Order Euplotida Small & Lynn, 1985
Hypotrich ciliates are more or less oval to disc- or cup-shaped in outline, with a cytostome that is usually subequatorial.
Ciliation bordering the buccal cavity is prominent, and in cludes paroral membrane, endoral cilia, and a series of polykinetids along the left side of the buccal cavity customarily referred to as the adoral zone of membranelles or AZM. The AZM is often differentiated into anterior collar and marginal lapel regions. These characters show extensive specialization and modification with in the group. During cortical morphogenesis associated with cell division, the proter inherits the parental buccal apparatus. The opisthe oral primordium develops from a subsurface pocket.
Somatic ciliation includes cilia and groups of cirri. Cilia occur dorsally in longitudinal kineties, and vary from long as motile (Diophryopsis) to short and stubby (Euplotes). They develop from a single dorsal equatorial band of ciliary rows.
Cirri in interphase cells appear in groups rather than rows although they also develop morphogenetically within row. They arise from three fields: frontventral-transverse, left marginal, and dorsal. Right marginal cirri are absent.
The frontoventral-transverse field gives rise to be paroral cirrus, endoral and paroral membranes, frontoventral cirri (FVC), and transverse cirri (TC). These ciliary structures arises from streaks numbered left to right across the cell's ventral surface in Roman numerals (I-VII) and in Arabic numbers from posterior to anterior. No more than five cirri develop from any streak within the proter or opisthe FVC-TC fields. Streak I gives rise to endoral monokinetal structure. Streak II forms the paroral kinetosomal structure and the paroral cirrus. Often the endoral and paroral membranes form a single functional unit. Streaks III-VII are sometimes termed the frontoventral-transverse (FVC-TC) field, a field that divides early in division forming the proter and opisthe FVC-TC fields.
Left marginal cirri arise from a field that forms lateral to parental marginal cirri. There is a trend toward reduction, to complete absence in many species, of left marginal cirri. Right marginal cirri are absent.
Right caudal cirri are often present. They are hypertrophied and dorsal in members of the Uronychiidae, and ventral in the Euplotidae. Right caudal cirri arise at the posterior ends of some of the right dorsal ciliary kineties.
There is a well-developed meshwork consisting of an interkinetal argentophilic system of polygons apparent in material prepared by the Chatton-Lwoff silver impregnation technique usually termed the argyrome. The macronucleus when single varies from spherical and compact to a variously C- or horseshoe-shaped structure. When multiple, the macronuclei may be spherical and compact, or are in an irregular moniliform grouping. Reorganization bands appear in macronuclei prior to division. Micronucleai are often single though are multiple in some species.
Some freshwater members of the genera Euplotoides, Euplotes, and Aspidisca, and some marine Diophrys encyst (Rawlinson and Gates 1985, 1986, Walker and Maugel 1980). Cysts contrast in numerous characters with cysts of members of the order Hypotrichia. The cysts exhibit a relatively thin cyst wall composed of two amorphous layers that are not desiccation-resistant. Recent evidence suggests that chitin is present in the cyst wall of Euplotopsis muscicola (Greco et al. 1990). Kinetosomal structures including cirri and AZM persist throughout encystment and the macronuclei do not fuse or round up in the cyst but remain extended. (Walker and Maugel 1980).
Euplotids differ from other hypotrichs in their ventral surface to ventral surface position during conjugation, involving an initial joining by a juxtaposition of the left anterior quadrant of their ventral surface (Luporini and Dallai 1980), a manner similar to that of most ciliates that join by their oral surfaces. The dorsal surface of Euplotes does not reorganize during conjugation, thus behaving differently from the reorganization of the dorsal surface that takes place during conjugation in Stylonychia (Tuffrau et al. 1981).
Relationship of the Euplotida to possible ancestors: Historically, protozoologists have linked Euplotes and its presumed relatives taxonomically with other ciliates with ventral cirri in the order Hypotrichida. However, considering the diversity and antiquity of the group, this may no longer be tenable. Based especially upon ultrastructural characteristics of the dikinetid. Small and Lynn aligned the Euplotida in the subclass Nassophorea (Small & Lynn 1985), however Schlegel (1991) considered Euplotes more closely allied to other hypotrichs than to Paramecium and Tetrahymena. Based on evidence from the sequence of the small subunit rRNA gene, several authors have suggested that Euplotes and Oxytricha are sister groups, but with a great evolutionary distance between them (Greenwood et al. 1991, Sogin et al. 1986). Ecological and ultrastructural similarities among Euplotids, Discocephalines, and some Urostylines continue to suggest to us that Euplotids may have derived from primitive discocephaline descendents of early urostyline hypotrichs. Martin (1982) suggested that Euplotes-like ciliates arose from Oxytricha-like hypotrichs by increase in size of the buccal cavity and reduction in number of cirri. Fleury et al. (1986) divided hypotrichs into two suborders, considering Aspidisca, Euplotes, Certesia, Diophrys, Uronychia, and Kiitricha in the suborder Pseudohypotrichina. From information obtained from rRNA sequences, Lynn & Sogin (1988) demonstrated that hypotrichs, stichotrichs, and choreotrichs may be of the same evolutionary branch. Considering the likelihood of undiscovered diversity among these ciliates, it may still be premature to understand exactly where the Euplotida "belongs" among other ciliates, but based on the foregoing, we assume here that the Euplotida derives from a common ancestor with members of the order Hypotrichia. Based also on the diversity of structure and habitat within the Euplotida, we surmise that an ancestral euplotid, may be presumed to have been marine, have been more elongate than present members of the order, and have lacked right marginal cirri. Numerous left marginal and right caudal cirri, and two macronuclear fragments may have been present. Dorsal cilia probably would have been shorter than in most hypotrichs. The frontoventral morphogenetic streaks including morphogenetic streaks that form endoral and paroral structures, and there would be little tendency toward the cephalization found in the Discocephalina. Our interpretation of the relationships among Euplotids suggests three trends involved in early differentiation of members of the other Euplotida from possible ancestors, including 1) a trend toward short and stubby dorsal cilia 2-3 um in length rather than full-length cilia, 2) decrease to a relatively stable number of five development streaks in the FVC field, and 3) shortening of the cell.
The ciliates in the family Aspidiscidae are nearly circular in frontal outline, flattened, disc-shaped, and strongly thigmotactic. There are no left marginal cirri. FVC and TC tend to be relatively thick, short, frayed, and thigmotactic. There are no apparent endoral or paroral structures. The macronucleus usually is C- or horseshoe-shaped. Aspidiscids are widely distributed in both freshwater and marine habitats, and a few are associated with invertebrate hosts. There are two genera in the family Aspidiscidae: Aspidisca Ehrenberg, 1831, and Euplotaspis Chatton & Seguela, 1936
Family Certesiidae n. fam.
Certesia quadrinucleata Fabre-Domergue, 1885 is the sole member of the family. It is from the marine environment and differs from members of the Aspidiscidae and Euplotidae in having a row of more than three left marginal cirri. The AZM is well developed, and the paroral polykinetid is particularly conspicuous. A unique granular inclusion, the condylopallium, is found near the anterior end of the cell. For a more complete description see (Wicklow 1983).
Members of the family Euplotidae generally are benthic and flattened ventrally. The few planktonic forms have a less hypertrophied anterior portion of the AZM than members of the Gastrocirrhidae. They are more parallel-sided and less flattened than members of the Aspidiscidae, and have a well developed argyrome and a single C-shaped or horseshoe-shaped macronucleus. Cirri include FVC, TC, varying numbers of non-hypertrophied left marginal cirri, and ventrally located right caudal cirri. There is a conspicuous AZM which shows little separation between the collar and lapel regions. The paroral ciliature is in the form of a patch of cilia deep within the posterior end of the buccal cavity situated beside an endoral monokinetal row of cilia. The family contains four genera: Euplotes Ehrenberg, 1839, Euplotoides n.g., Euplotopsis n.g., and Moneuplotes Jankowski, 1979.
Family Gastrocirrhidae Faure-Fremiet, 1961
Members of the family Gastrocirrhidae Faure-Fremiet, 1961 are marine planktonic members of the Euplotida with a relatively wide, anteriorly-opened buccal cavity. There is an elaborate, hypertrophied AZM but a reduced paroral membrane. Left marginal and right caudal cirri are sometimes present. The single macronucleus is usualy C-shaped, but organisms with it divided into moniliform segments have been described. There has been little information published on members of this family beyond descriptions of interphase cells. There are three genera; Cytharoides Tuffrau, 1974, Euplotidium Noland, 1937, and Gastrocirrhus Lepsi, 1928
Members of the Uronychiidae differ from members of the Euplotidae in having three hypertrophied dorsally located right caudal cirri and separated collar and lapel. The family contains four genera: Diophryopsis Hill & Borror, 1992, Diophrys Dujardin, 1840, Paradiophrys Jankowski, 1978, and Uronychia Stein, 1857.