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The Rotifera (="wheel bearers") are a group of tiny animals first observed by early microscopists in the late 1600s. Around 2,000 rotifer species have been described (Segers 2008). Most species are in the range of 0.1 to 1 mm in length, although a few species may reach 2 to 3 mm. Different rotifer species display a striking variety of body forms and the morphology of individuals may be further altered (e.g, by growth of spines) in response to ecological cues indicating the presence in the environment of particular types of prey or predators (Wallace and Snell 2001 and references therein). Most rotifers are solitary, but there are a small number of colonial (mostly sessile, i.e. attached to the substrate) species (reviewed in Wallace 1987). Although there are both solitary and colonial sessile rotifers, most rotifers are motile and very active. (Wallace and Snell 2001 and references therein; Wallace 2002; Brusca and Brusca 2003)
The anterior end of a rotifer bears a ciliary organ known as the corona. In action, the movements of the coronal cilia can give the impression of a pair of rotating wheels, giving this phylum its name. The ventral appendage known as the "foot" secretes a sticky cement for temporarily attaching the rotifer to a substrate. In larvae of sessile rotifers, the cement forms a bond with the substrate that is not easily broken and if sessile rotifers are dislodged they do not reattach (Wallace 1980). Most rotifer species display eutely, i.e., the adults of a given species all have the same fixed number of cells (in the case of rotifers, around a thousand cells, about a quarter of which make up the nervous system). (Wallace and Snell 2001 and references therein; Wallace 2002; Brusca and Brusca 2003)
Rotifers are best known from freshwater, but some live in damp soil or moss and there are many marine species, although only a fraction of these are exclusively marine (Wallace and Snell 2001 and references therein).
Parthenogenesis is very common among rotifers in general and is the exclusive mode of reproduction among the bdelloid rotifers. The bdelloid rotifers, which account for around a quarter of the roughly 2000 described rotifers (Segers 2008), have attracted particular attention from researchers because the bdelloid clade is the most ancient group of animals known in which males are unknown and reproduction is strictly parthenogenetic (i.e., eggs develop without fertilization). (Wallace and Snell 2001 and references therein; Wallace 2002; Brusca and Brusca 2003)
In addition to their striking lack of males, bdelloids have attracted attention for the fact that most of them can undergo anhydrobiosis, slowly drying out until they resemble a wrinkled barrel, with the head and foot retracted into the animals's trunk (Wallace and Snell 2001). Desiccated bdelloids may be revived after many years in this state. Wilson and Sherman (2010) proposed that the ability of bdelloids to dry up, as well as to be transported long distances by air currents in their desiccated state, may allow them to escape fungal parasites in both time and space. An important and well supported hypothesis for the widespread evolutionary maintenance of sex in organisms in general is that sexual recombination creates genetic variation that allows organisms to stay ahead of their parasites in a never ending coevolutionary arms race. Wilson and Sherman suggest that the alternative mechanism they have documented for bdelloid rotifers to escape their parasites may help explain how this clade could persist for tens of millions of years in the absence of sex.
The Rotifera were formerly believed to be sister to the endparasitic Acanthocephala. A variety of analyses now strongly suggest that the Acanthocephala are in fact a clade of rotifers, most likely sister to the bdelloids (Sørensen and Giribet 2006).