Two new nematode species of the genus Tobrilus Andrássy, 1959 (Nematoda, Triplonchida) from Lake Baikal, Russia

Two new nematode species of the genus Tobrilus Andrássy, 1959 from Lake Baikal are described and illustrated. The first species Tobrilus elginus sp. nov. was found in the littoral zone of Maloye More Bays. The second species Tobrilus juliae sp. nov. dwells on bodies of dead sponges Lubomirskia baicalensis (Pallas, 1776). Tobrilus elginus sp. nov. is most similar to T. amabilis Tsalolikhin, 1974 and T. bekmanae Tsalolikhin, 1975. In contrast to the first species it has a shorter body and spicules, longer gubernaculum and a shorter supplements row. Its body is shorter and thinner, tail and supplement row are shorter and the vulva is more posterior as compared to the second species. The body size of Tobrilus juliae sp. nov. is most similar to T. securus Gagarin & Naumova, 2011 and T. saprophagus Naumova & Gagarin, 2017. From the first of these species it differs by the thinner body, shorter tail, comparatively shorter outer labial setae and shorter spicules. It differs from the second species by a thinner body, shorter male tail and shorter labial setae.


Introduction
Lake Baikal is a fascinating freshwater reservoir known all over the Earth. Its shores and pure, crystal clean water attract numerous tourists as well as experts and researchers. The ancient age of the Lake (25-30 million years), the enormous depths of the water body (1637-1642 m) and its huge 7-8 km depression including sediment layers containing one fi fth of the world's surface fresh water and its high oxygen saturation (up to 9.5 mg/l at the 1600 m depth point) make it a wonder of our planet. In 1996, UNESCO announced Lake Baikal a World's Heritage Object.
Unique environments of the Lake inevitably provided favourable conditions for the formation of an abundant and diverse fauna including over 2600 species with a high level of endemism (more than 56%) (Timoshkin 2001). The fauna of free-living nematodes comprises 114 species in 32 genera, 16 families and 8 orders according to the latest records (Naumova & Gagarin 2019). These worms are encountered in rocky, sandy and silty substrates occupying all depths from the splash zone (above the water edge) to the abyssal zones. In terms of abunance, nematodes often dominate among other benthos groups.
A key for the identifi cation of Tobrilus species from Lake Baikal based on male characters was published by Gagarin & Naumova (2016).
The aim of this paper is to describe two new nematode species inhabiting Lake Baikal: T. elginus sp. nov. and T. juliae sp. nov.
The samples contained numerous free-living nematodes, including both the species described herein. Nematodes were fi xed by standard methods and mounted in glycerin on permanent slides. All observations were made using Olympus CX-21 and Nikon Eclipse 80i light microscopes with Nomarski DIC accessories. 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 ver. 3.2 program for analysis and documentation of images from the preparations.

Repositories
HM RAS = Helminthological Museum, Center for Parasitology, Institute of Ecology and Evolution, Russian Academy of Science, Moscow, Russia LIN-SB = Limnological Institute, Siberian Branch, Russian Academy of Sciences, Irkutsk, Russia.
Abbreviations a = body length / greatest body diameter b = body length / distance from anterior end to pharyngo-intestinal junction c = body length / tail length c' = tail length / tail diameter at anus or cloaca L = body length V = distance from body anterior end to vulva expressed as percentage (%) of the body length

Etymology
The specifi c epithet means 'from Elga', the type locality name.

Male
Body comparatively long and thin. Cuticle smooth under light microscope, 1.5-2.0 μm thick. Body diameter at the posterior pharynx end 1.7-2.0 times the width of the labial region. Crystalloids absent. Somatic setae rare and short, 5-7 μm long. Labial region slightly offset from the adjacent body; lips  well developed. Six inner labial sensillae papilliform. Six outer labial sensillae in the shape of smooth, non-articulated setae 14-17 μm long, 45-57% of labial region width. Four cephalic sensillae in the shape of thin and smooth setae, 6-8 μm long. Both circles of setae drawn close together. Cheilostom of the average size. Buccal cavity spacious, funnel-shaped, with thick walls. Dorsal pocket and its tooth absent. Both subventral pockets overlapping adjacent to each other. Its teeth small, close to each other (~ 5 μm away). Stoma 1.1-1.3 times as long as labial region width. Amphidial fovea cup-shaped, opening at the level of buccal cavity. Pharynx muscular, comparatively long, expanding gradually along entire length. Cardiac glands large, rounded, 20-23 μm in diameter. Ventral gland, its canal, ampulla and excretory pore not seen.

Female
General morphology similar to that of males in structure of cuticle and anterior body end. Cardia small, surrounded by three round glands. Prerectum not observed. Rectum length equal to or slightly greater than anal body diameter. Reproductive system didelphic, amphidelphic. Ovaries situated to the left of intestine, refl exed and comparatively short. Oocytes numerous. Vulva a transverse slit and situated slightly posterior to mid-body. Vulval lips not sclerotized and not protruding outside the body contour. Cuticular wrinkles around vulva and vulva glands not seen. Vagina short, with thick walls. Uterus containing numerous spermatozoa and 1-2 eggs, measuring 65-105 × 26-50 μm. Tail slender, long, elongate-conical, with terminal seta. Caudal glands well developed.

Etymology
The species named in gratitude to Ms Julia Zvereva.

Male
Body comparatively long and thin. Cuticle smooth under light microscope, 1.0-1.5 μm thick. Body diameter at the posterior pharynx end 2.1-2.5 times the width of the labial region. Crystalloids absent. Somatic setae rare and short, 3-4 μm long. Labial region only slightly offset from the adjacent body; lips well developed. Six inner labial sensillae papilliform. Six outer labial sensillae in the shape of smooth, non-articulated setae 4-6 μm long, 24-35% of labial region width. Four cephalic sensillae in the shape of thin and smooth setae 3-4 μm long. Both circles of setae drawn close together. Cheilostoma of the average size. Buccal cavity spacious, barrel-shaped, with thick walls. Dorsal pocket and it tooth absent. Both subventral pockets overlapping adjacent to each other. Its teeth small, close to each other (distance ~ 3 μm). Stoma 1.0-1.2 as long as labial region width. Amphidial fovea cup-shaped, opening at the level of buccal cavity. Pharynx muscular, comparatively long, expending gradually along entire length. Cardiac glands large, rounded, 15-17 μm in diameter. Ventral gland, its canal, ampulla and excretory pore not seen.
Testes paired, opposed, situated to the left of intestine; anterior testis outstretched, posterior testis refl exed. Vas deferens well developed. Spicules slightly curved, comparatively short, 1.2-1.5 times as long as the cloacal body diameter, apically bifurcate. Gubernaculum in the shape of 'gutter', 43-47% of the spicule length. Precloacal supplements 6-7 in number, small, submerged. Supplement ampulla fl attened, its contents concentrated at ampulla base. Supplement cap absent. Central thorn protruding slightly above cuticle. Supplements approximately identical in size and situated about equidistant from each other. The closest to cloaca supplement at 11-20 μm away from it; supplement row 175-290 μm long. Tail short, elongate-conical, with terminal seta 3-5 μm long. Caudal glands well developed; spinneret in the shape of a short, conical tube.

Female
General morphology similar to that of males in structure of cuticle and anterior body end. Six outer labial sensillae in the shape of smooth setae 5-6 μm long, 28-38% of labial region width; four cephalic sensillae in the shape of thin, smooth setae 2-4 μm long. Cardia small, surrounded by three round glands. Rectum length equal to or slightly greater than anal body diameter. Reproductive system didelphic, amphidelphic. Ovaries situated to the left of intestine, refl exed and comparatively short. Oocytes numerous. Vulva a transverse slit and situated slightly posterior to mid-body. Vulval lips not sclerotized and not protruding outside the body contour. Cuticular wrinkles around vulva and vulva glands not observed. Vagina short. Uterus containing numerous spermatozoa and 1-2 eggs, measuring 70-72 × 40-43 μm. Tail short, elongate-conical, with subterminal seta. Caudal glands well developed.

Discussion
Tobrilus is considered the most primitive genus within the family Tobrilidae; the small and simple supplements in variable (non-constant) number seem to support this opinion (Andrássy 2007). The genus is distributed in Europe, Asia, Africa, both Americas and Australia (Andrássy 2007). Half of the species in the genus Tobrilus (12 of 22-24 valid species) are inhabitants of Lake Baikal where they are encountered in the depth range of 0 to 1520 m, as well as in the splash zone (0.5 m above the water edge). Most of the species are endemic and only two have been found elsewhere in water bodies of the Baikal region (Gagarin 1993;Naumova et al. 2010). Evidently, Lake Baikal is the epicenter of Tobrilus speciation. The genus Tobrilus is considered a basic form for the whole Tobrilidae family. According to Hodda (2011), the world tobrilid fauna includes 162 species in 10 genera. At present, Baikal is the home of 43 species of the Tobrilidae family, i.e., ¼ of the world fauna (Naumova & Gagarin 2019).
The authors believe that the knowledge on the species diversity of Baikal representatives of the genus is far from complete and that new exciting contributions are to be expected. Use of molecular-genetic approaches would enable the researchers to highlight the phylogeny of this group and estimate the evolution rate within the genus. These are the objectives of future investigations.