Bdelloidea

Bdelloidea 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. 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.