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For the mite superfamily, see Bdelloidea (mites).
[Scanning electron microscope
Scientific classification
Kingdom: Animalia
Superphylum: Platyzoa
Phylum: Rotifera
Class: Bdelloidea

Bdelloidea /dɨˈlɔɪdiə/ (bdelloids) is a class of rotifers found in fresh water and moist soil all over the world. There are believed to be around 450 different species of bdelloid rotifer, with varied characteristics through morphology or behaviour.[1] The main characteristics shared by bdelloids are parthenogenetic reproduction and the ability to survive in harsh environments by anhydrobiosis (desiccation-tolerance).[2] The size of bdelloids varies between 150 and 700 µm, although there are some species such as the Rotaria neptunia that are particularly long at 1600 µm.

Classification and Identification[edit]

Bdelloidea is a class of the phylum Rotifera, consisting of three orders: Philodinavida, Philodinida and Adinetida. These orders are divided into four families and about 450 species.[3] 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.

Bdelloids can only be identified by eye whilst they are alive because many of the characteristics significant to classification are to do with feeding and crawling; however, genetic identification of bdelloids is possible on dead individuals. Once preserved, the individuals contract into "blobs" which restricts analysis.[4] There are currently three morphological identification methodologies, two of which are considered dated by Bartôs (1951)[5] and Donner (1965).[3] The third method is a diagnostic key developed in 1995 by Shiel.[4]


SEM] pictures of some species of the genus Rotaria with head (red), tail (white) and trunk (blue) areas highlighted

There are three main regions of the body of bdelloids: head, trunk and foot. The image on the right depicts each area to show how body parts can be very different although they are named the same depending on the species involved. Bdelloids typically have a well-developed corona, divided into two parts, on a retractable head.

Some identifiable features of the bdelloids include :

  • Well-developed foot glands[4]
  • A mouth opening with a long oesophagus[4]
  • Strong teeth (labelled by a tooth index)[4]
  • Many cilia[4]
  • Species-specific upper lip shape[4]
  • Order-specific corona type[2]
    • Philodinida consist of two ciliated discs
    • Adinetida consist of a ventral ciliated field
    • Philodinavida have a small corona

The bdelloid digestive and reproductive systems can be found within the trunk sections of their bodies, with the stomach being the most visible of the organs. 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. These pellets are a distinguishing characteristic since all the other genera release faeces as loose material.[2]

Most bdelloids retract the foot whilst they eat, but there are four genera that lack a foot - Adineta, Bradyscela, Henoceros and Philodinavus. This affects not only how they feed but also how they crawl; for instance Adineta and Bradyscela slide whereas the other genera loop.[2]

Evolutionary relationships[edit]

Since the phylum Rotifera only consists of three classes, Bdelloidea, Monogononota and Seisonidea, there are only three possible rooted solutions for the resulting taxon tree. All three taxon trees (Figure 2) have been suggested but tree A is generally the most accepted due to a number of characteristics that are synapomorphic between Bdelloidea and Monogononta. That is not to say however that trees B and C are wrong.[6]

Figure 2: Possible evolutionary trees for the phylum rotifera. B = Bdelloidea; M = Monogononota; and S = Seisonidea

It should be noted however that the basis behind these three orders is morphological and there has been recent evidence suggesting that all may not be as it appears. Many of the phylogenetic relationships within the phylum Bdelloidea itself are contested and so the situation is complication further when attempting to map the relationships to other phylum.[7]

More recent evidence (published 2014) using transcriptome data shows Bdelloidea, Monogononota and Seisonidea grouped with another class, the Acanthocephala, in a group termed the Syndermata. This gives various possible implications for the evolution of characters within this group, including the size of the corona and what this means in terms of parasitism.[8]


The behaviour of bdelloids can be split into four categories: feeding, locomotion, reproduction and stress-induced behaviours.


The specifics feeding behaviour of bdelloid feeding is varied but most use rings of cilia in the corona organ to create currents of water which blow food through the mouth to the mastrax organ which has been adapted specifically for grinding food.[9] Food includes suspended bacteria, algae and detritus amongst others as well.


There appears 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 βδελλα 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.[10] 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.

Stress induced behaviour[edit]

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.[11] The bdelloid can remain in this state, which is known as a 'tun' 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.[12] These forms of dormancy are also known as cryptobiosis or quiescence. Bdelloids have been known to survive in this state for up to 9 years whilst waiting for favourable conditions to return.[12]

When these creatures recover from hibernation, it has been shown that they undergo a possibly 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.[13] 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.

Bdelloid rotifers are extraordinarily resistant to damage from ionizing radiation due to the same DNA-preserving adaptations used to survive dormancy.[14]


Pictures of bdelloids to assist in visualising the anatomy.

See also[edit]


  1. ^ Segers, Hendrick (2007). Annotated checklist of the rotifers (Phylum Rotifera), with notes on nomenclature, taxonomy and distribution. Auckland: Magnolia Press. ISBN 978-1-86977-129-4. 
  2. ^ a b c d Ricci C (2000). "Key to the identification of the genera of bdelloid rotifers". Hydrobiologia 418: 73–80. doi:10.1023/A:1003840216827. 
  3. ^ a b Donner, Josef (1965). Ordnung Bdelloidea. Akademie-Verlag. p. 297. ISBN 9789031908851. 
  4. ^ a b c d e f g Shiel, R J (1995). A guide to identification of rotifers, cladocerans and copepods from Australian inland waters. Australia: Co-operative Research Centre for Freshwater Ecology. ISBN 0646224107. 
  5. ^ Bartôs B (1954). "The Czechoslovak Rotatoria of the order Bdelloidea". Memoires de la Societe Zoologique Tchecoslovaque de Prague 15: 241–500. 
  6. ^ Garey J (1998). "The evolutionary relationships of rotifers and acanthocephalans". Hydrobiologia. 387/388: 83–91. doi:10.1007/978-94-011-4782-8_12. 
  7. ^ Lasek-Nesselquist E (2012). "A Mitogenomic Re-Evaluation of the Bdelloid Phylogeny and Relationships among the Syndermata". Proceedings of the National Academy of Sciences 7: 3554. 
  8. ^ Wey-Fabrizius A R (2014). "Transcriptome Data Reveal Syndermatan Relationships and Suggest the Evolution of Endoparasitism in Acanthocephala via an Epizoic Stage". Proceedings of the National Academy of Sciences One 9: e88618. doi:10.1371/journal.pone.0088618. 
  9. ^ Klusemann J (1990). "The hard parts (trophi) of the rotifer mastax do contain chitin: evidence from studies on Brachionus plicatilis". Histochem. Cell Biol. 3: 277–283. 
  10. ^ Milius S. "Bdelloids: No sex for over 40 million years.". TheFreeLibrary. ScienceNews. Retrieved March 2014. 
  11. ^ Crowe J (1971). "Anhydrobiosis: an unsolved problem". American Naturalist 105: 563–573. doi:10.1086/282745. 
  12. ^ a b Guidetti R (2002). "Long-term anhydrobiotic survival in semi-terrestrial micrometazoans". Journal of zoology 257: 181–187. doi:10.1017/s095283690200078x. 
  13. ^ Boschetti C (20011). "Foreign genes and novel hydrophilic protein genes participate in the desiccation response of the bdelloid rotifer Adineta ricciae". The Journal of experimental biology 214: 59–68. doi:10.1242/jeb.050328. 
  14. ^ Gladyshev E (2008). "Extreme resistance of bdelloid rotifers to ionizing radiation". Proceedings of the National Academy of Sciences 105: 5139–5144. doi:10.1073/pnas.0800966105. 

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