Strobilidium rapulum, Xu, Dapeng, Song, Weibo, Sun, Ping & Chen, Xiangrui, 2006

Xu, Dapeng, Song, Weibo, Sun, Ping & Chen, Xiangrui, 2006, Morphology and infraciliature of the oligotrich ciliate Strombidium rapulum (Yagiu, 1933) Kahl, 1934 (Protozoa, Ciliophora, Oligotrichida) from the intestine of sea urchin Hemicentrotus pulcherrimus Agassiz, Zootaxa 1113, pp. 33-40 : 34-39

Strombidium rapulum ( Yagiu, 1933) Kahl, 1934 ( Figs. 1–2 ; Table 1 )

1933 Strobilidium rapulum n. sp. Yagiu, R., J. Sci. Hiroshima Univ., 2: 216. 1951 Strobilidium rapulum Poljansky, G. I., Mag. Parasit. Moscow, 13: 386.

This species has never been described based on modern methods, hence an improved diagnosis according to silver impregnation is supplied here.

Improved diagnosis: Large sized marine Strombidium in vivo 50–130 × 25–65 µm; dorsoventrally slightly flattened; outline ovate and turnip­shaped, tapering posteriorly and terminating in an elongate tail; apical protrusion about 5 µm long in live cells; one elongated macronucleus; girdle kinety composed of about 57 monokinetids; ventral kinety located subcaudally on right margin, composed of about 5 dikinetids; on average 16 anterior and 29 ventral membranelles.

Deposition of voucher slides: Since no silver impregnated specimens have previously been preserved, one slide (registration number 2005:08:24:03) is deposited in the Natural History Museum, London and two slides (registration numbers 05:05:17:01 and 05:05:17:02, respectively) are deposited in the Laboratory of Protozoology, Ocean University of China.

Description of Qingdao population: Cell size in vivo 80–130 × 45–60 µm. Dorsoventrally not or slightly flattened. The cell is turnip­shaped, tapering posteriorly and terminating in an elongate tail ( Figs. 1 A; 2A, B). The tail is about half to 2/3 of cell length, often curves towards left side of cell ( Figs. 1 A; 2A, D, arrow) and sometimes very thin ( Fig. 2 B). Different from those in the genera Tontonia and Spirotontonia , the tail of Strombidium rapulum is constant and not contractile. Apical protrusion (AP) is small, about 5 µm high and inconspicuous in vivo, but may disappear or be undetectable after protargol preparations ( Figs. 1 A; 2F, arrow). Buccal cavity deep, extends obliquely to the right cell side and terminates over to equatorial level ( Fig. 1 A). Cilia of anterior membranelles about 40–45 µm long, extend anteriorly as shown in Figs. 1 A and 2A–C. Cells not very fragile. Pellicle thin and hemitheca not detected. Cytoplasm colourless, often contains many large light­reflecting granules and food vacuoles (usually with algae including diatoms), making cell light­grey to greenish/brown colour ( Fig. 2 A, B). No extrusomes found. Single macronucleus left of mid­line, elongated ellipsoidal and containing numerous globular large chromatin aggregates ( Figs. 1 C, H, I; 2K). Micronucleus not observed. Neither a contractile vacuole nor a cytopyge recognized. Locomotion with two main patterns: very fast and irregular when swimming, or moderately fast when crawling obliquely over debris, using its adoral membranelles for attachment ( Fig. 1 B, D).

Oral apparatus occupies anterior end of cell. Anterior portion (AM) of adoral zone of membranelles (AZM) with 15–18 membranelles and the ventral portion (VM) with 28–31 membranelles ( Figs. 1 H, I; 2G, H). Bases of anterior membranelles about 8 µm long, each composed of three rows of basal bodies. Between and parallel to these membranelles, argentophilic fibres always present ( Figs. 1 E, 2H, arrowheads). When observed in vivo, protrusions often detected between anterior membranelles ( Fig. 2 C, arrowheads). Bases of ventral membranelles 3–5 µm long and distinctly shorter than anterior ones. Endoral membrane (EM) on inner wall of buccal cavity, often difficult to recognize in protargol stained specimens, possibly composed of a single row of basal bodies ( Fig. 1 E, H). Pharyngeal fibres range from 10–15 µm in length ( Fig. 1 H, arrowhead).

Somatic cilia arranged in a girdle and ventral kinety. Girdle kinety (GK) equatorially located, continuous and composed of 45–66 horizontally oriented monokinetids ( Figs. 1 H, I; 2I, arrowheads). Cilia of the girdle kinety are short, about 2 µm in length. Ventral kinety (VK) reduced, positioned at the anterior end of the tail, extending meridionally along right­lateral margin of cell and composed of 4–7 dikinetids ( Figs. 1 H; 2J, arrowheads), cilia relatively long, about 6 µm in length.

Occurrence and ecology: This species has been found repeatedly from different hosts of sea urchins. Yagiu (1933) discovered it from the intestine of Anthocidaris crassispina , collected from Pacific coast of Japan. Nie (1934) also found this species from Anthocidaris crassispina collected in Xiamen, southern China. Subsequently, this species was recorded from Strongylocentrotus nudus collected from Vladivostok, Russia ( Poljansky 1951). In China, this species was also isolated from in the intestine of Hemicentrotus pulcherrimus off Qingdao, north China ( Zhang 1959). Later, Zhang (1963) found this species from Anthocidaris crassispina collected in coastal regions of Zhejiang province, eastern China.

Comparison with original description and related species: Strombidium rapulum was originally reported by Yagiu (1933) from the intestine of Anthocidaris crassispina . Our population corresponds well with Yagius (1933) description regarding the basic morphology (viz. the habitat, cell size and shape, locomotion pattern, shape of macronucleus, etc.) ( Fig. 1 F). However, Yagiu (1933) found a micronucleus in his specimens, which is not observed by us. This is because that the micronucleus is not easily stained by protargol. Yagiu (1933) also stated that the tail bent, which often curves towards left side of cell in our specimens. This characteristic might be population dependent and thus it is reasonable to identify our organism to be conspecific with Yagius. Without knowledge of its infraciliature, Yagiu (1933) put the species into the genus Strobilidium . It was then transferred to Strombidium as S. rapulum by Kahl (1934) according to its open AZM. Based on the present work, the horizontally positioned girdle kinety, and longitudinally located ventral kinety proved that this species should be a member of Strombidium . The new combination made by Kahl (1934) is thus justified.

In 1951, Poljansky reported a population of S. rapulum from Vladivostok, Russia ( Poljansky 1951). Our population corresponds well with Poljanskys (1951) regarding the basic morphology (viz. the habitat, cell shape, shape and position of macronucleus, number of anterior and ventral membranelles, etc.). But the Russian population is relatively smaller (50–95 × 25–46 µm vs. 80–130 × 45–60 µm). We think this difference might be population dependent and thus should not be exaggerated. Since apical protrusion is inconspicuous in vivo and only can be detected under high magnification, this might explain why Poljanskys population has no apical protrusion ( Fig. 1 G). Hence, Poljanskys organism should be conspecific with ours.

In 1980, Jankowski designated Strobilidium rapulum as type of a new genus, Anthostrombidium ( Jankowski 1980). The infraciliature revealed by the present study proves it should be a member of Strombidium , thus the establishment of Anthostrombidium is not justified.

To date, only three species of oligotrich ciliates inhabiting sea urchins have ever been reported, Strombidium rapulum , Spirostrombidium echini Song et al., 1999 and Strombidium symbioticum Jankowski, 1974 ( Jankowski 1974; Song et al. 1999; Yagiu 1933).

Spirostrombidium echini was found from the digestive tracts of sea urchins Heliocidaris sp. and Hemicentrotus sp., which were collected from the Weddell Sea, Antarctica ( Song et al. 1999). It differs clearly from Strombidium rapulum mainly in: 1) cell shape (without tail vs. presence of the tail), and 2) totally different somatic ciliary pattern (pattern of Spirostrombidium vs. pattern of Strombidium ).

Strombidium symbioticum was described by Jankowski (1974), but the infraciliature remains unknown. However, it differs from Strombidium rapulum in the smaller size (39– 44 µm vs. 50–130 µm), cell shape (elongate, slender goblet without tail vs. turnip shaped with a conspicuous tail), and much lower number of ventral membranelles (about 10 vs. about 30).

Several other marine species of Strombidium having a long and pointed tail should also be compared with Strombidium rapulum .

Strombidium styliferum Levander, 1894 differs from S. rapulum by the combination of: 1) habitat (free living vs. endocommensal), 2) presence of extrusomes (vs. absence), 3) shape of macronucleus (globular vs. elongated), and 4) much lower number of ventral membranelles (9–11 vs. 28–31) ( Song & Packroff 1997).

Strombidium minor (Kahl, 1935) Maeda and Carey, 1985 can be clearly separated from S. rapulum by its habitat (free living vs. endocommensal), much smaller cell size (35 µm vs. 50–130 µm in length), presence of contractile vacuole (vs. absence) and extrusomes (vs. absence) ( Maeda & Carey 1985).

Strombidium longipes Meunier, 1910 exhibits a similar size and body shape, but can be separated from S. rapulum by its habitat (free living vs endocommensal), shape of macronucleus (oval vs. elongated), and less developed buccal field (about 1/6 cell length vs. about 1/3 cell length in living cells) ( Meunier 1910).

Figures

FIGURE 1. Strombidium rapulum from live cells (A, B, D, E – G) and after protargol impregnation (C, E, H, I). (A) Ventral view of a representative specimen, arrowheads mark the cilia of ventral kinety. (B) To show the creeping state, note the cell attached to the substrate with its adoral membranelles. (C) To demonstrate variations in macronucleus shape. (D) Swimming trace. (E) Apical view of the buccal apparatus, arrowheads note argentophilic fibres. (F, G) Ventral view of specimens (from Yagiu 1933 and Poljansky 1951, respectively). (H, I) Ventral and dorsal view of same specimen, to show the infraciliature, arrowhead in H marks the pharyngeal fibres. AM = anterior membranelles; AP = apical protrusion; EM = endoral membrane; GK = girdle kinety; Ma = macronucleus; VK = ventral kinety; VM = ventral membranelles. Scale bars in (A, G – I) = 40 µm, in (F) = 50 µm.

FIGURE 2. Microphotographs of Strombidium rapulum from live cells (A – D, F), after Methylgreen Pyronine staining and protargol impregnation (G – K). (A) A typical individual, arrowheads indicate light­reflecting granules in the cytoplasm while arrow marks the tail. (B) A slightly pressed specimens, to show light­reflecting granules in the cytoplasm (arrowheads). (C) Dorsal view, to show the protrusion between anterior membranelles (arrowheads). (D) To show details of the tail (arrow). (E) Ventral view, arrowhead marks the macronucleus. (F) Apical view, to show the apical protrusion (arrow). (G) Ventral view, to show the buccal apparatus. (H) Details of the anterior membranelles, arrowheads demonstrate the argentophilic fibres. (I) Ventral view, arrowheads indicate the girdle kinety, which is composed of monokinetids. (J) To show the ventral kinety (arrowheads). (K) Lateral view, to show the macronucleus. Scale bars = 60 µm.

Tables

TABLE 1. Morphometric data on Strombidium rapulum (Yagiu, 1933) Kahl, 1934. Data based on protargol impregnated, randomly selected specimens. Measurements in µm.

Characteristics Min	Max	Mean	M	SD	n
Cell, length* 70	120	88.7	88.0	11.9	30
Cell, width 35	50	41.8	42.0	4.3	30
Cell, length:width, ratio 2	3	2.1	2.2	0.3	30
Apex to macronucleus, distance 12	18	14.4	14.0	2.5	25
Apex to buccal vertex, distance 36	42	38.7	38.0	1.8	25
Apex to girdle kinety, distance 40	48	44.5	46.5	3.9	25
Macronucleus, length 28	37	31.1	31.0	2.9	30
Macronucleus, width 8	13	10.2	10.0	1.5	30
Anterior membranelles, number 15	18	15.8	15.0	1.1	20
Ventral membranelles, number 28	31	29.1	29.0	0.9	20
Girdle kinety, number of monokinetids 45	66	56.5	55.0	5.6	15
Ventral kinety, number of dikinetids 4	7	5.1	5.0	0.7	25