Pterobranchia

Pterobranchia
Temporal range: Sirius Passet–Recent[1]
Cephalodiscus dodecalophus
Scientific classification
Kingdom:Animalia
Phylum:Hemichordata
Class:Pterobranchia
Lankester 1877
Subgroups

See text

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.

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,[2][3] and phylogenetic analysis suggests that the pterobranchs are living members of the graptolite clade.[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 has 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

  • Order Cephalodiscida
    • Family Atubaridae
      • Genus Atubaria
        • Species Atubaria heterolopha Sato, 1935
    • Family Cephalodiscidae
      • Genus Cephalodiscus
        • Cephalodiscus agglutinans Harmer & Ridewood, 1914
        • Cephalodiscus atlanticus Bayer, 1962
        • Cephalodiscus australiensis author unknown
        • Cephalodiscus calciformis Emig, 1977
        • Cephalodiscus densus Andersson 1907
        • Cephalodiscus dodecalophus McIntosh 1882
        • Cephalodiscus evansi Ridewood
        • Cephalodiscus fumosus John, 1932
        • Cephalodiscus gilchristi Ridewood, 1908
        • Cephalodiscus gracilis Harmer 1905
        • Cephalodiscus graptolitoides Dilly 1993
        • Cephalodiscus hodgsoni Ridewood, 1907
        • Cephalodiscus indicus Schepotieff 1909
        • Cephalodiscus kempi John, 1932
        • Cephalodiscus levinsoni Harmer
        • Cephalodiscus nigrescens Lankester, 1905
        • Cephalodiscus sibogae Harmer 1905
        • Cephalodiscus solidus Andersson, 1907
  • Order Rhabdopleurida
    • Family Rhabdopleuridae
      • Genus Rhabdopleura
        • Rhabdopleura annulata Norman, 1921
        • Rhabdopleura compacta Hincks, 1880
        • Rhabdopleura grimaldi Julien
        • Rhabdopleura manubialis Julien 1903
        • Rhabdopleura normani Allmann, 1869
        • Rhabdopleura striata Schepotieff, 1909

Evolution

The earliest pterobranchs, including Yuknessia and Galeaplumosus, are known from mid-Cambrian Lagerstätten.[1][12]

Earlier Small carbonaceous fossils are known from the Buen Formation.[13]

Footnotes

  1. 1 2 Ben J. Slater; Sebastian Willman; Graham E. Budd; John S. Peel (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.
  2. ↑ 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.
  3. ↑ 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.
  4. ↑ 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.
  5. 1 2 3 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 (Stockholm). 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 (Stockholm). 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. ↑ 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.
  13. ↑ 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.

References

  • 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.
  •  This article incorporates text from a publication now in the public domain: Chisholm, Hugh, ed. (1911). "Pterobranchia". Encyclopædia Britannica (11th ed.). Cambridge University Press.
  • Cameron, C. B. Classification of the extant hemichordata
  • At Billie Swalla's site
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