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Class of parthenogenetic freshwater rotifers
the mite superfamily

Bdelloidea '/d//ɛ////l//ɔɪ//d//i//ə/ (Greek βδέλλα, bdella, "leech") is a class of rotifers found in freshwater habitats all over the world. There are over 450 described species of bdelloid rotifers (or 'bdelloids'), distinguished from each other mainly on the basis of morphology. The main characteristics that distinguish bdelloids from related groups of rotifers are exclusively parthenogenetic reproduction and the ability to survive in dry, harsh environments by entering a state of desiccation induced dormancy (anhydrobiosis) at any life stage. They are often referred to as "ancient asexuals" due to their unique asexual history that spans back to over 25 million years ago through fossil evidence. Bdelloid rotifers are microscopic organisms, typically between 150 and 700 μm in length.

The phylum Rotifera traditionally included three classes: Bdelloidea, Monogononta and Seisonidea. Prior to 1990, phylogenetic studies based on morphology seemed to indicate that the sister group to bdelloid rotifers was Monogononta, with seisonid rotifers as an early diverging outgroup. Modern molecular phylogenetic studies demonstrate that this classic understanding of 'Rotifera' is incomplete (paraphyletic), because it omits a fourth clade of closely related organisms: the Acanthocephala, or thorny headed worms. Originally classified as a separate phylum, molecular and morphological evidence accumulated between 1994 and 2014 to indicate that Acanthocephala forms a monophyletic group with Bdelloidea, Monogononta and Seisonidea. To accommodate this finding, some authors extend the term 'Rotifera' to include the highly modified, parasitic 'acanthocephalan rotifers' alongside bdelloid, monogonont and seisonid rotifers. Others refer to the grouping of the four taxa as Syndermata, a term derived from their shared syncytial epidermis. These orders are divided into four families and about 450 species. Since these organisms are asexual the usual definition of a species as a group of organisms capable of creating fertile offspring is inapplicable, therefore the species concept in these organisms is based on a mixture of morphological and molecular data instead. DNA studies suggest that the diversity is much greater than the original morphological classifications suggest. Bdelloids can only be identified by eye while they are alive because many of the characteristics significant to classification are related to feeding and crawling; however, genetic identification of bdelloids is possible on dead individuals. Once preserved, the individuals contract into "blobs" which restricts analysis. and Donner (1965).

There appear to be three main methods of movement: free swimming, inch worming along a substrate, or sessility. Inch worming (or crawling) involves taking alternate steps with the head and tail, as do certain leeches, which gives the group their name (Greek βδέλλα or bdella, meaning leech). This video demonstrates how bdelloids move in three different situations: locomotion and feeding of bdelloid rotifers.

Bdelloids are of interest in the study of the evolution of sex because a male has never been observed, and females reproduce exclusively by parthenogenesis, a form of asexual reproduction where embryos grow and develop without the need for fertilization; this is akin to the apomixis seen in some plants. Each individual has paired gonads. Despite having been asexual for millions of years, they have diversified into more than 450 species and are fairly similar to other sexually reproducing rotifer species. However, a new study provided evidence for interindividual genetic exchange and recombination in Adineta vaga, a species previously thought to be anciently asexual. Adineta vaga is capable of carrying out DNA repair by a nonreductional meiosis. Germline DNA repair occurs in a specific period of oogenesis during which homologous chromosomes take on a meiotic like juxtaposed configuration. One theory of how obligate parthenogenesis arose in bdelloid rotifers was that parthenogenic lineages lost the ability to respond to sex inducing signal, which is why these lineages retained their asexuality. The obligate parthenogenetic strains of bdelloid rotifers produce a sex inducing signal but have lost the ability to respond to that signal. It was later discovered that the inability to respond to sex inducing signals in obligate parthenogens was caused by simple Mendelian inheritance of the gene op.

Bdelloids are able to survive environmental stresses by entering a state of dormancy known as anhydrobiosis which enables the organism to rapidly dehydrate and thus resist desiccation. While preparing for this dormant state many metabolic processes are adjusted to equate for the change in state; e. g. the production of protective chemicals. The bdelloid can remain in this state, which is known as a 'xerosome' until the return of a sufficient amount of water, at which point they will rehydrate and become active within hours. Hatching of the young will only occur when conditions are at their most favourable. In addition to surviving desiccation through anhydrobiosis, desiccation stress on two bdelloid species actually helped to maintain fitness and even improved their species fecundity. The rotifers that were consistently kept hydrated fared worse than those desiccated and rehydrated. Bdelloidea have evolved a unique mechanism to help overcome one of the major perils of asexual reproduction. According to the Red Queen hypothesis of co evolution, obligate asexuals will be driven extinct by rapidly changing parasites and pathogens, because they cannot change their genotypes quickly enough to keep up in this never ending race. In populations of bdelloid rotifers, however, many parasites are destroyed during periods of extended desiccation. Moreover, desiccated bdelloid rotifers are easily blown away from parasite infested habitats by wind, and establish new, healthy populations elsewhere, which allows them to escape the Red Queen by moving in time and space instead of using sex to change their genotype. When these creatures recover from desiccation, it has been shown that they undergo a potentially unique genetic process where horizontal gene transfer occurs, resulting in a significant proportion of the bdelloid genome, up to 10%, having been obtained through horizontal gene transfer from bacteria, fungi and plants. How and why horizontal gene transfer occur in bdelloids is under much debate at present; particularly with regards to possible connections between the foreign genes and the desiccation process as well as possible connections to bdelloids' ancient asexuality. When they desiccate completely, their DNA breaks up into many pieces. And when they come back to life after being rehydrated, it creates an opportunity for alien DNA fragments to enter their genome. This process was improved 60 million years ago when they captured a bacterial gene this way, which gave them a new gene regulatory system. The new system was used to keep transposons in check. Bdelloid rotifers are extraordinarily resistant to damage from ionizing radiation due to the same DNA preserving adaptations used to survive dormancy. These adaptations include an extremely efficient mechanism for repairing DNA double strand breaks. This repair mechanism was studied in two Bdelloidea species, Adineta vaga, and appears to involve mitotic recombination between homologous DNA regions within each species.

Large scale horizontal transfer of bacterial, plant and fungal genes into bdelloid rotifers has been documented, and may represent an important factor in bdelloid evolution.