Mesodorylaimus rivalis, Gusakov, Vladimir A. & Gagarin, Vladimir G., 2016

Gusakov, Vladimir A. & Gagarin, Vladimir G., 2016, Two new species of nematodes (Nematoda) from highly mineralized rivers of Lake El'ton basin, Russia, Zootaxa 4161 (2), pp. 261-270 : 262-265

Mesodorylaimus rivalis sp. n.

( Figs 1 , 2 ; Table 1 )

Type material. Holotype male, inventory slide number 102/52, deposited in the nematode collection of the Helminthological Museum RAS, Institute of Ecology and Evolution , Center of Parasitology , Russian Academy of Sciences, Moscow, Russia.

Paratypes. One male and fourteen mature females deposited in the same collection as the holotype. One male and two females deposited in nematode collection in the Institute for Biology of Inland Waters, RAS ( Borok , Russia).

Measurements. Table 1 .

Type locality. Russia, Volgograd Province, Lake El’ton basin, river Khara , upper course (N 49°19.580, E 46°31.445), depth 0.15 m, gray-black clayey silt with brown bacterial-algal film on the surface, mineralization of water 21.1 g /l, temperature 19.8 °C , pH 10.0, dissolved oxygen 13.5 mg /l, current is absent. Collected on 27 May 2011 .

Etymology. The species name means “brook-inhabitant.

Description. Male. Body comparatively large, thin. Cuticle smooth and thin, without longitudinal ridges, 2.0– 2.5 µm thick at mid-body. Lateral chord 18–20 µm wide, ca one third of the mid-body diameter, without granular appearance. Somatic pores not observed. Labial region truncate and angular offset from adjacent body by a constriction and 3.9–4.6 times as wide as body diameter in posterior pharynx end. Labial and cephalic papillae large, distinct. Amphid fovea at level of the cephalic constriction and occupying ca 70–75% of the corresponding body diameter. Odontostyle straight, 8.7–9.3 times as long as wide and 1.2–1.3 times as long as the labial region width; aperture occupying ca 33% of the total odontostyle length. Odontophore rod-like, 0.6–0.7 times the odontostyle length. Guiding ring single. Pharynx muscular, expanding gradually along its whole length. Pharyngeal gland nuclei indistinct. Cardia conoid, nerve ring situated at 32–33% of the total pharynx length.

Genital system diorchic, with opposing testes. One adcloacal supplement and series of 16–17 contiguous ventromedial supplements present. All supplements in the shape of relatively large papillae. Prerectum from 4.8 to 5.0 cloacal body diameters long, beginning at 115–206 µm from the most anterior ventromedial supplement. Distances: from cloaca to adcloacal, 6–9 µm; from adcloacal to series of ventromedial supplements, 66–71 µm. Series of ventromedial supplements 89–96 µm long. Spicules relatively robust, curved ventrally and 1.5–1.6 times as long as cloacal body diameter. Lateral guiding pieces 12–13 µm long, spindle-shaped. Tail conoid-rounded, with round tip. Caudal papillae in four pairs: one subventral, one subdorsal and two median.

Female. General morphology similar to that of the male, but body larger and thicker. Structure of cuticle and anterior body end as in male. Reproductive system didelphic, amphidelphic. Ovaries comparatively long; oocytes numerous, arranged as a double and then a single row. Oviduct joining the ovary subterminally. Muscular sphincter separating oviduct and uterus not visible. Uterus a wide tube, two or three times as long as the corresponding body diameter and filled with spindle-shaped sperm. Eggs in uterus not observed. Vagina extending over half of the body diameter: pars proximalis vaginae in the shape of thin tube with muscular walls, 28–32 µm long; pars refringens vaginae consisting of two round sclerotisations; pars distalis vaginae very short. Vulva a longitudinal slit. One to six prominent papillae arranged anterior and posterior to vulva (in most cases one to three; in some cases anterior papillae are absent). Rectum 1.3–1.5 times as long as anal body diameter, prerectum 5.7–9.9 times as long as anal body diameter. Tail short, 19–22 µm; first dome-shaped then continuing in a short, slightly dorsally curved fingerlike portion and with rounded tip. Length of finger-like portion about 50% of total tail length.

Diagnosis and relationships. The new species is characterized by its comparatively long body (L = 1898– 2769 µm), smooth cuticle without somatic pores; labial region truncate and angular, offset by a constriction and 12–15 µm wide; odontostyle 14–19 µm long; vulva a longitudinal slit; pre- and postvulval papillae generally present; female prerectum 5.7–9.9 anal body diameter long; female tail with proximal conical and distal finger-like portion, which makes about 50% of total tail length; male tail conoid-rounded; males with 16–17 contiguous ventromedian supplements and one adcloacal supplement; spicules robust, arcuate, 47–49 µm long (measured along arc).

The new species resembles Mesodorylaimus vulvapapillatus Bagaturia & Eliava, 1966 in labial region shape, shape of female tail and presence of pre- and postvulval papillae, but can be separated from this species by the longer body (L = 1898–2769 µm vs L = 1309–1400 µm), shorter spicules (47–49 µm long vs 53 µm long) and longer female prerectum (5.7–9.9 anal body diameter vs 3.5 anal body diameter) ( Bagaturia & Eliava 1966).

Figures

FIGURE 1. Mesodorylaimus rivalis sp. n. Holotype male and paratype female. A: Entire male; B: Entire female; C: Male head; D: Vulva region; E: Posterior body end of male; F: Posterior body end of female. (Scale bars: A, B = 200 µm; C = 15 µm; D = 70 µm; E = 100 µm; F = 40 µm).

FIGURE 2. Mesodorylaimus rivalis sp. n. Holotype male and paratype female (microphotographs) A: Entire male; B: Entire female; C: Anterior body end of male; D: Anterior body end of female; E, F: Vulva region (arrows indicate position of papillae); G, H: Posterior body end of male; I, J: Posterior body end of female. (Scale bars: A, B = 250 µm; C, D, E, F = 10 µm; G, H, J = 50 µm; I = 20 µm).

Tables

TABLE 1. Morphometrics of Mesodorylaimus rivalis sp. n. All measurements are in µm (except for the ratios a, b, c, c', V; number of ventromedial supplements) and in the form: mean ± s. d. (range).

Character L	Holotype male 2020	Paratype males (n = 2) 1898, 2114	Paratype females (n = 16) 2315±196 (1974–2769)
a b c	34 5.8 98.2	32, 38 5.7, 5.8 97.4, 99.9	39±3 (32–45) 5.9±0.4 (5.2–6.9) 37.4±5.8 (30.8–54.9)
c' V, % labial region diameter	0.7 – 14.1	0.7, 0.7 – 13.8, 14.5	2.2±0.3 (1.5–2.9) 46.5±18.1 (43.3–49.6) 13.7±0.7 (12.4–14.9)
odontostyle length odontophore length pharynx length	16.8 11.8 349	16.3, 17.3 10.5, 12.8 333, 362	17.2±1.1 (14.3–18.9) 10.7±1.7 (7.8–14.0) 392±18 (356–418)
distance from pharynx base to vulva distance from vulva to anus distance from pharynx base to cloaca	– – 1650	– – 1546, 1730	677±81 (537–809) 1173±146 (967–1496) –
midbody diameter body diameter at level of anus or cloaca prerectum length	60 30 286	54, 66 29, 33 258, 332	60±5 (50–67) 28±2 (23–32) 230±34 (165–315)
tail length spicules length (along arc) number of ventromedial supplements	21 48 16	19, 22 47, 49 16, 17	63±9 (44–78) – –
length of supplement row	92	89, 96	–