{{Short description|Order of annelid worms}} {{Automatic taxobox |image = Phylo felix Kinberg 1866.jpg |image_caption = The orbiniid worm ''Phylo felix'' |taxon = Orbiniida |authority = Fauchald 1977<ref name="Fauchald Polychaetes"/> sensu Struck et al. 2015<ref name="Two adaptive routes Annelida"/> |subdivision_ranks = Families |subdivision_ref = <ref name="Two adaptive routes Annelida"/><ref name="Early Branching Orbiniida"/> |subdivision = *''Apharyngtus'' *Dinophilidae *Diurodrilidae *Nerillidae *Orbiniidae *Parergodrilidae }}
'''Orbiniida''' is an order of small polychaete worms in the phylum Annelida.<ref name="Early Branching Orbiniida"/> It is the earliest diverging clade in Sedentaria.<ref name="Two adaptive routes Annelida"/> Along with Protodriliformia (in Errantia), this order is composed of meiofaunal marine worms formerly known as "archiannelids".<ref name="AnnelidCurrent"/> These worms inhabit the marine interstitial ecosystem, the space between sand grains.<ref name="Two adaptive routes Annelida"/>
==Classification== First mentioned in 1977, Orbiniida was an order of the polyphyletic infraclass "Scolecida" of the class "Polychaeta". It was composed of the families Orbiniidae, Paraonidae and Questidae, all of which share in common the lack of antennae and palps, and the presence of an eversible pharynx and biramous parapodia with simple chaetae.<ref name="Fauchald Polychaetes"/>
However, through phylogenetic analysis it was found that Questidae and Paraonidae are more closely related to Clitellata and Cirratulida respectively. Because of this, the taxonomic composition of Orbiniida was changed to exclude these two families and include several interstitial groups previously belonging to the polyphyletic "Archiannelida": Orbiniidae, Parergodrilidae, Nerillidae, Diurodrilidae, and ''Apharyngtus'', and possibly Dinophilidae too.<ref name="Early Branching Orbiniida"/>
==Evolutionary history== Phylogenetic analysis of annelids has found Orbiniida to be the earliest diverging clade of Sedentaria. At the same time, the other half of "archiannelid" worms, Protodriliformia, was found to be the earliest diverging clade of Errantia. The convergence between these two groups occurred through progenesis and miniaturization, as a way to adapt to the marine interstitial ecosystem between sand grains (i.e. interstitium). This means that the larval or juvenile stages of a larger pleistoannelid ancestor temporarily inhabiting the interstitium stopped their somatic growth, became sexually mature and stayed in the interstitium permanently.<ref name="Two adaptive routes Annelida"/>
{{clade|style=font-size:90%;|label1=Pleistoannelida|1={{clade |label1=Errantia|1={{clade|2=Protodriliformia|barbegin2=green|1=Aciculata}} |label2=Sedentaria|2={{clade |label1='''Orbiniida'''|barend1=green|1={{clade |2={{clade|2=Nerillidae|1=Dinophilidae}} |1={{clade|2={{clade|1=Diurodrilidae|2=''Apharyngtus''}}|1={{clade|1=Orbiniidae|2=Parergodrilidae }}}}}} |2=other Sedentaria }} }} |grouplabel1={{clade labels|label1={{font color|green|'Archiannelids'}}|top1=45%}} }} Although Dinophilidae possibly belongs to Orbiniida,<ref name="Early Branching Orbiniida"/> a more recent analysis recovered several times the clade Dinophiliformia, containing the genera in Dinophilidae, as sister group to Sedentaria+Errantia. This would imply a third separate lineage of interstitial, miniaturized worms.<ref name="Genome compaction annelid"/>
==References== {{Reflist|refs=
<ref name="Early Branching Orbiniida">{{cite journal|vauthors=Meca MA, Zhadan A, Struck TH|title=The Early Branching Group of Orbiniida Sensu Struck et al., 2015: Parergodrilidae and Orbiniidae|journal=Diversity|date=2021|volume=13|issue=1|pages=29|doi=10.3390/d13010029|doi-access=free|bibcode=2021Diver..13...29M |hdl=10852/93589|hdl-access=free}}</ref>
<ref name="Two adaptive routes Annelida">{{cite journal|vauthors=Struck TH, Golombek A, Weigert A, Franke FA, Westheide W, Purschke G, Bleidorn C, Halanych KM|title=The evolution of annelids reveals two adaptive routes to the interstitial realm|journal=Curr Biol|date=3 August 2015|volume=25|issue=15|pages=1993–1999|doi=10.1016/j.cub.2015.06.007|pmid=26212885|doi-access=free|bibcode=2015CBio...25.1993S }}</ref>
<ref name="Fauchald Polychaetes">{{citation|last1=Fauchald|first1=Kristian|date=3 February 1977|title=The polychaete worms, definitions and keys to the orders, families and genera|publisher=Natural History Museum of Los Angeles County: Los Angeles, CA (USA)|series=Science Series|volume=28|pages=1–188|url=http://www.vliz.be/imisdocs/publications/123110.pdf}}</ref>
<ref name="AnnelidCurrent">{{cite journal|vauthors=Weigert A, Bleidorn C|title=Current status of annelid phylogeny|journal=Org Divers Evol|volume=16|pages=345–362|date=2016|issue=2 |doi=10.1007/s13127-016-0265-7|bibcode=2016ODivE..16..345W |url=http://publication.plazi.org/id/FFAA8650DB19FFA7AD16FFE6FFACFFDC }}</ref>
<ref name="Genome compaction annelid">{{cite journal|vauthors=Martín-Durán JM, Vellutini BC, Marlétaz F, etal.|title=Conservative route to genome compaction in a miniature annelid|journal=Nat Ecol Evol|volume=5|pages=231–242|date=2021|issue=2 |doi=10.1038/s41559-020-01327-6|doi-access=free|hdl=10230/46083|hdl-access=free|pmc=7854359}}</ref>
}}
{{Taxonbar|from=Q60982686}} {{Authority control}}
Category:Annelid orders Category:Polychaetes