Orbiniida

Orbiniida
	TheorbiniidwormPhylo felix.
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Annelida
Clade:	Pleistoannelida
Clade:	Sedentaria
Order:	Orbiniida; Fauchald 1977sensu Struck et al. 2015
Families
	Apharyngtus; Dinophilidae; Diurodrilidae; Nerillidae; Orbiniidae; Parergodrilidae;

Orbiniidais an order of smallpolychaeteworms in the phylumAnnelida.^[3]It is the earliest divergingcladeinSedentaria.^[2]Along withProtodriliformia(inErrantia), this order is composed ofmeiofaunal marineworms formerly known as "archiannelids".^[4]These worms inhabit themarineinterstitial ecosystem, the space betweensand grains.^[2]

Classification

First mentioned in 1977, Orbiniida was an order of thepolyphyleticinfraclass "Scolecida"of the class"Polychaeta".It was composed of the familiesOrbiniidae,ParaonidaeandQuestidae,all of which share in common the lack of antennae and palps, and the presence of an eversiblepharynxand biramousparapodiawith simplechaetae.^[1]

However, throughphylogenetic analysisit was found thatQuestidaeandParaonidaeare more closely related toClitellataandCirratulidarespectively. Because of this, the taxonomic composition of Orbiniida was changed to exclude these two families and include several interstitial groups previously belonging to thepolyphyletic"Archiannelida":Orbiniidae,Parergodrilidae,Nerillidae,Diurodrilidae,andApharyngtus,and possiblyDinophilidaetoo.^[3]

Evolutionary history

Phylogenetic analysisof annelids has found Orbiniida to be the earliest diverging clade ofSedentaria.At the same time, the other half of "archiannelid"worms,Protodriliformia,was found to be the earliest diverging clade ofErrantia.Theconvergencebetween these two groups occurred throughprogenesisandminiaturization,as a way to adapt to the marine interstitial ecosystem between sand grains (i.e. interstitium). This means that thelarvalor juvenile stages of a largerpleistoannelidancestor temporarily inhabiting the interstitium stopped theirsomatic growth,becamesexually matureand stayed in the interstitium permanently.^[2]

Pleistoannelida

Errantia

	Aciculata

	Protodriliformia

Sedentaria

Orbiniida

	Orbiniidae

	Parergodrilidae

	Diurodrilidae

	Apharyngtus

	Dinophilidae

	Nerillidae

other Sedentaria

'Archiannelids'

AlthoughDinophilidaepossibly belongs to Orbiniida,^[3]a more recent analysis recovered several times the cladeDinophiliformia,containing the genera inDinophilidae,as sister group toSedentaria+Errantia.This would imply a third separate lineage of interstitial, miniaturized worms.^[5]

References

^^a ^bFauchald, Kristian (3 February 1977),The polychaete worms, definitions and keys to the orders, families and genera(PDF),Science Series, vol. 28, Natural History Museum of Los Angeles County: Los Angeles, CA (USA), pp. 1–188
^^a ^b ^c ^d ^eStruck TH, Golombek A, Weigert A, Franke FA, Westheide W, Purschke G, Bleidorn C, Halanych KM (3 August 2015)."The evolution of annelids reveals two adaptive routes to the interstitial realm".Curr Biol.25(15): 1993–1999.doi:10.1016/j.cub.2015.06.007.PMID 26212885.
^^a ^b ^c ^dMeca MA, Zhadan A, Struck TH (2021)."The Early Branching Group of Orbiniida Sensu Struck et al., 2015: Parergodrilidae and Orbiniidae".Diversity.13(1): 29.doi:10.3390/d13010029.hdl:10852/93589.
^Weigert A, Bleidorn C (2016). "Current status of annelid phylogeny".Org Divers Evol.16:345–362.doi:10.1007/s13127-016-0265-7.
^Martín-Durán JM, Vellutini BC, Marlétaz F, et al. (2021)."Conservative route to genome compaction in a miniature annelid".Nat Ecol Evol.5:231–242.doi:10.1038/s41559-020-01327-6.hdl:10230/46083.

[Fauchald_Polychaetes-1] Fauchald, Kristian (3 February 1977),The polychaete worms, definitions and keys to the orders, families and genera(PDF),Science Series, vol. 28, Natural History Museum of Los Angeles County: Los Angeles, CA (USA), pp. 1–188

[Two_adaptive_routes_Annelida-2] Struck TH, Golombek A, Weigert A, Franke FA, Westheide W, Purschke G, Bleidorn C, Halanych KM (3 August 2015)."The evolution of annelids reveals two adaptive routes to the interstitial realm".Curr Biol.25(15): 1993–1999.doi:10.1016/j.cub.2015.06.007.PMID 26212885.

[Early_Branching_Orbiniida-3] Meca MA, Zhadan A, Struck TH (2021)."The Early Branching Group of Orbiniida Sensu Struck et al., 2015: Parergodrilidae and Orbiniidae".Diversity.13(1): 29.doi:10.3390/d13010029.hdl:10852/93589.

[AnnelidCurrent-4] Weigert A, Bleidorn C (2016). "Current status of annelid phylogeny".Org Divers Evol.16:345–362.doi:10.1007/s13127-016-0265-7.

[Genome_compaction_annelid-5] Martín-Durán JM, Vellutini BC, Marlétaz F, et al. (2021)."Conservative route to genome compaction in a miniature annelid".Nat Ecol Evol.5:231–242.doi:10.1038/s41559-020-01327-6.hdl:10230/46083.

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