Two new nematode species of the genus Paratrilobus Micoletzky, 1922 (Nematoda, Triplonchida) from the water area of Lake Baikal (Russia)

This paper describes and illustrates two new nematode species of the genus Paratrilobus Micoletzky, 1922. The species Paratrilobus tankhoyensis sp. nov. was found at the estuary of the Pereyomnaya River (water area of Lake Baikal, near the Tankhoy railway station). Paratrilobus tankhoyensis sp. nov. is most similar to P. expugnator (Tsalolichin, 1976) in the body size, but differs in the comparatively thin body, shorter and thicker tail, shorter stoma and spicules. Another new species, Paratrilobus aquaticus sp. nov., was found in Posolsk Bank (natural underwater elevation of the bottom between the southern and central basins of Lake Baikal). The species is similar to P. granulosus Gagarin & Naumova, 2011 and P. ultimus (Tsalolichin, 1977) in the structure of the precloacal supplements. It differs from the former in the absence of crystalloids, a comparatively longer pharynx, longer stoma and outer labial setae as well as the absence of subterminal seta. It differs from the latter in a longer pharynx, stoma and longer outer labial setae as well as a longer and more slender tail. We also discuss diagnostic features of the males of the genus Paratrilobus.

This paper aims to describe two new nematode species from the water area of Lake Baikal (P. tankhoyensis sp. nov. and P. aquaticus sp. nov.) and discuss some features of the genus Paratrilobus.

Material and methods
Nematodes were collected from the following two sites: water area of Lake Baikal, estuary of the Pereyomnaya River, near the Tankhoy railway station, (51.568452 N, 105.166531 E), 10 cm depth, collected on 19 July 2018; southern basin of Lake Baikal, Posolsk Bank (underwater elevation of the bottom between the southern and central basins, (52.07994 N, 105.90368 E), 51 m depth, sand and silt, collected on 5 July 2012.
The samples contained numerous free-living nematodes, including 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 3.2 program for analysis and documentation of images from the preparations.

Institutional abbreviations
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 cʹ = tail length / tail diameter at anus or cloaca diam.c.s. = body diameter at the level of cephalic setae, in μm gub.
= gubernaculum length, in μm L = body length, in μm o.l.s. = length of the outer labial setae, in μm o.l.s.% = length of the outer labial setae as percentage (%) of body diameter at the level of cephalic setae spic.
= number of supplements V = distance from body anterior end to vulva expressed as percentage (%) of the body length

Etymology
The species epithet means 'from Tankhoy', the type locality name.

Male
Body comparatively long, thin. Cuticle smooth under light microscope, 1.5-2.0 μm thick. Body diameter at posterior pharynx end 1.3-1.4 times as large as width of labial region. Crystalloids absent. Somatic setae sparse, short, 8-11 μm long. Labial region slightly offset from adjacent body; lips well developed. Six inner labial sensillae in shape of thick, short setae, 6-9 μm long. Six outer labial sensillae in shape of smooth non-articulated setae, long 54-67% of labial region width. Four cephalic sensillae in shape of thin, smooth setae. Cheilostom of average size. Buccal cavity spacious, barrel-shaped, with thick walls. One pocket offset from buccal cavity, on its base two small teeth. Stoma 0.9 times as long as labial region width. Amphidial fovea cup-shaped, opening at level of buccal cavity. Pharynx muscular, comparatively long, expanding gradually along entire length. Cardiac glands large, rounded, 22-25 μm in diameter. Ventral gland, its canal, ampulla, excretory pore not seen.

Female
General morphology similar to males in structure of cuticle, anterior body end. Six inner labial sensillae in shape of thick and short setae, 8-10 μm long. Outer labial sensillae in shape of smooth non-articulated setae, long 44-56% of labial region width. Cardia small, surrounded by three round glands. Prerectum not observed. Rectum length equal to or slightly less than anal body diameter. Reproductive system didelphic, amphidelphic. Ovaries situated to left of intestine, refl exed, comparatively short. Oocytes numerous. Vulva transverse slit, situated to mid-body or slightly anterior to mid-body. Vulval lips not sclerotized, not protruding outside body contour. Cuticular wrinkles around vulva, vulva glands not seen. Vagina short, with thick walls. Uterus containing numerous spermatozoa, 1-2 eggs, measuring 112-132 × 52-80 μm. Tail elongate-conical, comparatively thick, with subterminal seta. Caudal glands well developed.

Remarks
Paratrilobus tankhoyensis sp. nov. is most similar to P. expugnator (Tsalolichin, 1976) Table 2 Diagnosis Paratrilobus aquaticus sp. nov. is characterized by a 2015-2203 μm body length; cuticle fi nely annulated; crystalloids absent; six inner labial sensillae papilliform; six outer labial sensillae in the shape of smooth non-articulated setae 18-20 μm long (53-58% of labial region width); four cephalic sensillae in shape of thin and smooth setae 10-13 μm long; buccal cavity spacious, barrel-shaped; one pocket connected with buccal cavity by wide gleam and contains two small teeth. Spicules comparatively thin, 51 μm long, 1.2 times as long as cloacal body diameter; gubernaculum in shape of 'gutter'. Precloacal supplements 6 in number, about same size, located approximately at identical distance from each other; supplements echinate, not very protruded over body surface; contents of ampulla located at base; cap armed, numerous small thorns and one large central thorn. Tail slender, comparatively long; three caudal glands and spinneret well developed; subterminal seta not observed.

Description
Male Body comparatively short. Cuticle fi nely annulated, 1.5 μm thick. Crystalloids absent. Somatic setae sparse, short. Labial region comparatively high, slightly offset from adjacent body, lips well developed. Six inner labial sensillae papilliform. Six outer labial sensillae in shape of smooth setae long 53% of labial region width. Four cephalic sensillae in shape of thin setae. Cheilostom of average size. Buccal cavity spacious, barrel-shaped, with thick walls. One pocket connected with buccal cavity by wide gleam, containing two small teeth. Stoma 1.5 times as long as labial region width. Amphidial fovea cupshaped, opening at level of buccal cavity. Pharynx muscular, comparatively long, expanding gradually along length. Cardiac glands large, rounded, 30 μm in diameter. Ventral gland, its canal, ampulla, excretory pore not observed.
Testes paired, situated to left of intestine; anterior testis outstretched, posterior testis short, refl exed. Vas deferens well developed. Spicules slender, ventral curved, 1.2 times as long as cloacal body diameter.

Female
General morphology similar to that of males in structure of cuticle, anterior body end. Six outer labial sensillae in shape of smooth non-articulated setae long 53-58% of labial region width. Cardia surrounded by three round glands. Rectum length equal to or slightly greater than anal body diameter. Reproductive system didelphic, amphidelphic. Ovaries situated to left of intestine, refl exed. Oocytes numerous. Vulva transverse slit, situated slightly posterior to mid-body. Vulval lips not sclerotized, not protruded outside body contour. Cuticular wrinkles around vulva, vulva glands not observed. Vagina straight line, with well-expressed spherical vaginal chamber, thick walls. Generated eggs in uterus not observed. Tail slender, comparatively long. Subterminal seta not seen.

Discussion
We compared males of valid species of the genus Paratrilobus (except for P. rapis described only from females) according to morphological characters (Table 3). On grounds of the supplement structure, the genus can be divided into two species groups: 1) comparatively large supplements with 'shoulder', cap and ampulla contents located in the top part of ampulla (P. grandipapilloides, P. brevis, P. delicatus, P. expugnator, P. ponticus and P. tankhoyensis sp. nov., i.e., 'grandipapilloides' species group) 2) comparatively small supplements; 'shoulder' is absent, cap comparatively small, ampulla contents located in ampulla base (P. ultimus, P. granulosus, P. aquaticus sp. nov., i.e., 'ultimus' species group).
We cannot give the rank of subgenus to these groups, but we must take into account the heterogeneity of the genus.
The species P. strenuus was found in Lake Tajmyr (Siberia, Russia) and was originally described from 32 females as Eutobrilus strenuus Gagarin, 1991(Gagarin 1991). Subsequently, the species was transferred to the genus Quasibrilus Tsalolichin, 1976, because the buccal cavity in specimens of this species is small, one pocket with two small teeth is hardly visible and well isolated from the buccal Table 3. Diagnostic morphometric characters of males of valid species of the genus Paratrilobus Micoletzky, 1922. cavity (Gagarin 1993). Andrássy (2007) placed this species in the genus Paratrilobus, but we consider this an erroneous decision and return this species to the genus Quasibrilus, because a stoma structure in this species does not correspond to the morphological diagnosis of the genus Paratrilobus (Andrássy 2007). The main feature of the genus Paratrilobus is the buccal cavity structure: it is spacious, funnel-or barrel-shaped with well-sclerotized walls; pockets are almost reduced.
The distribution area of the genus is within the borders of Eurasia. Paratrilobus grandipapilloides was described from freshwater bodies of Germany (Micoletzky 1922), but later, it was also found in Lake Onega, Lake Tajmyr and Lake Baikal (Gagarin 1990;Zullini 2006;Naumova & Gagarin 2019a). Paratrilobus ponticus was known from Dnepro-Bugs estuary of the Black Sea (Tsalolichin 1981). Paratrilobus rapis was described from Lake Tajmyr (Siberia, Russia) (Gagarin 1991). Seven species of the genus were found and described from Lake Baikal (Naumova & Gagarin 2019a). Therefore, Lake Baikal is the largest natural centre of speciation of this genus.