INTRODUCTION

Fauna of free-living nematodes of Lake Baikal comprises more than 100 species belonging to 7 orders, 11 families, and 33 genera; about 65% of these species are endemic to the lake [4, 6, our data].

Studies of the fauna of nematodes living in the interstitial zone of the splash zone of Lake Baikal have started recently, and the first results of these researches were published in 2012 [13, 14]. In total, 20 nematode species belonging to 15 genera, 11 families, and 6 orders have been found [5, 13–15]. Most of the findings are known and widespread species and five are new to science.

The aim of the work is to study the fauna of nematodes of Lake Baikal, describe the species new to science, and compare those with the nematodes from other freshwater bodies.

MATERIALS AND METHODS

The meiobenthos samples were collected in the interstitial zone of Listvenichnyi Bay in vicinity of the settlement of Listvyanka, Lake Baikal, Russia. One part of the samples was collected on May 18, 2012, by a UWITEC sampling tube at sandy sediments at a distance of ~50 cm above the water edge (samples nos. 496, 497, and 500); the sediment height in the tube was ~10 cm. The other part of the samples was collected on September 4, 2012, at a distance of 1 m above the water edge: a 40-cm-deep hole was dug in the sand, and the groundwater accumulated there was filtered through a hydrobiological net (sample no. 614).

The samples were fixed with 4% formaldehyde solution. The worms were identified and measured under an Olympus CX-21 microscope on Trypan Blue glycerin smears framed with a colorless varnish. Photographs were taken with a Nikon Eclipse 80i microscope equipped with a DIC phase contrast mode, a Nikon DS-Fil digital camera, and a PC with NIS-Elements D 3.2 software for analyzing and documenting images from the smears.

RESULTS

Order Monhysterida Filipjev, 1929. Family Monhysteridae de Man, 1876. Genus Eumonhystera (Bastian, 1865) Andrássy, 1981. Eumonhystera arenosa sp. n. (Figs. 1, 2).

Fig. 1.
figure 1

Eumonhystera arenosa sp. n.: (a) body in the vulva area; (b) general view of the male; (c) head of the male; (d) anterior end of the male; and (e, f) tails of the male and female, respectively.

Full size image
Fig. 2.
figure 2

Male (a, c, e, h, i) and female (b, d, f, g, j, k) Eumonhystera arenosa sp. n.: (a, b) general view, (c, d) head, (e) anterior end of the body, (f) body in the area of the cardium, (g) body in the vulva area, (h) the body in the area of cloaca, (i, j) tail, and (k) the tip of the tail.

Full size image

Morphological characteristics of the holotype and paratypes are given in Table 1.

Table 1.   Morphometric characteristics of Eumonhystera arenosa sp. n.
Full size table

Material. Holotype ♂, smear no. 102/54, paratypes: 4♂♂, 5♀♀. The smears of holotype and three paratypes (2♂♂ and 1♀) are stored in the Helminthological Museum of the Russian Academy of Sciences, Center for Parasitology of the Severtsov Institute for Ecology and Evolution of the Russian Academy of Sciences (RAS) (Moscow, Russia). The rest of the smears of the paratypes are stored in the collection of the Laboratory of Biology of Aquatic Invertebrates of the Limnological Institute of the Siberian Branch (SB), RAS (Irkutsk, Russia).

Location. Russia, Eastern Siberia, Lake Baikal, the interstitial zone of the splash zone of the Listvenichnyi Bay (settlement of Listvyanka, opposite the post office, 51°30ʹ21ʺ N, 104°31′15ʺ E), ~50 cm above the water edge.

Male. Fine thin worm. Both ends of the body are narrowed. Cuticle thin, smooth, without somatic setae and cuticular pores. No crystalline bodies are found in the body cavity. The anterior margin of the head is slightly flattened. Lips are poorly expressed. The labial area is not separated from the rest of the body. Around the osculum, there are six internal labial sensillae in the form of small papillae. At a distance of ~3 μm from the anterior end of the body, six external labial sensillae are localized in the form of small, fine setae; their length is 1/5–1/6 of the diameter of labial area. No head or neck sensillae are found. The amphidial foveae form a circle with a diameter of 41–46% of the body diameter at this section; they are located at a distance from the anterior end of the body that exceeds the width of the labial area 1.8–2.0 times. Eyes are absent. Stoma in the form of a short funnel, its walls thin, not cuticular. Pharynx is slender, muscular. The basal end of the pharynx forms a bulbous extension, the walls of the inner cavity of which are cuticularized. Cardium is small, surrounded by three petal-like glands. Renette, its duct and excretory pore were not found. One straight testis. Spicules thin, curved, with poorly expressed heads. The length of the spicules is 2.4–2.6 times larger than the diameter of the body in the area of the cloaca. The gubernaculum is a plate-shaped, sharpened from both edges. The gubernaculum length is 11–13 μm. Precloaca supplementary organs and precloaca setae are absent. The tail is relatively long, gradually narrowing. Caudal glands are present. Spinneret short, “beak-shaped.”

Females. General morphology is similar to that in male. Structure of cuticle and anterior end of body is similar to that in males. Cuticle smooth, without somatic setae. Lips are poorly developed. There are no crystalloid bodies. Six internal labial sensillae in the form of papillae and six external labial sensillae in the form of short setae. Head and neck sensillae are not found. The amphidial fovea form of a circle with a diameter of 40–44% of the corresponding body diameter; they are located at a distance exceeding the width of the labial area 2.0–2.1 times. Eyes are absent. Stoma has the shape of a short funnel. Pharynx muscular; there are bulbous-shaped thickenings at its basal end. Around the cardium, there are three petal-shaped glands. Renette, its duct and excretory pore were not found. The length of the rectum is equal to or slightly larger than the diameter of the body at the anus.

One anterior straight ovary located to the right of the mid-gut. Vulva is postequatorial. The vulva lips are not sclerotized and do not protrude beyond the contours of the body. The vagina is short, inclined to the anterior end of the body; its walls are thin. In the uterus, numerous spermatozoa and one egg size of (34–40) × (38–41) μm are found often. Postvulvar cell is present. The tail is relatively long, gradually narrowing. Tail length is equal to 0.9–1.0 of the distance from the vulva to the anus. The caudal glands are well-developed. Spinneret short, beak-shaped.

Diagnosis. Relatively small and slender worms (L = 785–910 μm, a = 26–38). Cuticle smooth. Somatic setae and crystalloid bodies are absent. The labial region is not separated from the rest of the body. Six internal sensillae look like small papillae and six external labial sensillae in the shape of 2-μm short setae. Head and neck sensillae not found. The amphidial fovea form a circle and are located at a distance of 1.8–2.1 of the labial area width from the anterior end of the body. Eyes absent. Pharynx muscular, forms a basal bulbous-shape extension. Three cardiac glands. Vulva is postequatorial (V = 61–63%). The vagina is short; the walls are thin. Postvulvar cell is present. The tail length in the females is 0.9–1.0 of the distance from the vulva to the anus. Spicules thin, with poorly expressed heads; their length is 53–56 μm, exceeding the diameter of the body in the cardiac area in 2.4–2.6 times. Gubernaculum has a shape of a short plate; the dorsal spur is absent. The tail is relatively long, gradually narrowing. Spinneret short, beak-shaped.

Differential diagnosis. The species is morphologically close to the widely distributed species Eumonhystera filiformis (Bastian, 1865) Andrássy, 1981 (Table 2). E. arenosa sp. n. differs from E. filiformis by longer body (L = 785–910 μm against L = 500–790 μm in E. filiformis), by the presence of basal bulbus-shape expansion of pharynx (absent in E. filiformis), by shorter external labial setae (2.0 μm versus 2.5–3.0 μm in E. filiformis), by a different ratio of the tail length to the distance from the vulva to the anus in females (0.9–1.0 vs. 1.3–1.5 in E. filiformis), and by longer spicules (53–56 μm, 2.4–2.6 times greater of the body diameter in the cloaca area, versus 25–35 μm and 1.5–1.7 times in E. filiformis, respectively) [7, 8].

Table 2. Morphometric characteristics of Eumonhystera filiformis (Bastian, 1865) Andrássy, 1981
Full size table

Morphological and taxonomic notes. The genus Eumonhystera (Bastian, 1865) Andrássy, 1981 has been isolated from the genus Monhystera Bastian, 1865, because the latter was clearly a composite genus that included a rather large number of morphologically diverse Monhysteridae worms inhabiting the freshwater bodies and soil [7]. The genus Eumonhystera combined relatively small nematodes with a body length of 0.5 mm to 1.5 mm characterized by the absence of crystalloid bodies. The amphidial fovea are located from the anterior end of the body at a distance of 1.0–3.5 of the diameter of the labial area. Males are very rare, and the females, apparently, reproduce by parthenogenesis. The tail length in females is approximately equal to or greater than the distance from the vulva to the anus. Spicules in males are relatively short; their length, as a rule, does not exceed the double body diameter in the area of the cloaca. Currently, the genus Eumonhystera comprises 38 valid species [8, 12]. E. filiformis is a cosmopolitan species; it belongs to the most common species of the genus. It inhabits the inland waters, as well as the damp moist soils and moss. Morphologically, the species is characterized by the presence of a postvulvar glandular cell in females; the amphidial fovea are located at a distance of 1.7–2.1 of the diameter of labial area from the anterior end of the body; the tail length in females is 1.3–1.5 times longer than the distance from the vulva to the anus; the length of the spicules is 25–35 μm, which exceeds the body diameter at the cloaca area 1.5–1.7 times. Males are very rare and are found only in the freshwater bodies of the Netherlands [17], lakes of Poland [18], Lake Sevan (Armenia), and in a mineral spring at Sakhalin Island (Russia) [2].

Etymology. Species name means “sandy,” “sand.”

Order Triplonchida Cobb, 1920. Family Tripylidae de Man, 1876. Genus Tripyla Bastian, 1965. Tripyla alekseevi sp. n. Morphological characteristics of the holotype and paratypes are given in Table 3.

Table 3. Morphometric characteristics of Tripyla alekseevi sp. n.
Full size table

Material. Holotype ♂, smear no. 102/55, paratypes: 10♂♂, 10♀♀. Smears of the holotype and three paratypes (1♂ and 2♀♀) are stored in the Helminthological Museum of the Russian Academy of Sciences, Center for Parasitology of the Severtsov Institute of Ecology and Evolution of the RAS (Moscow, Russia). The rest of the smears of the paratypes are stored in the collection of the Laboratory of Biology of Aquatic Invertebrates of the Limnological Institute of the SB RAS (Irkutsk, Russia).

Location. Russia, Eastern Siberia, Lake Baikal, interstitial zone of the splash zone in Listvenichnyi Bay (settlement of Listvyanka, opposite the sealarium, 51°30ʹ16ʺ N, 104°31ʹ23ʺ E), ~1 m above the water edge.

Male. A relatively small fine worm. Cuticle annulate, somatic setae are absent. The thickness of the cuticle in the middle part of the body is 2.0–2.5 μm. The anterior end of the body is slightly narrowed. Three low lips. The labial area is not separated from the rest of the body. Six internal labial sensillae have the shape of small papillae, ~1-μm-long. Six external labial sensillae have the shape of large papillae, 4.0- to 5.0-μm long. At a distance of 5 μm from the external labial sensillae, there are four head sensillae shaped like thin bristles, 1.5- to 2.0-μm-long. The amphidial fovea are pocket-shaped, localized slightly below the level of the head setae. Stoma is not expressed. Pharynx muscular, only slightly widens to its base. In the pharynx lumen, a small dorsal tooth is located at a distance of 17–20 μm from the anterior end of the body. Subventral teeth are not found. The cardium is small, surrounded by the three round-shape cardiac glands. Renette, its duct and excretory pore are not found.

Paired testes are opposite each other. Spicules are relatively thick, with no heads. The length of the spicules exceeds the width of the body in the area of the cloaca 1.4–1.5 times. Gubernaculum has the shape of a simple plate, 14- to 18-μm-long. Supplementary organs has the shape of small, barely noticeable papillae. The number of supplements has been counted only in three males from Lake Baikal. One male had nine supplements (one in the pharynx region), the other had eight supplements (also one in the pharynx region), and the third had eleven supplements (two in the pharynx region). The tail is relatively long, gradually narrowing. The caudal glands and the spinneret are well-developed.

Females. General morphology is similar to that in males. Structure of cuticle and anterior end of body same as in males. Cuticle annulate, without somatic setae. Three low lips. Six internal labial sensillae have the shape of small papillae. Six external labial sensillae have the shape of large papillae, 4.0-μm-long. Four head sensillae have the shape of thin bristles, 2-μm-long. Amphidial fovea pocket-shaped and located just below the head setae. Stoma not expressed. Pharynx muscular, slightly dilated in basal area. In the lumen of the esophagus, there is a small dorsal tooth located 13–20 μm from the anterior end of the body. There are three round glands around the cardium. The length of the rectum is less than the diameter of the body in the anus area.

Ovaries are paired, with bends. Bends are short, not reaching the vulva. Vulva is equatorial, and it has the shape of a transverse slit. The vulva lips protrude slightly behind the contours of the body. The vagina is straight; its walls are thin, nonmuscular. In the uterus, numerous spermatozoa and 1–3 rounded eggs (30–60) × (26–48) μm in size. The tail is relatively long, gradually narrowing. The caudal glands and the spinneret are well developed.

Diagnosis. Slender, small worms (L = 1050–1400 μm, a = 25–39). Cuticle annulate, without somatic setae. Six internal labial sensillae have the shape of small papillae; six external labial sensillae have the shape of large papillae, 4.0- to 5.0-μm-long; four head sensillae in the shape of thin bristles, 1.5- to 3.0-μm-long. There is a small dorsal tooth in the lumen of the esophagus. Spicules are 40- to 45-μm-long, which is 1.4–1.5 times larger than the diameter of the body in the area of the cloaca. The gubernaculum is 14- to 18-μm-long. Seven to eleven supplementary organs have the shape of small, hardly noticeable papillae; one or two of them are located in the pharynx region. The tail is relatively long, gradually narrowing. The caudal glands and the spinneret are well developed.

Fig. 3.
figure 3

Tripyla alekseevi sp. n.: (a) general view of a male, (b) head of a male, (c) tail of a female, (d) tail of a male, and (e) body in the vulva area with two eggs.

Full size image

Differential diagnosis. The species Tripyla alekseevi sp. n. is morphologically close to the species T. affinis de Man, 1880; the latter is fairly widespread in moist soils and freshwater bodies around the world [16, 19]. T. alekseevi sp. n. differs from T. affinis by larger external labial papillae (4.0–5.0 μm versus 1.0–2.0 μm in T. affinis), a narrower labial area (width of 17–22 μm versus 23–29 μm in T. affinis), a smaller number of supplementary organs (7–11, including 1–2 in the pharynx region versus 14–19 and 4–7 in T. affinis, respectively) and a longer gubernaculum (14–18 μm versus 9–14 μm in T. affinis) [11].

Fig. 4.
figure 4

Male (a, b, d, f, g) and female (c, e, h) Tripyla alekseevi sp. n.: (a) general view, (b, c) anterior end of the body, (d) body in the area of cardium, (e) body in the vulva area, (f) body in the cloaca area, and (g, h) tail.

Full size image

Morphological and taxonomic notes. The species Tripyla affinis de Man, 1880 was first found and described in soils in the Netherlands [17]. It was then repeatedly found in soils and freshwater bodies around the world [16]. The main morphological features of this species are a relatively small body size and very short external labial and head sensillae; their length does not exceed 1–2 μm. The species was described twice and illustrated during the revision of the genus [10, 11]. In 1991, Russian nematologist Valentin M. Alekseev described a fairly large population of nematodes from the genus Tripyla from several reservoirs of the Far East, he attributed these nematodes to T. affinis [1]. Morphologically, these nematodes differed in some aspects from the generally accepted description of T. affinis. The main distinguishing feature was the length of the external labial sensillae; they were more than twice as long as that of T. affinis. Morphologically similar specimens of the genus Tripyla were found within this study in the interstitial zone of Lake Baikal (Table 3). They are morphologically similar to the worms from the freshwater bodies of the Far East (Table 3) and differed from T. affinis in four morphological features. Therefore, the authors attribute these worms to an independent species of the genus Tripyla: T. alekseevi sp. n.

Etymology. The species is named in honor of the Russian nematologist V.M. Alexeev.

Valid species of Tripyla genus inhabiting Lake Baikal. Five valid species of the genus Tripyla are currently found in Lake Baikal: T. dybowskii Tsalolikhin, 1976, T. filicaudata de Man, 1880, T. setifera Bütschli, 1873, T. infia Brzeski, Winiszewska-Slipinska, 1993, and T. alekseevi sp. n. [3–6, 13]. The key for species identification is presented below.

Taxonomic key for identifying valid species of the genus Tripyla from Lake Baikal

1. External labial sensillae are long, their length >1/4 of the labial area width

proceed to 2

(or) external labial sensillae are shorter, their length ≤1/4 of the labial area width

proceed to 3

2. Tail length exceeds the body diameter in the anus area 4–6 times

T. setifera

(or) tail length exceeds the body diameter in the anus area 9–13 times

T. filicaudata

3. Body length 2.58–3.53 mm; length of the spicules 85–92 μm

T. dybowskii

(or) body length <2.5 mm; length of the spicules <60 μm

proceed to 4

4. Body length 1.28–2.20 mm; length of the external labial sensillae ~2 μm; 17–20 supplemental organs

T. infia

(or) body length 1.05–1.24 mm; length of the external labial sensillae 4–5 μm; 7–10 supplemental organs

T.alekseevi sp. n

ACKNOWLEDGMENTS

We are grateful to the staff of the Laboratory of Biology of Aquatic Invertebrates of the Limnological Institute, SB RAS: O.V. Medvezhonkova for sampling and T.A. Podkorytova for sample processing.

This work was supported by state budget topic “Large-Scale Changes in the Ecology and Biodiversity of the Communities of the Coastal Zone of Lake Baikal: Interdisciplinary Research, Identification of Causes, and Developmental Forecast” (no. AAAA-A16-116122110067-8).

COMPLIANCE WITH ETHICAL STANDARDS

Сonflict of interests. The authors declare that they have no conflict of interest.

Statement on the welfare of animals. All applicable international, national, and/or institutional guidelines for the care and use of animals were followed.