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Bdelloidea is a class of rotifers found in fresh water and moist soil all over the world. There are believed to be around 350 different species of bdelloid rotifer, each of which has varying characteristics whether through morphology or behaviour. This class of rotifer has many of the phylum characteristics as well as certain unique features; the two main features being (1) parthenogenesis reproduction and (2) the ability to survive in harsh environments through anhydrobiosis.

=Classification and Identification= Bdelloids are a class found within the phylum Rotifer, and consists of three orders: Philodinavida, Philodinida and Adinetida. These three orders can then be broken down into four families and consequently about 350 species. The size of the bdelloids varies between 150 and 700 µm, although there are some species such as the Rotaria neptunia that are particularly long at 1600 µm.

Bdelloids can only be identified whilst they are alive because many of the characteristics that are significant are to do with feeding and crawling, also once preserved the individuals contract into "blobs". This means that it can be rather restrictive when trying to analysis bdelloids especially since the current methods that are available for identification can be rather difficult to use correctly. There are currently three methods used in the scientific community, two of which can be considered dated by Bartôs (1951) and Donner (1965). The third method is a diagnostic key developed in 1995 by Shiel.

Morphology
There are three main regions of the body of bdelloids: The head, trunk and foot. The image on the right depicts each area to show how each body part 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
 * A mouth opening with a long oesophagus
 * Strong teeth (labelled by a tooth index)
 * Many cilia
 * Species-specific upper lip shape
 * Order-specific corona type
 * Philodinida consist of two ciliated discs
 * Adinetida consist of a ventral ciliated field
 * Philodinavida have a small corona

The bdelloids 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 eventually released as faeces. These pellets are distinguishable since all the other genera release faeces as loose material instead of packed pellets.

Most bdelloid retract the foot whilst they eat, but four genera don't because they lack a foot entirely - Adineta, Bradyscela, Henoceros and Philodinavus. This lack of a foot affects not only how they feed but also how they crawl; for instance Adineta and Bradyscela slide whereas the other genera loop.

Evolutionary relationships
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 of these taxon trees (Figure 2) have been suggested at various times although 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 tree B and C are wrong.

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

Feeding
The feeding behaviour of bdelloids is much similar to that of the rest of the phylum. They use rings of cilia in the corona organ to create currents of water which blow plankton through the mouth to the mastrax organ mastrax organ. The mastrax organ has been adapted specifically for grinding food and has a feature similar to jaws called trophi.

Locomotion
As with the feeding behaviour of bdelloids, the movement of bdelloids is the same of much of the rotifer phylum. There tends to be three main methods a species will move: free swimming, inchworming along a substrate or sessility. Free-swimming and inchworming species 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 (Greek βδελλα or bdella, meaning leech). This video shows a demonstration of how bdelloids move in three different situations: locomotion and feeding of bdelloid rotifers

Reproduction
Bdelloids have been of interest to those interested in the evolutionary role of sexual reproduction, because a male has never been observed from the group, and females reproduce exclusively by parthenogenesis which is a form of asexual reproduction where embryos grow and develop without the need for fertilization; this is part of the apomixis process. 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 reproducing rotifer species.

Stress induced behaviour
Bdelloids respond to environmental stresses by entering a state of dormancy known as anhydrobiosis. This dormancy form enables the organism to rapidly dehydrate itself and thus resist desiccation. During this dormant stage many metabolic processes are adjusted to equate for the change in state; an example of this is the production of protective chemicals The bdelloid will remain in this cystic state until optimal environmental conditions re-occur 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. These forms of dormancy are also known as cryptobiosis or quiescence. Bdelloids have been known to survive in this state successfully for up to 9 years whilst waiting for favourable conditions to return

When these creatures spring from hibernation, it has been suggested that they undergo a fascinating and possibly unique genetic process. This area of research is currently under much debate and controversy particularly with regards to possible horizontal gene transfer and tetraploidy degeneration.

Bdelloid rotifers have recently been shown to be extraordinarily resistant to damage from ionizing radiation. The same DNA-preserving adaptations used to survive dormancy are thought to work in this case, and may have also helped the organisms to thrive despite their totally asexual mode of reproduction.

=Images= Below are some pictures of bdelloids to assist in visualising the anatomy of bdelloids.

=External links=


 * Introduction to rotifers
 * Rotifer World Catalog, by Jersabek C.D. & Leitner M.F.
 * The Weird Sisters
 * Bdelloids: No sex for over 40 million years
 * An Evolutionary Scandal, from Harvard Magazine
 * Who Needs Sex (or Males) Anyway?