Theristus flevensis Schuurmans Stekhoven, 1935

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 : 377-381

Theristus flevensis Schuurmans Stekhoven, 1935

Figure 3 , 4 ; Table 2

Material examined: five males, five females. Females and males mounted on slides (FlN/BS-33 and FlN/BS-34), in glycerin-gelatin. One male and one female mounted on slide (FlN/BS-46), in glycerin.

Type locality: Black Sea, Crimea, Lake Chersonesskoye, depths 0.3–0.4 m, algal Cladophora mats and mud sediments, water salinity from 45 to 71 g ̕ l–1. Geographical coordinates 44°35’10’’N, 33°23’32’’E .

Description. Male. Body long and slender. Cuticle annulated, width of rings in the middle of the body 2 μm. Head blunt; labial region low, about 6 (5–6) μm in height. Head with six setiform outer labial papillae, each about 2 μm long and fourteen cephalic setae: 6 longer (11–16 μm) and 8 shorter (8–14 μm). Somatic setae 7–8 μm long, arranged irregularly along body. Amphidial fovea circular, 8–9 μm in diameter, occupying about 32 (30.7–37.5) % of the corresponding body diameter; anterior margin of amphids 18–20 μm from anterior body end. Buccal cav-ity cup-shaped, 11–12 μm wide, funnel-shaped posterior half surrounded by pharynx. Pharynx muscular, almost cylindrical. Cardia well developed, 18 (11–19) μm long, surrounded with intestinal tissue. Nerve ring situated at 43.7 (44.1–45.4) % of pharynx length from anterior. Ventral gland and excretory pore not observed. Reproductive system diorchic with outstretched testes. Anterior testis to the left of and posterior to the right of intestine. Spicules large (1.4–1.7 of the anal body diameter) and complicated in shape, twisted and curved along middle portion with well-pronounced capitulum. Gubernaculum consists of two parts: a distal well-cuticularised claw-shaped formation (9 μm long); a proximal thin fitting plate and two relatively long and narrow dorsal and ventral apophyses. Three or four medioventral (maybe more), preanal supplementary papillae, appearing as very minute tubercles with thin canals. Precloacal setae 3 μm long, located at a distance of 9–12 μm from cloaca. Ejaculatory glands ( Fig. 3B , 4H ) present on lateral side but exact number not defined. Tail elongated,, 5.8 (5.1–6.4) of the anal body diameter. Three caudal glands are present.

Female. Similar to male in general characteristics. Cuticle structure and head end similar to those in males. Cephalic setae: 6 longer (12–17 μm) and 8 shorter (8–14 μm). Amphidial fovea 6–8 μm in diameter, occupying about 21.4–32 % of the corresponding body diameter. Reproductive system is monodelphic, prodelphic. Ovary outstretched, on left of intestine, extending near to cardia. Vulva transverse, located at 66.7–74.9 % from the anterior end, vagina well sclerotized. Intrauterine eggs 53–60 × 27–34 μm. The spermatheca is large and distinct. Tail elongated, 6.4–8.1 of the anal body diameter, conical in shape, gradually narrowing.

Discussion. T. flevensis is distributed nearly world wide (seas and continental basins of Europe, Atlantic, Indian, Pacific Oceans) inhabiting biotopes with wide gradient of salinity (from freshened to hypersaline water bodies with salinity up to 128‰) (Chesunov, 1981). Specimens from hypersaline Lake Chersonesskoye in general fit the description of a large forms T. flevensis from Chile (Murhy 1966) and Caspian Sea (Chesunov 1981). However, they differ from the Caspian Sea specimens by having a larger amphidial fovea (6–9 μm vs. 5.5–6 μm), size of cephalic setae (8–17 μm vs. 10.5–11 μm) and large size of spicules (57–65 μm vs. 53.2 μm). In Table 3 some morphometric characteristics of similar species: T. flevensis , T. macroflevensis , T. metaflevensis and T. parambronensis are given. Specimens of T. flevensis from Lake Chersonesskoye are morphologically and morphometrically similar to T. pratti Murthy & Canaris (1964) from the Indian Ocean. They have identical structure of the reproductive system in females and the presence of precloacal papillae and ejaculatory glands in males. Therefore, T. pratti should be considered as a synonym of T. flevensis . T. flevensis differ to T. macroflevensis in having shorter somatic setae (7–8 μm vs. 30 μm), location of vulva (66.7–74.9 % vs. 74–76 %) and the presence of ventral outgrowth of the plate part of gubernaculum. T. flevensis differs from T. metaflevensis and T. parambronensis by position of the vulva (66.7–74.9 % vs. 83–87 % and 83.4 %). According to Chesunov (1981) T. ambronensis Schulz, 1937 and T. flevensis are practically similar species on morphology, and morphometrics of the first one are within of variability for T. flevensis . In this regard, I consider T. ambronensis to be a synonym of T. flevensis .

Figures

FIGURE 3. Theristus flevensis Schuurmans Stekhoven, 1935: A. general view of female, B. general view of male, C. tail end of male, D. spicula and gubernaculum, E. tail end of female, F. female head, G. male head, H. vulval region. Scale bar: A, B = 200 μm; C, E = 50 μm; D, F, G, H = 20 μm.

FIGURE 4. Theristus flevensis Schuurmans Stekhoven, 1935: A. general view of male, B. general view of female, C. amphid of female, D. amphid of male, E. spicule and gubernaculum, F. caudal glands in male (arrows), G. vulval region, H. Ejaculatory glands in male (arrows). Scale bar: A, B = 100 μm; H = 40 μm; C, D, E, F, G = 10 μm.