Rotifera Cuvier, Dataset English Wikipedia - Species Pages Rank PHYLUM Classification phylum Rotifera In popular culture "The Rotifers" is a science fiction short story by Robert Abernathy, about a young boy who observes rotifers in a microscope that his father gave him, discovering that just as he observes them, they are able to observe him as well. It was first published in the March issue of If and has appeared in various anthologies of classic science fiction since. It appears on Project Gutenberg. John Harris first described the rotifers in particular a bdelloid rotifer in 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". In , Antonie van Leeuwenhoek gave a detailed description of Rotifer vulgaris and subsequently described Melicerta ringens and other species. He was also the first to publish observations of the revivification of certain species after drying.
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Around 2, rotifer species have been described Segers Most species are in the range of 0. Different rotifer species display a striking variety of body forms and the morphology of individuals may be further altered e.
Most rotifers are solitary, but there are a small number of colonial mostly sessile, i. Although there are both solitary and colonial sessile rotifers, most rotifers are motile and very active.
Wallace and Snell and references therein; Wallace ; Brusca and Brusca The anterior end of a rotifer bears a ciliary organ known as the corona. In action, the movements of the coronal cilia can give the impression of a pair of rotating wheels, giving this phylum its name.
The ventral appendage known as the "foot" secretes a sticky cement for temporarily attaching the rotifer to a substrate. In larvae of sessile rotifers, the cement forms a bond with the substrate that is not easily broken and if sessile rotifers are dislodged they do not reattach Wallace Most rotifer species display eutely, i. Rotifers are best known from freshwater, but some live in damp soil or moss and there are many marine species, although only a fraction of these are exclusively marine Wallace and Snell and references therein.
Parthenogenesis is very common among rotifers in general and is the exclusive mode of reproduction among the bdelloid rotifers.
The bdelloid rotifers, which account for around a quarter of the roughly described rotifers Segers , have attracted particular attention from researchers because the bdelloid clade is the most ancient group of animals known in which males are unknown and reproduction is strictly parthenogenetic i. In addition to their striking lack of males, bdelloids have attracted attention for the fact that most of them can undergo anhydrobiosis, slowly drying out until they resemble a wrinkled barrel, with the head and foot retracted into the animals's trunk Wallace and Snell Desiccated bdelloids may be revived after many years in this state.
Wilson and Sherman proposed that the ability of bdelloids to dry up, as well as to be transported long distances by air currents in their desiccated state, may allow them to escape fungal parasites in both time and space. An important and well supported hypothesis for the widespread evolutionary maintenance of sex in organisms in general is that sexual recombination creates genetic variation that allows organisms to stay ahead of their parasites in a never ending coevolutionary arms race.
Wilson and Sherman suggest that the alternative mechanism they have documented for bdelloid rotifers to escape their parasites may help explain how this clade could persist for tens of millions of years in the absence of sex. The Rotifera were formerly believed to be sister to the endparasitic Acanthocephala. Depending on the species, colonies may range in size from fewer than 5 individuals to large colonies of 50 to individuals or, in the case of a few species, more than a thousand individuals.
The ventral appendage known as the "foot" usually has 2 toes range 0 to 4 and pedal glands, with ducts exiting near the toes, that secrete a sticky cement for temporarily attaching the rotifer to substrates. The rotifer's pharynx is modified as a "mastax" consisting of sets of internal jaws. Rotifers are best known from freshwater, but some live in damp soil or moss and there are many marine species, although only a fraction of these are exclusively marine see review of saltwater rotifers by Fontaneto et al.
Densities in freshwater generally range up to around rotifers per liter, but with abundant food densities can exceed individuals per liter and, reportedly, in some water bodies at certain times densities may exceed , individuals per liter Wallace and Snell and references therein. Rotifers vary in their feeding habits. Some rotifers are ciliary suspension feeders; others grasp prey mainly small animals, but some plant material as well with protrusible, pincer-like mastax jaws; and still others draw prey into a funnel-shaped trap formed by spines or setae on the corona Brusca and Brusca A few rotifers have evolved symbiotic relationships with other species, for example living on the gills of crustacean arthropods or on snail egg masses see May for a review of the literature on commensal and parasitic relationships between rotifers and other organisms.
Although parasitism is not common among rotifers in general, there is one major exception: The Acanthocephala, until recently generally treated as a phylum but now believed to be a clade of parasitic rotifers, most likely sister to the free-living bdelloids Garcia-Varela and Nadler ; Min and Park ; Witek et al.
Most rotifers are oviparous, releasing eggs outside their bodies, but a a few are ovoviviparous, retaining the embryo inside the body until it hatches. Although most rotifers other than the bdelloids are dioecious i. The bdelloid rotifers have attracted particular attention from researchers because the bdelloid clade is the most ancient group of animals known in which males are unknown and reproduction is strictly parthenogenetic i.
Bdelloids account for nearly a quarter of the roughly 2, described rotifers Segers The life cycle of some rotifer species may include both a parthenogenetic amictic phase and a mictic phase in which haploid eggs are produced, some of which may parthenogenetically produce haploid males, which can then fertilize haploid eggs with their mitotically produced haploid sperm. The Rotifera were formerly believed to be sister to the endparasitic Acanthocephala, with the two groups together forming a clade that has been referred to as the Syndermata.
Based on a large-scale analysis of molecular data amino acid sequences , Witek et al. The Gnathifera was originally proposed as a monophyletic clade based on jaw morphology and may also include the Micrognathozoa and, conceivably, the Cycliophora Funch et al.
Wallace and Snell provide a thorough review of rotifer biology, including a dichotomous key to families. Smith provides a dichotomous key to the freshwater genera of the United States. Segers reviews the global diversity of rotifers. They were first described by Rev. John Harris in , and other forms were described by Antonie van Leeuwenhoek in Some rotifers are free swimming and truly planktonic , others move by inchworming along a substrate, and some are sessile , living inside tubes or gelatinous holdfasts that are attached to a substrate.
About 25 species are colonial e. 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. In some recent treatments, rotifers are placed with acanthocephalans in a larger clade called Syndermata. John Harris first described the rotifers in particular a bdelloid rotifer in 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 ".
Other forms were described by other observers, but it wasn't until the publication of Christian Gottfried Ehrenberg 's Die Infusionsthierchen als vollkommene Organismen in that the rotifers were recognized as being multicellular animals. About species of rotifers have 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. There are only two known genera with three species of Seisonidea. 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. The Rotifera, strictly speaking, are confined to the Bdelloidea and the Monogononta. Rotifera, Acanthocephala and Seisonida make up a clade called Syndermata.
The word rotifer is derived from a Neo-Latin word meaning " wheel -bearer",  due to the corona around the mouth that in concerted sequential motion resembles a wheel though the organ does not actually rotate. 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. There is a well-developed cuticle , 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 called loricate and illoricate.
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 most distinctive feature of rotifers is the presence of a ciliated structure, called the corona , on the head. In the more primitive species, this 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 bdelloids , 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.
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.
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.
In some ectoparasitic rotifers, the mastax is adapted to grip onto the host, although, in others, the foot performs this function instead. 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 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 produce digestive enzymes. A pair of protonephridia open into a bladder that drains into the cloaca. These organs expel water from the body, helping to maintain osmotic balance. Rotifers have a small 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. Close to the brain lies a retrocerebral organ , consisting of two glands either side of a medial sac. The sac drains into a duct that divides into two before opening through pores on the uppermost part of the head.
The function of the retrocerebral organ is unclear. Rotifers typically possess one or two pairs of short antennae and 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.
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 cells within a species, usually on the order of 1, Bdelloid rotifer genomes contain two or more divergent copies of each gene , suggesting a long-term asexual evolutionary history. Each is different and found on a different chromosome excluding the possibility of homozygous sexual reproduction. Rotifers eat particulate organic detritus, dead bacteria, algae, and protozoans.
Rotifera Cuvier, 1798