Euplotes balteatus ( Dujardin, 1841 ) Kahl, 1932

Chen, Xiangrui, Zhao, Yan, Al-Farraj, Saleh A., Al-Quraishy, Saleh A., El-Serehy, Hamed A., Shao, Chen & Al-Rasheid, Khaled A. S., 2013, Taxonomic Descriptions of Two Marine Ciliates, Euplotes dammamensis n. sp. and Euplotes balteatus (Dujardin, 1841) Kahl, 1932 (Ciliophora, Spirotrichea, Euplotida), Collected from the Arabian Gulf, Saudi Arabia, Acta Protozoologica 52 (2), pp. 73-89 : 82-87

Euplotes balteatus ( Dujardin, 1841) Kahl, 1932 (Figs 37–66; Tables 1, 2)

Some new data, especially in respect to the molecular information, have been obtained from the population studied here and we therefore present an updated description and an improved diagnosis based on present and previous works ( Dujardin 1841, Kahl 1932, Tuffrau 1964, Pan et al. 2012).

Improved diagnosis. Marine Euplotes with 3–5 dorsal ridges, 30–150 × 30–105 μm in vivo. Adoral zone about 67–75% of cell length, with 25–80 membranelles. 10 frontoventral, five transverse, 2–3 caudal and two marginal cirri, 7–8 dorsal kineties bearing 10–16 dikinetids in mid-dorsal row. Macronucleus C-shaped or horseshoe-shaped. Micronucleus ellipsoidal, and attached to left arm of macronucleus. Dorsal silverline system in a double-eurystomus pattern.

Voucher slides. Two voucher slides of protargolstained cells have been deposited in the Laboratory of Protozoology, Ocean University of China, Qingdao (registration number CXR- 20111216 -21-01, 02).

Morphological description of the Saudi population. Cells in vivo usually 70–100 × 50–75 μm in size. Body outline ellipsoidal with anterior and posterior ends slightly narrowed, as shown in Figs 37, 54. Left margin of anterior end truncated in some individuals (Figs 38, 55). Buccal field approximately three-quarters of total cell length. Three conspicuous ridges on ventral side, two right ridges extending posteriorly to transverse cirri, leftmost ridge interrupted by paroral membrane (Figs 37, 38, 55, 57). Five dominant ridges on dorsal side extending over entire cell length (Figs 39, 56, arrowheads).

Cytoplasm colourless, highly transparent at posterior part and oral area, but opaque in central part where it is packed with many different-sized gray granules and a few food vacuoles. Contractile vacuole under the fourth right transverse cirrus (Figs 37, 38, 54, arrow). Macronucleus variable in shape: usually typical horseshoe-shaped (Fig. 44); occasionally, approximately ring-shape, with two ends expanded and almost connected to each other ( Fig. 59 ). Micronucleus spherical, and attached to left arm of macronucleus (Figs 48, 58, arrowhead).

Movement slow, involving crawling on substrate and remaining stationary for long periods.

Cirri and adoral membranelles fine and long. Adoral membranelles, frontoventral, and left marginal cirri approximately 25 μm long, caudal cirri about 20 μm, and transverse cirri 30–35 μm.

Infraciliature as shown in Figs 47, 48, 58–62. Paroral membrane small, typically composed of many irregularly arranged kinetosomes, positioned close to surface of buccal lip and easily revealed in vivo (Figs 37, 38, 57, double-arrowheads). Adoral zone of membranelles resembling that of Euplotes vannus , proximal portion curved at about 90° angle to right, and composed of 31–39 membranelles. Consistently, 10 frontoventral and five strong transverse cirri forming a normal pattern, two left marginal cirri separated and aligned evenly with two caudal cirri. Seven dorsal kineties extending over almost entire length of cell, with middle row containing 10–13 dikinetids, but leftmost row including only 4–7 dikinetids (Figs 47, 60). Dorsal silverline system in a double-eurystomus pattern (Figs 40, 62).

Morphogenesis. Only one specimen in the middle stage of morphogenesis was obtained. The morphogenesis of this species can be summarized as follows: 1) the oral primordium form in a subcortical pouch; 2) nine frontoventral and five transverse cirri develop from the five frontoventral transverse anlagen which fragment in a “3:3:3:3:2” pattern; 3) the leftmost frontal cirrus develops de novo on the cell surface in both dividers; and 4) the left marginal cirri develop from the marginal anlagen which are formed de novo (Figs 49, 50, 63–66).

Comparison with different populations. This species was first described by Dujardin (1841) and subsequently redescribed several times ( Kahl 1932, Tuffrau 1964, Pan et al. 2012). We identify our form by the basic ciliature on the ventral and dorsal surfaces, the marine habitat, the five dorsal ridges, and the double-eurystomus silverline system pattern. Nonetheless, there are some small differences between the Arabian Gulf population and historic populations, such as cell size, the number of dorsal kineties and dikineties in the mid-dorsal kinety, as well as the shape of the macronucleus: for details see Table 2.

The original description was brief and incomplete: oval shaped body with the anterior end slightly narrowed, five inconspicuous dorsal ridges ( Dujardin 1841). About a century later, Kahl (1932) gave a good succinct description: cell length about 60–80 μm in vivo, dorsoventrally flattened, three to five slight dorsal ridges, ten frontoventral, five transverse, four caudal cirri (Figs 43, 44; Table 2). The morphology of the silverline system and infraciliature were supplied by Tuffrau (1964) (Figs 41, 42, 45; Table 2) and Pan et al. (2012) (Figs 46, 51–53; Table 2). In the present Arabian Gulf population, it is noticeable that the five

Figs 37–53. Euplotes balteatus in vivo (37–39, 43, 44, 51), after silver nitrate (41, 42, 45, 52, 53) and protargol (40, 46, 47–50) impregnation. 37, 38, 43, 44, 51 – ventral views of different individuals (43, 44 from Kahl 1932; 51 from Pan et al. 2012); 39 – dorsal view, showing the five conspicuous ridges; 40 – double-eurystomus type of silverline system on the dorsal side; 41, 42, 52, 53 – silverline system on the ventral and dorsal sides (41, 42 after Tuffrau 1964; 52, 53 from Pan et al. 2012); 45 – macronucleus hook-shaped (from Tuffrau 1964); 46 – macronucleus horseshoe-shaped (from Pan et al. 2012); 47, 48 – ventral and dorsal views of the infraciliature; 49, 50 – ventral and dorsal views of an middle divider, showing the frontoventral-transverse cirral anlagen, the migratory cirri anlagen in the proter (49, doublearrowhead) and opisthe (49, arrowhead), the marginal cirral anlagen (49, arrows), the dorsal kineties anlagen (50, arrowheads) and the replication bands (50, double-arrowheads). AZM – adoral zone of membranelle, CC – caudal cirri, DK – dorsal kineties, FVC – frontoventral cirri, MC – marginal cirri, PM – paroral membrane, TC – transverse cirri, 1–7 – dorsal kineties. Scale bars: 50 μm.

dorsal ridges are dominant. Because this diagnostic trait usually varies in relation with the cell’s nutritive conditions, it is regarded as a population-level difference.

Supposed synonym. In the original description ( Kahl 1932), Euplotes alatus is a medium marine species (75–90 μm long) with 10 frontoventral, two caudal, and two marginal cirri, and five conspicuous dorsal ridges (the leftmost 2 nd ridge being huge and prominent). In Borror’s redescription, it has eight dorsal kineties, 10–12 dikinetids in the mid-dorsal kinety, about 26 adoral membranelles, and the double-eurystomus silverline system pattern ( Table 2; Figs 67–72 ; Borror 1968). We incline to consider these characters as innerspecific differences and agree with Song and Wilbert (2002) that E. alatus is possibly a junior synonym of E. balteatus . For the same reason, E. quinquecarinatus Borror, 1968 may also be a junior synonym of E. balteatus ( Table 2; Figs 80–83 ; Borror 1968, Song and Wilbert 2002).

Comparison with related congeners. In terms of the 10 frontoventral, two caudal, two marginal cirri, and several dorsal ridges, there are ten potentially related species that should be compared with E. balteatus : E. trisulcatus Kahl, 1932 ; E. cristatus Kahl, 1932 , sensu Tuffrau (1960), E. magnicirratus Carter, 1972 , E. euryhalinus Valbonesi and Luporini, 1990 , E. plicatum Valbonesi et al., 1997 , E. petzi Wilbert and Song, 2008 , E. orientalis Jiang et al., 2010 , E. wilberti Pan et al., 2012 , E. parabalteatus Jiang et al., 2010 , E. dammamensis n. sp. ( Table 2). Of these, E. cristatus differs from E. balteatus in its single-vannus silverline system pattern ( Tuffrau 1960). For the same reason, E. orientalis and E. petzi can be easily distinguished from E. balteatus through their double-patella silverline system pattern ( Petz et al. 1995, Wilbert and Song 2008, Jiang et al. 2010b).

Euplotes magnicirratus is similar to E. balteatus in its cell size, the basic ciliature on both ventral and dorsal sides, the marine habitat, and the features of its dorsal ridges. The former differs from the latter, however, in having more adoral membranelles (49–52 vs. generally 25–39) and, particularly, stronger cirri on the ventral side ( Table 2; Figs 73–75 ; Carter 1972). In addition, the SSU-rRNA gene sequences of E. magnicirratus and E. balteatus differ in 165 nucleotides and exhibit 91.1% similarity [ E. magnicirratus GenBank accession number: AJ549210; submitted by Petroni (2003)] ( Table 3; Petroni et al. 2002).

Euplotes plicatum is a small (42–55 × 24–40 μm) organism collected from the harbour of Lyttelton (Christchurch, New Zealand) whose diagnosis closely resembles that of E. balteatus : marine habitat, 10 frontoventral, five transverse, two caudal, and two marginal cirri, 22–25 adoral membranelles, double-eurystomus silverline system pattern ( Table 2; Valbonesi et al. 1997). It can be distinguished from E. balteatus , however, through several morphological differences, i.e. the number of dorsal ridges (7–8 vs. 3–5) and dorsal kineties (10 vs. 7–8). Also, the SSU-rRNA gene sequences of the two species differ in 14 nucleotides and exhibit 99.2% similarity [ E. plicatum GenBank accession number: EF094966; submitted by Giuseppen and Dini (2006)] ( Table 3; Vallesi et al. 2008).

Euplotes euryhalinus differs from E. balteatus in having more dorsal kineties (11 vs. generally 7–8), and more dikinetids in the mid-dorsal kinety (18 vs. 10–16) ( Table 2; Valbonesi and Luporini 1990). The two species can also be easily distinguished by their molecular information, with 228 different nucleotides between their SSU-rRNA gene sequences and a similarity of 87.8% [ E. euryhalinus GenBank accession number: EF094968; submitted by Giuseppen and Dini (2006)] ( Table 3; Vallesi et al. 2008).

Euplotes parabalteatus is similar to E. balteatus in cell shape and size, cirral pattern and silverline system. The former, however, can be separated from the latter by the feature of dorsal ridges (absent vs. 3–5 conspicuous), having fewer adoral membranelles (19–23 vs. 25–80) and dorsal kineties (6–7 vs. 7–8), the shape of macronucleus (slightly curved-bar-shaped vs. inverted C-shaped) ( Figs 87–90 ; Jiang et al. 2010a). The divergence of these two forms is supported by SSU-rRNA gene sequence data, as they differ in 317 nucleotides and exhibit 82.3% similarity [ E. parabalteatus GenBank accession number: FJ346568; submitted by Li and Song (2008)] ( Table 3; Jiang et al. 2010a).

Euplotes wilberti was collected from King George Island, Antarctica and originally reported by Song and Wilbert (2002) as another population of E. balteatus . The Antarctica population differs from E. balteatus in several respects, however, including: the number of dorsal ridges (6–7 vs. 3–5) and dorsal kineties (8–10 vs. 7–8) and the character of the marginal cirri (densely ranged vs. widely separated). Based on these differences, Pan et al. (2012) identified it as a new species, E. wilberti ( Figs 84–86 ; Table 2; Song and Wilbert 2002, Pan et al. 2012).

Euplotes trisulcatus is a “slim” organism, with a ratio of body length to width of about 2:1. It differs from E. balteatus in two respects: the number of dorsal ridges (stable three vs. 3–5) and the shape of the macronucleus (slightly curved vs. inverted C-shaped) ( Figs 76– 79 ; Table 2; Kahl 1932, Tuffrau 1960). Based on these morphological features alone it should be synonymised with E. balteatus . The SSU-rRNA gene sequences of the two species, however, differ in 196 nucleotides and exhibit 89.5% similarity [ E. trisulcatus GenBank accession number: EF690810; submitted by Schwarz and Stoeck (2007); unpublished] ( Table 3). In view of this, detailed redescription of E. trisulcatus is necessary, especially to ensure that the morphometric and molecular data are based on the same population.

Euplotes dammamensis , meanwhile, the new species described in this work, can be clearly distinguished from the present population of E. balteatus by its body size (100–170 μm vs. 60–100 μm), the number of dorsal ridges (10 vs. 3–5), and dorsal kineties (11 vs. 7–8) ( Table 2). The dissimilarity between them is also supported by the molecular data, since their SSU-rRNA gene sequences differ by 337 nucleotides and exhibit 80.3% similarity ( Table 3).

Figures

Figs 54–66. Photomicrographs of Euplotes balteatus in vivo (54–57) and after protargol-impregnation (58–66). 54, 55 – ventral views of different individuals, arrow (54) denotes the contractile vacuole; 56 – dorsal view, arrowheads mark dominant ridges; 57 – ventral view, arrowheads mark ventral ridges, double-arrowhead denote paroral membrane; 58, 59 – different shapes of macronucleus, arrowhead (58) marks the micronucleus; 60, 61 – infraciliature on the ventral and dorsal sides; 62 – double-eurystomus type of silverline system on the ventral side; 63–66 – ventral and dorsal views of an early divider, showing the marginal cirral anlagen (63, arrowheads), frontoventral-transverse cirral anlagen (64, arrowheads), the dorsal kineties anlagen (65, arrowheads), and replication bands (66, arrowheads). Scale bars: 50 μm.

Figs 67–90. Morphologically similar marine Euplotes species with double-eurystomus silverline system pattern, and 10 frontoventral, two caudal, two marginal cirri. 67–72 – E. alatus Kahl, 1932 (67, 68 from Kahl 1932; 69–72 from Borror 1968); 73–75 – E. magnicirratus Carter, 1972 (from Carter 1972); 76–79 – E. trisulcatus Kahl, 1932 (76, 77 from Kahl 1932; 78, 79 from Carter 1972); 80–83 – E. quinquecarinatus Gelei, 1950 sensu Borror (1968) (from Borror 1968); 84–86 – E. wilberti Pan et al., 2012 (from Song and Wilbert 2002); 87–90 – E. parabalteauts Jiang et al., 2010 (from Jiang et al. 2010a). Scale bars: 25 μm.