TWO NEW NEMATODE SPECIES OF THE GENUS EUTOBRILUS (NEMATODA, TRIPLONCHIDA) FROM LAKE BAIKAL (RUSSIA)

This paper describes and illustrates two new nematode species of the genus Eutobrilus found in Lake Baikal. Eutobrilus tsalolikhini sp. nov. is most similar to E. mirandus and E. obesus in body size and spicules length. It differs from E. mirandus in its thicker body, longer outer labial setae, larger stoma, and more supplements. It differs from E. obesus in a shorter tail, farther location of vulva from the anterior body end, longer outer labial setae, and more supplements. Eutobrilus olkhonensis sp. nov. is most similar to E. fortis and E. godlewskii in body size and supplements location but differs from both species in a thicker body, longer tail in males, shorter spicules, and fewer supplements.


Material and Methods
The nematodes were collected in the littoral zone of Lake Baikal from the following sites: central basin of Lake Baikal (Maloye More Strait), Elginsky Bay (53.154010°N, 107.174021°E), 30-cm depth, sand, collected on 16.07.2018;central basin of Lake Baikal (Maloye More Strait), Olkhon Island, near Saraysky Beach (52.12865°-53.12670° N, 107.21023°-107.20789° E), 15-20-m depth, sand with mica, collected on 17.06.2021.The samples contained numerous free-living nematodes, including both species described herein.Nematodes were preserved in 4% formaldehyde, tinted with Rose Bengal dye and mounted in glycerin on permanent slides (Williams & Williams, 1974;Ryss, 2003).All observations were made with the Olympus CX-21 and Nikon Eclipse 80i light microscopes with Nomarski DIC accesso-ries.Images were taken using a Nikon DS-Fil digital camera and Intel Pentium Dual CPU E 2200 Processor Series for Desktop with the NIS-Elements D 3.2 programme for analysis and documentation of images from the preparations.Phylum Nematoda Potts, 1931Class Enoplea Inglis, 1983Order Triplonchida Cobb, 1920Family Tobrilidae de Coninck, 1965Genus Eutobrilus Tsalolikhin, 1981 Type species.Eutobrilus grandipapillatus (Brakenhoff, 1914) Tsalolikhin, 1981-Trilobus grandipapillatus Brakenhoff, 1914.Diagnosis.It is made according to Andrássy (2007) and Holovachov & Shoshin (2014).The body length varies from 1.5 mm to 4.7 mm.Cuticle is smooth or very finely annulated.Buccal cavity is cup-or funnel-shaped, with an overlapping subventral pocket and teeth close to each other.Vagina is moderately strong.Spicules are comparatively short.Supplements are of 5 to 9 (rarely 13), echinate, protrusible; the first and last supplements are generally smaller than the others; two posterior supplements are often more remote from the others.
Type locality and habitats.Russia, central basin of Lake Baikal (Maloye More Strait), Elginsky Bay.Samples were collected on 16.07.2018(leg.T.V. Naumova).Individuals of the new species were found at 30-cm depth.
Etymology.The species is named in honour of the Russian scientist, nematologist, Professor, Dr. Sc. Semyon Ya.Tsalolikhin.
Type locality and habitats.Russia, central basin of Lake Baikal (Maloye More Strait), Olkhon Island, near Saraysky Beach.Samples were collected on 17.06.2021(leg.T.V. Naumova).Individuals of the new species were found at 15-20-m depth.
Etymology.The specific epithet means from «Olkhon Island», the type locality name.
Description of males.Body is comparatively short and thick (Table 2, Fig. 3, Fig. 4).Cuticle is faintly annulated, 2-µm thick.Somatic setae are sparse, short, 4-7 µm long.Crystalloids are absent.Labial region is slightly offset from the adjacent body; lips are well developed.Six inner labial sensillae are papilliform.Six outer labial sensillae in the shape of smooth non-articulated setae are 7-8 µm long, 26-30% of labial region width.Four cephalic sensillae are in shape of smooth setae.Cheilostom is of average size.Buccal cavity is spacious, funnel-shaped, with thick walls.Dorsal pocket and its tooth are absent.Subventral pockets are located close to each other or less often overlap each other.Stoma is 1. 1-1.2 times longer than labial region width.Amphidial fovea is cup-shaped, opening at the level of the buccal cavity.Pharynx is muscular, comparatively long, gradually expanding along the entire length.Cardial glands are large, rounded.Ventral gland, its canal ampulla and excretory pore are not seen.Testes are paired, opposed, situated to the left of the intestine; anterior testis is outstretched, posterior testis reflexed.Vas deferens is well developed.Spicules are slender, slightly curved, 55-58 µm long, 1. 1-1.3 times longer than the cloacal body diameter.Gubernaculum is in the shape of "gutter", 25-27 µm long.Precloacal supplements are echinate, located in two groups (2 + 3), supplement closest to the cloaca smaller than others.Supplementary pads are armed with small thorns and one longer and thicker thorn.Ampulla contents are at the top of the ampullae.Tail is slender, long, elongate-conical, with subterminal seta.Caudal glands and spinneret are well developed.
Description of females.General morphology is similar to that of males in the structure of cuticle and the anterior body end.Six inner labial sensillae are papilliform.Six outer labial sensillae are in the shape of smooth, non-articulated setae, 8-10 µm long; four cephalic sensillae are smooth, 4-5 µm long.Cardia is small, surrounded by three round glands.Prerectum is not observed.Rectum length is equal to or slightly less than anal body diameter.Reproductive system is didelphic.Ovaries are located to the left of intestine, reflexed, comparatively short.Oocytes are numerous.Vulva is transverse slit, located in mid-body.Vulval lips are not sclerotised, not protruding outside the body contour.Cuticular wrinkles near vulva and vulva glands are not seen.Vagina is comparatively long, with thick walls.Uterus contains numerous spermatozoa and 1-2 eggs, measuring 58-75 × 35-55 µm.Tail is slender, elongate-conical, with subterminal seta.Caudal glands and spinneret are well developed.
Diagnosis.Eutobrilus olkhonensis sp.nov. is characterised by its small size (body 1251-1570 µm long).Cuticle is faintly annulated.Crystalloids are absent.Inner labial sensillae are papilliform.Six outer labial sensillae are in the shape of smooth, non-articulated setae, 7-10 µm long, 21-30% of labial region width.Four cephalic sensillae are in the shape of thin and smooth setae 4-5 µm long.Buccal cavity is spacious, funnel-shaped.Dorsal pocket and its tooth are absent.Subventral pockets are located close to each other or less often overlap each other.Spicules are slender, slightly curved, short (55-58 µm), 1.1-1.2times longer than cloacal body diameter.Gubernaculum is in the shape of «gutter» with curved distal end.Precloacal supplements are echinate are in two groups (2 + 3) and the supplements closest to the cloaca smaller than the outers.Supplementary pads are armed with small thorns and one longer and thicker thorn.Ampulla contents are at its top.Tail is slender, elongate-conical, with subterminal seta.
The combination of the shape and structure of stoma (pockets less separated from each other), the simple vagina and the protrusible supplements characterise the genus Eutobrilus within the family Tobrilidae.Regarding the number of species, it is the largest genus in this family (Andrássy, 2007).The genus occurs in Europe, Asia, Africa, North America, Australia, and Antarctica.In the genus Eutobrilus, more than half of the species (19 of 36) are inhabitants of Lake Baikal, where they occur in the depth range of 0-1610 m as well as in the splash zone (0.5 m above the water edge).Most of the species (15 of 19) are endemic (Naumova & Gagarin, 2019).
We believe that the knowledge on the species diversity of Baikal representatives of the genus Eu-tobrilus is far from complete awaiting new exciting contributions.The use of molecular-genetic approaches would enable researchers to highlight the phylogeny of this group and estimate the evolution rate within the genus.These are the objectives of future investigations.
from the pharynx base to the cloaca, µm 1725 1535-1750 1655 --Distance from the vulva to anus, µm --body length; a -ratio of body length to body width in its middle part; b -ratio of body length to pharynx length; c -ratio of body length to tail length; cʹ -ratio of tail length to body width at the anus or cloacal region; V -ratio of the distance from the anterior end of the body to the vulva to the total body length.
times longer than the cloacal body diameter, apically bifurcate.Gubernaculum is in the shape of a straight «gutter», 27-30 µm long.Precloacal supplements are 8-11 in number, echinate, protrusible.Supplementary pads are armed with small thorns and one longer thorn.Ampullae contents are at the top of ampullae.Supplements are approximately identical in size and located almost equidistant from each other.The supplement closest to cloaca is at 50-82 µm away from it; supplement row is 450-650 µm long.Tail is slender, elongate-conical.Caudal glands are well developed; spinneret is present.Subterminal seta is absent.Description of females.General morphology is similar to that of males in the structure of the cuticle and anterior body.Cardia is small, surrounded by three round glands.Prerectum is not observed.Rectum length is equal to or slightly greater than anal body diameter.Reproductive system is didelphic, amphidelphic.Ovaries are located to the left of the intestine, reflexed and comparatively short.Oocytes are numerous.Vulva is a transverse slit and equa-Distance from the pharynx base to the vulva, µm

Table 1 .
Morphometrics of males and females of Eutobrilus tsalolikhini sp.nov.torial.Vulval lips are not sclerotised and not protruding outside the body contour.Cuticular wrinkles near the vulva and vulva glands are not seen.Vagina is short, with thin walls.Uterus contains numerous spermatozoa and 2-7 eggs, measuring 70-75 × 42-62 µm.Tail is slender, long, elongate-conical.Caudal glands are well developed.Subterminal seta is absent.