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The phylum Rotifera contains more than 2000 species. Rotifers are a group of microscopic multicellular animals, ranging in size from 50μm to 2mm, and widely distributed in freshwater (although there are a few saltwater species as well). Whilst many species are planktonic, others creep over a surface or are sessile. Rotifers are also known as 'wheel animals', as they possess a so-called corona, a ciliated structure on the head. The cilia generate a water current that propels the animal and transports food particles into its mouth. Most rotifers are suspension feeders, but there are also some carnivorous and ectoparasitic species. Food is processed in the muscular pharynx that is equipped with calcified trophi, jaw-like structures that are variable in shape. Similar feeding structures have evolved in different groups that employ similar feeding mechanisms, and particularly the trophi are highly convergent. The diverse morphology of rotifers presents other examples of convergence. For instance, a foot structure was lost in several groups independently, leading to the development of other structures for locomotion.
Rotifers can reproduce very rapidly when conditions are good, and sexually reproducing species are able to produce resting eggs that can survive unfavourable conditions. In bdelloid rotifers, however, sexual reproduction is unknown - females reproduce asexually by apomictic parthenogenesis. The closest relatives of the Bdelloidea, a group known as the Monogononta, alternate between asexual and sexual reproduction, and the asexual bdelloids most likely descended from female monogononts that lost the capacity for reproducing sexually. Molecular phylogenetic analyses have suggested that this loss of sexual reproduction occurred independently at least three times. Given the general notion that asexual organisms are more likely to go extinct and less likely to speciate than those with sexual reproduction, the success and diversity of bdelloids is surprising. It has, however, been suggested that the microniches they inhabit change so little that substantial genetic change is not necessary. Intriguingly, the biological diversity of these rotifers shows a pattern that 'mimics' that of sexually reproducing species, which might be explained by selection for a common ecological niche.
Bdelloid rotifers are furthermore remarkable in that they show extensive horizontal gene transfer (HGT). Such transfer of genes from one organism to another that is not a direct descendant is rare in metazoans, but seems to be a general feature and presumably an important evolutionary force in bdelloids (although the adaptive significance remains yo be shown). Many of their genes, particularly near telomeres, are of foreign origin, stemming from bacteria, fungi or plants, and may encode simple enzymatic functions. HGT has also been demonstrated in prokaryotes as well as in unicellular eukaryotes that are phagocytic or parasitic.