvaceans transport microplastics into the deep sea". Science Advances 3 (8): e1700715. doi:10.1126/sciadv.1700715. PMID 28835922. Bibcode: 2017SciA....3E0715K. 
  24. ↑ R. E. Sherlock, K. R. Walz, K. L. Schlining, and B. H. Robison (15 December 2016). "Morphology, ecology, and molecular biology of a new species of giant larvacean in the eastern North Pacific: Bathochordaeus mcnutti sp. nov.". Marine Biology 164 (1): 20. doi:10.1007/s00227-016-3046-0. PMID 28042175. 
  25. ↑ Satoshi Kimura, Takao Itoh (2001). "Occurence [sic of High Crystalline Cellulose in the Most Primitive Tunicate, Appendicularian"]. Progress in Biotechnology 18: 121–125. doi:10.1016/S0921-0423(01)80063-0. ISBN 9780444509581. https://www.sciencedirect.com/science/article/abs/pii/S0921042301800630. Retrieved 19 May 2023. 
  26. ↑ Sagane, Yoshimasa; Zech, Karin; Bouquet, Jean-Marie; Schmid, Martina; Bal, Ugur; Thompson, Eric M. (23 February 2010). "Functional specialization of cellulose synthase genes of prokaryotic origin in chordate larvaceans". Development 137 (9): 1483–1492. doi:10.1242/dev.044503. PMID 20335363. 
  27. ↑ 27.0 27.1 Masunaga, Aki; Liu, Andrew W.; Tan, Yongkai; Scott, Andrew; Luscombe, Nicholas M. (16 June 2020). "Streamlined Sampling and Cultivation of the Pelagic Cosmopolitan Larvacean, Oikopleura dioica". J. Vis. Exp. (160): e61279. doi:10.3791/61279. PMID 32628172. https://www.jove.com/fr/t/61279/streamlined-sampling-cultivation-pelagic-cosmopolitan-larvacean. Retrieved 18 May 2023. 
  28. ↑ "Blue-tailed giant larvacean Bathochordaeus mcnutti". https://www.montereybayaquarium.org/animals/animals-a-to-z/blue-tailed-giant-larvacean. 
  29. ↑ 29.0 29.1 Robison, Bruce H.; Sherlock, Rob E.; Reisenbichler, Kim R. (15 August 2010). "The bathypelagic community of Monterey Canyon". Deep Sea Research Part II: Topical Studies in Oceanography 57 (16): 1551–1556. doi:10.1016/j.dsr2.2010.02.021. Bibcode: 2010DSRII..57.1551R. https://www.sciencedirect.com/science/article/abs/pii/S0967064510000871. Retrieved 19 May 2023. 
  30. ↑ "A review of the life cycles and life-history adaptations of pelagic tunicates to environmental conditions". https://academic.oup.com/icesjms/article/69/3/358/598151?login=false. 
  31. ↑ Robison, B.H.; Reisenbichler, K.R.; Sherlock, R.E. (2005). "Giant larvacean houses: Rapid carbon transport to the deep sea floor". Science 308 (5758): 1609–1611. doi:10.1126/science.1109104. PMID 15947183. Bibcode: 2005Sci...308.1609R. http://www.sciencemag.org/cgi/content/abstract/308/5728/1609. Retrieved 15 December 2005. 
  32. ↑ 32.0 32.1 Tokioka, Takasi (30 June 1971). "Phylogenetic Speculation of the Tunicata". Publications of the Seto Marine Biological Laboratory 19 (1): 47. doi:10.5134/175655. https://core.ac.uk/download/pdf/39301635.pdf. Retrieved 13 May 2023. 
  33. ↑ "A phylogenomic framework and timescale for comparative studies of tunicates". BMC Biology 16 (1): 39. April 2018. doi:10.1186/s12915-018-0499-2. PMID 29653534. 
  34. ↑ Zhang, Aiyun (10 August 1987). "Fossil appendicularians in the Early Cambrian". Scientia Sinica, B 30 (8): 888–896. doi:10.1360/YB1987-30-8-888. 
  35. ↑ Blieck, Alain (1992). "At the orgin of chordates". Geobios 25 (1): 101–113. doi:10.1016/S0016-6995(09)90039-0. Bibcode: 1992Geobi..25..101B. https://www.sciencedirect.com/science/article/abs/pii/S0016699509900390. Retrieved 15 May 2023. 
  36. ↑ Dominguez, Patricio; Jeffries, Richard (2003). "Fossil evidence on the origin of appendicularians". International Urochordate Meeting 2003. http://depts.washington.edu/ascidian/UromeetingAbstracts.html. Retrieved 15 May 2023. 
  37. ↑ Brena, Carlo; Cima, Francesca; Burighel, Paolo (31 July 2003). "Alimentary tract of Kowalevskiidae (Appendicularia, Tunicata) and evolutionary implications". Journal of Morphology 258 (2): 225–238. doi:10.1002/jmor.10145. PMID 14518015. https://onlinelibrary.wiley.com/doi/10.1002/jmor.10145. Retrieved 14 May 2023. 
  38. ↑ 38.0 38.1 Masunaga, Aki (2022). "The cosmopolitan appendicularian Oikopleura dioica reveals hidden genetic diversity around the globe". Marine Biology 169 (12): 157. doi:10.1007/s00227-022-04145-5. Bibcode: 2022MarBi.169..157M. 
  • 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. 

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