INTRODUCTION

The study of the fauna of free-living nematodes in the water bodies and watercourses of Vietnam began about 15 years ago in connection with the compilation of a data bank on the aquatic fauna of Vietnam. Currently, large-scale research into the fauna of nematodes is being conducted. Data on the fauna of free-living nematodes in mangroves and in the mouths of some rivers have been published in a number of articles (Gagarin, 1987; Tchesunov and Nguyen, 2010; Nguyen et al., 2011; Gagarin, 2018).

This study aims to provide an illustrated description of two new species of free-living nematodes.

MATERIALS AND METHODS

Twenty samples of nematodes were collected in May 2016 by colleagues from the Institute of Ecology and Biological Resources of the Vietnam Academy of Sciences and Technologies (Hanoi, Vietnam) in the Cấm River mouth. Samples were taken from a boat using a bottom cylindrical core; the nematodes in the samples were isolated by centrifuging in the salt solution LUDOX-TM 50. The nematodes were defined and measured under an MBB-1 light microscope (Seinhorst, 1959). A Nikon Eclipse 80i light microscope equipped with a DIC contrast, a Nikon DS-Fil digital camera, and a PC equipped with NIS-Elements D 3.2 software for analysis and documentation were used for measuring, species definition, photographing, and drawings of the nematodes.

RESULTS

Species description. Order Monhysterida Filipjev, 1929. Family Monhysteridae de Man, 1876. Genus Eumonhystera Andrassy, 1984.

Eumonhystera rivalis Gagarin sp. n. (Figs. 1, 2).

Fig. 1.
figure 1

Details of the structure of Eumonhystera rivalis sp. n .: (a) head, male; (b) body in the cardium area; (c) posterior end, male; and (d) posterior end, female.

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Fig. 2.
figure 2

Micrographs of Eumonhystera rivalis sp. n. (a) general view, male; (b) general view, female; (c) head, male; (d, i) anterior end, male; (e) anterior end, female; (f) body in the vulva area; (g) posterior end, female; and (h) posterior end, male.

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Material. Holotype ♂, slide no. Vu 2.2.16, paratypes: 3♂♂, 2♀♀. The holotype preparation has been deposited in the collection of the Museum of Nature of the Vietnam Academy of Sciences and Technologies (Hanoi, Vietnam); preparations of paratypes are in the collection of nematodes of the Department of Nematology of the Institute of Ecology and Biological Resources of the Vietnam Academy of Sciences and Technologies (Hanoi, Vietnam).

Fig. 3.
figure 3

Details of the structure of Daptonema borealis sp. n.: (a) head, male; (b) spicules and gubernaculum; (c) posterior end, male; and (d) posterior end, female.

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Fig. 4.
figure 4

Micrographs of Daptonema borealis sp. n.: (a) general view, male; (b) general view, female; (c) anterior end, male; (d, f) anterior end, male; (e) head, female; (g) body in the vulva area; (h, k) body in the cloaca area; (i) tail, male; and (j) tail, female.

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Location. Northern Vietnam, Hai Phong Province, Cấm River mouth, depth 1 m, water salinity 12–15‰, sandy bottom sediments. Coordinates: 20°40ʹ22ʹʹ–20°40ʹ57ʹʹ N, 106°42ʹ48ʹʹ–106°42ʹ58ʹʹ E.

Description. The morphometric characteristics of the holotype and paratypes are given in Table 1.

Table 1.   Morphometric characteristics of Eumonhystera rivalis sp. n.
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Males. Small, rather thick worms. The anterior end of the body is narrowed. The width of the labial area is 3–4 times narrower than the diameter of the body in the cardium area. The cuticle is smooth. Somatic setae are short and sparse. Biocrystal-like bodies are small and numerous. Six internal papilla-shaped sensillae. Six external labial sensillae and four head sensillae have the shape of fine bristles, both of their circles close together. Length of bristles is approximately equal to half the width of the labial area. Amphid fovea are arranged in a circle whose diameter is half the diameter of the body at this level; they are located 4–5 μm from the anterior end of the body. Cheilostoma is small; its walls are smooth. Pharyngostoma has a shape of a small funnel; its walls are weakly cuticulized. Pharynx is muscular, equally thickened along its entire length. Cardium is small; it protrudes into the lumen of the middle intestine. Renette and its excretory pore were not found. One straight testis. The spicules are small, strongly curved, headed. Their length exceeds the body diameter in the cloaca area 1.3 times. Gubernaculum with a dorsal process. Precloacal supplemental organs are absent. The tail is slender, elongated-conical. The caudal glands and spinneret are well developed.

Females. Similar to males by general morphology, the cuticle structure, and the anterior end of the body. The cuticle is smooth. Biocrystal-like bodies are small. Internal labial sensillae have the shape of papillae. External labial sensillae and head sensillae have the shape of fine bristles, their circles are located very close to each other. Amphid fovea are circular, located 4.5–5.0 μm from the anterior end of the body. Pharyngostoma has the shape of a small funnel; its walls are weakly cuticulized. Pharynx is muscular, equally thickened along its entire length. Cardium is small, muscular. Rennet and its excretory pore were not found. One ovary, anterior, straight, without bend, located to the right of the middle intestine. Oocytes are numerous. The vulva is postequatorial, has the shape of a transverse fissure; vulvar flaps are not sclerotized. The uterus is extensive. Vulvar glands and the posterior uterus were not found. Distance from vulva to anus exceeds the tail length 1.4–1.6 times. The tail is slender, elongated-conical. The caudal glands and spinneret are well developed.

Differential diagnosis. Currently, the genus Eumonhystera includes 49 species, of which six were found in Vietnam (Gagarin, 2018; Bezerra et al., 2019).

In females of most species of this genus, the distance from the vulva to the anus is smaller than the tail length. Only three species, E. dispar (Bastian, 1865); E. sudanensis Zeidan et al., 1989; and E. maxima Gagarin, 1996, have a tail that is which shorter than the distance from vulva to anus. Females of E. rivalis sp. n. differ from these three species by the presence of biocrystal-like bodies (compared species do not have these) and by the closer location of the amphid fovea to the anterior end of the body (at a distance of ~0.5 of the labial area width; in the compared species, 1.5–2.5 labial area width); the head setae are longer (their length is ~40–44% of the labial area width, versus 17–30% in the compared species) (Zeidan et al., 1989; Andrássy, 2005).

Males in the species of the genus Eumonhystera are very rarely found; they are known only for seven species of this genus. In males of four species, gubernaculum is shaped like a short plate or groove; in males E. similis (Bütschli, 1873); E. borealis Turpeenniemi, 1997; and E. vulgaris (de Man, 1889), it has a dorsal process similarly to that in E. rivalis. Male E. rivalis sp. n. has biocrystal-like bodies, in contrast to the males of the three species lacking this one. In addition, there are differences in morphometric characteristics. The new species differs from E. similis males by a longer body (L = 650–717 μm versus L = 400–600 μm in E. similis), a shorter and less slender tail (c = 7.2–8.7, c' = 3.7 –4.5 versus с = 4.5–5.5, с' = 7–9 in E. similis), amphid fovea located closer to the anterior end of body (at a distance of ~0.5% of the labial area width versus 2.0–2.6 of the labial area width in E. similis), and longer spicules (their length is 26–29 μm, versus 18–23 μm in E. similis) (Andrássy, 2005). The males of the new species differ from the males of E. vulgaris by a relatively shorter tail (c = 7.2–8.7 versus c = 5.9–6.0 in E. vulgaris), amphid fovea located closer to the anterior end of the body (at a distance of ~0.5% the labial area width versus 1.1–1.3 of the labial area width in E. vulgaris), and shorter spicules (their length is 26–29 μm, versus 43–45 μm in E. vulgaris) (Hernandez and Jordana, 1988). From E. borealis males, E. rivalis males differ by a longer body (L = 650–717 μm versus L = 290–400 μm in E. borealis), a shorter and less slender tail (c = 7.2–8.7, c' = 3.7–4.5 versus с = 4.4–4.8, с' = 9.3–10.8 in E. borealis), and longer spicules (their length is 26–29 μm versus 13–14 μm in E. borealis) (Turpeenniemi, 1997).

Etymology. The species name means riverine or inhabiting the rivers.

Family Xyalidae Chitwood, 1951

Genus Daptonema Cobb, 1920

Daptonema borealis Gagarin sp. n.

Material. Holotype ♂, slide no. PL 3.1.7, paratypes: 3 ♂♂, 1 ♂. Holotype and two paratype (1 ♂ and 1 ♀) preparations are deposited in the collection of the Museum of Nature of the Vietnam Academy of Sciences and Technologies (Hanoi, Vietnam). Other preparations of paratypes (2♂) are deposited in the collection of nematodes of the Department of Nematology of the Institute of Ecology and Biological Resources of the Vietnam Academy of Sciences and Technologies (Hanoi, Vietnam).

Location. Northern Vietnam, Hai Phong Province, the Cấm River mouth, depth 1.5 m, water salinity 1.5‰, sandy bottom sediments. Coordinates: 20°48′84″ N, 106°55′02″ E.

Description. The morphometric characteristics of the holotype and paratypes are given in Table 2.

Table 2. Morphometric characteristics of Daptonema borealis sp.n.
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Males. Relatively thick worms of medium length. The anterior end is narrowed. The width of the labial area is ~2.5 times less than the width of the body in the cardium area. The cuticle is finely annulated. The cuticle thickness in the middle part of the body is 1.0–1.5 μm. Somatic setae are short and sparse. Six internal labial sensillae have the shape of papillae. Six external labial sensillae and four head sensillae have the shape of fine setae 5.0–5.5 μm in length arranged in one circle. No sub-cephalic setae were found. Six to eight cervical setae, 7.0–8.5 μm in length, arranged in one circle and located in 20–22 μm from the anterior end of the body. The amphid foveae are circular, 5.0–5.5 μm in diameter, located 7.0–7.6 μm from the anterior end of the body. Cheilostoma is small. Pharyngostoma is also small; its shape resembles a small funnel. Pharynx is muscular, equally thick along its entire length. The cardium is short, protrudes into the lumen of the middle intestine, and is surrounded by three rounded glands. Renette and its excretory pore were not found. Spicules are thin, headed, curved at almost right angles. Their length exceeds the body diameter in the cloaca area 1.5–1.6 times. Gubernaculum is complex, with a small caudal process. Its main body encloses the apical ends of the spicules like a sleeve. In addition, the main body of gubernaculum has two thin ventral processes ending as clawlike formations. Pericloacal supplementary organs were not found. The tail is slender, elongated-conical. Caudal setae are short. The caudal glands and spinneret are well developed. There are three terminal setae at the tip of the tail.

Female. Similar to males by general morphology, cuticle structure, and anterior end of the body. The cuticle is finely annulated. Internal labial sensillae have the shape of papillae. External labial sensillae and head sensillae have the shape of fine setae 5.0 µm in length arranged in one circle. Subcephalic setae 8‑µm-long, located 18 µm from the anterior end of the body. Amphid fovea 5.0 µm in diameter, located 7.0 µm from the anterior end of the body. Cheilostoma is small. Pharyngostoma has the shape of a small funnel. Pharynx is equally thick along its entire length. The cardium is surrounded by three rounded glands. The ovary is single, anterior, straight, and located to the left of the middle intestine. Oocytes are numerous. The vulva is postequatorial and has the shape of a transverse fissure. The vulvar flaps are not sclerotized. The vagina is short, with thin walls. The uterus is extensive, filled with numerous sperm. Posterior uterus and vulvar glands were not found. The tail is slender, elongated-oval, its length 1.6 times shorter than the distance from the vulva to the anus. The caudal glands and spinneret are well developed. Terminal setae are present on tail.

Differential diagnosis. At present, the genus Daptonema Cobb, 1920 includes 117–136 valid species. In the water bodies of Vietnam, 22 species of the genus were found (Riemann, 1966; Fonseca, Bezerra, 2014). Such a large number of species in the genus is explained by the diversity of the spicular apparatus in males. According to the structure of the spicular apparatus of D. borealis sp. n., it is similar to D. vicinus (Riemann, 1966); D. aquaedulcis (Gagarin, 1987); and D. salinae Gagarin, Gusakov, 2014. It differs from D. vicinus by a shorter body (♂♂ L = 796–887 μm versus ♂♂ L = 1160–1180 μm in D. vicinus), shorter head setae (5-μm – long, or 25–28% of the labial area width versus 11 μm and 70% of the labial area in D. vicinus), and longer spicules (their length is 50–52 μm versus 34–37.5 μm in D. vicinus) (Riemann, 1966). The new species differs from D. aquaedulcis by a shorter body (L = 1400–1650 μm in D. aquaedulcis), shorter head setae (5 μm in length, 25–28% of the labial area width versus 10.5–12.0 μm and 50–52% of the labial area width in D. aquaedulcis) and shorter spicules (in D. aquaedulcis, their length is 67 μm) (Gagarin, 1987). The new species differs from D. salinae by a slender body (a = 18–24 versus a = 11–16 in D. salinae), a relatively shorter tail (c = 6.7–7.3 versus c = 5.3–5.7 in D salinae), shorter head setae (their length is 5 μm or 25–28% of the labial area width versus 8.8–10.0 μm or 45–67% of the labial area width in D. salinae), and longer spicules (their length is 50–52 μm versus 41–42 μm in D. salinae) (Gagarin and Gusakov, 2014).

Etymology. The species name means eastern or from the east.