[2] 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.

[3] Most are slightly too small to be seen with the naked eye, but appear as tiny white dots through even a weak hand lens, especially in bright light.

[7] 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.

[10] 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.

As of 2014, the "most comprehensive phylogenomic analysis of syndermatan relationships" to date was based on transcriptome data from all four groups,[8] and provided "strong support" for the hypothesis illustrated in the bottom left of the figure, in which Seisonidea and Acanthocephala are sister taxa.

In certain genera, (Habrotrocha, Otostephanos and Scepanotrocha) the bdelloid can actually be identified by the appearance of distinct spherical pellets within the stomach, which will be released as faeces.

Bdelloids are of interest in the study of the evolution of sex because a male has never been observed,[20] and females reproduce exclusively by parthenogenesis,[broken anchor] 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.

However, a new study provided evidence for interindividual genetic exchange and recombination in Adineta vaga, a species previously thought to be anciently asexual.

[23] Germline DNA repair occurs in a specific period of oogenesis during which homologous chromosomes take on a meiotic-like juxtaposed configuration.

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.

[26] 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.

[31] 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.

[32] When these creatures recover from desiccation, it has been shown that they undergo a potentially unique genetic process where horizontal gene transfer occurs,[citation needed] resulting in a significant proportion of the bdelloid genome, up to 10%, having been obtained through horizontal gene transfer from bacteria, fungi and plants.

[34] Bdelloid rotifers are extraordinarily resistant to damage from ionizing radiation due to the same DNA-preserving adaptations used to survive dormancy.