Theristus siwaschensis, Revkova, 2020

Revkova, Tatiana N., 2020, Two species of the genus Theristus Bastian, 1865 (Nematoda: Xyalidae) from the hypersaline water bodies of the Crimea (Azov-Black Sea basin), Zootaxa 4881 (2), pp. 372-382 : 373-376

Theristus siwaschensis sp. n.

Figure 1 , 2 ; Table 1

Material examined: five males, six females. Holotype. Male mounted on slide (FlN/BS-35), in glycerin-gelatin. Paratypes. Males, slides (FlN/BS-36 and FlN/BS-37); females, slides (FlN/BS-38–FlN/BS-42), in glycerin-gelatin. Males, slides (FlN/BS-43 and FlN/BS-44); female, slide (FlN/BS-45), in glycerin.

Type locality: The Sea of Azov, Crimea, Sivash Bay, depths 0.3–0.6 m, Cladophora algal mats and muddy sand sediments, water salinity 75–77 g ̕ l–1. Geographical coordinates 45°31’13.7’’N, 35°11’12.9’’E (holotype); 45°27’19.5’’N, 35°13’27.9’’E; 45°37’48.3’’N, 35°01’54.8’’E.

Etymology. The species name siwaschensis refers to the type locality.

Description. Male. Body slender, about 1756–1916 μm long, almost uniform diameter except for pharyngeal and tail regions. Cuticle annulated, thickness 2 μm in the middle of body. Head blunt, labial region 7 (6–7) μm in height. Head with six setiform outer labial papillae, each about 3 μm long; 16 cephalic setae in one circle: 8 longer (15–18 μm) and 8 shorter (10–14 μm). Somatic setae 5–6 μm long, arranged irregularly along body. Amphidial fovea circular, 7 (6–7) μm in diameter, occupying about 26.9 (24.1–29.6) % of the corresponding body diameter; anterior margin of amphids 17–22 μm from anterior body end. Buccal cavity cup-shaped (12–15 μm wide), funnelshaped posterior half surrounded by pharynx. Pharynx muscular, almost cylindrical. Cardia elongate and conical, 17 (14–17) μm long. Nerve ring situated at 40 (40.3–43.4) % of pharynx length from anterior. Ventral gland and excretory pore not observed. Reproductive system diorchic with outstretched testes. Anterior testis situated to left of and smaller posterior testis to right of intestine. Spicules large, 2.1 (1.5–2.4) of the anal body diameter, complex in shape; twisted and curved at middle part with a poorly development capitulum. Gubernaculum consists of two parts: a distal well-cuticularised claw-shaped formation (12–14 μm long) with two minute ridges ( Fig. 2H ); a proximal thin fitting plate and relatively long and narrow dorsal apophyses. In one male spines on the distal “claw” absent ( Fig. 2G ). Six medioventral, precloacal supplementary papillae, appearing as very minute tubercles with thin canals. Precloacal setae 4 μm long, located at a distance of 12 (14–15) μm from cloaca. Tail elongated, 5.1 (4.2–6.1) of the anal body diameter; conical in shape with thin and short setae, 4–6 μm long. Two caudal glands present.

Female. Similar to males in most features. Amphidial fovea 6 (5–7) μm in diameter, occupying about 23.1 (20–28) % of the corresponding body diameter. Reproductive system monodelphic. Ovary outstretched, on left of intestine. Vulva transverse, located at 74.1 (68.9–75.4) % from the anterior end. Vaginal canal is very long (55–64 μm), consisting from two parts: thin (30–39 μm long) and well muscularized (22–25 μm long) ( Fig. 1G ; Fig. 2F ). Two ventral glands surrounding the vulva. Only one female (FlN/BS-42) carry one big egg 93×25 μm ( Fig. 1F ). Tail elongated, 5.8 (4.5–6.1) of the anal body diameter, conical in shape, gradually narrowing. Two caudal glands present.

Differential diagnosis. In accordance with the new key to species by Tu & Gagarin (2017) Theristus siwaschensis sp. n. is morphologically closest to T. flevensis Schuurmans Stekhoven, 1935 , T. parambronensis Timm, 1952 , T. macroflevensis Gerlach, 1954 , T. metaflevensis Gerlach, 1955 . However, new species differs from them by the structure of reproduction system of females: vaginal canal is very long, consisting of thin and well muscularized parts; presence of 16 cephalic setae; large spicules and a poorly development capitulum. T. siwaschensis sp. n. differs from T. flevensis (including new data) by having longer spicules (71–77 μm vs. 34–65 μm); new species differs from T. parambronensis by having larger body (1487–1916 μm vs. 790–1350 μm) and position of vulva (68.9–75.4 % vs. 83.4 %); it differs from T. macroflevensis by shorter somatic setae (5–6 μm vs. 30 μm) and presence of pre-cloacal supplements. The new species differs from T. metaflevensis by having longer cephalic setae (10–18 μm vs. ♀ 9 μm and ³11–14 μm), smaller amphidial fovea (5–7 μm vs. 9 μm), position of vulva (68.9–75.4 % vs. 83–87 %), presence of precloacal supplements and large value of de Man’s index “c” (8.7–10.2 vs. 6.7–6.8).

Figures

FIGURE 1. Theristus siwaschensis sp. n. Holotype male and paratype female.A. tail end of male, B. male head, C. female head, D. general view of male, E. general view of female, F. vulval region, G. vulval region and egg in paratype female, H. spicule and gubernaculum. Scale bar: D, E = 100 μm; A = 50 μm; G = 40 μm; B, C, F, H = 20 μm.

FIGURE 2. Theristus siwaschensis sp. n. A. general view of male, B. amphid of female, C. amphid of male, D. precloacal papillae, E. general view of female, F. vulval region, G. spicule of paratype male 1, H. spicule of male. Scale bar: A, E = 100 μm; B, C, E, F, H = 20 μm; G = 10 μm.