By Martin Sorensen, Niels Bohr Institute & Biological Institute, University of Copenhagen
Rotifera is a phylum of microscopic metazoans, characterized by the presence of cephalic ciliary bands, called the corona or wheel organ, and a complex pharyngeal apparatus, referred to as the mastax. The general appearance of a rotifer varies greatly, but typically the body can be divided into a head, a trunk and a foot region. The head carries the ciliary wheel organ that is used for feeding and locomotion. A ventral mouth leads to the pharynx with the mastax that consists of chitinous jaws, called the trophi, and surrounding musculature. A short esophagus connects the pharynx with the stomach and the gut. The latter terminates into a dorsal cloaca posteriorly on the trunk. A flexible foot is often present posteriorly or ventrally on the trunk, and in most species it terminates into two toes with adhesive glands.
Unlike in most other metazoans, the rotifer integument lacks an outer protective cuticle. Instead, their syncytial epidermal cells have an intracellular protein lamina located in the cytoplasm close to the apical membrane. This special feature is shared with their close relatives, the endoparasitic acanthocephalans.
All rotifers are dioecious, but in many species the reproduction is solely based on parthenogenesis (in subclass Bdelloidea) or on a cyclic switch between parthenogenesis and gamogenesis. During the latter, short-lived and strongly reduced haploid males are present in a short period.
Rotifers mostly inhabit freshwater, but some taxa are present in marine and brackish habitats as well. Only about 50-100 species are known as exclusively marine. Marine rotifers may be planktonic, periphytic, epibenthic or interstitial, but they are never found in soft sediments, and they have only rarely been recorded from depths below 30 m.
The rotifers have been studied by scientists since the first microscopes were invented, but in the beginning they were grouped among the protozoans. More recently, they have been assigned to the obviously polyphyletic Aschelminthes, together with other pseudocoelomate taxa such as Gastrotricha, Nematoda and Priapulida. However, today most studies support a close relationship between Rotifera, Acanthocephala, Micrognathozoa and Gnathostomulida, and the four taxa are usually united in a clade named Gnathifera. Rotifera consist of ca. 1900 described species, and the phylum can be divided into two classes: the Eurotatoria and the Pararotatoria. The latter contains only three species that are distributed between the two genera Seison and Paraseison. Common for all Seisonidae is that they are commensals or ectoparasites on the leptostracan Crustacea Nebalia. The Eurotatoria consist of the subclasses Bdelloidea and Monogononta. The Bdelloidea are characterized by reproduction based solely on parthenogenesis, and the group is only rarely present in marine environments. The Monogononta comprises more than 1500 species and display by far the greatest morphological disparity. The endoparasitic acanthocephalans have traditionally been considered sister group to the Rotifera, but recent studies indicate that they could be a rotifer ingroup.