Pterobranchia

Pterobranchia
Temporal range: Cambrian stage 3–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 genus Atubaria is unusual in lacking the tubes typical of other pterobranchs.[5]

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) [6][7] and from Bermuda (R. normani).[8][9] 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".[10] 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

Evolution

The earliest pterobranchs, including Yuknessia and Galeaplumosus, are known from mid-Cambrian Lagerstätten.[1][11]

Footnotes

  1. 1 2 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.
  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 0-375-40119-9.
  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 0-03-056747-5.
  6. ↑ Stebbing, ARD (1970). "Aspects of the reproduction and life cycle of Rhabdopleura compacta (Hemichordata)". Marine Biology. 5 (3): 205–212. doi:10.1007/BF00346908.
  7. ↑ Dilly, PN (1973). "The larva of Rhabdopleura compacta (Hemichordata)". Marine Biology. 18: 69–86. doi:10.1007/BF00347923.
  8. ↑ 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.
  9. ↑ 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.
  10. ↑ Sato, A; Bishop JDD and Holland PWH (2008). "Developmental biology of pterobranch hemichordates: history and perspectives". Genesis. 46 (11): 587–91. PMID 18798243. doi:10.1002/dvg.20395.
  11. ↑ 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. PMID 21439828. doi:10.1016/j.cub.2011.03.005.

References

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