Pterobranchia

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Pterobranchia
Temporal range: Cambrian Stage 3–Recent
Cephalodiscus nigrescens.jpg
Cephalodiscus nigrescens (collected from the Weddell Sea)
Scientific classification OOjs UI icon edit-ltr.svg
Domain: Eukaryota
Kingdom: Animalia
Phylum: Hemichordata
Class: Pterobranchia
Lankester 1877
Orders
Synonyms
  • Graptolithoidea Lapworth sensu Beklemishev, 1951

Pterobranchia, members of which are often called pterobranchs, is a class 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 25 known living pterobranch species in three genera, which are Rhabdopleura , Cephalodiscus , and Atubaria . On the other hand, there are several hundred extinct genera, some of which date from the Cambrian Period.

Contents

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 with the Enteropneusta.

Electron microscope studies 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] living as a naked zooid on corals. [6] Recently, Atubaria has been regarded as a questionable species by Tassia et al. (2016) and is no longer considered valid. [7]

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 ( Rhabdopleura compacta ) [8] [9] and from Bermuda ( Rhabdopleura normani ). [10] [11] 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". [12] 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]

Evolution

Paleontology

The earliest pterobranchs, including Yuknessia and Galeaplumosus , are known from mid-Cambrian Lagerstätten. [13] [14] Earlier small carbonaceous fossils are known from the Buen Formation. [15]

Taxonomy

Comparison of 18S ribosomal RNA sequences indicated that pterobranchs are closely related to enteropneust hemichordates. [16]

Phylogeny of Pterobranchia [4]
Pterobranchia

Cephalodiscida

Graptolithina

Rhabdopleurida

Eugraptolithina

Dendroidea

Graptoloidea

Class PterobranchiaLankester 1877 [4]

Genomics

Genetic code

The two pterobranch taxa Rhabdopleura compacta and Cephalodiscus use alternative genetic codes in their mitochondrial genome. [17] [18]

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 mitochondrial 24AGAAGASer(S)Arg(R)
AGGAGGLys(K)Arg(R)
TGAUGATrp(W)STOP = Ter(*)
Cephalodiscidae mitochondrial 33AGAAGASer(S)Arg(R)
AGGAGGLys(K)Arg(R)
TGAUGATyr(Y)STOP = Ter(*)
TAAUAATrp(W)STOP = Ter(*)
Amino acids biochemical propertiesnonpolarpolarbasicacidicTermination: stop codon

Related Research Articles

<span class="mw-page-title-main">Hemichordate</span> Phylum of marine deuterostome animals

Hemichordata is a phylum which consists of triploblastic, enterocoelomate, and bilaterally symmetrical marine deuterostome animals, generally considered the sister group of the echinoderms. They appear in the Lower or Middle Cambrian and include two main classes: Enteropneusta, and Pterobranchia. A third class, Planctosphaeroidea, is known only from the larva of a single species, Planctosphaera pelagica. The class Graptolithina, formerly considered extinct, is now placed within the pterobranchs, represented by a single living genus Rhabdopleura.

<span class="mw-page-title-main">Graptolite</span> Subclass of Pterobranchs in the phylum Hemichordata

Graptolites are a group of colonial animals, members of the subclass Graptolithina within the class Pterobranchia. These filter-feeding organisms are known chiefly from fossils found from the Middle Cambrian through the Lower Carboniferous (Mississippian). A possible early graptolite, Chaunograptus, is known from the Middle Cambrian. Recent analyses have favored the idea that the living pterobranch Rhabdopleura represents an extant graptolite which diverged from the rest of the group in the Cambrian. Fossil graptolites and Rhabdopleura share a colony structure of interconnected zooids housed in organic tubes (theca) which have a basic structure of stacked half-rings (fuselli). Most extinct graptolites belong to two major orders: the bush-like sessile Dendroidea and the planktonic, free-floating Graptoloidea. These orders most likely evolved from encrusting pterobranchs similar to Rhabdopleura. Due to their widespread abundance, planktonic lifestyle, and well-traced evolutionary trends, graptoloids in particular are useful index fossils for the Ordovician and Silurian periods.

<span class="mw-page-title-main">Ambulacraria</span> Clade of deuterostomes containing echinoderms and hemichordates

Ambulacraria, or Coelomopora, is a clade of invertebrate phyla that includes echinoderms and hemichordates; a member of this group is called an ambulacrarian. Phylogenetic analysis suggests the echinoderms and hemichordates separated around 533 million years ago. The Ambulacraria are part of the deuterostomes, a clade that also includes the many Chordata, and the few extinct species belonging to the Vetulicolia.

Yuknessia is an early pterobranch, known from the Burgess shale, the Chengjiang and the Wheeler shale. Long, unbranched fronds emerge from a central holdfast-like body covered in small conical plates. 23 specimens of Yuknessia are known from the Greater Phyllopod bed, where they comprise < 0.1% of the community. The genus contains two species: the type species Y. simplex and Y. stephenensis. It was originally interpreted as a green alga, and has since been reinterpreted it as a colonial pterobranch.

<span class="mw-page-title-main">Deuterostome</span> Superphylum of bilateral animals

Deuterostomes are bilaterian animals of the superphylum Deuterostomia, typically characterized by their anus forming before the mouth during embryonic development. The three major clades of extant deuterostomes include chordates, echinoderms and hemichordates.

<span class="mw-page-title-main">Cephalodiscida</span> Order in the class Pterobranchia in the phylum Hemichordata

Cephalodiscida is one of two orders in the class Pterobranchia, which are small, worm-shaped animals. Members belong to the hemichordates. Species in this order are sessile, living in clear water and secrete tubes on the ocean floor.

<span class="mw-page-title-main">Rhabdopleurida</span> Order of hemichordates in the pterobranchian class

Rhabdopleurida is one of three orders in the class Pterobranchia, which are small, worm-shaped animals, are the only surviving graptolites. Members belong to the hemichordates. Species in this order are sessile, colonial, connected with a stolon, living in clear water and secrete tubes called tubarium. They have a single gonad, the gill slits are absent and the collar has two tentaculated arms. Rhabdopleura is the best studied pterobranch in developmental biology. Rhabdopleura is the only extant graptolite.

<i>Rhabdopleura</i> Genus of hemichordates

Rhabdopleura is a genus of colonial sessile hemichordates belonging to the Pterobranchia class. As one of the oldest living genera with a fossil record dating back to the Middle Cambrian, it is also considered to be the only living genus of graptolites.

<i>Rhabdopleura normani</i> Species of hemichordate in the pterobranchian class

Rhabdopleura normani is a small, marine species of worm-shaped animal known as a pterobranch. It is a sessile suspension feeder, lives in clear water, and secretes tubes on the ocean floor.

The pterobranchia mitochondrial code is a genetic code used by the mitochondrial genome of Rhabdopleura compacta (Pterobranchia). The Pterobranchia are one of the two groups in the Hemichordata which together with the Echinodermata and Chordata form the three major lineages of deuterostomes. AUA translates to isoleucine in Rhabdopleura as it does in the Echinodermata and Enteropneusta while AUA encodes methionine in the Chordata. The assignment of AGG to lysine is not found elsewhere in deuterostome mitochondria but it occurs in some taxa of Arthropoda. This code shares with many other mitochondrial codes the reassignment of the UGA STOP to tryptophan, and AGG and AGA to an amino acid other than arginine. The initiation codons in Rhabdopleura compacta are ATG and GTG.

<i>Cephalodiscus</i> Genus of hemichordates in the pterobranchian class

Cephalodiscus is a genus of hemichordates in the monotypic family Cephalodiscidae of the order Cephalodiscida.

Atubaria heterolopha is a species of hemichordates in the monotypic genus Atubaria and in the monotypic family Atubaridae. This taxon belongs to the pterobranchian order Cephalodiscida. It was described by Tadao Sato in 1936 from specimens found feeding on a colony of the hydrozoan Dycoryne conferta in Sagami Bay, Japan.

Rhabdopleura annulata is a sessile hemichordate. It is a suspension feeder that secretes tubes on the ocean floor.

<i>Rhabdopleura compacta</i> Species of hemichordates in the pterobranchian class

Rhabdopleura compacta is a sessile hemichordate. It is a suspension feeder that secretes tubes on the ocean floor.

Rhabdopleura striata is a sessile hemichordate. It is a suspension feeder that secretes tubes on the ocean floor.

Rhabdopleura recondita is a sessile hemichordate. It is a suspension feeder that secretes tubes on the ocean floor. It is found in the Adriatic and Ionian Seas.

Didymograptidae is an extinct family of graptolites.

Cysticamaridae is an extinct family of graptolites.

Cephalodiscus calciformis is a sessile hemichordate belonging to the order Cephalodiscida. It is the only pterobranch species with funnel-shaped tubaria.

Cephalodiscus sibogae is a sessile hemichordate belonging to the order Cephalodiscida. Sightings of the species has been reported only once.

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. 1 2 3 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. 1 2 3 Barnes, Robert D. (1982). Invertebrate Zoology. Philadelphia, PA: Holt-Saunders International. pp. 1026–1027. ISBN   978-0-03-056747-6.
  6. ↑ Maletz, Jörg (2017). Graptolite Paleobiology. Wiley-Blackwell. ISBN   9781118515617.
  7. ↑ Tassia, MG; Cannon, JT; Konikoff, CE; Shenkar, N; Halanych, KM; Swalla, BJ (2016). "The Global Diversity of Hemichordata". PLOS ONE. 11 (10): e0162564. Bibcode:2016PLoSO..1162564T. doi: 10.1371/journal.pone.0162564 . PMC   5049775 . PMID   27701429.
  8. ↑ Stebbing, ARD (1970). "Aspects of the reproduction and life cycle of Rhabdopleura compacta (Hemichordata)". Marine Biology. 5 (3): 205–212. doi:10.1007/BF00346908. S2CID   84014156.
  9. ↑ Dilly, PN (1973). "The larva of Rhabdopleura compacta (Hemichordata)". Marine Biology. 18: 69–86. doi:10.1007/BF00347923. S2CID   86563917.
  10. ↑ 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.
  11. ↑ 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.
  12. ↑ 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.
  13. ↑ 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 .
  14. ↑ 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.
  15. ↑ 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.
  16. ↑ 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.
  17. ↑ 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.
  18. ↑ 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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