Mesodorylaimus ushkaniensis, Naumova & Gagarin, 2021

Naumova, Tatyana V. & Gagarin, Vladimir G., 2021, Mesodorylaimus ushkaniensis sp. n. and Allodorylaimus baikalensis sp. n (Nematoda, Dorylaimida) from Lake Baikal, Russia, Zootaxa 4985 (2), pp. 285-293 : 286

Mesodorylaimus ushkaniensis sp. n.

( Figs 1 , 2 ; Table 1 )

Type material. Holotype male, inventory slide number 102/72 (1554-1) deposited in the nematode collection of the Helminthological Museum of the Russian Academy of Sciences (RAS), Institute of Ecology and Evolution , Center for Parasitology RAS (Moscow, Russia)

Paratypes. 4 paratype females with the same collection data as holotype ; nine paratype males and six paratype females deposited in the collection of the Limnological Institute , Siberian Branch of the Russian Academy of Sciences (Irkutsk, Russia)

Measurements. Table 1 .

Type locality. Lake Baikal, Siberia, Russia, Bolshoy Ushkaniy Island, depth 3.0– 8.3 m, silt and stones. Collected on 27–28 September 2016.

Etymology. The specific epithet means “from Ushkaniy Island”, the type locality.

Description. Male. Body comparatively large, slender. Cuticle smooth and thin, without longitudinal ridges, 4.0–5.0 µm thick at mid-body. Lateral chord 35–37 µm wide, ca one third of the mid-body diameter; without granular appearance. Somatic pores not observed. Labial region truncate and offset from adjacent body; 5.4–6.4 times narrower than body diameter the base of the pharynx. Labial and cephalic papillae large, district. Amphidial fovea at level of the cephalic constriction and occupying ca 60–65% of the corresponding body diameter. Odontostyle straight, 1.5–1.6 times as long as the labial region wide; aperture occupying ca 23–25% of the total odontostyle length. Odontophore rod-like, 0.5–0.6 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 32–35% of the total pharynx length.

Genital system diorchic, with opposing testes. One adcloacal supplement and series of 23–27 contiguous ventromedial supplements present. All supplements in the shape of relatively large papillae. Prerectum 200–288 µm long, beginning at level of supplement row. Distance from cloaca to adcloacal supplement 5–6 µm; from adcloacal supplement to the series of ventromedial supplements 92–140 µm. Adcloacal space (i.e., hiatus) present. Series of ventromedial supplements 230–325 µm in extent. Spicules relatively robust, curved ventrally; at 1.5–1.7 times as long as cloacal body diameter. Lateral guiding pieces spindle-shaped. Tail conoid-rounded with round tip. Caudal papillae in three-four pairs.

Female. General morphology similar to that of the male, but body larger and thicker. Structure of cuticle and anterior body end as in males. 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; filled to the end with spindle-shaped sperm. Uterus containing one or three eggs measuring 108–125 x 53–62 µm. Vagina extending ca half of the body diameter: pars proximalis vaginae in the shape of thin tube with muscular walls 48–51 µm long; pars refringens vaginae consisting of two round sclerotized pieces; pars distalis vaginae very short. Vulva a transverse slit. Vulva lips not sclerotized, not protruding outside the body contour. Vulvar papillae not observed. Rectum 1.2–1.3 times as long as anal body diameter, prerectum 4.0–5.2 times as long as anal body diameter. Tail long, consisting of anterior conical segment and posterior thin, whip-like segment. Length of posterior segment making up 70–75% of total tail length.

Diagnosis. Body long and comparatively thin (L = 2095–2824 µm, a = 19–23 in males, L = 2283–3780, a = 21– 29 in females); cuticle smooth; labial region offset from the adjacent body; odontostyle straight and slender, 34–36 µm long, aperture occupying 23–25% of odntostyle length; odontophore 16–25 µm long; guiding ring single; vulva a transverse slit; uterus spacious, with numerous sperm and 1– 3 eggs, measuring 108–125 x 53–62 µm; female tail consisting of anterior conical segment and posterior thin whip-like segment, length of posterior segment reaching 70–75% of total tail length; spicules 80–85 µm long; adcloacal supplement and row of 23–27 contiguous supplements present, precloacal space present; male prerectum 200–288 µm long, beginning at level of supplement row.

Differential diagnosis. Mesodorylaimus ushkaniensis sp. n. is the largest species of the genus Mesodorylaimus ( Loof 1969, Ahmad 1993, Andrássy 2009) thus far described. It resembles Mesodorylaimus cryptosperma Loof, 1969 , but can be separated from this species by the longer body (L = 2095–3780 µm vs L = 1800–2300 µm in M. cryptosperma ), comparatively shorter tail in females (c = 12.1–16.3, cʹ = 3.3–5.3 vs c = 10–12, cʹ = 6.0–7.0 in M. cryptosperma ), longer odontostyle (34–35 µm long vs 20–22 µm long in M. cryptosperma ), and longer spicules (80–85 µm long vs 52–58 µm long in M. cryptosperma ) ( Loof 1969).

Figures

FIGURE 1. Mesodorylaimus ushkaniensis sp. n. Holotype male and paratype female. A: entire body of male; B: male head; C: vulva region; D: posterior body end of male; E: posterior body end of female. Scale bars: A—300 µm; C—200 µm; E—100 µm; D—60 µm; B—20 µm

FIGURE 2. Mesodorylaimus ushkaniensis sp. n. Holotype male and paratype female. A: male, entire body; B: female, entire body; C: male anterior body end; D: male head; E: female head; F: vulva region; G: male, posterior body end; H: cloaca region; I: female, posterior body end. Scale bars: A, B—200 µm; C, F, G, I—100 µm; D, E, H—10 µm