Pterobranchia

Pterobranchia is a clade of small worm-shaped animals. They belong to the Hemichordata, and live in secreted tubes on the ocean floor. Pterobranchia feed by filtering plankton out of the water with the help of cilia attached to tentacles. There are about 30 known living species in the group.

Pterobranchia
Temporal range: Cambrian Stage 3 to Recent
Cephalodiscus dodecalophus
Scientific classification
Kingdom: Animalia
Phylum: Hemichordata
Class: Pterobranchia
Lankester 1877
Orders
Synonyms
  • Graptolithoidea Lapworth sensu Beklemishev, 1951

The class Pterobranchia was established by Ray Lankester in 1877. It contained, at that time, the single genus Rhabdopleura. Rhabdopleura was at first regarded as an aberrant polyzoon, but when the Challenger report on Cephalodiscus was published in 1887, it became clear that Cephalodiscus, the second genus now included in the order, had affinities in the direction of the Enteropneusta.

Studies under an electron microscope have suggested that pterobranchs belong to the same clade as the extinct graptolites,[1][2] and phylogenetic analysis suggests that the pterobranchs are living members of the graptolite clade.[3][4]

Biology

Pterobranchs are small worm-like filter-feeders, living on the ocean floor, often in relatively deep waters. Like their relatives, the acorn worms, their body is divided into three parts: an anterior proboscis, a collar, and a trunk. The proboscis is wide and flattened at the tip, and in most species contains glands that secrete a tube of organic material in which the pterobranch spends its adult life. The animals are mostly colonial, with several zooids living together in a cluster of tubes. In some species, the individual zooids within the colony are connected by stolons. The single member in the genus Atubaria is unusual in lacking the tubes typical of other pterobranchs,[5] but is regarded as a questionable species and is no longer considered valid.[6]

The collar bears a number of large arms, each of which includes a row of tentacles along one side. The number of arms varies between species, with anything from one to nine pairs. The tentacles are covered in cilia and aid in filtering food from the water. The trunk includes a simple tubular gut, and is curved over so that the anus projects upwards, lying dorsal to the collar. Cephalodiscus and Atubaria have a single pair of gill slits in the pharynx, although Rhabdopleura has none.[5]

Development of pterobranchs have been studied only in Rhabdopleura from Plymouth (R. compacta) [7][8] and from Bermuda (R. normani).[9][10] Both of these species are dioecious, with the fertilised egg hatching to produce a free-swimming ciliated larva. Despite the close relationship between the two groups, the larva does not resemble that of the acorn worms; they are "planula-like".[11] Eventually, the larva settles onto the substrate and metamorphoses to an adult. Alternatively, they also reproduce asexually by budding to create a new colony.[5]

Taxonomy

The class is a small one, with only three known families, each containing a single genus.

Class Pterobranchia Lankester 1877[4]

  • Subclass Cephalodiscida Fowler 1892 stat. nov.
  • Subclass Graptolithina Bronn 1849
    • Order †Camaroidea Kozlowski 1928 sensu Kozlowski 1949
    • Order †Crustoidea Bulman 1970
    • Order †Dithecoidea Obut, 1960
    • Order Rhabdopleurida Fowler 1892 sensu Beklemishev 1951
    • Order †Tuboidea Kozlowski 1938 sensu Kozlowsk 1949
    • Clade †Eugraptolithina Mitchell et al., 2013
      • Order †Dendroidea Nicholson 1872
      • Order †Graptoloidea Maletz, Carlucci and Mitchell 2009

Evolution

Paleontology

The earliest pterobranchs, including Yuknessia and Galeaplumosus, are known from mid-Cambrian Lagerstätten.[12][13] Earlier small carbonaceous fossils are known from the Buen Formation.[14]

Phylogeny

Comparison of 18S ribosomal RNA sequences indicated that pterobranchs are closely related to enteropneust hemichordates.[15]

Genomics

Genetic code

The two pterobranch taxa Rhabdopleura compacta and Cephalodiscus use alternative genetic codes in their mitochondrial genome.[16][17]

Table of alternative codons in pterobranchs and comparison with the standard genetic code
Genetic codeTranslation
table		DNA codonRNA codonTranslation
with this code		Standard translation
Pterobranchia mitochondrial24AGAAGASer (S)Arg (R)
AGGAGGLys (K)Arg (R)
TGAUGATrp (W)STOP = Ter (*)
Cephalodiscidae mitochondrial33AGAAGASer (S)Arg (R)
AGGAGGLys (K)Arg (R)
TGAUGATyr (Y)STOP = Ter (*)
TAAUAATrp (W)STOP = Ter (*)
Amino acids biochemical properties nonpolar polar basic acidic Termination: stop codon

References
  1. Sato, A; Rickards RB; Holland PWH (2008). "The origins of graptolites and other pterobranchs: a journey from 'Polyzoa'". Lethaia. 41 (4): 303–316. doi:10.1111/j.1502-3931.2008.00123.x.
  2. Fortey, Richard A. (1998). Life: A Natural History of the First Four Billion Years of Life on Earth. New York: Alfred A. Knopf. p. 129. ISBN 978-0-375-40119-0.
  3. Mitchell, C. E.; Melchin, M. J.; Cameron, C. B.; Maletz, J. R. (2012). "Phylogenetic analysis reveals that Rhabdopleura is an extant graptolite". Lethaia. 46: 34–56. doi:10.1111/j.1502-3931.2012.00319.x.
  4. Maletz, Jörg (2014). "The classification of the Pterobranchia (Cephalodiscida and Graptolithina)". Bulletin of Geosciences. 89 (3): 477–540. doi:10.3140/bull.geosci.1465. ISSN 1214-1119.
  5. Barnes, Robert D. (1982). Invertebrate Zoology. Philadelphia, PA: Holt-Saunders International. pp. 1026–1027. ISBN 978-0-03-056747-6.
  6. Tassia, MG; Cannon, JT; Konikoff, CE; Shenkar, N; Halanych, KM; Swalla, BJ (2016). "The Global Diversity of Hemichordata". PLoS One. 11: e0162564. doi:10.1371/journal.pone.0162564. PMC 5049775. PMID 27701429.
  7. Stebbing, ARD (1970). "Aspects of the reproduction and life cycle of Rhabdopleura compacta (Hemichordata)". Marine Biology. 5 (3): 205–212. doi:10.1007/BF00346908.
  8. Dilly, PN (1973). "The larva of Rhabdopleura compacta (Hemichordata)". Marine Biology. 18: 69–86. doi:10.1007/BF00347923.
  9. Lester, SM (1988). "Settlement and metamorphosis of Rhabdopleura normani (Hemichordata: Pterobranchia)". Acta Zoologica. 69 (2): 111–120. doi:10.1111/j.1463-6395.1988.tb00907.x.
  10. Lester, SM (1986). "Ultrastructure of adult gonads and development and structure of the larva of Rhabdopleura normani". Acta Zoologica. 69 (2): 95–109. doi:10.1111/j.1463-6395.1988.tb00906.x.
  11. Sato, A; Bishop JDD and Holland PWH (2008). "Developmental biology of pterobranch hemichordates: history and perspectives". Genesis. 46 (11): 587–91. doi:10.1002/dvg.20395. PMID 18798243.
  12. Loduca, S. T.; Caron, J. B.; Schiffbauer, J. D.; Xiao, S.; Kramer, A. (2015). "A reexamination of Yuknessia from the Cambrian of British Columbia and Utah". Journal of Paleontology. 89: 82–95. doi:10.1017/jpa.2014.7.
  13. Hou, X. G.; Aldridge, R. J.; Siveter, D. J.; Siveter, D. J.; Williams, M.; Zalasiewicz, J.; Ma, X. Y. (2011). "An Early Cambrian Hemichordate Zooid". Current Biology. 21 (7): 612–6. doi:10.1016/j.cub.2011.03.005. PMID 21439828.
  14. Slater, Ben J; Willman, Sebastian; Budd, Graham E; Peel, John S (2017). "Widespread preservation of small carbonaceous fossils (SCFs) in the early Cambrian of North Greenland". Geology. 46 (2): 107–110. doi:10.1130/G39788.1.
  15. Halanych KM. (1995). "The Phylogenetic Position of the Pterobranch Hemichordates Based on 18S rDNA Sequence Data". Molecular Phylogenetics and Evolution. 4 (1): 72–76. doi:10.1006/mpev.1995.1007. PMID 7620637.
  16. Perseke, Marleen; Hetmank, Joerg; Bernt, Matthias; Stadler, Peter F; Schlegel, Martin; Bernhard, Detlef (2011-05-20). "The enigmatic mitochondrial genome of Rhabdopleura compacta(Pterobranchia) reveals insights into selection of an efficient tRNA system and supports monophyly of Ambulacraria". BMC Evolutionary Biology. 11 (1): 134. doi:10.1186/1471-2148-11-134. ISSN 1471-2148. PMC 3121625. PMID 21599892.
  17. Li, Yuanning; Kocot, Kevin M.; Tassia, Michael G.; Cannon, Johanna T.; Bernt, Matthias; Halanych, Kenneth M. (2019-01-01). "Mitogenomics Reveals a Novel Genetic Code in Hemichordata". Genome Biology and Evolution. 11 (1): 29–40. doi:10.1093/gbe/evy254. PMC 6319601. PMID 30476024.
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