AskDefine | Define rotifer

Dictionary Definition

rotifer n : minute aquatic multicellular organisms having a ciliated wheel-like organ for feeding and locomotion; constituents of freshwater plankton

User Contributed Dictionary

English

Noun

  1. Any of many minute aquatic multicellular organisms, of the phylum Rotifera, that have a ring of cilia resembling a wheel

Extensive Definition

The rotifers make up a phylum of microscopic and near-microscopic pseudocoelomate animals. They were first described by John Harris in 1696 (Hudson and Gosse, 1886). Leeuwenhoek is mistakenly given credit for being the first to describe rotifers but Harris had produced sketches in 1703. Most rotifers are around 0.1-0.5 mm long, and are common in freshwater throughout the world with a few saltwater species. Rotifers may be free swimming and truly planktonic, others move by inchworming along the substrate whilst some are sessile, living inside tubes or gelatinous holdfasts. About 25 species are colonial (e.g. Sinantherina semibullata), either sessile or planktonic.

Structure and form

Rotifers get their name (derived from Greek and meaning "wheel-bearer"; they have also been called wheel animalcules) from the corona, 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 (called the mastax) containing a tiny, calcified, jaw-like structure called the trophi. The cilia also pull the animal, when unattached, through the water. Most free-living forms have pairs of posterior toes to anchor themselves while feeding. Rotifers have bilateral symmetry and a variety of different shapes. 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.
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.
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.
Bdelloid rotifer genomes contain two or more divergent copies of each gene, suggesting a long term asexual evolutionary history. Four copies of hsp82 are, for example, found. Each is different and found on a different chromosome excluding the possibility of homozygous sexual reproduction.

Taxonomy

There are about 2000 species of rotifers, divided into three classes, Monogononta, Bdelloidea, and Seisonidea. The parasitic Acanthocephala is closely related to these groups as well.
rotifer in Arabic: دولابية
rotifer in Catalan: Rotífer
rotifer in Czech: Vířníci
rotifer in Danish: Hjuldyr
rotifer in German: Rädertierchen
rotifer in Estonian: Keriloomad
rotifer in Spanish: Rotifera
rotifer in French: Rotifera
rotifer in Galician: Rotífero
rotifer in Croatian: Kolnjaci
rotifer in Italian: Rotifera
rotifer in Lithuanian: Verpetės
rotifer in Dutch: Raderdieren
rotifer in Japanese: 輪形動物
rotifer in Norwegian: Hjuldyr
rotifer in Norwegian Nynorsk: Hjuldyr
rotifer in Occitan (post 1500): Rotifera
rotifer in Polish: Wrotki (biologia)
rotifer in Portuguese: Rotífero
rotifer in Russian: Коловратки
rotifer in Slovak: Vírniky
rotifer in Finnish: Rataseläimet
rotifer in Swedish: Hjuldjur
rotifer in Vietnamese: Luân trùng
rotifer in Turkish: Tekerlekli hayvanlar
rotifer in Chinese: 轮形动物门
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