Sterkiella Foissner, Blatterer, Berger and Kohmann, 1991
Family Oxytrichidae Ehrenberg (ref. ID; 4813)
Family Oxytrichidae Ehrenberg, 1838: Subfamily Stylonychinae Berger & Foissner, 1997 (ref. ID; 4894 subfamily original paper)

Synonym Histriculus, Histrio

[ref. ID; 2857]
Genus erected to contain some oxytrichid stylonychids erroneously assigned to Histrio and Histriculus. (ref. ID; 2857)

[ref. ID; 4894]
Improved characterization; Undulating membranes in Oxytricha pattern. One right and one left row of marginal cirri, distinctly separate posteriorly. Six dorsal kineties. Caudal cirri present. Primordia V and VI of the opisthe originate de novo. Dorsal morphogenesis in Oxytricha pattern. (ref. ID; 4894)
Remarks; Sterkiella histriomuscorum is very common in freshwater and soil (Foissner et al. 1991) and has often been misidentified as Oxytricha sp. because it is not as rigid as the other members of the Stylonychinae. The autapomorphy has been confirmed in the type species only (W. F., unpubl. data). (ref. ID; 4894)
Type species (original designation); Oxytricha cavicola Kahl, 1935 (ref. ID; 4894)

Sterkiella histriomuscorum Foissner, Blatterer, Berger & Kohmann, 1991 (ref. ID; 2846, 2857, 4488, 4609) or (Foissner, Blatterer, Berger & Kohmann, 1991) Foissner, Blatterer, Berger & Kohmann, 1991 (ref. ID; 4813) reported author and year? (ref. ID; 1629, 2128)
Syn; Histriculus muscorum (Kahl, 1932) Corliss, 1960 (ref. ID; 2857 replacement name for genus because Histrio was preoccupied) reported author and year? (ref. ID; 2846) or (Kahl, 1932) Berger, Foissner & Adam, 1985 (ref. ID; 4813); Histrio muscorum Kahl, 1932 (ref. ID; 2857, 4609, 4813); Opistotricha terrestris Horvath, 1956 (ref. ID; 2857); Oxytricha histrioides Gellert, 1957 (ref. ID; 2857); Oxytricha trifallax (ref. ID; 2857); Stylonychia curvata Giese & Alden, 1938 (ref. ID; 2857)
Diagnosis; Morphology and morphogenesis very similar to Sterkiella nova (= sibling species). Differences in nucelotide sequences of the complete pol alpha gene and the small subunit rDNA significant, however. Complete nucelotide sequence of macronuclear DNA pol alpha gene described in Hoffman and Prescott (1997) and deposited in the Gene Bank sequence data base, accession number U59426. (ref. ID; 2857)
Description; Size in vivo slightly smaller (70-110 x 40-60 um) than in other well-studied population (100-160 x 40-70 um). Marginal cirri about 15 um long, frontal cirri about 17 um, transverse and caudal cirri 20-24 um, and dorsal bristles about 3 um. Adoral zone of membranelles extends over about 45% of body length, longest bases 7-9 um; cilia 17-20 um long. (ref. ID; 2846)
[Cyst]: Several-days-old resting cysts spherical and entirely filled by cell; wall about 2.5 um thick, surface smooth or slightly uneven. Macronuclei fused; however, when cyst wall is ruptured two (disintegrated?) nodules sometimes stain with methyl green-pyronin. Cytoplasm contains numerous granules and some globules up to 4 um across. (ref. ID; 2846)
Comments; The original description of S. histriomuscorum (Histrio muscorum in Kahl, 1932, p. 617): "Length in vivo 100-150 um, length:width ratio slightly variable. Body rather rigid and distinctly flattened parallel-sided with posterior end broadly rounded. Rightmost transverse cirri 1/3-1/2 projecting beyond posterior body margin. Last three cirri of left marginal row form rather distinct bristles, which, however, are soft and only slightly elongated. Frequently found in mosses from the German Alps and California". Fortunately, Kahl (1932) provided an excellent figure (Fig. 71), which not only perfectly matches later redescriptions (Foissner 1982, Shin and Kim 1994) but also Oxytricha nova and Oxytricha trifallax. Kahl's description of the caudal cirri, which he misinterpreted as 'the last three cirri of the left marginal row', exactly matches our observations; they are indeed inconspicuous and easily misidentified as marginal cirri. (ref. ID; 2857)
Sterkiella nova Foissner & Berger, 1999 (ref. ID; 2857 original paper)
Syn; Oxytricha nova Klobutcher et al., 1981 (ref. ID; 2857)
Diagnosis; Size in vivo about 120 x 60 um, ellipsoidal. Two macronuclear nodules. On average 34 adoral membranelles, 21 right and 20 left marginal cirri, and 5 transverse cirri. 6 dorsal kineties with 1 caudal cirrus each associated with kineties 1, 2, 4. Undulating membranes intersecting (Oxytricha pattern). Proter and opisthe cirral anlagen separate, proter anlagen 4, 5, 6 originate from cirrus IV/3, opsithe anlagen originate from oral primordium (anlagen 1-3), cirrus IV/2 (anlage 4) and cirrus V/4 (anlagen 5, 6). Dorsal kineties generated in Oxytricha pattern. Complete nucleotide sequence of macronuclear DNA pol alpha gene described in Mansour et al. (1994) and deposited in the Gene Bank sequence data base, accession number U 02001. Complete sequence of small subunit rRNA in Elwood et al. (1985) (ref. ID; 2857)
Description (cultivated material); Size in flourishing cultures in vivo about 100-140 x 45-65 um, usually around 120 x 60 um, very small specimens (< 100 um) occur in declining cultures. Body ellipsoidal, right margin usually less convex than left, sometimes even concave, both ends broadly rounded, rarely bluntly pointed posteriorly; dorsoventrally flattened up to 2:1, depending on nutritional state, ventral side flat, dorsal convex. Body rather rigid, specimens with sharp-cornered injuries have been observed, very much like those known from Stylonychia mytilus. However, cells become rather flexible when overfed and, especially, when slightly squeezed by the cover glass. Hence, body rigidity must be observed in freely motile, untouched specimens and compared with that of common, flexible species, such as Oxytricha and Urostyla. Macronuclear nodules in central portion of cell slightly left of midline, ellipsoidal (about 2:1), number slightly variable, contain many 1-2.5 um sized nucleoli. Micronuclei globular, near or attached to macronuclear nodules in variable positions, number highly variable. Contractile vacuole slightly above mid-body at left margin of cell, with one lacunar collecting canal each extending anteriorly and posteriorly. No specific cortical granules. Cells colourless, however, well-fed specimens often appear dark in posterior half at low magnification (< / _ 100 x) due to food inclusions and many fat globules 1-4 um (usually 2-3 um) across; similarly, small cells from declining cultures usually contain black patches composed of hundreds of colourless to slightly yellowish, variably shaped crystals, which develop in small vacuoles from granular precursors and grow to a size of 2-5 um. Feeds on green algae (Chlorogonium), bacteria, and wheat starch. Movement moderately rapid, usually gliding to and for on slide surface and bottom of culture dish, never rests. Ventral and dorsal ciliary pattern (infraciliature) very constant, that is, 18 front-ventral-transverse cirri on ventral side and 6 kineties (ciliary rows) on dorsal. Shape and size of cirral bases, as well as number of basal bodies (cilia) in individual cirri, in contrast, highly variable; specimens entirely identical in this respect were not observed, common structure. Marginal cirri in vivo about 18 um long, size of cirral bases decreases posteriad, rather evenly spaced in one row each near right and left margin of cell, rows separated posteriorly right of midline by small, difficult to recognize gap seemingly occupied by caudal cirri, which, however, insert at posterior margin of dorsal side. Fronto-ventral-transverse cirri of similar size and length: frontal cirri about 20 um long, rightmost (third) cirrus very near to and thus easily confused with distalmost adoral membranelle, especially in protargol preparations; fronto-ventral cirri and buccal cirrus in vivo about 18 um long, form V-shaped pattern because posterior cirri closer together than anterior ones; buccal cirrus in area where paroral and endoral membrane optically intersect, that is, slightly above mid of buccal cavity; uppermost postoral ventral cirri underneath buccal vertex, close together, left one invariably smallest than right, separated by large gap from third (posterior) postoral ventral cirrus distinctly underneath mid-body; anterior pretransverse cirrus smaller than posterior one, which is very near to the rightmost transverse cirrus; transverse cirri near posterior body end, in vivo 25-30 um long and thus distinctly projecting beyond posterior body margin, distally frayed, form hook-like pattern. Dorsal cilia in vivo 3-4 um long, originate from anterior basal body of dikinetids comprising dorsal bristle rows. Rows 1-3 in left half of dorsal side, almost as long as body, follow curvature of body margin, except row 3, which curves right in posterior half producing rather large, barren area between kineties 2 and 3; row 4 commences subapically near midline of cell, curved to right margin in mid-body, and continues posteriad to right caudal cirrus; row 5 slightly shortened anteriorly, ends somewhat above or below mid-body; row 6 very short, on average comprising 6 dikinetids only, terminates in anterior third of cell. Caudal cirri at posterior body margin right of midline, narrowly spaced, associated with dorsal kineties 1, 2 and 4, inconspicuous because slender and only slightly longer (22 um) than marginal cirri. Oral apparatus in anterior left quadrant of cell, conspicuous because occupying about 41% of body length. Adoral zone of membranelles commences subapically at right margin of body, curves along anterior body margin, and extends obliquely posteriad to midline of cell; adoral cilia in vivo about 20 um long, bases of largest membranelles 11 um wide, each membranelle composed of four ciliary rows with anterior rows successively shortened from left to right, frontal (distal) membranelles of different structure, as described by Augustin and Foissner (1992) in S. histriomuscorum: the distalmost membranelle, which is composed of three rows of equal length, is followed by four to five membranelles, which are also composed of three rows but have a fourth, shorter row attached to right mid-portion. Buccal cavity narrow and rather flat, slightly curved anteriorly, almost entirely covered by hyaline, lanceolate lip widening from anterior to posterior. Paroral and endoral membrane at right margin of buccal cavity, paroral near level of cell surface in deep cleft of buccal lip, endoral on bottom of buccal cavity, both slightly curved and possibly composed of tightly spaced dikinetids, intersect optically in anterior third of buccal cavity, as also evident from ontogenesis. Paroral cilia in vivo about 10 um long, endoral cilia at least 15 um long, form bundle beating into cytopharynx. Pharyngeal fibres inconspicuous, originate from posterior portion of endoral membrane and adoral zone of membranelles. (ref. ID; 2857)
[Resting cyst]: Permanent resting cysts spherical to slightly ellipsoidal, colourless, old cysts slightly smaller than young ones. Ectocyst 1.5-3.5 um, usually 2-3 um thick, appears to be composed of many tightly spaced membranes, colourless and hyaline, surface smooth in very young cysts, distinctly wrinkled when finished, stains lilac with methyl green-pyronin. Endocyst about 1 um thick and with brownish shimmer, compact, separate from cortex of cell by narrow, hyaline zone. Cyst content comprises countless fat globules 1-2 um across and some 3-4 um sized vacuoles with granular, yellowish content, possibly food remnants. Macronuclear nodules fused to reniform or dumb-bell shaped mass. (ref. ID; 2857)
Etymology; "nova" (new) refers to a new isolate of an Oxytricha sp. (ref. ID; 2857)
Type locality; Freshwater in North Carolina, USA. (ref. ID; 2857)
Type specimens; One holotype slide and seven paratype slides (all protargol-impregnated) with morphostatic and dividing specimens of S. nova have been deposited in the Oberosterreichische Landesmuseum in Linz (LI), Austria, accession numbers 1999/111 - 118. Relevant specimens are marked by black ink circle on the cover glass. (ref. ID; 2857)
Sterkiella thompsoni Foissner, 1996 (ref. ID; 2128 original paper)
Description; Three macronuclear nodules, On average 34 adoral membranelles, 22 right marginal cirri, 17 left marginal cirri, and 5 transverse cirri. Four dorsal kineties with 1 caudal cirrus each associated with kineties 1 and 3. Usually broadly parallel-sided, rarely slightly bursiform, anterior end broadly rounded, posterior narrowly rounded and often inconspicuously pointed. Rather rigid like, e.g., Sterkiella histriomuscorum and Stylonychia pustulata, dorsoventrally flattened up to 2 : 1. Usually 3, very rarely 2 (in 6 out of 120 specimens, possibly postdividers) or 4 slightly to distinctly ellipsoidal macronuclear nodules in median of middle third of cell; middle nodule often slightly smaller than anterior and posterior ones. Micronuclei globular, number high variable, however, most specimens have only one. Contractile vacuole in mid-body at left margin, with 2 long collecting canals extending anteriorly and posteriorly, anterior canal often with small dilatations at level of buccal cavity. Cortex colourless, rigid, without special granules. Cytoplasm colourless, contains many small, yellowish crystals, some 2-6 um sized, colourless fat globules, and food vacuoles up to 10 um in diameter. Feeds on bacteria, heterotrophic flagellates, green algae, diatoms and wheat starch. Moves moderately fast, often resting for some time and thus easy to observe. Anterior frontal cirri, transverse cirri and caudal cirri about 30 um, other cirri 25 um long. Marginal rows open at posterior end, gap occupied by caudal cirri right of cell median. Ventral cirral pattern as in other oxytrichids s. str. Dorsal cilia in vivo about 3 um long, associated with distinct, oblique fibrillar structures; arranged in 4 rows which, according to some divisional stages found, originate as follows: row 1 slightly shortened anteriorly and associated with right caudal cirrus; row 2 as long as body; row 3 also extends along whole body length but is associated with left caudal cirrus; row 4 slightly but invariably shortened posteriorly, originates close to right marginal row. Dorsal rows 1-3 simply divide, i.e. none originates by fragmentation as in many other oxytrichids, for instance, S. histriomuscorum. Oral apparatus and adoral zone of membranelles conspicuous, occupy about 42% of body length. Buccal cavity rather large and deep, right margin thickened, possibly by fibres or backwardly directed in dorsal cilia, right half of cavity covered by hyaline lip. Paroral and endoral membrane slightly curved, inconspicuously to distinctly intersecting optically, both very likely composed of dikinetids. Pharyngeal fibres inconspicuous. (ref. ID; 2128)
Note; Skerkiella thompsoni differs from its congeners by the unusual number, viz. 3, of macronuclear nodules. All other oxytrichids s. str. either have 2, 4 or more nodules. Thus, at first Foissner supposed that S. thompsoni could be a teratological population of S. histriomuscorum (2 nodules) or S. cavicola (4 nodules), especially because some specimens with 2 or 4 micronuclei were found. However, some variability is also apparent in binucleate and quadrinucleate species, when a large number of cells is analysed. Furthermore, the Antarctic population remained constant over 4 weeks and some dividing cells showed that only 3 macronuclear nodules are generated. Finally, a detailed literature search showed that oxytrichids with 3 macronulear nodules have been reported several times and, most surprisingly, mainly from Antarctica. Thompson (1972) described and figured and Oxytricha sp. from a rock pool of the Antarctic Peninsula having the same characteristics as Foissner specimens. Likewise, Sudzuki (1964) mentions an Antarctic Opisthotricha with 2-3 macronuclei and a size 96-120 x 40-80 um. All these forms are very likely conspecific and S. thompsoni is thus obviously wide-spread in Antarctica. Seshacher & Kasturi Bai (1963) described an Oxytricha sp. from a fish tank in India, which "differs markedly in its nuclear constitution from the other known species of the genus". This still unnamed and very briefly described population has, like S. thompsoni, 3 macronuclear nodules propagated through many generations in ordinary laboratory cultures. However, the Indian species is much larger than S. thompsoni, viz. 200-450 x 100-150 um. It is thus very likely that several distinct oxytrichids with 3 macronuclear nodules exist. The generic classification of S. thompsoni is difficult and tentative because it possesses characters of several oxytrichid genera, and Sterkiella (formerly Histriculus; Foissner et al. 1991) is still vaguely separated from Stylonychia and Oxytricha. However, at least the general appearance, the inflexibility of the body, and the structure of the oral apparatus strongly resembles S. histriomuscorum and S. cavicola. On the other hand, the simple dorsal infraciliature of S. thompsoni is completely different from that of Sterkiella. Stylonychia and Oxytricha, but highly reminiscent of Urosomoida and Urosoma. Finally, the distinct fibres around the dorsal bristles and the rigidity of the cortex resemble stylonychid oxytrichids and Stylonychia which, however, has parallel undulating membranes. To sum up, S. thompsoni is a further example of the bewildering and still poorly understood diversity of oxytrichid hypotrichs. (ref. ID; 2128)
Etymology; Named in honour of Jesse C. Thompson Jr. (Roanoke College, Virginia, USA), who provided the first reliable description of this species, but did not establish it as a new taxon because he considered has data as insufficient; an honorable practice which should be applied more often!. (ref. ID; 2128)
Measurements; In vivo about 90-130 x 40-60 um. (ref. ID; 2128)
Sterkiella tricirrata Buitkamp (ref. ID; 2857)
Comments; This species differs from S. histriomuscorum by a slightly reduced number of transverse cirri (3 vs. 3-5) and dorsal kineties (5 vs. 6). (ref. ID; 2857)