Caryotricha orientalis, Hu & Suzuki & Hu, 2025

Caryotricha orientalis nov. spec.

urn:lsid:zoobank.org:act:350CCA3A-1F88-4257-B4FE-0258355764DB

Diagnosis. Body size in vivo 40–65 × 25–35 µm, oval in outline. Minute cortical granules densely arranged between ciliary rows. One macronucleus and usually two micronuclei. Adoral zone composed of about 19 membranelles. About eight cirral rows; the first cirri of the rightmost four rows enlarged significantly; about six transverse cirri, arranged in two rows posterior to the second and third inner cirral row; about four migratory cirri and eight dorsal kineties. Seawater habitat.

Type locality. New Fishing Port of Nagasaki, Japan .

Etymology. The species-group name orientalis recalls the fact that this species was first collected from oriental (Japanese) coastal waters.

Ecological data. Water temperature 23.7 ºC, pH 8.2, salinity 34.0‰, DO 5.82 mg /L.

Deposition of specimens. One protargol slide (registration number: NagC2003101601) containing the holotype and paratype specimens was deposited at the Marine Biological Museum , Chinese Academy of Sciences, China . Three other slides with paratype specimens (registration numbers: NagC2003101602, -03, -04) were deposited in the Laboratory of Protozoology , Ocean University of China , Qingdao, China.

Description. Body size in vivo 40–65 × 25–35 µm, with the ratio of body length to body width ca 1.5–1.8; oval in outline with anterior end narrowed and posterior end round ( Figs. 1A View FIGURE 1 , 2A – F View FIGURE 2 ); dorsoventrally somewhat flattened, with ventral side flat and dorsal side convex. Buccal field dominant, extending about 65% of the cell length in vivo ( Figs. 1A View FIGURE 1 , 2F and H View FIGURE 2 ). Pellicle rigid, with cortical granules rod-shaped, about 1 µm long and 0.2–0.25 µm wide, arranged perpendicularly underneath in dense packs between cirral or ciliary rows ( Figs. 1B, C View FIGURE 1 , 2G, I View FIGURE 2 ). Cytoplasm full of many inclusions and several food vacuoles containing diatoms, especially in the posterior half of the cell, rendering cell opaque under low magnification except the transparent buccal cavity that is broad, about half of body width with the margin of buccal lip concaved where cirri are located ( Figs. 1A View FIGURE 1 , 2A – F, H View FIGURE 2 ). Macronucleus globular to ellipsoidal in shape, distributed in the anterior half of the cell, up to 15 µm across in vivo ( Figs.1A, D View FIGURE 1 , 2C, F, I View FIGURE 2 ); mostly two, rarely one or three micronuclei recognized in stained specimens, globular to slightly oval, about 1.6 µm long ( Figs. 1D View FIGURE 1 , 2K View FIGURE 2 ). No contractile vacuole observed. Locomotion moderately quick, either by swimming around the body axis or crawling rapidly on the substrate.

Adoral zone of membranelles slightly curved, extending about 68% of body length in protargol preparations and consisting of about 19 membranelles ( Figs. 1D View FIGURE 1 , 2J View FIGURE 2 ; Table 1 View TABLE 1 ). Endoral membrane double-rowed in zig-zag pattern, while paroral membrane consisting of obliquely arranged rows of kinetosomes and three or four kinetosomes in each row; both being almost equally long and parallel to each other ( Figs. 1D View FIGURE 1 , 2J View FIGURE 2 ).

On both ventral and dorsal sides, most cirri arranged in 8 or 9 longitudinally curved rows, of which the longest one extends to mid-body ( Figs. 1D, E View FIGURE 1 , 2J, L View FIGURE 2 ). Cirri generally uniform and non-grouped with exception of transverse cirri that are generally arranged in two short rows behind the second and third inner cirral rows ( Figs. 1D View FIGURE 1 , 2J View FIGURE 2 ), with cilia about 18 µm long in vivo. The bases of transverse cirri mostly long rectangular and larger than other cirri and thus clearly recognizable. Always the first cirrus of each of the rightmost four cirral rows on dorsal side conspicuously larger than other cirri ( Figs. 1E, L View FIGURE 1 ). One short migratory row posterior to buccal field, comprising about four cirri ( Figs. 1D View FIGURE 1 , 2J View FIGURE 2 ), with cilia 10 µm long in vivo.

About eight dorsal kineties on the dorsal and ventrolateral sides, with dikinetids sparsely arranged; each dikinetid bearing two cilia, about 3 µm long in vivo ( Figs. 1D, E View FIGURE 1 , 2I, J, L View FIGURE 2 ). The leftmost dorsal kinety commencing subapically on dorsal side and extending to ventral side; the middle kineties shortened anteriorly and the rightmost one on dorsal side continuing anteriorly with the rightmost cirral row (counted clockwise from ventral side to dorsal side), thus making a mixed structure.

Species identification of Japan form. As regards the simple, ancestral pattern of the dorsal dikinetids (both basal bodies of each dikinetid bearing one short cilium), uniform distribution of most cirri on the ventral and dorsolateral sides, and the presence of a short migratory row just behind peristome as well as rod-shaped cortical granules, the Japan form undoubtedly belongs to the genus Caryotricha .

Up to now, there are only four species reported in Caryotricha , i.e., C. convexa Kahl, 1932 (type species), C. minuta ( Xu et al., 2008) Miao et al., 2009 , C. rariseta Jiang et al., 2013 , and C. sinica Lian et al., 2020 ( Kahl 1932; Xu et al. 2008; Miao et al. 2009; Jiang et al. 2013; Lian et al. 2020; Table 2 View TABLE 2 ).

No morphometric data are available for C. convexa . According to the description by Kahl (1932) and illustration of Borror (1972), the cirral rows of C. convexa extend to the posterior end of the cell, which is very distinct from those of the present form (vs. about half of body length).

Among the remaining three congeners, C. minuta is most similar to the present form in terms of body size and shape, the number and arrangement of transverse cirri, and the number of dorsal kineties ( Xu et al. 2008; Miao et al. 2009; Tables 1 View TABLE 1 and 2 View TABLE 2 ), however, the latter can be clearly distinguished by a combination of the following morphological characters: relative shorter adoral zone (ca. 68% of body length vs. on average 80% of body length in C. minuta ), the longest cirral row extending to mid-body with about 15 cirri (vs. two-thirds of body length, composed of 21 cirri for the holotype), the number and arrangement of migratory cirri (on average 4, sparsely spaced vs. 7 or 8, close-set) and lower number of adoral membranelles (17–20, on average 19 vs. 22–27, on average 25 in the Korea population, and 22–25, on average 23 in the China population). Furthermore, for the Japan isolates, the first cirrus of each of the rightmost four cirral rows has extremely larger base than others, a character not described for C. minuta . Actually, according to line illustration ( Fig. 1g View FIGURE 1 ) and micrograph ( Fig. 2n View FIGURE 2 ) given for China population of C. minuta , the first cirrus of each of the rightmost six cirral rows is slightly enlarged when compared with the remain cirri in these rows ( Miao et al. 2009). Unfortunately, this feature cannot be determined for the Korea (type) population due to excessive staining. Additionally, the current form has lower number of cirral rows (8–9, mostly 8 vs. 9–12, on average 10, and invariably 11) and fewer cirri in cirral rows 1–3 (3–5, ca. 4; 5–7, on average 6; and 5–9, on average 8 vs. 7, 11–13, and 10–13 counted from illustrations; Tables 1 View TABLE 1 and 2 View TABLE 2 ). Therefore, both forms cannot be conspecific from morphological perspective.

Xu et al. (2008) described rod-shaped, 3–4 µm long extrusomes, which cannot be justified owing to the lack of micrographs of live ciliates; the colorless cortical granules illustrated are ca. 1 µm long (estimated from Figure 6 in the original publication), similar to cortical granules in the present form in the length and arrangement mode, but they are different in the shape (ellipsoidal vs. rod-shaped). Miao et al. (2009) recorded bar-shaped cortical ‘granules’ up to 5 µm long, which was, however, not substantiated by a micrograph provided ( Figure 2g View FIGURE 2 in the original paper); according to the size of macronucleus (ca. 25 µm across), these cortical granules are about 1 µm long. These cortical granules in C. minuta show the same pattern as in the current form and two other congeners with data available ( Jiang et al. 2013, Lian et al. 2020), suggesting it might be a plesiomorphic character for the genus Caryotricha .

Caryotricha sinica Lian et al., 2020 is also closely related to the present form in terms of body size and shape, the numbers of adoral membranelles and cirral rows as well migratory cirri, but both can differ by the arrangement and number of transverse cirri (3 or 4 in a single pseudo-row in the former vs. 5–7, two- rowed in the latter), the number of dorsal kineties (5 vs. 8), and the number and arrangement of cirri in the rightmost cirral row (17–30, distinctly smaller and more narrowly spaced than others vs. 9–12, usual as others ( Lian et al. 2020).

Our form can be separated from C. rariseta Jiang et al., 2013 by having higher numbers of adoral membranelles (17–20, on average 19 vs. 13–15) and dorsal kineties (ca. 8 vs. 5), and in the number and arrangement of transverse cirri (5–7, arranged in two rows vs. 3 in a pseudo-row) ( Jiang et al. 2013).

Conclusively, the present form cannot be any extant species in the genus and a new species has to be proposed for it.