Pterobranchia

{{Short description|Class of hemichordates}} {{Automatic taxobox | fossil_range = {{Geological range/linked|Cambrian Stage 3|Recent|refs=<ref name="ReferenceA">{{Cite journal |title=Widespread preservation of small carbonaceous fossils (SCFs) in the early Cambrian of North Greenland |author1=Ben J. Slater |author2=Sebastian Willman |author3=Graham E. Budd |author4=John S. Peel |journal=Geology |volume=46 |issue=2 |pages=107–110 |year=2017 | doi = 10.1130/G39788.1}}</ref>}} | image = Cephalodiscus nigrescens.jpg | image_caption = ''Cephalodiscus nigrescens'' (collected from the Weddell Sea) | taxon = Pterobranchia | authority = Lankester 1877 | subdivision_ranks = Subclasses | subdivision = * Cephalodiscida * Graptolithina | 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.

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,<ref>{{cite journal|last=Sato|first=A|author2=Rickards RB |author3=Holland PWH |title=The origins of graptolites and other pterobranchs: a journey from 'Polyzoa'|journal=Lethaia|year=2008|volume=41|issue=4|pages=303–316|doi=10.1111/j.1502-3931.2008.00123.x}}</ref><ref name=Fortey>{{cite book |author=Fortey, Richard A. |author-link=Richard Fortey |title=Life: A Natural History of the First Four Billion Years of Life on Earth |publisher=Alfred A. Knopf |location=New York |year=1998 |isbn=978-0-375-40119-0 |page=[https://archive.org/details/lifenaturalhisto00fort/page/129 129] |title-link=Life: A Natural History of the First Four Billion Years of Life on Earth }}</ref> and phylogenetic analysis suggests that the pterobranchs are living members of the graptolite clade.<ref name="Mitchell2012">{{Cite journal | last1 = Mitchell | first1 = C. E. | last2 = Melchin | first2 = M. J. | last3 = Cameron | first3 = C. B. | last4 = Maletz | first4 = J. R. | title = Phylogenetic analysis reveals that Rhabdopleura is an extant graptolite | journal = Lethaia | volume = 46 | pages = 34–56 | year = 2012 | doi = 10.1111/j.1502-3931.2012.00319.x }}</ref><ref name="Maletz">{{cite journal |last=Maletz |first=Jörg| title=The classification of the Pterobranchia (Cephalodiscida and Graptolithina) |journal=Bulletin of Geosciences |year=2014 |volume=89 |issue=3 |pages=477–540 |doi=10.3140/bull.geosci.1465 |issn=1214-1119|doi-access=free }}</ref>

==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,<ref name=IZ>{{cite book |author= Barnes, Robert D. |year=1982 |title= Invertebrate Zoology |publisher= Holt-Saunders International |location= Philadelphia, PA|pages= 1026–1027|isbn= 978-0-03-056747-6}}</ref> living as a naked zooid on corals.<ref name= Maletzgp>{{cite book|last=Maletz|first=Jörg|title=Graptolite Paleobiology|year=2017|publisher=Wiley-Blackwell|url=https://www.wiley.com/en-us/Graptolite+Paleobiology-p-9781118515617|isbn=978-1-118-51561-7}}</ref> Recently, ''Atubaria'' has been regarded as a questionable species by Tassia et al. (2016) and is no longer considered valid.<ref>{{cite journal| pmc=5049775 | pmid=27701429 | doi=10.1371/journal.pone.0162564 | volume=11 | title=The Global Diversity of Hemichordata | year=2016 | journal=PLOS ONE | article-number=e0162564 | last1 = Tassia | first1 = MG | last2 = Cannon | first2 = JT | last3 = Konikoff | first3 = CE | last4 = Shenkar | first4 = N | last5 = Halanych | first5 = KM | last6 = Swalla | first6 = BJ| issue=10 | bibcode=2016PLoSO..1162564T | doi-access=free }}</ref>

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.<ref name=IZ/>

Development of pterobranchs have been studied only in ''Rhabdopleura'' from Plymouth (''Rhabdopleura compacta'')<ref>{{cite journal|last=Stebbing|first=ARD|title=Aspects of the reproduction and life cycle of ''Rhabdopleura compacta'' (Hemichordata)|journal=Marine Biology|year=1970|volume=5|issue=3|pages=205–212|doi=10.1007/BF00346908|s2cid=84014156}}</ref><ref>{{cite journal|last=Dilly|first=PN|title=The larva of ''Rhabdopleura compacta'' (Hemichordata)|journal=Marine Biology|year=1973|volume=18|pages=69–86|doi=10.1007/BF00347923|s2cid=86563917}}</ref> and from Bermuda (''Rhabdopleura normani'').<ref>{{cite journal|last=Lester|first=SM|title=Settlement and metamorphosis of ''Rhabdopleura normani'' (Hemichordata: Pterobranchia)|journal=Acta Zoologica|year=1988|volume=69|issue=2|pages=111–120|doi=10.1111/j.1463-6395.1988.tb00907.x}}</ref><ref>{{cite journal|last=Lester|first=SM|title=Ultrastructure of adult gonads and development and structure of the larva of ''Rhabdopleura normani''|journal=Acta Zoologica|year=1986|volume=69|issue=2|pages=95–109|doi=10.1111/j.1463-6395.1988.tb00906.x}}</ref> 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",<ref>{{cite journal|last=Sato|first=A|author2=Bishop JDD and Holland PWH|title=Developmental biology of pterobranch hemichordates: history and perspectives|journal=Genesis|year=2008|volume=46|pages=587–91|doi=10.1002/dvg.20395|pmid=18798243|issue=11|doi-access=}}</ref> and do not feed (lecithotrophic).<ref>[https://oxfordre.com/neuroscience/display/10.1093/acrefore/9780190264086.001.0001/acrefore-9780190264086-e-204?d=%2F10.1093%2Facrefore%2F9780190264086.001.0001%2Facrefore-9780190264086-e-204&p=emailA6xW1XgJvLKrU Hemichordate Nervous System]</ref> Eventually, the larva settles onto the substrate and metamorphoses to an adult. Alternatively, they also reproduce asexually by budding to create a new colony.<ref name=IZ/>

==Evolution== ===Paleontology=== The earliest pterobranchs, including ''Yuknessia'' and ''Galeaplumosus'', are known from mid-Cambrian Lagerstätten.<ref name="ReferenceA">{{Cite journal | doi = 10.1017/jpa.2014.7| title = A reexamination of Yuknessia from the Cambrian of British Columbia and Utah| journal = Journal of Paleontology| volume = 89| pages = 82–95| year = 2015| last1 = Loduca | first1 = S. T. | last2 = Caron | first2 = J. B. | last3 = Schiffbauer | first3 = J. D. | last4 = Xiao | first4 = S. | last5 = Kramer | first5 = A. | doi-access = free }}</ref><ref>{{Cite journal | doi = 10.1016/j.cub.2011.03.005| pmid = 21439828| title = An Early Cambrian Hemichordate Zooid| journal = Current Biology| volume = 21| issue = 7| pages = 612–6| year = 2011| last1 = Hou | first1 = X. G. | last2 = Aldridge | first2 = R. J. | last3 = Siveter | first3 = D. J. | last4 = Siveter | first4 = D. J. | last5 = Williams | first5 = M. | last6 = Zalasiewicz | first6 = J. | last7 = Ma | first7 = X. Y. | doi-access = free }}</ref> Earlier small carbonaceous fossils are known from the Buen Formation.<ref>{{cite journal | doi = 10.1130/G39788.1| title = Widespread preservation of small carbonaceous fossils (SCFs) in the early Cambrian of North Greenland| journal = Geology| volume = 46| issue = 2| pages = 107–110| year = 2017| last1 = Slater| first1 = Ben J| last2 = Willman| first2 = Sebastian| last3 = Budd| first3 = Graham E| last4 = Peel| first4 = John S}}</ref>

===Taxonomy=== Comparison of 18S ribosomal RNA sequences indicated that pterobranchs are closely related to enteropneust hemichordates.<ref>{{cite journal | last = Halanych |first = K. M. |author-link=Kenneth M. Halanych | year = 1995 | title = The Phylogenetic Position of the Pterobranch Hemichordates Based on 18S rDNA Sequence Data | journal = Molecular Phylogenetics and Evolution | volume =4 | issue = 1 | pages = 72–76 | doi =10.1006/mpev.1995.1007 | pmid = 7620637 }}</ref>

Class '''Pterobranchia''' Lankester 1877<ref name="Maletz" /> * Subclass '''Cephalodiscida''' Fowler 1892 stat. nov. ** Order Cephalodiscida Fowler 1892 * Subclass '''Graptolithina''' Bronn 1849 ** Order ?†Camaroidea Kozlowski 1928 sensu Kozlowski 1949 ** Order ?†Crustoidea Bulman 1970 ** Order ?†Dithecoidea Obut, 1960 ** Order ?†Tuboidea Kozlowski 1938 sensu Kozlowsk 1949 ** Order Rhabdopleurida Fowler 1892 sensu Beklemishev 1951 ** Clade †Eugraptolithina Mitchell et al., 2013 *** Order †Dendroidea Nicholson 1872 *** Order †Graptoloidea Maletz, Carlucci and Mitchell 2009

==Genomics== ===Genetic code=== The two pterobranch taxa ''Rhabdopleura compacta'' and ''Cephalodiscus'' use alternative genetic codes in their mitochondrial genome.<ref name="Perseke2011">{{Cite journal |last1=Perseke |first1=Marleen |last2=Hetmank |first2=Joerg |last3=Bernt |first3=Matthias |last4=Stadler |first4=Peter F |last5=Schlegel |first5=Martin |last6=Bernhard |first6=Detlef |date=2011-05-20 |title=The enigmatic mitochondrial genome of Rhabdopleura compacta(Pterobranchia) reveals insights into selection of an efficient tRNA system and supports monophyly of Ambulacraria |url= |journal=BMC Evolutionary Biology |language=en |volume=11 |issue=1 |page=134 |doi=10.1186/1471-2148-11-134 |issn=1471-2148 |pmc=3121625 |pmid=21599892 |doi-access=free }}</ref><ref name="Li2019">{{Cite journal |last6=Halanych |first6=Kenneth M. |last5=Bernt |first5=Matthias |last4=Cannon |first4=Johanna T. |last3=Tassia |first3=Michael G. |last2=Kocot |first2=Kevin M. |last1=Li |first1=Yuanning |date=2019-01-01 |title=Mitogenomics Reveals a Novel Genetic Code in Hemichordata |journal=Genome Biology and Evolution |language=en |volume=11 |issue=1 |pages=29–40 |doi=10.1093/gbe/evy254 |pmc=6319601 |pmid=30476024}}</ref>

{|class="wikitable" style="border: none; text-align: center;" |+ Table of alternative codons in pterobranchs and comparison with the standard genetic code |- ! style="width: 250px;" | Genetic code !! style="width: 25px;" | Translation table !! style="width: 50px;" | DNA codon !! style="width: 50px;" | RNA codon !! colspan="3" style="width: 100px;" | Translation with this code !! style="border: none; width: 1px;" | !! style="width: 125px;" | Standard translation |- | rowspan="3" | Pterobranchia mitochondrial || rowspan="3" | 24 || <code>AGA</code> || <code>AGA</code> || colspan="3" style="background-color:#b3dec0;" | <code>Ser</code> <code>(S)</code> || style="border: none; width: 1px;" | || style="background-color:#bbbfe0;" | <code>Arg</code> <code>(R)</code> |- | <code>AGG</code> || <code>AGG</code> || colspan="3" style="background-color:#bbbfe0;" | <code>Lys</code> <code>(K)</code> || style="border: none; width: 1px;" | || style="background-color:#bbbfe0;" | <code>Arg</code> <code>(R)</code> |- | <code>TGA</code> || <code>UGA</code> || colspan="3" style="background-color:#ffe75f;" | <code>Trp</code> <code>(W)</code> || style="border: none; width: 1px;" | || style="background-color:#B0B0B0;" | <code>STOP = Ter</code> <code>(*)</code> |- | rowspan="4" | Cephalodiscidae mitochondrial || rowspan="4" | 33 || <code>AGA</code> || <code>AGA</code> || colspan="3" style="background-color:#b3dec0;" | <code>Ser</code> <code>(S)</code> || style="border: none; width: 1px;" | || style="background-color:#bbbfe0;" | <code>Arg</code> <code>(R)</code> |- | <code>AGG</code> || <code>AGG</code> || colspan="3" style="background-color:#bbbfe0;" | <code>Lys</code> <code>(K)</code> || style="border: none; width: 1px;" | || style="background-color:#bbbfe0;" | <code>Arg</code> <code>(R)</code> |- | <code>TGA</code> || <code>UGA</code> || colspan="3" style="background-color:#ffe75f;" | <code>Trp</code> <code>(W)</code> || style="border: none; width: 1px;" | || style="background-color:#B0B0B0;" | <code>STOP = Ter</code> <code>(*)</code> |- | <code>TAA</code> || <code>UAA</code> || colspan="3" style="background-color:#b3dec0;" | <code>Tyr</code> <code>(Y)</code> || style="border: none; width: 1px;" | || style="background-color:#B0B0B0;" | <code>STOP = Ter</code> <code>(*)</code> |}

{| class="wikitable" style="border: none; text-align: center;" | style="width: 250px;" | '''Amino acids''' biochemical properties | style="background-color:#ffe75f; width: 60px;" | nonpolar | style="background-color:#b3dec0; width: 60px;" | polar | style="background-color:#bbbfe0; width: 60px;" | basic | style="background-color:#f8b7d3; width: 60px;" | acidic | style="border: none; width: 1px;" | | style="background-color:#B0B0B0;" | Termination: stop codon |}

==References== {{Reflist|2}}

== External links == {{Commons category}} {{Wikispecies}} * {{EB1911|wstitle=Pterobranchia}} * [https://www.webdepot.umontreal.ca/Usagers/cameroc/MonDepotPublic/Cameron/Species.html Cameron, Christopher B. (2016). A comprehensive list of extant hemichordate species with links to images] * [http://faculty.washington.edu/bjswalla/Hemichordata/Pterobranchia/class%20pterobranchia.html Swalla Billie J. (2007). Class Pterobranchia]

Category:Pterobranchia