They were first described byRev. John Harris in 1696, and other forms were described byAntonie van Leeuwenhoek in 1703.[2] Most rotifers are around 0.1–0.5 mm (0.0039–0.0197 in) long (although their size can range from 50 μm (0.0020 in) to over 2 mm (0.079 in)),[1] and are common infreshwater environments throughout the world with a fewsaltwater species.
Some rotifers are free swimming and trulyplanktonic, others move by inchworming along a substrate, and some aresessile, living inside tubes or gelatinousholdfasts that are attached to a substrate. About 25 species are colonial (e.g.,Sinantherina semibullata), either sessile or planktonic. Rotifers are an important part of the freshwaterzooplankton, being a major foodsource and with many species also contributing to the decomposition of soil organic matter.[3] Most species of the rotifers arecosmopolitan, but there are also someendemic species, likeCephalodella vittata toLake Baikal.[4] Recentbarcoding evidence, however, suggests that some 'cosmopolitan' species, such asBrachionus plicatilis,B. calyciflorus,Lecane bulla, among others, are actuallyspecies complexes.[5][6] In some recent treatments, rotifers are placed withacanthocephalans in a largerclade calledSyndermata.
In June 2021, biologists reported the restoration ofbdelloid rotifers after being frozen for 24,000 years in theSiberianpermafrost.[7][8] Early purported fossils of rotifers have been suggested in Devonian[9] and Permian[10] fossil beds.
John Harris first described the rotifers (in particular abdelloid rotifer) in 1696 as "an animal like a largemaggot 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 anearwig".[2] In 1702,Antonie van Leeuwenhoek gave a detailed description ofRotifer vulgaris and subsequently describedMelicerta ringens and other species.[11] 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 ofChristian Gottfried Ehrenberg'sDie Infusionsthierchen als vollkommene Organismen in 1838 that the rotifers were recognized as beingmulticellular animals.[11]
In the landmark monograph onThe Rotifera (1886-9) byC.T. Hudson, assisted byP.H. Gosse,[12] 400 British and foreign species were included; by 1912, the total reached 607 species.[13] About 2,200species 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 andMonogononta.[14] 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 three species of Seisonidea.[15]
TheAcanthocephala, 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.[16] 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 thecladogram below.
The Rotifera, strictly speaking, are confined to the Bdelloidea and the Monogononta. Rotifera, Acanthocephala and Seisonida make up aclade called Syndermata.[17]
The wordrotifer is derived from aNeo-Latin word meaning 'wheel-bearer'[18] due to the corona around the mouth that in concerted sequential motion resembles a wheel (although the organ does not actually rotate).
Rotifers havebilateral 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. There is a well-developedcuticle, which may be thick and rigid, giving the animal a box-like shape, or flexible, giving the animal a worm-like shape; such rotifers are respectively calledloricate andilloricate. Rigid cuticles are often composed of multiple plates, and may bear spines, ridges, or other ornamentation. Their cuticle is nonchitinous and is formed from sclerotized proteins.
The two most distinctive features of rotifers (in females of all species) are the presence ofcorona on the head, a structureciliated in all genera exceptCupelopagis and 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 asCollotheca, the corona is modified to form a funnel surrounding the mouth. In many species, such as those in the genusTestudinella, the cilia around the mouth have disappeared, leaving just two small circular bands on the head. In thebdelloids, 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.[19]
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.[19]
Rotifers have a small cerebral ganglion, effectively its brain, located just above the mastax, from which a number of nerves extend throughout the body. The number of nerves varies among species, although the nervous system usually has a simple layout.[19]
The nervous system comprises about 25% of the roughly 1,000 cells in a rotifer.[20]
Rotifers typically possess one or two pairs of shortantennae and up to five eyes. The eyes are simple in structure, sometimes with just a singlephotoreceptor 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.[19]
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.[21] In some species it is reduced or may even be absent completely.Benthic species have larger RCO's thanplanktonic species. Despite this diversity, positional correspondence of RCOs strongly suggestshomology.[19][20][22]
A 2023 study usingtransmission electron microscopy andconfocal 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 asyncytial organ, composed of aposterior glandular region, an expansive reservoir, and an anteriorduct. Theglandular portion has an active cytoplasm with pairednuclei, abundantrough ER,ribosomes,Golgi, andmitochondria. Secretion granules accumulate at theanterior end of the gland where they undergohomotypic fusion to create larger granules with numerous "mesh-like" contents. These contents gradually fuse into tubular secretions that accumulate in the reservoir, awaiting secretion.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.[22]
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 ofT. similis secretions showed them to be a series of tube-like secretions with a highly filamentous framework. This is highly suggestive of aglycosaminoglycan structure- proteins with negatively chargedpolysaccharide chains formingproteoglycan molecules. These molecules are standard in vertebrate and invertebrategelatins such asmucus.[22]
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 inbenthic 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.[22]
Scanning electron micrographs showing morphological variation of bdelloid rotifers and their jaws.
Rotifer colonies
Colonial rotifers, tentatively identified asConochilus. The colony is less than 1 mm in diameter, but visible to the naked eye.
Colony ofSinantheria socialis on anElodea 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 chewingpharynx (called themastax), sometimes via a ciliated tube, and sometimes directly. The pharynx has a powerful muscular wall and contains tiny, calcified, jaw-like structures calledtrophi, 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 likeforceps to help bite into prey. In someectoparasitic rotifers, the mastax is adapted to grip onto the host, although, in others, the foot performs this function instead.[19]
Behind the mastax lies anoesophagus, which opens into astomach where most of the digestion and absorption occurs. The stomach opens into a shortintestine that terminates in acloaca on the posterior dorsal surface of the animal. Up to sevensalivary glands are present in some species, emptying to the mouth in front of the oesophagus, while the stomach is associated with two gastric glands that producedigestive enzymes.[19]
A pair ofprotonephridia open into a bladder that drains into the cloaca. These organs expel water from the body, helping to maintainosmotic balance.[19]
The coronalcilia pull the animal, when unattached, through the water.
Like many other microscopic animals, adult rotifers frequently exhibiteutely—they have a fixed number ofcells within a species, usually on the order of 1,000.
Bdelloid rotifer genomes contain two or more divergent copies of eachgene, suggesting a long-term asexualevolutionary history.[23] For example, four copies of hsp82 are found. Each is different and found on a different chromosome excluding the possibility ofhomozygoussexual reproduction.
Video of rotifer feeding, probably of the genusCephalodellaVideo of abdelloid rotifer feeding
Rotifers eat particulate organic detritus, dead bacteria, algae, and protozoans. They eat particles up to 10 micrometres in size. Likecrustaceans, 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 withcladocera andcopepods forplanktonic food sources.
Rotifers aredioecious and reproduce sexually orparthenogenetically. They aresexually 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, or absent altogether.[19]
The female reproductive system consists of one or twoovaries, each with avitellarium gland that supplies the eggs withyolk. Together, each ovary and vitellarium form a singlesyncitial structure in the anterior part of the animal, opening through anoviduct into thecloaca.[19]
Males do not usually have a functional digestive system, and are therefore short-lived, often beingsexually fertile at birth. They have a singletesticle andsperm duct, associated with a pair of glandular structures referred to asprostates (unrelated to the vertebrateprostate). The sperm duct opens into agonopore at the posterior end of the animal, which is usually modified to form apenis. The gonopore ishomologous to the cloaca of females, but in most species has no connection to the vestigial digestive system, which lacks ananus.[19]
In the genusAsplanchna also the females lacks an anus, but have kept the cloacal opening for excretion and the release of eggs.[24]
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").[25] 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.[25] 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.
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 theRotaria, areovoviviparous, retaining the eggs inside their body until they hatch.[19]
Most species hatch as miniature versions of the adult. Sessile species, however, are born as free-swimminglarvae, 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.[19]
The life span ofmonogonont females varies from two days to about three weeks.
'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 byparthenogenesis.
However, a new study provided evidence for interindividual genetic exchange and recombination inAdineta vaga, a species previously thought to be anciently asexual.[26]
Recent transitions: Loss ofsexual reproduction can be inherited in a simpleMendelian fashion in the monogonont rotiferBrachionus 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.[27]
Resting eggs enclose an embryo encysted in a three-layered shell that protects it from external stressors.[28][29] They are able to remain dormant for several decades and can resist adverse periods (e.g., pond desiccation or presence of antagonists).[30][31] When favourable conditions return and after an obligatory period ofdiapause which varies among species, resting eggs hatch releasing diploid amictic females that enter into the asexual phase of the life cycle.[25][32]
Bdelloid rotifer females cannot produce resting eggs, but many can survive prolonged periods of adverse conditions afterdesiccation. This facility is termedanhydrobiosis, 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.[33] While in other anhydrobionts, such as thebrine shrimp, this desiccation tolerance is thought to be linked to the production oftrehalose, 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.[34] This repair mechanism likely involves mitotic recombination between homologous DNA regions.[34]
The genome size of a bdelloid rotifer,Adineta vaga, was reported to be around 244 Mb.[36] 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.[37] Haploid "1C" genome sizes inBrachionus species range at least from 0.056 to 0.416 pg.[38]
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^Wurdak, Elizabeth S.; Gilbert, John J.; Jagels, Richard (January 1978). "Fine Structure of the Resting Eggs of the Rotifers Brachionus calyciflorus and Asplanchna sieboldi".Transactions of the American Microscopical Society.97 (1):49–72.doi:10.2307/3225684.JSTOR3225684.PMID564567.
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^García-Roger, Eduardo M.; Carmona, María José; Serra, Manuel (January 2005). "Deterioration patterns in diapausing egg banks of Brachionus (Müller, 1786) rotifer species".Journal of Experimental Marine Biology and Ecology.314 (2):149–161.Bibcode:2005JEMBE.314..149G.doi:10.1016/j.jembe.2004.08.023.
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