Bdelloidea

Live observation required for morphological identification, as key characteristics involve feeding and crawling behaviors. Three orders distinguished by corona structure: Philodinida (two ciliated discs), Adinetida (ventral ciliated field), Philodinavida (small corona). Four footless genera identified by sliding locomotion (Adineta, Bradyscela) versus looping. Genetic identification possible on dead specimens. Three morphological methodologies exist: Bartoš (1951) and Donner (1965) are dated; Shiel's 1995 diagnostic key is current standard. DNA studies indicate diversity exceeds morphological classifications.

Microscopic organisms 150–700 μm in length, appearing as tiny white dots under weak magnification. Body divided into three regions: head, trunk, and foot. Head bears a retractable, well-developed corona divided into two ciliated parts used for feeding and locomotion. Foot typically present with well-developed foot glands, though absent in four genera (Adineta, Bradyscela, Henoceros, Philodinavus). Mouth opening connects to long esophagus leading to mastax with strong teeth. Stomach visible as prominent organ in trunk; some genera (Habrotrocha, Otostephanos, Scepanotrocha) produce distinct spherical fecal pellets rather than loose material. Covered in many cilia. Preserved specimens contract into unidentifiable 'blobs'.

Freshwater habitats worldwide. Capable of colonizing temporary and ephemeral water bodies due to desiccation resistance. Survives harsh, dry conditions through anhydrobiosis at any life stage.

Global distribution in freshwater habitats.

Suspended bacteria, algae, detritus, and other particulate organic matter. Food captured by ciliary currents in corona and processed by mastax adapted for grinding.

Development occurs without fertilization through obligate parthenogenesis. Embryos develop directly without larval stages. Hatching of young delayed until favorable conditions return following dormancy. Germline DNA repair occurs during specific period of oogenesis via nonreductional meiosis with meiotic-like juxtaposition of homologous chromosomes.

Feeding: Most create water currents via ciliary rings in corona to draw food to mastax. Locomotion: Three methods—free swimming, inch-worming/crawling (alternate head and tail steps, like leeches), or sessility. Foot retracted during feeding in most genera; footless genera slide or loop. Reproduction: Exclusively parthenogenetic; each individual has paired gonads. Stress response: Enters anhydrobiosis (xerosome state) with metabolic adjustments and protective chemical production; can survive up to 9 years. Rehydration and activity resume within hours when water returns. Desiccation facilitates escape from parasites and enables wind dispersal to establish new populations.

Consumer of bacteria, algae, and detritus in freshwater food webs. Survives environmental fluctuations that eliminate other organisms, contributing to ecosystem resilience in temporary waters. Parasite populations destroyed during extended desiccation, with wind dispersal enabling colonization of new habitats.

Subject of extensive research in evolutionary biology due to ancient asexuality, desiccation tolerance, and horizontal gene transfer. Model organism for studying DNA repair mechanisms, including double-strand break repair and resistance to ionizing radiation. 2021 study demonstrated revival after 24,000 years in permafrost, highlighting extreme longevity potential. Potential applications in understanding cryptobiosis and stress tolerance.

• MonogonontaOther major rotifer class with sexual reproduction and cyclic parthenogenesis; males present. Previously considered sister group to Bdelloidea but now understood to be more distantly related within Syndermata.
• SeisonideaSmall marine rotifer class; now considered sister group to Acanthocephala, with this clade being sister to Bdelloidea—making seisonids closest living relatives despite highly modified morphology.
• AcanthocephalaThorny-headed worms, highly modified parasitic organisms; molecular evidence places them as sister to Seisonidea, with this combined group sister to Bdelloidea. Formerly separate phylum, now included in expanded Rotifera/Syndermata.

• parthenogenesis
• anhydrobiosis
• desiccation tolerance
• horizontal gene transfer
• ancient asexual
• cryptobiosis
• freshwater
• microscopic

• BugGuide
• Wikipedia
• GBIF taxonomy match
• iNaturalist taxon
• NCBI Taxonomy
• Catalogue of Life