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 100 to 1000 µm (=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). Depending on the species, colonies may range in size from fewer than 5 individuals to large colonies of 50 to 200 individuals or, in the case of a few species, more than a thousand individuals. 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" usually has 2 toes (range 0 to 4) and pedal glands, with ducts exiting near the toes, that secrete a sticky cement for temporarily attaching the rotifer to substrates. 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). The rotifer's pharynx is modified as a "mastax" consisting of sets of internal jaws. (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 (see review of saltwater rotifers by Fontaneto et al. 2006). Densities in freshwater generally range up to around 1000 rotifers per liter, but with abundant food densities can exceed 5000 individuals per liter and, reportedly, in some water bodies at certain times densities may exceed 100,000 individuals per liter (Wallace and Snell 2001 and references therein).
Rotifers vary in their feeding habits. Some rotifers are ciliary suspension feeders; others grasp prey (mainly small animals, but some plant material as well) with protrusible, pincer-like mastax jaws; and still others draw prey into a funnel-shaped trap formed by spines or setae on the corona (Brusca and Brusca 2003). A few rotifers have evolved symbiotic relationships with other species, for example living on the gills of crustacean arthropods or on snail egg masses (see May 1989 for a review of the literature on commensal and parasitic relationships between rotifers and other organisms). Although parasitism is not common among rotifers in general, there is one major exception: The Acanthocephala, until recently generally treated as a phylum but now believed to be a clade of parasitic rotifers, most likely sister to the free-living bdelloids (Garcia-Varela and Nadler 2006; Min and Park 2009; Witek et al. 2009), are obligate intestinal parasites of vertebrates as adults, with arthropod intermediate hosts.
Most rotifers are oviparous, releasing eggs outside their bodies, but a a few are ovoviviparous, retaining the embryo inside the body until it hatches. Although most rotifers other than the bdelloids are dioecious (i.e., have both males and females), with the exception of the genus Seison males are generally reduced in abundance, size, and complexity. Parthenogenesis is very common among rotifers in general and is the exclusive mode of reproduction among the bdelloid rotifers. The bdelloid rotifers 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). Bdelloids account for nearly a quarter of the roughly 2,000 described rotifers (Segers 2008). The life cycle of some rotifer species may include both a parthenogenetic (amictic) phase and a mictic phase in which haploid eggs are produced, some of which may parthenogenetically produce haploid males, which can then fertilize haploid eggs with their mitotically produced haploid sperm. (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, with the two groups together forming a clade that has been referred to as the Syndermata. A variety of analyses now strongly suggest that the Acanthocephala are in fact a clade of rotifers, most likely sister to the bdelloids, making the name Syndermata a junior synonym of Rotifera (Sørensen and Giribet 2006). Based on a large-scale analysis of molecular data (amino acid sequences), Witek et al. (2009) concluded that Syndermata and Gnathostomulida together comprise a monophyletic clade known as the Gnathifera. The Gnathifera was originally proposed as a monophyletic clade based on jaw morphology and may also include the Micrognathozoa and, conceivably, the Cycliophora (Funch et al. 2005; Sørensen and Giribet 2006).
Wallace and Snell (2001) provide a thorough review of rotifer biology, including a dichotomous key to families. Smith (2001) provides a dichotomous key to the freshwater genera of the United States. Segers (2008) reviews the global diversity of rotifers.
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