View source for Rotifer

{{Short description|Phylum of pseudocoelomate invertebrates}}
{{Use dmy dates|date=May 2020}}
{{Automatic taxobox
| name = Rotifera
| fossil_range = {{Fossil range|earliest=Devonian|Middle Jurassic|Recent}} Possible Devonian and Permian records
| image = Mikrofoto.de-Raedertier-14.jpg
| image_caption = Bdelloid rotifer ([[Bdelloidea]])
| image2 = Pulchritia dorsicornuta - ZooKeys-342-001-g001.jpg
| image2_caption = ''Pulchritia dorsicornuta'' ([[Monogononta]])
| display_parents = 7
| taxon = Rotifera
| authority = [[Georges Cuvier|Cuvier]], 1798
| subdivision_ranks = Classes and other subgroups
| subdivision = 
* [[Bdelloidea]]
* [[Monogononta]]
* [[Seisonidae]]
*[[Acanthocephala]]{{small|(cladistically included)}}
*{{extinct}}''[[Juracanthocephalus]]''
and [[#Taxonomy and naming|See text]].
}}

The '''rotifers''' ({{IPAc-en|ˈ|r|oʊ|t|ᵻ|f|ər|z}}, from [[Latin]] {{lang|la|[[wikt:rota#Latin|rota]]}} 'wheel' and {{lang|la|[[wikt:-fer#Latin|-fer]]}} 'bearing'), sometimes called '''wheel animals''' or '''wheel animalcules''',<ref name="Howey">{{cite web |last=Howey |first=Richard L. |year=1999 |title=Welcome to the Wonderfully Weird World of Rotifers |url=http://www.microscopy-uk.org.uk/mag/indexmag.html?http://www.microscopy-uk.org.uk/mag/artnov99/rotih.html |access-date=2010-02-19 |publisher=Micscape Magazine |language=en-US}}</ref> make up a [[phylum]] ('''Rotifera''' {{IPAc-en|r|oʊ|'|t|I|f|ər|@}}) of microscopic and near-microscopic [[Coelom#Pseudocoelomates|pseudocoelomate]] [[animal]]s.

They were first described by [[John Harris (writer)|Rev. John Harris]] in 1696, and other forms were described by [[Antonie van Leeuwenhoek]] in 1703.<ref name="cambridge1896">{{cite book | title = The Cambridge Natural History |author1=Harmer, Sidney Frederic  |author2=Shipley, Arthur Everett  |name-list-style=amp | year = 1896 | publisher = The Macmillan company | url = https://archive.org/details/cambridgenatura00shipgoog | quote = john harris rotifer. | pages = [https://archive.org/details/cambridgenatura00shipgoog/page/n9 197] | access-date = 2008-07-25}}</ref> Most rotifers are around {{cvt|0.1–0.5|mm}} long (although their size can range from {{cvt|50|μm}} to over {{cvt|2|mm}}),<ref name="Howey" /> and are common in [[freshwater]] environments throughout the world with a few [[Seawater|saltwater]] species.

Some rotifers are free swimming and truly [[plankton]]ic, others move by inchworming along a substrate, and some are [[Sessility (zoology)|sessile]], living inside tubes or gelatinous [[holdfast (biology)|holdfast]]s that are attached to a substrate. About 25 species are [[Colony (biology)|colonial]] (e.g., ''[[Sinantherina semibullata]]''), either sessile or planktonic. Rotifers are an important part of the freshwater [[zooplankton]], being a major foodsource and with many species also contributing to the decomposition of soil organic matter.<ref>{{cite web|url=http://www.freshwaterlife.org/servlet/CDSServlet?status=ND01MDU3JmN0bl9pbmZvX3ZpZXdfc2l6ZT1jdG5faW5mb192aWV3X2Z1bGwmNj1lbiYzMz0qJjM3PWtvcw~~|archive-url=https://archive.today/20120801144708/http://www.freshwaterlife.org/servlet/CDSServlet?status=ND01MDU3JmN0bl9pbmZvX3ZpZXdfc2l6ZT1jdG5faW5mb192aWV3X2Z1bGwmNj1lbiYzMz0qJjM3PWtvcw~~|archive-date=2012-08-01|title=Rotifers|work=Freshwater Life |access-date=2010-02-19}}</ref> Genetic evidence indicates that the parasitic [[acanthocephala]]ns are a highly specialised group of rotifers.<ref name=":0">{{Cite journal |last1=Luo |first1=Cihang |last2=Parry |first2=Luke A. |last3=Boudinot |first3=Brendon E. |last4=Wang |first4=Shengyu |last5=Jarzembowski |first5=Edmund A. |last6=Zhang |first6=Haichun |last7=Wang |first7=Bo |date=2025-04-09 |title=A Jurassic acanthocephalan illuminates the origin of thorny-headed worms |url=https://www.nature.com/articles/s41586-025-08830-5 |journal=Nature |volume=641 |issue=8063 |pages=674–680 |language=en |doi=10.1038/s41586-025-08830-5 |bibcode=2025Natur.641..674L |issn=0028-0836|url-access=subscription }}</ref>

Most species of the rotifers are [[Cosmopolitan distribution|cosmopolitan]], but there are also some [[Endemism|endemic]] species, like ''[[Cephalodella vittata]]'' to [[Lake Baikal]].<ref>Hendrik Segers (2007). Annotated checklist of the rotifers (Phylum Rotifera), with notes on nomenclature, taxonomy</ref> Recent [[DNA barcoding|barcoding]] evidence, however, suggests that some 'cosmopolitan' species, such as ''[[Brachionus plicatilis]]'', ''[[Brachionus calyciflorus|B. calyciflorus]]'', ''[[Lecane bulla]]'', among others, are actually [[species complex]]es.<ref>{{Cite journal|title = Speciation in ancient cryptic species complexes: evidence from the molecular phylogeny of ''Brachionus plicatilis'' (Rotifera)|journal = Evolution; International Journal of Organic Evolution|date = July 2002|issn = 0014-3820|pmid = 12206243|pages = 1431–1444|volume = 56|issue = 7|first1 = Africa|last1 = Gómez|first2 = Manuel|last2 = Serra|first3 = Gary R.|last3 = Carvalho|first4 = David H.|last4 = Lunt|doi=10.1111/j.0014-3820.2002.tb01455.x|doi-access = free}}</ref><ref>Dec 2011 4th Internat. Barcode of Life conference, University of Adelaide</ref> In some recent treatments, rotifers are placed with [[acanthocephalan]]s in a larger [[clade]] called '''Syndermata'''.

In June 2021, biologists reported the restoration of [[Bdelloidea|bdelloid rotifers]] after being frozen for 24,000 years in the [[Siberia]]n [[permafrost]].<ref name="NYT-20210607">{{cite news |last=Renault |first=Marion |title=This Tiny Creature Survived 24,000 Years Frozen in Siberian Permafrost - The microscopic animals were frozen when woolly mammoths still roamed the planet, but were restored as though no time had passed. |url=https://www.nytimes.com/2021/06/07/science/frozen-rotifers-siberia.html |date=7 June 2021 |work=[[the New York Times]] |access-date=8 June 2021 }}</ref><ref name=":1">{{cite journal |last1=Shmakova |first1=Lyubov |last2=Malavin |first2=Stas |last3=Iakovenko |first3=Nataliia |last4=Vishnivetskaya |first4=Tatiana |last5=Shain |first5=Daniel |last6=Plewka |first6=Michael |last7=Rivkina |first7=Elizaveta |title=A living bdelloid rotifer from 24,000-year-old Arctic permafrost |journal=Current Biology |date=June 2021 |volume=31 |issue=11 |pages=R712–R713 |doi=10.1016/j.cub.2021.04.077 |pmid=34102116 |s2cid=235365588 |doi-access=free |bibcode=2021CBio...31.R712S }}</ref> The earliest record of the rotifer clade is of an acanthocephalan from the [[Middle Jurassic]] of China.<ref name=":0" /> Earlier purported fossils of rotifers have been suggested in Devonian<ref>{{cite web|url=http://chertnews.de/attack.html|title=Spoilt attack in the Lower Devonian}}</ref> and Permian<ref>{{cite journal|url=https://www.researchgate.net/publication/236255968|title=The Oldest Bdelloid Rotifera from Early Permian sediments of Chamba Valley: A New Discovery|journal=International Journal of Geology, Earth and Environmental Science}}</ref> fossil beds.

== Taxonomy and naming ==
{{See also|List of bilaterial animal orders}}

[[John Harris (writer)|John Harris]] first described the rotifers (in particular a [[Bdelloidea|bdelloid rotifer]]) in 1696 as "an animal like a large [[maggot]] which could contract itself into a spherical figure and then stretch itself out again; the end of its tail appeared with a forceps like that of an [[earwig]]".<ref name="cambridge1896"/> In 1702, [[Antonie van Leeuwenhoek]] gave a detailed description of ''[[Rotaria rotatoria|Rotifer vulgaris]]'' and subsequently described ''[[Floscularia ringens|Melicerta ringens]]'' and other species.<ref name="brit1907">{{cite book | title = Encyclopædia Britannica | edition = Ninth | year = 1907 | page = 8 | volume = XXI | author = Bourne, A.G. | editor = Baynes, Spencer and W. Robertson Smith | publisher = The Werner Company | location = Akron, Ohio | url = https://books.google.com/books?id=yAsEAAAAYAAJ}}</ref> He was also the first to publish observations of the revivification of certain species after drying. Other forms were described by other observers, but it was not until the publication of [[Christian Gottfried Ehrenberg]]'s {{lang|de|Die Infusionsthierchen als vollkommene Organismen}} in 1838 that the rotifers were recognized as being [[multicellular]] animals.<ref name="brit1907"/>

In the landmark monograph on ''The Rotifera'' (1886-9) by [[Charles Thomas Hudson|C.T. Hudson]], assisted by [[Philip Henry Gosse|P.H. Gosse]],<ref>Charles Thomas Hudson, ''The Rotifera; or Wheel-Animalcules.'' Assisted by P.H. Gosse, [https://books.google.com/books?id=Lt5L5lxgpDIC&pg=RA1-PA5&dq=charles+thomas+hudson,+%22the+rotifera%22&hl=en&newbks=1&newbks_redir=0&sa=X&ved=2ahUKEwjzg57h5ZSMAxXUv4kEHeAKCIoQ6AF6BAgMEAM#v=onepage&q=charles%20thomas%20hudson%2C%20%22the%20rotifera%22&f=fa volume 1] and [https://books.google.com/books?id=9dsgDrxr-NIC&pg=PP196&dq=charles+thomas+hudson,+%22the+rotifera%22&hl=en&newbks=1&newbks_redir=0&sa=X&ved=2ahUKEwjzg57h5ZSMAxXUv4kEHeAKCIoQ6AF6BAgNEAM#v=onepage&q=charles%20thomas%20hudson%2C%20%22the%20rotifera%22&f=fals volume 2], 1886; Hudson and Gosse, ''[https://books.google.com/books?id=PZ5aAAAAQAAJ&printsec=frontcover&dq=charles+thomas+hudson,+%22the+rotifera%22&hl=en&newbks=1&newbks_redir=0&sa=X&ved=2ahUKEwjzg57h5ZSMAxXUv4kEHeAKCIoQ6AF6BAgOEAM#v=onepage&q=charles%20thomas%20hudson%2C%20%22the%20rotifer The Rotifera; or Wheel-Animalcules, both British and Foreign. Supplement with illustrations].'' Assisted by P.H. Gosse, 1889.</ref> 400 British and foreign species were included; by 1912, the total reached 607 species.<ref>Wertheimer, Douglas (2024), ''[https://search.worldcat.org/title/1495001167 Philip Henry Gosse: A Biography],'' Glasgow: Brethren Archivists and Historians Network, p.575 fn.250.</ref> About 2,200 [[species]] of rotifers have now been described. Their taxonomy is currently in a state of flux. One treatment places them in the phylum Rotifera, with three classes: [[Seisonidea]], [[Bdelloidea]] and [[Monogononta]].<ref>{{Citation|last1=Barnes |first1=R.S.K. |last2=Calow |first2=P. |last3=Olive |first3=P.J.W. |last4=Golding |first4=D.W. |last5=Spicer |first5=J.I. |year=2001 |title=The Invertebrates: a synthesis |location=Oxford; Malden, MA |publisher=Blackwell |isbn=978-0-632-04761-1 |name-list-style=amp }}, p. 98</ref> The largest group is the Monogononta, with about 1,500 species, followed by the Bdelloidea, with about 350 species. There are only two known genera with four species of Seisonidea.<ref name="Berkeley">{{cite web | url = http://www.ucmp.berkeley.edu/phyla/rotifera/rotifera.html | title = Introduction to the Rotifera | access-date = 2008-07-27 | date = 1 May 2000 |author1=Baqai, Aisha |author2=Guruswamy, Vivek |author3=Liu, Janie |author4=Rizki, Gizem |name-list-style=amp | publisher = University of California Museum of Paleontology}}</ref>

The [[Acanthocephala]], previously considered to be a separate phylum, have been demonstrated to be modified rotifers. The exact relationship to other members of the phylum has not yet been resolved.<ref name="shimek">{{cite web | url = http://reefkeeping.com/issues/2006-01/rs/index.php | author = Shimek, Ronald | access-date = 2008-07-27 | title = Nano-Animals, Part I: Rotifers | publisher = Reefkeeping.com |date=January 2006 }}</ref> One possibility is that the Acanthocephala are closer to the Bdelloidea and Monogononta than to the Seisonidea; the corresponding names and relationships are shown in the [[cladogram]] below.

{{clade|style=line-height:100%
|1={{clade
|label1=[[Syndermata]]
|1={{clade
   |1=[[Seisonida]]
   |2={{clade
   |label1=[[Eurotatoria]]
   |1={{clade
   |1=[[Bdelloidea]]
   |2=[[Monogononta]]
  }}
   |2=[[Acanthocephala]]
   }}
 }}
}}
}}

The Rotifera, strictly speaking, are confined to the Bdelloidea and the Monogononta. Rotifera, Acanthocephala and Seisonida make up a [[clade]] called Syndermata.<ref>{{Citation |last1=Ruppert |first1=Edward E. |last2=Fox |first2=Richard S |last3=Barnes |first3=Robert D. |year=2004 |title=Invertebrate zoology: a functional evolutionary approach |edition=7th |location=Belmont, CA |publisher=Thomson-Brooks/Cole |isbn=978-0-03-025982-1 |name-list-style=amp |url-access=registration |url=https://archive.org/details/isbn_9780030259821 }}, p. 788ff. – see particularly p. 804</ref> The findings of a fossil called ''[[Juracanthocephalus]]'' shares features with both Seisonidea and Acanthocephala, suggesting that they are sister groups.<ref>{{cite journal | last1=Luo | first1=Cihang | last2=Parry | first2=Luke A. | last3=Boudinot | first3=Brendon E. | last4=Wang | first4=Shengyu | last5=Jarzembowski | first5=Edmund A. | last6=Zhang | first6=Haichun | last7=Wang | first7=Bo | title=A Jurassic acanthocephalan illuminates the origin of thorny-headed worms | journal=Nature | date=2025 | volume=641 | issue=8063 | pages=674–680 | doi=10.1038/s41586-025-08830-5 | pmid=40205043 | bibcode=2025Natur.641..674L | url=https://www.nature.com/articles/s41586-025-08830-5 | url-access=subscription }}</ref>

Giribet & Edgecombe (2020)<ref>{{Cite book |last1=Giribet |first1=Gonzalo |title=The invertebrate tree of life |last2=Edgecombe |first2=Gregory D. |publisher=Princeton University Press |year=2020 |isbn=978-0-691-17025-1 |page=296 |language=en}}</ref> and Brusca ''et al''. (2023)<ref>{{Cite book |last1=Brusca |first1=Richard C. |title=Invertebrates |last2=Giribet |first2=Gonzalo |last3=Moore |first3=Wendy |publisher=Oxford University Press |year=2023 |isbn=978-0-19-755441-8 |edition=4th |pages=286 |language=en}}</ref> accept the following classification:

* class Hemirotatoria/Hemirotifera
** subclass [[Bdelloidea]]
** subclass [[Acanthocephala]]
** subclass [[Seisonidea]]/Seisonacea
* class Eurotifera
** subclass [[Monogononta]]

=== Etymology ===
The word ''[[wikt:rotifer|rotifer]]'' is derived from a [[Neo-Latin]] word meaning '[[wheel]]-bearer'<ref>{{cite book |title=Biology of the invertebrates |last=Pechenik |first=Jan A.|year=2005 |publisher=McGraw-Hill, Higher Education |location=Boston |isbn=978-0-07-234899-6|page=178}}</ref> due to the corona around the mouth that in concerted sequential motion resembles a wheel (although the organ does not actually rotate).

== Anatomy ==
[[File:Figure 28 03 05.jpg|thumb|upright=1.5| {{center|Anatomy of a bdelloid rotifer}}]]

Rotifers have [[bilateral symmetry]] and a variety of different shapes. The body of a rotifer is divided into a head, trunk, and foot, and is typically somewhat cylindrical. The trunk contains visceral organs, and often, sensory antennae.  There is a well-developed [[cuticle]], found everywhere except in the corona, which is secreted by a fibrous layer in the syncytial epidermis.  This fibrous layer may be thick and rigid, made of giving the animal a box-like shape, or flexible, giving the animal a worm-like shape; such rotifers are respectively called ''loricate'' and ''illoricate''. Loricate fibrous layers are often composed of multiple plates or rings, and may bear spines, ridges, or other ornamentation.  Certain species have superficial rings in the body wall imitating segments.  Also, sub-epidermal muscles, which may be circular, longitudinal, or traversing the pseudocoel to the visceral organs.  This large fluid-filled pseudocoel contains certain muscles and mesenchymal ameboid cells.<ref name=":2">{{Cite book |title=Integrated principles of zoology |date=2008 |publisher=McGraw-Hill/Higher Education |isbn=978-0-07-297004-3 |editor-last=Hickman |editor-first=Cleveland P. |edition=14th |location=Boston |pages=317–318}}</ref>  Their cuticle is nonchitinous and is formed from [[Sclerite|sclerotized]] proteins.

The two most distinctive features of rotifers (in females of all species) are the presence of ''corona'' on the head, a structure [[cilia]]ted in all genera except ''Cupelopagis,'' and the presence of mastax. In the more primitive species, the corona forms a simple ring of cilia around the mouth from which an additional band of cilia stretches over the back of the head. In the great majority of rotifers, however, this has evolved into a more complex structure.

Modifications to the basic plan of the corona include alteration of the cilia into bristles or large tufts, and either expansion or loss of the ciliated band around the head. In genera such as ''[[Collotheca]]'', the corona is modified to form a funnel surrounding the mouth. In many species, such as those in the genus ''[[Testudinella]]'', the cilia around the mouth have disappeared, leaving just two small circular bands on the head. In the [[bdelloid]]s, this plan is further modified, with the upper band splitting into two rotating wheels, raised up on a pedestal projecting from the upper surface of the head.<ref name=IZ>{{cite book |author= Barnes, Robert D. |year=1982 |title= Invertebrate Zoology |publisher= Holt-Saunders International |location= Philadelphia, PA|pages= 272–286|isbn= 978-0-03-056747-6}}</ref>

The trunk forms the major part of the body, and encloses most of the internal organs. The foot projects from the rear of the trunk, and is usually much narrower, giving the appearance of a tail. The cuticle over the foot often forms rings, making it appear segmented, although the internal structure is uniform. Many rotifers can retract the foot partially or wholly into the trunk. The foot ends in from one to four toes, which, in sessile and crawling species, contain adhesive glands to attach the animal to the substratum. In many free-swimming species, the foot as a whole is reduced in size, and may even be absent.<ref name=IZ/>  Rotifers move by swimming with the coronal cilia and/or foot-assisted leech-like creeping.<ref name=":2" />

=== Nervous system ===
Rotifers have a small bilobed cerebral ganglion, effectively its brain, located just above the mastax, from which a number of paired nerves extend throughout the body, namely, sense organs, mastax, muscles, and viscera.<ref name=":2" />  The number of nerves varies among species, although the nervous system usually has a simple layout.<ref name=IZ/>

The nervous system comprises about 25% of the roughly 1,000 cells in a rotifer.<ref name="wallace" />

Rotifers typically possess one or two pairs of short dorsal [[antenna (biology)|antennae]], and with usually paired eyespots (and possibly up to five eyes). The eyes are simple in structure, sometimes with just a single [[photoreceptor cell]]. In addition, the bristles of the corona are sensitive to touch, and there are also a pair of tiny sensory pits lined by cilia in the head region, as well as bristles and papillae.<ref name=":2" /><ref name=IZ/>

=== Retrocerebral organ ===
Despite over 100 years of research, rotifer anatomy still has many poorly understood components. One of the more mysterious organs in rotifers is the "retrocerebral organ" (RCO), which still remains very enigmatic in its morphology, function, development, and evolution. Lying close to the brain, this organ usually consists of one or more glands and a sac or reservoir. The sac drains into a duct before opening through pores on the uppermost part of the head. Current data shows a wide diversity in structure and potential function.<ref>Fontaneto, D., & De Smet, W. H. (2015). Rotifera, chapter 4 Handbook of Zoology, Gastrotricha, Cycloneuralia and Gnathifera, Volume 3, Gastrotricha and Gnathifera Schmidt-Rhaesa, Andreas.</ref>  In some species it is reduced or may even be absent completely. [[Benthic zone|Benthic]] species have larger RCO's than [[plankton]]ic species. Despite this diversity, positional correspondence of RCOs strongly suggests [[Homology (biology)|homology]].<ref name=IZ/><ref name="wallace">
Robert Lee Wallace.
[https://academic.oup.com/icb/article/42/3/660/724027/Rotifers-Exquisite-Metazoans1 "Rotifers: Exquisite Metazoans"].
2002.
{{doi | 10.1093/icb/42.3.660 }}
quote:
"What is the function of the retrocerebral organ?"
</ref><ref name=Hochberg/>

A 2023 study using [[transmission electron microscopy]] and [[Confocal microscopy|confocal laser scanning microscopy]] has illuminated the fine structure of this organ further. The study, the first of its kind, investigated the RCO in one species, ''Trichocerca similis''. It was determined to be a [[Syncytium|syncytial organ]], composed of a [[Anatomical terms of location|posterior]] glandular region, an expansive reservoir, and an anterior [[Duct (anatomy)|duct]]. The [[gland]]ular portion has an active cytoplasm with paired [[Cell nucleus|nuclei]], abundant [[Endoplasmic reticulum|rough ER]], [[ribosome]]s, [[Golgi apparatus|Golgi]], and [[Mitochondrion|mitochondria]]. Secretion granules accumulate at the [[Anatomical terms of location|anterior]] end of the gland where they undergo [[Cell fusion|homotypic fusion]] to create larger granules with numerous "mesh-like" contents.  These contents gradually fuse into tubular secretions that accumulate in the reservoir, awaiting secretion. [[Striated muscle tissue|Cross-striated longitudinal muscles]] form a partial sleeve around the reservoir and may function to squeeze the secretions through the gland's duct that often penetrates through the cerebral ganglion.<ref name=Hochberg>Hochberg, R., Araújo, T. Q., Walsh, E. J., Mohl, J. E., & Wallace, R. L. (2023). Fine structure of the retrocerebral organ in the rotifer Trichocerca similis (Monogononta). Invertebrate Biology, 142(1), e12396. https://doi.org/10.1111/ivb.12396.</ref>

====Retrocerebral organ secretions====
Much like the organ itself, the precise function and biochemical makeup of the secretions is still unknown. The small size of rotifers and small volume of the secretions makes isolation immensely difficult. The secretions have some similarities to the hydrogel secretions that form gelatinous housings in some rotifer species.
[[Ultrastructure]] analysis of ''T. similis'' secretions showed them to be a series of tube-like secretions with a highly filamentous framework. This is highly suggestive of a [[glycosaminoglycan]] structure- proteins with negatively charged [[polysaccharide]] chains forming [[proteoglycan]] molecules. These molecules are standard in vertebrate and invertebrate [[gelatin]]s such as [[mucus]].<ref name=Hochberg/>

Despite recent advancements in understanding RCO organ and secretion ultrastructure, the exact function of the organ is still ultimately unclear. The leading hypotheses are that the RCO secretes a mucus-like substance that aids in [[Benthic boundary layer|benthic]] locomotion, adhesion, and/or reproduction (i.e., attachment of eggs to a substrate), although more research is needed to explore function and evaluate the homology between species.<ref name=Hochberg/>

=== Digestive system ===
[[File:Bdelloid.JPG|thumb|upright=1.5|right|Scanning electron micrographs showing morphological variation of bdelloid rotifers and their jaws.]]

{{Multiple image
| direction = vertical
| align = right
| width = 220
| header = Rotifer colonies
| image1 = 20100708 colonial rotifer Conochilius.JPG
| caption1 = Colonial rotifers, tentatively identified as ''Conochilus''. The colony is less than 1 mm in diameter, but visible to the naked eye.
| image2 = Sinantheria-socialis.jpg
| caption2 = Colony of ''Sinantheria socialis'' on an ''[[Elodea densa]]'' leaf. Note heart-shaped corona of individuals.
}}

The coronal cilia create a current that sweeps food into the mouth. The mouth opens into a characteristic chewing [[pharynx]] (called the ''mastax''), sometimes via a ciliated tube, and sometimes directly. The pharynx has a powerful muscular wall and contains tiny, calcified, jaw-like structures called ''trophi'', which are the only fossilizable parts of a rotifer. The shape of the trophi varies between different species, depending partly on the nature of their diet. In suspension feeders, the trophi are covered in grinding ridges, while in more actively carnivorous species, they may be shaped like [[forceps]] to help bite into prey, pierce it inside the pharynx, and retain only the edible portions to be consumed.  The diet of carnivorous species comprises mainly protozoa and small metazoans.  In rotifer species that trap their prey, there is a funnel shaped structure around the mouth, and lobes turn inward to contain the prey and draw it to the mouth and pharynx.<ref name=":2" />  In some [[ectoparasite|ectoparasitic]] rotifers, the mastax is adapted to grip onto the host, although, in others, the foot performs this function instead.<ref name=IZ/>

Behind the mastax lies an [[oesophagus]], which opens into a [[stomach]] where most of the digestion and absorption occurs. The stomach opens into a short [[intestine]] that terminates in a [[cloaca]] on the posterior dorsal surface of the animal. Up to seven [[salivary gland]]s are present in some species, emptying to the mouth in front of the oesophagus, while the stomach is associated with two gastric glands that produce [[digestive enzyme]]s.<ref name=IZ/> Digestion is [[Extracellular digestion|extracellular]] and the stomach absorbs the nutrients.<ref name=":2" />

A pair of [[protonephridium|protonephridia]] open into a bladder that drains into the cloaca. These organs expel water from the body, helping to maintain [[osmoregulation|osmotic balance]].<ref name=IZ/>

{{Multiple image
| caption_align     = center
| direction         = horizontal
| align             = left
| header            = Individual rotifers
| total_width       = 660
| image1            = Philodina rugosa 756859.jpg
| caption1          = ''Philodina rugosa''
| image2            = Mikrofoto.de-Raedertier Ptygura pilula 2.jpg
| caption2          = ''Ptygura pilula''
| image3            = Mikrofoto.de-Brachionus quadridentatus 6.jpg
| caption3          = ''[[Brachionus|Brachionus quadridentatus]]''
}}

{{clear}}

===Biology===
The coronal [[cilia]] pull the animal, when unattached, through the water.

Like many other microscopic animals, adult rotifers frequently exhibit [[eutely]]—they have a fixed number of [[cell (biology)|cell]]s within a species, usually on the order of 1,000.

Bdelloid rotifer genomes contain two or more divergent copies of each [[gene]], suggesting a long-term asexual [[evolution]]ary history.<ref>{{cite journal |author= Jessica L. Mark Welch, David B. Mark Welch & Matthew Meselson |date=10 February 2004 |title=Cytogenetic evidence for asexual evolution of bdelloid rotifers |journal=[[Proceedings of the National Academy of Sciences]] |volume=101 |issue=6 |pages=1618–1621 |doi=10.1073/pnas.0307677100 |pmid=14747655 |pmc=341792 |bibcode=2004PNAS..101.1618W |doi-access=free }}</ref> For example, four copies of hsp82 are found. Each is different and found on a different chromosome excluding the possibility of [[homozygous]] [[sexual reproduction]].

==Feeding==
[[File:Movie of Rotifer, feeding (probably of the genus Cephalodella).theora.ogv|thumb|Video of rotifer feeding, probably of the genus ''[[Cephalodella]]'']]
[[File:Feeding rotifer.ogv|thumb|Video of a [[bdelloid]] rotifer feeding]]
Rotifers eat particulate organic detritus, dead bacteria, algae, and protozoans. They eat particles up to 10 micrometres in size. Like [[crustacean]]s, rotifers contribute to nutrient recycling. For this reason, they are used in fish tanks to help clean the water, to prevent clouds of waste matter. Rotifers affect the species composition of algae in ecosystems through their choice in grazing. Rotifers may compete with [[cladocera]] and [[copepod]]s for [[plankton]]ic food sources.

==Reproduction and life cycle==
Rotifers are [[dioecious]] and reproduce sexually or [[parthenogenesis|parthenogenetically]]. They are [[sexual dimorphism|sexually dimorphic]], with the females always being larger than the males. In some species, this is relatively mild, but in others the female may be up to ten times the size of the male. In parthenogenetic species, males may be present only at certain times of the year (Monogononta), or absent altogether (Bdelloidea).<ref name=":2" /><ref name=IZ/>

The female reproductive system consists of one or two [[ovary|ovaries]], each with a ''[[vitellarium]]'' gland that supplies the eggs with [[yolk]]. Together, each ovary and vitellarium form a single [[syncitium|syncitial]] structure in the anterior part of the animal, opening through an [[oviduct]] into the [[cloaca]].. Unlike ectolecithal groups like Platyhelminthes, yolk is supplied to the ovum not by sepаrate yolk cells but by flow-through cytoplasmic bridges.  Males do not usually have a functional digestive system, and are therefore short-lived, often being [[sexually fertile]] at birth. They have a single [[testicle]] and [[sperm duct]], associated with a pair of glandular structures referred to as ''prostates'' (unrelated to the vertebrate [[prostate]]). The sperm duct opens into a [[gonopore]] at the posterior end of the animal, which is usually modified to form a [[penis]]. The gonopore is [[homology (biology)|homologous]] to the cloaca of females, but in most species has no connection to the vestigial digestive system, which lacks an [[anus]].<ref name=":2" /><ref name="IZ" />

In the genus [[Asplanchna]] also the females lacks an anus, but have kept the cloacal opening for excretion and the release of eggs.<ref>{{Cite journal |last1=Hejnol |first1=Andreas |last2=Martín-Durán |first2=José M. |date=May 2015 |title=Getting to the bottom of anal evolution |url=https://linkinghub.elsevier.com/retrieve/pii/S004452311500011X |journal=Zoologischer Anzeiger - A Journal of Comparative Zoology |language=en |volume=256 |pages=61–74 |doi=10.1016/j.jcz.2015.02.006|bibcode=2015ZooAn.256...61H |hdl=1956/10848 |hdl-access=free }}</ref>

The phylum Rotifera encloses three classes that reproduce by three different mechanisms: Seisonidea only reproduce sexually; Bdelloidea reproduce exclusively by asexual parthenogenesis; Monogononta reproduce alternating these two mechanisms ("cyclical parthenogenesis" or "heterogony"). Parthenogenesis (amictic phase) dominates the monogonont life cycle, promoting fast population growth and colonization. In this phase males are absent and amictic females produce diploid eggs by mitosis which develop parthenogenetically into females that are clones of their mothers. Some amictic females can generate mictic females that will produce haploid eggs by meiosis. Mixis (meiosis) is induced by different types of stimulus depending on species. Haploid eggs develop into haploid dwarf males if they are not fertilized and into diploid "resting eggs" (or "diapausing eggs") if they are fertilized by males. Such eggs are often dispersed by winds or birds.<ref name=":2" /><ref name="Nogrady, T. 1993">Nogrady, T., Wallace, R.L., Snell, T.W., 1993. Rotifera vol.1: biology, ecology and
systematics. Guides to the identification of the microinvertebrates of the continental
waters of the world 4. SPB Academic Publishing bv, The Hague.</ref>

Fertilization is internal. The male either inserts his penis into the female's cloaca or uses it to penetrate her skin, injecting the sperm into the body cavity. The egg secretes a shell, and is attached either to the substratum, nearby plants, or the female's own body. A few species, such as members of the ''[[Rotaria]]'', are [[ovoviviparous]], retaining the eggs inside their body until they hatch.  The zygote undergoes modified [[spiral cleavage]].<ref name=":2" /><ref name=IZ/>

Most species hatch as miniature versions of the adult. Sessile species, however, are born as free-swimming [[larva]]e, which closely resemble the adults of related free-swimming species. Females grow rapidly, reaching their adult size within a few days, while males typically do not grow in size at all.<ref name=IZ/>

The life span of [[Monogononta|monogonont]] females varies from two days to about three weeks.

===Loss of sexual reproduction system===
''''Ancient asexuals':''' [[Bdelloid]] rotifers are assumed to have reproduced without sex for many millions of years. Males are absent within the species, and females reproduce only by [[parthenogenesis]].

However, a new study provided evidence for interindividual genetic exchange and recombination in ''[[Adineta]] vaga'', a species previously thought to be anciently asexual.<ref>{{cite journal |author=Vakhrusheva, O.A. |author2=Mnatsakanova, E.A. |author3=Galimov, Y.R. |display-authors=etal|title=Genomic signatures of recombination in a natural population of the bdelloid rotifer Adineta vaga |date=18 December 2020|journal=Nature|volume=11 |issue=1 |page=6421 |doi=10.1038/s41467-020-19614-y |pmid=33339818 |pmc=7749112 |bibcode=2020NatCo..11.6421V }}</ref>

'''Recent transitions:''' Loss of [[sexual reproduction]] can be inherited in a simple [[Mendelian inheritance|Mendelian fashion]] in the monogonont rotifer ''[[Brachionus calyciflorus]]'': This species can normally switch between sexual and asexual reproduction (cyclical parthenogenesis), but occasionally gives rise to purely asexual lineages (obligate parthenogens). These lineages are unable to reproduce sexually due to being homozygous for a recessive allele.<ref>{{cite journal |author=Claus-Peter Stelzer |author2=Johanna Schmidt |author3=Anneliese Wiedlroither |author4=Simone Riss |date=20 September 2010 |title=Loss of sexual reproduction and dwarfing in a small metazoan |journal=[[PLoS ONE]] |volume=5 |issue=9 |article-number=e12854 |doi=10.1371/journal.pone.0012854 |pmid=20862222 |pmc=2942836|bibcode=2010PLoSO...512854S |doi-access=free }}</ref>

===Resting eggs===
Resting eggs enclose an embryo encysted in a three-layered shell that protects it from external stressors.<ref>{{cite journal |last1=Wurdak |first1=Elizabeth S. |last2=Gilbert |first2=John J. |last3=Jagels |first3=Richard |title=Fine Structure of the Resting Eggs of the Rotifers Brachionus calyciflorus and Asplanchna sieboldi |journal=Transactions of the American Microscopical Society |date=January 1978 |volume=97 |issue=1 |pages=49–72 |pmid=564567 |jstor=3225684 |doi=10.2307/3225684 }}</ref><ref>{{cite book |last1=Clément |first1=P. |last2=Wurdak |first2=E. |year=1991 |chapter=Rotifera |editor1-last=Harrison |editor1-first=F.W. |editor2-last=Ruppert |editor2-first=E.E. |title=Microscopic Anatomy of Invertebrates |series=Aschelminthes, vol. IV |publisher=Wiley-Liss |pages=219–97 }}</ref> They are able to remain dormant for several decades and can resist adverse periods (e.g., pond desiccation or presence of antagonists).<ref>{{cite journal |last1=Marcus |first1=Nancy H. |last2=Lutz |first2=Robert |last3=Burnett |first3=William |last4=Cable |first4=Peter |title=Age, viability, and vertical distribution of zooplankton resting eggs from an anoxic basin: Evidence of an egg bank |journal=Limnology and Oceanography |date=January 1994 |volume=39 |issue=1 |pages=154–158 |doi=10.4319/lo.1994.39.1.0154 |bibcode=1994LimOc..39..154M |doi-access=free }}</ref><ref>{{cite book |doi=10.1007/978-94-010-0756-6_37 |chapter=Reproductive isolation among geographically and temporally isolated marine Brachionus strains |title=Rotifera IX |pages=283–290 |year=2001 |last1=Kotani |first1=T. |last2=Ozaki |first2=M. |last3=Matsuoka |first3=K. |last4=Snell |first4=T. W. |last5=Hagiwara |first5=A. |isbn=978-94-010-3820-1 }}</ref> When favourable conditions return and after an obligatory period of [[diapause]] which varies among species, resting eggs hatch releasing diploid amictic females that enter into the asexual phase of the life cycle.<ref name="Nogrady, T. 1993"/><ref>{{cite journal |last1=García-Roger |first1=Eduardo M. |last2=Carmona |first2=María José |last3=Serra |first3=Manuel |title=Deterioration patterns in diapausing egg banks of Brachionus (Müller, 1786) rotifer species |journal=Journal of Experimental Marine Biology and Ecology |date=January 2005 |volume=314 |issue=2 |pages=149–161 |doi=10.1016/j.jembe.2004.08.023 |bibcode=2005JEMBE.314..149G }}</ref>

===Anhydrobiosis===
Bdelloid rotifer females cannot produce resting eggs, but many can survive prolonged periods of adverse conditions after [[desiccation]]. This facility is termed [[anhydrobiosis]], and organisms with these capabilities are termed anhydrobionts. Under drought conditions, bdelloid rotifers contract into an inert form and lose almost all body water; when rehydrated they resume activity within a few hours. Bdelloids can survive the dry state for long periods, with the longest well-documented dormancy being nine years. Rotifers can also undergo other forms of cryptobiosis, notably cryobiosis which results from decreased temperatures. In 2021, researchers collected  samples from remote Arctic locations containing rotifers which when thawed revealed living specimens around 24,000 years old.<ref name=":1" /> While in other anhydrobionts, such as the [[brine shrimp]], this desiccation tolerance is thought to be linked to the production of [[trehalose]], a non-reducing disaccharide ([[sugar]]), bdelloids apparently cannot synthesise trehalose. In bdelloids, a major cause of the resistance to desiccation, as well as resistance to ionizing radiation, is a highly efficient mechanism for repairing the DNA double-strand breaks induced by these agents.<ref name="pmid25105197">{{cite journal |vauthors=Hespeels B, Knapen M, Hanot-Mambres D, Heuskin AC, Pineux F, LUCAS S, Koszul R, Van Doninck K |title=Gateway to genetic exchange? DNA double-strand breaks in the bdelloid rotifer Adineta vaga submitted to desiccation |journal=J. Evol. Biol. |volume=27 |issue=7 |pages=1334–45 |date=July 2014 |pmid=25105197 |doi= 10.1111/jeb.12326|hdl=2013/ULB-DIPOT:oai:dipot.ulb.ac.be:2013/324107 |url=https://hal-pasteur.archives-ouvertes.fr/pasteur-01420001/file/hespeels%20et%20al%202014.pdf|doi-access=free }}</ref> This repair mechanism likely involves mitotic recombination between homologous DNA regions.<ref name="pmid25105197" />

==Predators==
Rotifers fall prey to many animals, such as [[copepod]]s, fish (e.g. [[herring]], [[salmon]]), [[bryozoa]], [[comb jellies]], [[jellyfish]], [[starfish]], and [[tardigrade]]s.<ref>Wallace, R.L., T.W. Snell, C. Ricci & T. Nogrady (2006). Rotifera Vol. 1: Biology, ecology and systematics. ''Guides to the identification of the microinvertebrates of the continental waters of the world'' '''23''', 299 pp. Kenobi, Ghent/Backhuys, Leiden</ref>

==Genome size==
The genome size of a bdelloid rotifer, ''[[Adineta vaga]]'', was reported to be around 244 Mb.<ref>{{cite journal |last1=Flot |first1=Jean-François |last2=Hespeels |first2=Boris |last3=Li |first3=Xiang |last4=Noel |first4=Benjamin |last5=Arkhipova |first5=Irina |last6=Danchin |first6=Etienne G. J. |last7=Hejnol |first7=Andreas |last8=Henrissat |first8=Bernard |last9=Koszul |first9=Romain |last10=Aury |first10=Jean-Marc |last11=Barbe |first11=Valérie |last12=Barthélémy |first12=Roxane-Marie |last13=Bast |first13=Jens |last14=Bazykin |first14=Georgii A. |last15=Chabrol |first15=Olivier |last16=Couloux |first16=Arnaud |last17=Da Rocha |first17=Martine |last18=Da Silva |first18=Corinne |last19=Gladyshev |first19=Eugene |last20=Gouret |first20=Philippe |last21=Hallatschek |first21=Oskar |last22=Hecox-Lea |first22=Bette |last23=Labadie |first23=Karine |last24=Lejeune |first24=Benjamin |last25=Piskurek |first25=Oliver |last26=Poulain |first26=Julie |last27=Rodriguez |first27=Fernando |last28=Ryan |first28=Joseph F. |last29=Vakhrusheva |first29=Olga A. |last30=Wajnberg |first30=Eric |last31=Wirth |first31=Bénédicte |last32=Yushenova |first32=Irina |last33=Kellis |first33=Manolis |last34=Kondrashov |first34=Alexey S. |last35=Mark Welch |first35=David B. |last36=Pontarotti |first36=Pierre |last37=Weissenbach |first37=Jean |last38=Wincker |first38=Patrick |last39=Jaillon |first39=Olivier |last40=Van Doninck |first40=Karine |title=Genomic evidence for ameiotic evolution in the bdelloid rotifer Adineta vaga |journal=Nature |date=August 2013 |volume=500 |issue=7463 |pages=453–457 |doi=10.1038/nature12326 |pmid=23873043 |bibcode=2013Natur.500..453F |url=https://hal.archives-ouvertes.fr/hal-01282538/document |doi-access=free |hdl=1721.1/87072 |hdl-access=free }}</ref> The genomes of Monogononts seem to be significantly smaller than those of Bdelloids. In Monogononta the nuclear DNA content (2C) in eight different species of four different genera ranged almost fourfold, from 0.12 to 0.46 pg.<ref>{{cite journal |last1=Stelzer |first1=Claus-Peter |title=A first assessment of genome size diversity in Monogonont rotifers |journal=Hydrobiologia |date=1 March 2011 |volume=662 |issue=1 |pages=77–82 |doi=10.1007/s10750-010-0487-1 |pmid=34764494 |pmc=7611972 |bibcode=2011HyBio.662...77S }}</ref> Haploid "1C" genome sizes in ''[[Brachionus]]'' species range at least from 0.056 to 0.416 pg.<ref>{{cite journal |last1=Stelzer |first1=Claus-Peter |last2=Riss |first2=Simone |last3=Stadler |first3=Peter |title=Genome size evolution at the speciation level: The cryptic species complex Brachionus plicatilis(Rotifera) |journal=BMC Evolutionary Biology |date=7 April 2011 |volume=11 |issue=1 |page=90 |doi=10.1186/1471-2148-11-90 |pmid=21473744 |pmc=3087684 |doi-access=free |bibcode=2011BMCEE..11...90S }}</ref>

==Gallery==
<gallery>
File:Pair of Rotifers, likely Euchlanis, from Northeast US Pond sample.jpg| Pair of ''Lepadella'' rotifers from pond water
File:Fish01.png| Locula of the rotifer ''[[Keratella cochlearis]]''
</gallery>

==References==
{{Reflist}}

==External links==
{{Commons category|Rotifera}}
{{Wikispecies|Rotifera}}
{{Wikibooks|Dichotomous Key|Rotifera}}
* Jersabek, C. D. & Leitner, M. F. (2013): [http://www.rotifera.hausdernatur.at/ ''The Rotifer World Catalog'']. World Wide Web electronic publication.
* [http://www.ucmp.berkeley.edu/phyla/rotifera/rotifera.html Introduction to the Rotifera]
* [https://www.plingfactory.de/Science/Atlas/KennkartenTiere/Rotifers/01RotEng/E-TL/TL5Rotifera.html Rotifers of Germany and Neighbouring Countries (Website with high-quality photos)]
* [http://www.micrographia.com/specbiol/rotife/homebdel/bdel0100.htm Rotifers]
* [http://www.tolweb.org/Rotifera/2480 Tree of Life Web Project]
* [http://www.alinweb.com/video/ Rotifer Videos]
* [https://web.archive.org/web/20061003205918/http://icb.oxfordjournals.org/cgi/content/full/42/3/660 Detailed description of Rotifers]
* [http://www.gutenberg.org/files/35879/35879-h/35879-h.html ''The Rotifers'', by Robert Abernathy, on Project Gutenberg]
* {{Cite EB1911|wstitle=Rotifera|short=x}}

{{Animalia}}
{{Life on Earth}}
{{Taxonbar|from=Q180148}}
{{Authority control}}

[[Category:Rotifers| ]]
[[Category:Gnathifera (clade)]]
<!-- [[Category:Animal phyla]] moved to "Rotifera" redirect -->
[[Category:Articles containing video clips]]
[[Category:Extant Eocene first appearances]]
[[Category:Taxa named by Georges Cuvier]]