Therotifers are aphylum of tiny animals which are common infreshwater 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]
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 insoil. Rotifers eat fish waste, deadbacteria, andalgae. They eat particles up to 10 micrometres in size. A rotiferfilters 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 2200species of rotifers have been described. They are placed in the phylum Rotifera. This phylum is subdivided into threeclasses,Monogononta,Bdelloidea, andSeisonidea. 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 speciesHabrotrocha angusticollis have been found in 6000 year oldPleistocene peat deposits.[9] The oldest known fossil rotifers have been found inEoceneDominicanamber.[10]
Habrotrocha rosa, a bdelloid rotifer that lives only within the pitcher leaves ofSarracenia purpurea.
The front has a ring ofcilia circling the mouth. This gave the rotifers their old name of "wheel animalules". There is a protectivelorica 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.
Rotifers have a number of unusual features. Biologists suppose that these peculiarities areadaptations to their small size and the transient (fast changing) nature of itshabitats.
Rotifers are specialists at living inhabitats where water dries up regularly.
The Monogononta, which have males, produce fertilised 'resting eggs' which can resist desiccation (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 ascryptobiosis. Bdelloids can also survive the dry state for long periods: the longest well-documented dormancy is 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]
Rotifers are hatched with a standard number ofcell nuclei, exactly the same number for every rotifer in aspecies. This is calledeutely. Nocell division whatsoever takes place during adult life.[13] 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 asyncytium.[14]
The absence of cell division is probably one reason they are extraordinarily resistant to ionisingradiation. Also, repairingDNA is one of the things they are known to do after desiccation.[15]
In one of the classes, the freshwater Bdelloid rotifers,no males have ever been seen. It is the largest group of whollyparthenogenetic species in theAnimalia.
The females in this group produce eggs by parthenogenesis (virgin birth). In some species these eggs develop into small juveniles before they are released from their parent. The offspring areclones of their mother.
Bdelloid rotifer genomes contain two or more non-identical copies of eachgene. This suggests their asexual reproduction is of long standing.[18] For example, there are four copies of genehsp82. Each is different and on a differentchromosome. This cannot be explaned by normalgene duplication, which produces two or more near-identical genes next to each other. By contrast, in a monogont rotifer, most genes were single-copy.[19]
There are genes in bdelloid rotifers that seem to have come frombacteria,fungi, andplants. This suggests they arrived byhorizontal gene transfer (HGT). The capture and use of exogenous (~foreign) genes seems to be important in bdelloid evolution.[20][21] The team led by Matthew S. Meselson atHarvard University showed that, despite the lack ofsexual reproduction, bdelloid rotifers do engage in genetic (DNA) transfer within aspecies orclade. The method used is not known at present. Bdelloid rotifers currently hold the 'record' for HGT in animals with ~8% of their genes from bacterial origins.[22]
TheAcanthocephala, a group ofparasitic worms previously considered to be a separate phylum, have been shown to be modified rotifers. The exact relationship to the normal, freeliving, members of the phylum is not resolved.[14]
↑They live inside tubes or gelatinous holdfasts that are attached to a substrate
↑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.
↑Nogrady, Thomas, Wallace R.L. & Snell T.W. 1993.Rotifera, vol. 1: biology, ecology and systematics. The Hague: SPB Academic Publishing.
↑Baqai, Aisha; Guruswamy, Vivek; Liu, Janie; and Rizki, Gizem (2000)."Introduction to the Rotifera". University of California Museum of Paleontology.{{cite web}}: CS1 maint: multiple names: authors list (link)
↑Warner B.G. et al. 1988. Holocene fossilHabrotrocha angusticollis (Bdelloidea: Rotifera) in North America.Journal of Paleolimnology1: 141-147.
↑Waggoner B.M. & Poinar G.O. Jr. 1993. Fossil habrotrochid rotifers in Dominican amber.Experientia (Basel)49: 354-357.
↑Örstan A. 1995. Desiccation survival of the eggs of the rotiferAdineta vaga (Davis 1873).Hydrobiologia313/314:373-375
↑Kirk, Kevin L. et al. 1999. Physiological responses to variable environments: storage and respiration in starving rotifers.Freshwater Biology42 637-644.
↑Their eggs are already present in the adult rotifer.
↑E. Gladyshev and M. Meselson. Extreme resistance of bdelloid rotifers to ionizing radiation.Proc. Nat. Acad. Sci., 10.1073/pnas.0800966105 (published online 3/24/08)
