Tobrilus methanus, Gagarin, Vladimir G. & Naumova, Tatyana V., 2016

Gagarin, Vladimir G. & Naumova, Tatyana V., 2016, Tobrilus methanus sp. n. and Tripyla posolskii sp. n. (Nematoda, Triplonchida) from Lake Baikal, Russia, Zootaxa 4196 (1), pp. 95-106 : 96-100

Tobrilus methanus sp. n.

( Figs 1 , 2 ; Table 1,2)

Type material. Holotype male, slide reference number 102/56, deposited in the helminthological museum of the Russian Academy of Sciences ( RAS), Institute of Ecology and Evolution , Center for Parasitology RAS (Moscow, Russia) .

Paratypes. Seven males and ten females deposited in the collection of the Limnological Institute Siberian Branch of the Russian Academy of Sciences (Irkutsk, Russia).

Type locality. Posol’skaya Banka elevation ( South Baikal , N 52°02ʹ08ʺ, E 105°50ʹ36ʺ) methane seep, Lake Baikal, Russia, depth 500 m, silt. Nematodes were collected on 3 July 2015.

Etymology. The specific epithet means “methane’, “from area of methane seep”.

Description. Male. Body comparatively short and thin. Cuticle finely annulated, about 1 µm thick at midbody. Cuticular rings narrow, poorly visible. Crystalloids numerous; somatic setae short and rare. Labial region slightly offset from the adjacent body; lips well developed. Body diameter at the posterior pharynx end 3.0–4.7 times the width of the labial region. Inner labial sensillae papilliform. Six outer labial sensillae in the shape of thin setae 3.0–4.0 µm long, 20–27 % of labial region width. Four cephalic setae in the shape of thin setae 2.5–3.0 µm long. Both circles of setae drawn close together. Cheilostom comparatively large. Buccal cavity narrow, funnelshaped. Dorsal pocket hardly visible. Both subventral pockets overlapping adjacent to each other, poorly developed. Teeth in pockets small. Stoma 1.5–1.8 times as long as labial region width. Amphid fovea cup-shaped, opening at the level of buccal cavity. Pharynx muscular, comparatively long, expanding gradually along its entire length. Cardiac gland large, rounded, 20–25 µm long. Ventral gland, its canal, ampulla and excretory pore not observed.

Testes paired, opposed, situated to the left of intestine; anterior testis outstretched, posterior testis reflexed. Vas deferens well developed. Spicules comparatively short, slightly curved, 1.1–1.3 times as long as the cloacal body diameter. Gubernaculum in the shape of a thin plate, 1.9–2.4 times less than spicule length. Precloacal supplements 6 in number, small, submerged. Ampulla of supplement flattened, its contents concentrated at ampulla base. Cap of supplement absent. Central thorn protruding slightly above cuticle. Distance from cloaca to first supplement 35–55 µm. Length of supplement row is 170–260 µm. Tail slender, long, elongate-conical, with subterminal seta. Caudal glands poorly visible. Spinneret in the shape of a short and thin tube.

Female. General morphology is similar to that of males in structure of cuticle and anterior body end. Cuticle finely annulated. Labial region slightly offset from the adjacent body; lips very well developed. Inner labial sensillae papilliform. Outer labial sensillae and cephalic sensillae in the shape of thin setae. Both circles of setae drawn close together. Cheilostom small; buccal cavity narrow, funnel-shaped. Both subventral pockets overlapping adjacent to each other. Teeth in pockets small. Pharynx muscular, expanding gradually towards base. Cardia small, surrounded by 3 oval glands. Prerectum not observed. Rectum length equal to or slightly greater than anal body diameter. Reproductive system didelphic, amphidelphic. Ovaries situated to the left of intestine, reflexed and comparatively short. Oocytes numerous. Vulva a transverse slit and situated slightly anterior to mid-body. Vulva lips not sclerotized and not protruding outside the body contour. Cuticular wrinkles round vulva and vulva glands not seen. Vagina short, with thick walls. Uterus containing one or two eggs, measuring 65– 70 x 28 –35 µm. Tail long, elongate-conical, with subterminal seta. Three caudal glands present, opening through a short, tube-like spinneret.

Characters Holotype Paratype males (n = 7) Paratype females (n = 10)

male

range mean range mean Diagnosis. Body comparatively short and thin (L = 1040–1505 µm, a = 20–29). Cuticle finely annulated. Crystalloids numerous. Outer labial setae and cephalic arranged in single circle. Outer labial setae longer than cephalic setae and as long as 20–27% of labial region width. Cheilostom comparatively large. Buccal cavity narrow, funnel-shaped. Both subventral pockets overlapping adjacent to each other, poorly developed. Stoma 1.5– 1.8 as long as labial region width. Spicules comparatively short, slightly curved, 1.1–1.3 times as long as the cloacal body diameter. Gubernaculum in the shape of a thin plate. Precloacal supplementary organs 6 in number, vesiculate, small, completely submerged under cuticle except for a short thorn protruding beyond cuticle. Subterminal seta at tail present.

Differential diagnosis. Tobrilus methanus sp. n. similar to T. modestus Gagarin, 1996 and T. incognitus Tsalolikhin, 1972 in body size and shape and structure of stoma. From the first species it differs in the thinner body (a = 20–29 vs a = 15.2–18.5 in T. modestus ), longer pharynx (b = 3.4–4.4 vs b = 5.9–6.6 in T. modestus ), comparatively longer tail (c ʹ = 3.4–5.0 vs c ʹ = 2.0– 2.5 in T. modestus ), lower number of supplementary organs (6 vs 7–8 in T. modestus ) and presence of subterminal setae on the tail ( Gagarin 1996). From the second species T. methanus sp. n. differs in the thicker body (a = 20–29 vs a = 35–57 in T. incognitus ), longer pharynx (b = 3.4–4.4 vs b = 5.1–5.8 in T. incognitus ), shorter outer labial setae (3–4 µm long vs 10 µm long in T. incognitus ) and presence of crystalloids ( Tsalolikhin 1972).

Faunistic remarks. At present, the genus Tobrilus Andrássy, 1959 contains 21–23 valid species ( Andrássy 2007, Zullini 2006). Nine species have been found in the Lake Baikal: T. amabilis Tsalolikhin, 1974 , T. bekmanae Tsalolikhin, 1975 , T. incognitus Tsalolikhin, 1972 , T. latens Tsalolikhin, 1974 , T. longisetosus Gagarin & Naumova, 2011 , T. macramphis Tsalolikhin, 1977 , T. securus Gagarin & Naumova, 2011 , T. undophylus Shoshin, 1988 and T. methanus sp. n. Morphometrics of these nine species are given in Table 2 ; a dichotomous key for determination of species is given below.

Figures

FIGURE 1. Tobrilus methanus sp. n. Holotype male and paratype female. A: male, entire body; B: male head; C: vulva region; D: male tail; E: female tail. Scale bars: A— 100 µm; C – E— 40 µm; B— 15 µm.

FIGURE 2. Tobrilus methanus sp. n. Paratype male and female. A: male, entire body; B: female, entire body; C: male, anterior body end; D: female head; E: male head; F: vulva region; G: cloaca region; H: male tail; I: female tail. Scale bars: A, B— 100 µm; C, H— 50 µm; F, I— 20 µm; D, E, G— 10 µm.