Actinotricha saltans Cohn, 1866

Hu, Yue, Suzuki, Toshikazu & Hu, Xiao-Zhong, 2025, Taxonomic study on marine ciliates from Japan, with description of a new species, Caryotricha orientalis nov. spec. (Alveolata: Ciliophora), Zootaxa 5584 (2), pp. 243-258 : 251-254

Actinotricha saltans Cohn, 1866

Description. Cell size in vivo 60–85 × 25–40 μm (estimated from live and stained specimens), elongate lanceolate with anterior end tapering and posterior end round; length:width ratio ca. 2.5–3:1 ( Fig. 5A, B ). Flexible but not distinctly contractile. Cytoplasm greyish at low magnification, and the posterior cell portion often packed with different sized globules. Constantly two ellipsoid macronuclear nodules distinctly separated, 11–21 × 8–13 μm after protargol staining, with one spherical micronucleus between ( Figs. 4B , 5E and G ). Movement characteristic, that is, often immobile for several minutes with radiating distal adoral membranelles lying, then jumping suddenly.

Buccal field less than 30 % of body length ( Fig. 5A, B ); buccal cavity rather short and narrow; cytopharyngeal fibers extending posteriorly to mid-body ( Fig. 5C ). Adoral zone of membranelles divided into two parts: usually five apical membranelles with cilia up to 25 μm long, spine-shaped and radiating ( Fig. 5A–C ), proximal part with nine or ten membranelles with short cilia, the posterior-most two often covered by lip, detached from other proximal membranelles in protargol preparations ( Figs. 4A , 5C, G ). Undulating membranes located at the mid-portion of the buccal field, parallel to each other, and monokinetidal, with paroral membrane ahead of endoral membrane, and the former about half of the latter in length ( Figs. 4A , 5G ). All cirri relatively fine, 15–20 μm long, except transverse cirri that are strong with cilia up to 25 μm long, projecting beyond the rear end of body ( Fig. 5A ). Cirral pattern rather stable ( Figs. 4A , 5G ). Cirri in frontal field arranged in fixed pattern but shifted backward when compared with typical oxytrichids, of which three frontal cirri located near distal end of adoral zone, one buccal cirrus behind endoral membrane, and four frontoventral cirri behind the level of cytostome. Other cirri on ventral side including three postoral ventral cirri (about in mid body), and two pretransverse ventral cirri just anterior to five transverse cirri. The left marginal row comprised of nine cirri, which starts behind the cytostome and extends to the posterior end of the cell; the right marginal row, commencing in the mid-body, composed of only five cirri. Consistently six dorsal kineties with stiff cilia 7 μm long ( Fig. 5D ). Among them kineties 1, 2, 3 and 5 are almost bipolar. Kinety 4 quite shortened anteriorly, with only four pairs of basal bodies. Kinety 6 shortened posteriorly, extending to mid-body ( Figs. 4B , 5 F ). Constantly three fine caudal cirri ( Fig. 4B ) located at ends of kineties 1, 2 and 4, consisting of two basal body pairs each.

A late divider was recognized ( Fig. 5H – J ), from which some traits can be deduced as follows: A new adoral zone of membranelles develops independently in opisthe; five streaks of cirral anlagen (II-VI) plus undulating membranes anlage were formed in both proter and opisthe, respectively. These streaks begin to fragment and (finally) differentiate into new cirri in the following mode: 1: 3: 3: 3: 4: 4 (from left to right), which confirms the presence of 18 frontal-ventral-transverse cirri in interphasic cell of the present form. Two sets of anlagen occur ahead of the parental right marginal row. Dorsal kineties anlagen seem to originate within some old structures.

Ecological data. Water temperature 14.8 ºC, pH 8.2, salinity 34.9‰, DO 8.1 mg /L.

Comparison with some populations. The species was dated to more than 150 years ago, when Cohn first recorded it under the present name. After that, it has been redescribed many times with some details under different names, namely Oxytricha (Actinotricha) saltans , and Oxytricha saltans ( Kahl 1932; Berger 1999). Detailed redescriptions with ciliature data of O. saltans were provided by Song et al (1991) and Song & Wilbert (1997). Berger (1999) comprehensively reviewed the research history of Oxytricha saltans . Until 2011, Actinotricha saltans was reactivated by Shao et al. (2011).

Only a few live and stained specimens are available for the current form. However, with reference to the bipartite adoral zone with the crown-like apical membranelles, short and parallel undulating membranes, one left and one extremely shortened right marginal row, more than three dorsal kineties, and caudal cirri present, it should be classified into Actinotricha , a monotypic genus with only A. saltans included. Prior to this study, the species had never been recorded in the coastal waters of Japan despite its cosmopolitan distribution. The Japan population resembles previous populations of the species very much in body size and shape, nuclear apparatus, extremely long dorsal cilia as well as the arrangement of cirri (three frontal, one buccal, four frontoventral, three postoral, two pretransverse ventral and five transverse cirri) on ventral size, which was distinctly illustrated in Rees (1884) (redrawn as Fig. 87e and g in Berger, 1999) and its identification is unquestionable. In terms of ciliature, only Qingdao populations can be used for comparison ( Song et al. 1991; Song & Wilbert 1997). It is curious that two Qingdao populations have almost identical morphometric data. But the present form corresponds well in most diagnostic features (see above), except minute difference in the number of frontoventral cirri (4 vs. 3 as described). The presence of four frontoventral cirri in the Japan isolate can be confirmed by a late divider (see above). By contrast, the record of three frontoventral cirri was not substantiated by any micrograph in Qingdao population of Song et al. (1991). Considering all species of genus Oxytricha , in which this species was placed for a long time, constantly have four frontoventral cirri, misinterpretation cannot be excluded for the 1991 population. Of course, another possibility occurs, i.e. the form reported in 1991 did possess the slight reduction of frontoventral cirri, which can be attributed to the resorption of kinetosomes during pattern formation. Song & Wilbert (1997) also described another Qingdao population with invariable seven frontal cirri (including buccal and frontoventral cirri; see Fig. 5b and d in the original publication). However, the line illustrations are inconsistent with the micrograph of one stained specimen provided in the same paper (the original Fig. 20, unfortunately, not cited), which show the presence of four frontoventral cirri. Therefore, misinterpretation existed for the description of 1997 population. As regards the details about dorsal ciliature, the location and shortness of dorsal kineties 4 and 6 recognized in current study may infer that this species shows dorsal kinety fragmentation and dorsomarginal kinety, a feature also illustrated by Song et al. (1991) and Song & Wilbert (1997) though these authors did not mention them clearly.

Figures

FIGURE 4. Morphology of Actinotricha saltans after protargol staining. (A–B) Ventral and dorsal view of the same specimen, displaying ciliature and nuclear apparatus, arrows and arrowheads indicate frontal cirri and frontoventral cirri, respectively. AZM, adoral zone of membranelles; BC, buccal cirrus; CC, caudal cirri; EM, endoral membrane; LMR, left marginal row; Ma, macronuclear nodule; Mi, micronucleus; PM, paroral membrane; PTC, pretransverse ventral cirri; PVC, postoral ventral cirri; RMR, right marginal row; TC, transverse cirri; 1−6, dorsal kineties 1−6. Scale bars = 40 µm.

FIGURE 5. Actinotricha saltans from life (A–D) and after protargol staining (E–J). (A, B) Ventral view showing general body shapes and the cirral arrangement on the ventral side, the arrows in A and B indicate the right marginal row starting at midbody and cirral-like apical membranelle, respectively. (C) Ventral view of the anterior body showing adoral membranelles. (D) Lateral view showing conspicuous dorsal cilia (arrowheads).(E) Macronuclear nodules.(F) Dorsal view showing dorsal kineties (arrowheads). (G) Ventral view showing ciliature, arrow indicates the micronucleus. (H–J) Ventral (H, I) and dorsal sight of the same late divider, presenting development of oral primordium and anlagen, arrowheads indicate a small anlage detached from undulating membranes anlage, arrows mark dorsal kineties anlagen. AZM, adoral zone of membranelles; BC, buccal cirrus; EM, endoral membrane; LMR, left marginal row; Ma, macronuclear nodule; PM, paroral membrane; PTC, pretransverse ventral cirri; PVC, postoral ventral cirri; RMR, right marginal row; TC, transverse cirri; 1~6, dorsal kineties. Scale bars = 30 µm.