Rotifers comprise a phylum, Rotifera, of microscopic and near-microscopic, multicellular aquatic animals. The name rotifer is derived from the Latin word for "wheel-bearer," referring to a characteristic crown of cilia surrounding the mouth of most rotifers, with the cilia movement in some species appearing under the microscope to whirl like a wheel (Baqai et al. 2000). Rotifers are pseudocoelomate invertebrates—that is, they have a fluid filled "false body cavity" that is only partly lined by mesoderm rather than a cavity within the mesoderm.
Rotifers are important in aquatic food chains, being widely distributed in these enviroments, where they consume various microorganisms and detritus and in turn are consumed by small crustaceans, among other animals. Thus, in seeking their own individual purpose (survival, maintenance, reproduction, development), they also provide value to the ecosystem as a whole. Such "dual purposes" underlies the harmony in nature. One of the classes of rotifers, Bdelloidea, is unique in being a higher taxa that is only known to reproduce asexually, which is very rare even at the genus level, given the ubiquity of sexual reproduction.
Leeuwenhoek is considered to have mentioned rotifers in a September 7, 1674 letter and October 9, 1676 letter; he gives his first clear description of rotifers in a letter of October 17, 1687 (Swart 1964). John Harris described them in 1696 (in particular a bdelloid rotifer) 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" (Hudson and Gosse 1886). In 1702, Leeuwenhoek gave a detailed description of Rotifer vulgaris and subsequently described Melicerta ringens and other species (Swart 1964; Bourne 1907). 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 wasn't until the publication of Christian Gottfried Ehrenberg's Die Infusionsthierchen als vollkommene Organismen in 1838 that the rotifers were recognized as being multicellular animals.
About 2200 species of rotifers have been described. Most rotifers are around 0.1-0.5 mm long (Towle 1989), but a few species, such as Rotaria neptunia, may exceed a millimeter (Baqau et al. 2000, Orstan 1999).
Rotifers are common in freshwater throughout the world, with a few saltwater species. They can be found in both still water (lake bottoms) and flowing water (rivers and streams) environments, as well as in moist soil in the films of water around soil particles, on mosses and lichens, in rain gutters and puddles, in leaf litter, on mushrooms, and even on freshwater crustaceans and larvae of aquatic insects (Baqai et al. 2000; Orstan 1999).
Most rotifers are free swimming, but others move by inchworming along the substrate, and some are sessile, living inside tubes or gelatinous holdfasts. About 25 species are colonial (i.e. Sinantherina semibullata), either sessile or planktonic.
In addition to their name meaning "wheel-bearer," rotifers also have been called wheel animalcules) from the corona (crown), which is composed of several ciliated tufts around the mouth that in motion resemble a wheel. These create a current that sweeps food into the mouth, where it is chewed up by a characteristic pharynx (mastax) containing tiny jaws. It also pulls the animal, when unattached, through the water. Most free-living forms have pairs of posterior toes to anchor themselves while feeding.
Rotifers feed on unicellular algae, bacteria, protozoa, and dead and decomposing organic materials, and are preyed upon by shrimp and crabs, among other secondary consumers (Towle 1989; Baqai et al. 2000).
Rotifers have bilateral symmetry. They lack any skeleton (Towle 1989); however, they have a variety of different shapes because of a well-developed cuticle and hydrostatic pressure within the pseudocoelom. This cuticle 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.
Rotifers have specialized organ systems. The rotifer nervous system is composed of anterior ganglia, two anterior eyespots, and two long nerves that transverse the length of the body (Towle 1989). Rotifers have a complete digestive tract with a mouth and anus.
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 one thousand.
Rotifers belong to the superphylum Platyzoa.
Rotifers are typically divided into three classes—Monogononta, Bdelloidea, and Seisonidea—although Acanthocephala (spiny-headed worms, thorny-headed worms) are sometimes placed with Rotifera as well. Monogononta is the largest class, with about 1,500 species, while Seisonidea has only two (or three) known species (Baqai et al. 2000, TOL 2006). Bdelloidea comprises about 350 species placed into four families (Habrotrochidae, Philodinidae, Philodinavidae, and Adinetidae) and 19 genera (TOL 2006).
Both sexual reproduction and asexual reproduction are found in Rotifera. 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").
The entire class Bdelloidea appears to be especially remarkable with their apparent lack of sexual reproduction challenging the view that genetic transfer between individuals within a species is essential for its long-term evolutionary success (Meselson). Among plants and animals, few species reproduce only asexually and even these rarely comprise an entire genus, let alone a taxon of higher rank as with bdelloid rotifers. Instead of sexual reproduction, these rotifers produce eggs from oocytes by two mitotic divisions with no chromosome pairing and no decrease in chromosome number (Meselson). Bdelloid rotifer genomes contain two or more divergent copies of each gene, suggesting a long term asexual evolutionary history (Welch et al. 2004). Four copies of “hsp82” are, for example, found. Each is different and found on a different chromosome, excluding the possibility of homozygous sexual reproduction.
In the class Monogononta, rotifers have the ability to alternate reproduction by sexual or asexual means, though most times asexually. Males in the class Monogononta may be either present or absent depending on the species and environmental conditions. In the absence of males, reproduction is by parthenogenesis and results in clonal offspring that are genetically identical to the parent. Individuals of some species form two distinct types of parthenogenetic eggs; one type develops into a normal parthenogenetic female, while the other occurs in response to a changed environment and develops into a degenerate male that lacks a digestive system, but does have a complete male reproductive system that is used to inseminate females, thereby producing fertilized "resting eggs." Resting eggs develop into zygotes that are able to survive extreme environmental conditions, such as may occur during winter or when the pond dries up. These eggs resume development and produce a new female generation when conditions improve again. The life span of monogonont females varies from a couple of days to about three weeks.
Bdelloid rotifers are unable to 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, however, they resume activity within a few hours. Bdelloids can survive the dry state for prolonged periods, with the longest well-documented dormancy being nine years. 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 lack the ability to synthesise trehalose.
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