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Bdelloids
Scanning electron micrographs showing morphological variation of bdelloid rotifers and their jaws.
Scientific classification
Kingdom: Animalia
Phylum: Rotifera
Class: Bdelloidea
Hudson, 1884
Order: Bdelloida
Families

Adinetidae
Habrotrochidae
Philodinavidae
Philodinidae

Bdelloidea (pronounced /dɨˈlɔɪdiə/) 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 unknown, 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-reproducting 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.[1]

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Notes

  1. ^ 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)
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