|Scanning electron micrographs showing morphological variation of bdelloid rotifers and their jaws.|
Bdelloidea (pron.: //) is a class of rotifers found in fresh water and moist soil. Bdelloids typically have a well-developed corona, divided into two parts, on a retractable head. They may move by swimming or crawling. The latter commonly involves taking alternate steps with the head and tail, as do certain leeches, which gives the group their name (Greek βδελλα or bdella, meaning leech).
Bdelloids have been of interest to those interested in the evolutionary role of sexual reproduction, because it has disappeared entirely from the group: males are not present within the species, and females reproduce exclusively by parthenogenesis. Each individual has paired gonads. Despite the fact that they have been asexual for millions of years, they have diversified into more than 300 species and are fairly similar to other sexually reproducing rotifer species.
Bdelloids respond to environmental stresses by entering a state of dormancy known as anhydrobiosis. This dormancy form enables the organism to rapidly dehydrate itself. The Bdelloid will remain in this cysted state until optimal environmental conditions re-occur at which point they will rehydrate and become active within hours. Diapause is the ability of the organism to produce offspring in a dormant and unhatched state. Hatching of the young will only occur when conditions are at their most favourable. These forms of dormancy are also known as cryptobiosis or quiescence.
When these unusual creatures spring from hibernation, they undergo a fascinating and possibly unique genetic process. A study conducted by Matthew Meselson of Harvard University suggests that when bdelloid rotifers recover from suspended animation, they incorporate foreign DNA when patching up their own ruptured cell membranes. Any DNA in proximity to the organisms can be included in the new genome, including semi-digested food. This may be interpreted as an intermediate between true asexual and sexual reproduction.
Bdelloid rotifers have recently been shown to be extraordinarily resistant to damage from ionizing radiation. The same DNA-preserving adaptations used to survive dormancy are thought to work in this case, and may have also helped the organisms to thrive despite their totally asexual mode of reproduction.
See also 
|Wikimedia Commons has media related to: Bdelloidea|
- The Weird Sisters
- Bdelloids: No sex for over 40 million years
- Eighty million years without sex
- The benefits of 80 million years without sex
- An Evolutionary Scandal, from Harvard Magazine
- Who Needs Sex (or Males) Anyway?
- Organisms Gave up Sexual Reproduction for DNA Theft - Bdelloids able to incorporate foreign DNA
- "Bdelloids: No sex for over 40 million years.". TheFreeLibrary. ScienceNews. Retrieved 30 April 2011.
- Gladyshev, Eugene A.; Meselson, Matthew; Arkhipoval, Irina R. (May 30, 2008). "Massive Horizontal Gene Transfer in Bdelloid Rotifers". Science 320 (5880): 1210–1213. doi:10.1126/science.1156407. PMID 18511688. Retrieved November 19, 2012.
- LiveScience Staff (November 16, 2012). "'Odd little creature' skips sex and eats DNA". LiveScience. Retrieved November 19, 2012.
- 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)