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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 2 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 (Ruttner-Kolisko 1974, Nogrady, Wallace, and Snell 1993, Wallace 2002). 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. Rotifers do not have any commercial importance, and as they often occur in relatively low numbers, their ecological importance must be considered as minor. 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 533 27 Rotifera of the Gulf of Mexico Martin V. Sørensen  Rotifera. After Jennings 1901. 534 ~ Rotifera rotifers. When it apparently occurs, it is probably primarily due to our limited information about distributional ranges for marine rotifers, rather than examples of true endemism . Instead, there is a high fraction of cosmopolitan species. Several of the recorded species display a peculiar andapparentlyextremelypatchydistribution.Forexample, Encentrum striatum has been recorded from the Black Sea and the Gulf of Mexico only. This is, however, not uncommon for marine rotifers and again, it is merely a reflection of our insufficient information about their distributional patterns. Many of the recorded species, such as Encentrum marinum, Notholca salina, and Trichocerca marina are common marine inhabitants, whereas others are euryhaline , brackish-water species. Typical euryhaline species are Brachionus plicatilis, Colurella adriatica, C. colurus, Lecane inermis, L. luna, and Notholca bipalium. A few of the recorded species, inclusive Brachionus plicatilis, Colurella salina, and Proales minima, have also been recorded from inland salt springs. The latter species is, however, probably synonymous with Proales halophila, which is a common brackish-water species (De Smet 1996). Abbreviations The following abbreviations are used in this chapter’s checklist: ben = benthic; bsl = beach and shoreline; cep = coastal surface and epipelagic; ins = interstitial; plk = planktonic; sft = soft substrates (mud, sands, clays); svg = submergent vegetation. References 1. Ahlrichs, W. H. 1995. Ultrastruktur und Phylogenie von Seison nebaliae (Grube 1859) und Seison annulatus (Claus 1876). Cuvillier Verlag, Göttingen, Germany. Rotifera, Acanthocephala, Micrognathozoa, and Gnathostomulida , and the 4 taxa are usually united in a clade named Gnathifera (Ahlrichs 1995, Rieger and Tyler 1995, Giribet et al. 2000, Kristensen and Funch 2000, Sørensen et al. 2000). Rotifera consist of about 1900 described species , and the phylum can be divided into 2 classes: the Eurotatoria and the Pararotatoria. The latter contains only 3 species that are distributed between the 2 genera Seison and Paraseison. Common for all Seisonidae is that they are commensals or ectoparasites on the leptostracan Crustacea Nebalia (Ricci, Melone, and Sotgia 1993, Segers and Melone 1998, Sørensen, Funch, and Segers 2005). 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...

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