|SEM showing morphological variation of bdelloid rotifers and their jaws|
Bdelloidea // (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. The main characteristics shared by Bdelloids are parthenogenetic reproduction and the ability to survive in harsh environments by anhydrobiosis (desiccation-tolerance). 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
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. 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. There are currently three morphological identification methodologies, two of which are considered dated by Bartôs (1951) and Donner (1965). The third method is a diagnostic key developed in 1995 by Shiel.
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 
- 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 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.
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.
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.
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.
More recent evidence (published 2014) using transcriptome data shows Bdelloidea, Monogononota and Seisonidea grouped with another class, theAcanthocephala, 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.
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. 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. 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
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 '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. 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.
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. 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.
Pictures of bdelloids to assist in visualising the anatomy.
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