Pleistoannelida

Pleistoannelida
Examples of Pleistoannelida (clockwise from upper left corner): Sabellida, Echiura, Eunicida, Clitellata, Phyllodocida, Siboglinidae.
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
(unranked):	Spiralia
Superphylum:	Lophotrochozoa
Phylum:	Annelida
Clade:	Pleistoannelida Struck 2011[1]
Clades[2][3][4]
Errantia Aciculata Eunicida Phyllodocida Protodriliformia Polygordiidae Protodrilida Sedentaria Orbiniida Cirratuliformia Siboglinidae (=Pogonophora) Sabellida Spionida Opheliida Capitellida Echiura Capitellidae Terebelliformia Maldanomorpha Psammodrilidae Clitellata Incertae sedis Myzostomida Spinther Aberranta

Pleistoannelida is a group of annelid worms that comprises the vast majority of the diversity in phylum Annelida. Discovered through phylogenetic analyses, it is the largest clade of annelids, comprised by the last common ancestor of the highly diverse sister groups Errantia and Sedentaria (Clitellata and related polychaetes) and all the descendants of that ancestor.^[1][4] Most groups in the Clade find their ancestors within the Cambrian explosion when Annelid diversity expanded dramatically.^[5] The Pleistoannelida clade covers a variety of traits. However, the evolution of simple to complex eyes, developed papillae for burrowing, and for some specialized radioles for feeding can be seen universally across every species.^[6][5] New findings have discovered the range of Annelid diversity have led to uncertainty if groups with developed ancestral traits should remain within the clade. Furthermore, there's been a lack of recently discovered Annelid traits being used in the categorization of groups within the clade, leading to many hypothesis on how to do so and which should remain within the clade.^[7] Currently three smaller clades that were originally a part of the groups Errantia and Sedentaria have been proven to fall outside while still being connected to the basal groups.^[6]

Half of a fossil containing the species of worm Esconites zelus.

Phylogeny

Pleistoannelida is composed by two big clades: Errantia (eunicids, siboglinids and related polychaetes) and Sedentaria (spoon worms, tubeworms, clitellates and related polychaetes). There are also smaller groups of difficult placement, such as Myzostomida and Spintheridae, that belong to either one of them.^[2] Closely related to Pleistoannelida is a grade of basal annelids: the Amphinomida/Sipunculida/Lobatocerebrum clade, Chaetopterida and Palaeoannelida.^[4] The clade Amphinomida was once originally included within Errantia, but has since been removed into its own taxa.

A possible, closer sister group to Pleistoannelida could be Dinophiliformia, a clade containing interstitial (meiofaunal) genera previously found in Orbiniida (Sedentaria).^[8]

Annelida	Palaeoannelida Chaetopterida Amphinomida Lobatocerebrum Sipunculida ? Dinophiliformia Pleistoannelida Errantia Aciculata Eunicida Phyllodocida Protodriliformia ? Myzostomida ? Spinther ? Aberranta Sedentaria Orbiniida Cirratuliformia Siboglinidae Sabellida Spionida Opheliida Capitellida Echiura Capitellidae Terebelliformia Maldanomorpha Clitellata

A 2019 analysis recovered the mesozoan group Orthonectida as the sister group to Pleistoannelida.^[9] However, a more recent 2022 analysis found a monophyletic Orthonectida+Dicyemida clade localized outside of the annelids, between Gnathifera and Platyhelminthes.^[10]

Research

Morphological

(A) Anterior end of Magelona mirabilis, showing prostomium, palps, achaetous first segment and first five chaetigers (dorsal view)

There has been research conducted to help determine the taxonomy of the clade Pleistoannelida. The research includes the morphological characteristics of the clade as well as the phylum Annelida as a whole. For example, there are studies of gene expression in annelids. It was supported that larval eyes of annelids are considered homologous to pigmented eyes of bilaterians. It is believed that annelid adult eyes evolved in a common ancestor of Pleistoannelida.^[11] Besides the eyes, it has also been studied that commissures in the brain, glanglia, and nuchal organs were to have also evolved in the lineage of Pleistonannelida (Errantia and Sedentaria) by observing Magelona mirabilis.^[12] At a cellular level, it was found that the highly conserved mitochondrial gene order could be only depicted to the clade Pleistoannelida.^[13]

Genetic

There has been much research done on the genetics of Pleistoannelida and Annelida as a whole to determine the phylogeny of the clade, because many taxa do not share ancestral characteristics. Genetic studies have led to the current phylogenetic tree, which separates the clade into the two sister groups of Errantia and Sedentaria.^[14] Genetic data was also used in the placement of groups of Myzostomida, Nerilliade, and Aberranta within Pleistoannelida. These groups were previously thought to be anywhere from annelids to flatworms, but genetics studies have found strong evidence to place them solidly in Annelida, with the closest relations to groups in the clade Pleistoannelida.^[15][16] The most current research puts Myzostomida in Errantia, however the data is not strong enough to make this a certainty.^[13] Research places Nerillidae closest to Eunicida (supported by certain morphology) and places Aberranta near Nerillidae or syllids and nereidids.^[15]

References

1. Struck TH (November 2011). "Direction of evolution within Annelida and the definition of Pleistoannelida". Journal of Zoological Systematics and Evolutionary Research. 49 (4): 340–345. doi:10.1111/j.1439-0469.2011.00640.x.
2. Weigert A, Bleidorn C (2016). "Current status of annelid phylogeny". Org Divers Evol. 16 (2): 345–362. doi:10.1007/s13127-016-0265-7.
3. 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. Bibcode:2015CBio...25.1993S. doi:10.1016/j.cub.2015.06.007. PMID 26212885.
4. Struck TH (2019). "Phylogeny". In Purschke G, Böggemann M, Westheide W (eds.). Handbook of Zoology: Annelida. Vol. 1: Annelida Basal Groups and Pleistoannelida, Sedentaria I. De Gruyter. pp. 37–68. doi:10.1515/9783110291582-002. ISBN 9783110291469.
5. Zhang, ZhiFei; Smith, Martin R.; Ren, XinYi (2023-02-08). "The Cambrian cirratuliform Iotuba denotes an early annelid radiation". Proceedings of the Royal Society B: Biological Sciences. 290 (1992). doi:10.1098/rspb.2022.2014. ISSN 0962-8452. PMC 9890102. PMID 36722078.
6. Purschke, Günter; Vodopyanov, Stepan; Baller, Anjilie; von Palubitzki, Tim; Bartolomaeus, Thomas; Beckers, Patrick (2022-01-25). "Ultrastructure of cerebral eyes in Oweniidae and Chaetopteridae (Annelida) – implications for the evolution of eyes in Annelida". Zoological Letters. 8 (1): 3. doi:10.1186/s40851-022-00188-0. ISSN 2056-306X. PMC 8787891. PMID 35078543.
7. Weigert, Anne; Connard, Helm; Meyer, Matthias; Birgit, Nickel; Arendt, Detlev; Hausdorf, Bernhard; Santos, Scott R.; Halanych, Kenneth M.; Purschke, Günter; Bleidorn, Christoph; Struck, Torsten H. (2014). "Illuminating the Base of the Annelid Tree Using Transcriptomics". Molecular Biology and Evolution. 31 (6): 1391–1401. doi:10.1093/molbev/msu080 – via Oxford academy.
8. Martín-Durán JM, Vellutini BC, Marlétaz F, et al. (2021). "Conservative route to genome compaction in a miniature annelid". Nat Ecol Evol. 5 (2): 231–242. doi:10.1038/s41559-020-01327-6. hdl:10230/46083. PMC 7854359. PMID 33199869.
9. Zverkov, Oleg A.; Mikhailov, Kirill V.; Isaev, Sergey V.; Rusin, Leonid Y.; Popova, Olga V.; Logacheva, Maria D.; Penin, Alexey A.; Moroz, Leonid L.; Panchin, Yuri V.; Lyubetsky, Vassily A.; Aleoshin, Vladimir V. (24 May 2019). "Dicyemida and Orthonectida: Two Stories of Body Plan Simplification". Front. Genet. 10: 443. doi:10.3389/fgene.2019.00443. PMC 6543705. PMID 31178892.
10. Drábková, Marie; Kocot, Kevin M.; Halanych, Kenneth M.; Oakley, Todd H.; Moroz, Leonid L.; Cannon, Johanna T.; Kuris, Armand; Garcia-Vedrenne, Ana Elisa; Pankey, M. Sabrina; Ellis, Emily A.; Varney, Rebecca; Štefka, Jan; Zrzavý, Jan (2022). "Different phylogenomic methods support monophyly of enigmatic 'Mesozoa' (Dicyemida + Orthonectida, Lophotrochozoa)". Proc. R. Soc. B. 289 (1978): 20220683. doi:10.1098/rspb.2022.0683. PMC 9257288. PMID 35858055.
11. Purschke, Günter; Bleidorn, Christoph; Struck, Torsten (2014). "Systematics, evolution and phylogeny of Annelida – a morphological perspective". Memoirs of Museum Victoria. 71: 247–269. doi:10.24199/j.mmv.2014.71.19.
12. Beckers, Patrick; Helm, Conrad; Bartolomaeus, Thomas (2019). "The anatomy and development of the nervous system in Magelonidae (Annelida) – insights into the evolution of the annelid brain". BMC Evolutionary Biology. 19 (1): 173. Bibcode:2019BMCEE..19..173B. doi:10.1186/s12862-019-1498-9. ISSN 1471-2148. PMC 6714456. PMID 31462293.
13. Weigert, Anne; Golombek, Anja; Gerth, Michael; Schwarz, Francine; Struck, Torsten H.; Bleidorn, Christoph (January 2016). "Evolution of mitochondrial gene order in Annelida". Molecular Phylogenetics and Evolution. 94: 196–206. Bibcode:2016MolPE..94..196W. doi:10.1016/j.ympev.2015.08.008.
14. Weigert, Anne; Bleidorn, Christoph (2016-06-01). "Current status of annelid phylogeny". Organisms Diversity & Evolution. 16 (2): 345–362. doi:10.1007/s13127-016-0265-7. ISSN 1618-1077.
15. Worsaae, Katrine; Nygren, Arne; Rouse, Greg W.; Giribet, Gonzalo; Persson, Jenny; Sundberg, Per; Pleijel, Fredrik (May 2005). "Phylogenetic position of Nerillidae and Aberranta (Polychaeta, Annelida), analysed by direct optimization of combined molecular and morphological data". Zoologica Scripta. 34 (3): 313–328. doi:10.1111/j.1463-6409.2005.00190.x. ISSN 0300-3256.
16. Bleidorn, Christoph; Eeckhaut, Igor; Podsiadlowski, Lars; Schult, Nancy; McHugh, Damhnait; Halanych, Kenneth M.; Milinkovitch, Michel C.; Tiedemann, Ralph (August 2007). "Mitochondrial genome and nuclear sequence data support myzostomida as part of the annelid radiation". Molecular Biology and Evolution. 24 (8): 1690–1701. doi:10.1093/molbev/msm086. ISSN 0737-4038. PMID 17483114.