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Rotifera
Fossil range: EoceneRecent
Rotaria
Scientific classification
Domain: Eukarya
Kingdom: Animalia
Subkingdom: Eumetazoa
Superphylum: Platyzoa
Phylum: Rotifera
Cuvier, 1798
Classes

Monogononta
Digononta
Bdelloidea
Seisonidea

The rotifers make up a phylum of microscopic and near-microscopic pseudocoelomate animals. They were first described by Rev. John Harris in 1696, and other forms were described by Anton van Leeuwenhoek in 1703.[1] Most rotifers are around 0.1–0.5 mm long (although their size can range from 50μm to over 2 millimeters),[2] and are common in freshwater environments throughout the world with a few saltwater species; for example, those of genus Synchaeta. Some rotifers are free swimming and truly planktonic, others move by inchworming along the substrate, and some are sessile, living inside tubes or gelatinous holdfasts that are attached to a substrate. About 25 species are colonial (e.g., Sinantherina semibullata), either sessile or planktonic. Rotifers play an important part of the freshwater zooplankton, being a major foodsource and with many species also contributing to the decomposition of soil organic matter.[3]

Contents

Taxonomy and naming

Rev. John Harris first described the rotifers (in particular the 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".[1] In 1702, Anton van Leeuwenhoek gave a detailed description of Rotifer vulgaris and subsequently described Melicerta ringens and other species.[4] 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.[4]

About 2200 species of rotifers have been described. Taxonomically, they are placed in the phylum Rotifera. This phylum is subdivided into four classes: Monogononta, Digononta, Bdelloidea, and Seisonidea. The largest group is the Monogononta, with about 1500 species, followed by the Bdelloidea, with about 350 species. There are only two known species of Seisonidea.[5] The Acanthocephala, previously considered to be a separate phylum, have been unequivocally demonstrated to be modified rotifers. However, the exact relationship to other members of the phylum has not yet been resolved.[6]

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Etymology

The word "rotifer" is derived from a Latin word meaning "wheel-bearer,[7] due to the corona around the mouth that in motion resemble a wheel (though the organ does not actually rotate).

Structure and form

The corona of a rotifer is composed of several ciliated tufts around the mouth. 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.

Scanning electron micrographs showing morphological variation of bdelloid rotifers and their jaws.

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.[8] Four copies of hsp82 are, for example, found. Each is different and found on a different chromosome excluding the possibility of homozygous sexual reproduction.

Like nemertine worms, the rotifers have not been proven to be derived from cilia. Its thin lamellae resemble cabbage leaves, and are composed from their creases of the receptor membrane. [9]

Feeding

Rotifers eat fish waste, dead bacteria, and algae. They eat particles up to 10 micrometres in size. Like crustaceans, 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 be in competition with cladocera and copepods for phytoplanktonic food sources.

Images

References

  1. ^ a b Harmer, Sidney Frederic and Shipley, Arthur Everett (1896). The Cambridge Natural History. The Macmillan company. pp. 197. http://books.google.com/books?id=bvoEAAAAYAAJ&pg=PA197&lpg=PA197&dq=%22john+harris%22+rotifer&source=web&ots=hhpqBzp2L4&sig=XXsKKHav5eiRZVu104NbDQvtvwI&hl=en&sa=X&oi=book_result&resnum=3&ct=result. Retrieved 2008-07-25.  
  2. ^ [1] Welcome to the Wonderfully Weird World of Rotifers by Richard L. Howey
  3. ^ [2]
  4. ^ a b Bourne, A.G. (1907). Baynes, Spencer and W. Robertson Smith. ed. Encyclopaedia Britannica. XXI (Ninth Edition ed.). Akron, Ohio: The Werner Company. pp. 8. http://books.google.com/books?id=yAsEAAAAYAAJ&printsec=titlepage#PPA8,M1. Retrieved 2008-07-27.  
  5. ^ Baqai, Aisha; Guruswamy, Vivek; Liu, Janie; and Rizki, Gizem (2000-05-01). "Introduction to the Rotifera". University of California Museum of Paleontology. http://www.ucmp.berkeley.edu/phyla/rotifera/rotifera.html. Retrieved 2008-07-27.  
  6. ^ Shimek, Ronald Ph.D (January 2006). "Nano-Animals, Part I: Rotifaers". Reefkeeping.com. http://reefkeeping.com/issues/2006-01/rs/index.php. Retrieved 2008-07-27.  
  7. ^ Pechenik, Jan A. (2005). Biology of the invertebrates. Boston: McGraw-Hill, Higher Education. pp. 178. ISBN 0072348992.  
  8. ^ J.L.M. Welch, D.B.M Welch, and M. Meselson. Cytogenic evidence for asexual evolution of bdelloid rotifers. Proc. Nat. Acad. Sci., Feb. 2004 vol. 101, no. 6, pp.1618–1621
  9. ^ "Photoreception."Encyclopædia Britannica from Encyclopædia Britannica 2006 Ultimate Reference Suite DVD . 2009.

External links


Simple English

Rotifera
Scientific classification
Domain: Eukarya
Kingdom: Animalia
Subkingdom: Eumetazoa
Superphylum: Platyzoa
Phylum: Rotifera
Cuvier, 1798
Classes

Monogononta, Bdelloidea, Seisonidea

[[File:|thumb|right|180px|]] The rotifers are a phylum of tiny animals which are common in freshwater environments, such as ponds and puddles.[1] Some rotifers are free swimming, others move by inching along, and some are fixed.[2] A few species live in colonies.[3][4]

Contents

History and taxonomy

Rotifers were first described when early microscopes became available, around 1700AD.[5] They are an important part of the freshwater zooplankton. Also, many species help decompose organic matter in soil. Rotifers eat fish waste, dead bacteria, and algae. They eat particles up to 10 micrometres in size. A rotifer filters 100,000 times its own volume of water per hour. They are used in fish tanks to help clean the water, to prevent clouds of waste matter.

About 2200 species of rotifers have been described. They are placed in the phylum Rotifera. This phylum is subdivided into three classes, Monogononta, Bdelloidea, and Seisonidea. The largest group is the Monogononta, with about 1500 species, followed by the Bdelloidea, with about 350 species.[6] There are only two known species of Seisonidea.[7][8]

Fossils of the species Habrotrocha angusticollis have been found in 6000 year old Pleistocene peat deposits.[9] The oldest known fossil rotifers have been found in Eocene Dominican amber.[10]

Appearance

The front has a ring of cilia circling the mouth. This gave the rotifers their old name of "wheel animalules". There is a protective lorica round its body, and a foot. Inside the lorica are the usual organs in miniturised form: a brain, an eye-spot, jaws, stomach, kidneys, urinary bladder.

Peculiarities

Biologists suppose that these peculiarities are adaptations to its small size and the transient (fast changing) nature of its habitats.

Resisting drought

Rotifers are specialists at living in habitats where water dries up regularly. The Monogononta, which have males, produce zygotes which can resist dessication (drought) for long periods.[11] The Bdelloids, who have no males, contract into an inert form and lose almost all body water, a process known as cryptobiosis. Bdelloids can survive the dry state for prolonged periods, with the longest well-documented dormancy being nine years. After they have dried, they may be revived by adding water. In this, and several other ways, they are a unique group of animals.[12]

Cell number

Rotifers are hatched with a standard number of cell nuclei, exactly the same number for every rotifer in a species. This is called eutely. No cell division whatsoever takes place during adult life. Not only that, but the number of nuclei in each tissue is constant. Furthermore, most of the nuclei do not have cell walls: rotifer tissue is largely or wholly a syncytium.[13]

Genetics

Bdelloid rotifer genomes contain two or more divergent copies of each gene, suggesting a long term asexual evolutionary history.[14] Four copies of hsp82 are, for example, found. Each is different and found on a different chromosome excluding the possibility of homozygous sexual reproduction.

Genes in bdelloid rotifers have been found which appear to have originated in bacteria, fungi, and plants. The capture and functional assimilation of exogenous genes may represent an important force in bdelloid evolution.[15][16] The team led by Matthew S. Meselson at Harvard University has also shown that, despite the lack of sexual reproduction, bdelloid rotifers do engage in genetic (DNA) transfer within a species or clade. The method used is not known at present.

Parthenogenesis

In one of the classes, the freshwater Bdelloid rotifers, no males have ever been seen. The females in this group produce eggs by parthenogenesis (virgin birth). These eggs develop into small juveniles before they are released from their parent. They are clones of their mother. When the juvenile exits the mother, it already contains developing embryos.

Cytological and molecular genetic studies have provided evidence that bdelloids evolved from a common ancestor that lost sexual recombination (meiosis and syngamy) about 100 million years ago.[17] Research has also been done on the implications of parthenogenesis for speciation.[18]

The Acanthocephala

The Acanthocephala, a group of parasitic worms previously considered to be a separate phylum, have been shown to be modified rotifers. However, the exact relationship to other members of the phylum has not yet been resolved.[13]

Rotifer web sites

  • Microscopy UK Gallery of Rotifers [3]
    • Live birth [4]
  • Micrographia.com Rotifer guide [5]
  • Wheelbase [6]

References

  1. There are a few saltwater species.
  2. They live inside tubes or gelatinous holdfasts that are attached to a substrate
  3. Clément P. and Wurdak E. 1991. Rotifera. In: Harrison F.W. and E.E. Ruppert eds. Microscopic Anatomy of Invertebrates vol 4. pages 219-297. Wiley-Liss, New York.
  4. Nogrady, Thomas, Wallace R.L. & Snell T.W. 1993. Rotifera, vol. 1: biology, ecology and systematics. The Hague: SPB Academic Publishing.
  5. Harmer, Sidney Frederic and Shipley, Arthur Everett (1896). The Cambridge Natural History. Macmillan, London.. http://books.google.com/books?id=bvoEAAAAYAAJ&pg=PA197&lpg=PA197&dq=%22john+harris%22+rotifer&source=web&ots=hhpqBzp2L4&sig=XXsKKHav5eiRZVu104NbDQvtvwI&hl=en&sa=X&oi=book_result&resnum=3&ct=result. 
  6. Ricci, Claudia & Melone, Guilio 2000. Key to the identification of the genera of bdelloid rotifers. Hydrobiologia 418: 73-80.
  7. Hudson C.T. and P.H. Gosse. 1889. The Rotifera: or, wheel-animalcules. Longmans Green, London.
  8. Baqai, Aisha; Guruswamy, Vivek; Liu, Janie; and Rizki, Gizem (2000). "Introduction to the Rotifera". University of California Museum of Paleontology. http://www.ucmp.berkeley.edu/phyla/rotifera/rotifera.html. 
  9. Warner B.G. et al. 1988. Holocene fossil Habrotrocha angusticollis (Bdelloidea: Rotifera) in North America. Journal of Paleolimnology 1: 141-147.
  10. Waggoner B.M. & Poinar G.O. Jr. 1993. Fossil habrotrochid rotifers in Dominican amber. Experientia (Basel) 49: 354-357.
  11. Örstan A. 1995. Desiccation survival of the eggs of the rotifer Adineta vaga (Davis 1873). Hydrobiologia 313/314:373-375
  12. Kirk, Kevin L. et al. 1999. Physiological responses to variable environments: storage and respiration in starving rotifers. Freshwater Biology 42 637-644.
  13. 13.0 13.1 Shimek, Ronald 2006. "Nano-Animals, Part I: Rotifaers". Reefkeeping.com. http://reefkeeping.com/issues/2006-01/rs/index.php. 
  14. Welch J.L.M. Welch D.B.M and Meselson M. 2004. Cytogenic evidence for asexual evolution of bdelloid rotifers. PNAS 101 pp1618–1621
  15. Gladyshev E.A. Meselson M. & Arkhipova I.R. 2008. Massive horizontal gene transfer in Bdelloid rotifers. Science 320, pp1210 - 1213
  16. Arkhipova I.R. and Meselson M. 2005. Diverse DNA transposons in rotifers of the class Bdelloidea. PNAS 102: 11781-11786
  17. Welch, Mark [1]
  18. Fontaneto D. et al 2007. Independently evolving species in asexual Bdelloid rotifers. PLoS [2]


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