Female [Based on 10 specimens from 3 localities; see Table 2 for measurements]. Body short, 0.70–1.15 mm, J- or C-shaped upon relaxation, body diameter at mid-body 47–53. Cuticle smooth under light microscope (very faint striation observed in one specimen, Fig. 2H), 2–3 thick along most of body, 3–4 thick in post-anal region. Lip region offset, cephalic and labial papillae prominent, conical, of almost same size. Amphid apertures oval, 5 ± 0.4 (4–5) (n = 6) wide, situated anterior to dorsal tooth apex, at 10–14 from anterior end. Buccal capsule oval, somewhat flattened at base, 1.6–1.9 as long as wide or 0.9–1.2 times as long as lip region width; its ventral wall 1.5–2.5 thick, dorsal wall posterior to dorsal tooth ~ 3 thick. Dorsal tooth small, its anterior margin 3.0 ± 0.5 (2–4) wide, located near middle of buccal capsule, tooth apex at 9 ± 1 (8–11) from anterior end of buccal capsule. Nerve-ring at 97 ± 6 (90–103) (n = 6) from anterior end of body. Excretory pore posterior to nerve-ring, small, well visible. Reproductive system amphidelphic. Genital branches almost symmetrical; anterior branch 134 ± 50 (80–253) (n = 9) long; posterior branch 115 ± 19 (80–133) (n = 9) long. Ovaries well developed; anterior ovary 60–105 (n = 7) long; posterior ovary 70–140 (n = 7) long. Oviduct with marked pars dilatata oviductus, ~ 30 wide. Uteri very short. Two uterine eggs present in one female measuring 81 × 40 and 90 × 38. Vagina with straight walls, its length representing 25–33% of corresponding body width; pars refringens vaginae as 2 oval to drop-shaped smooth sclerotised pieces, 3–4 long and 2–3 wide; pars distalis vaginae ~ 3 long. Vulva a transverse slit; pars refringens vaginae protruding in some specimens (Fig. 2I). Rectum 0.7–0.8 times as long as body diameter at anus. Tail conoid, ventrally arcuate, with finely rounded tip. Caudal glands and spinneret absent. Caudal pores two pairs. Male: Not found.

Figure 1. 

Line drawings of Coomansus parvus (de Man, 1880) Jairajpuri & Khan, 1977. Female specimens from populations collected from riverbanks of the rivers Vedena (A, B, D) and Arda (C): A anterior region B posterior genital branch C, D tail end. Scale bar: 25 µm.

Figure 2. 

Photomicrographs of Coomansus parvus (de Man, 1880) Jairajpuri & Khan, 1977. Female specimens from populations collected from riverbanks of the rivers Lopushnitsa (A, B, E–G, M), Vedena (C, D, H, I, K, L), and Arda (J): A entire body B–D, G anterior region (amphid opening arrowed in G) E, F, I reproductive system (E anterior genital branch F, I vulval region showing pars refringens vaginae) H, J–M tail (cuticle striation arrowed in H; caudal pores arrowed in J and K). Scale bars: 200 µm (A); 20 µm (B–D, F–I, M); 50 µm (E, J–L).

Ten specimens are deposited in the Nematological Collection of the Institute of Biodiversity and Ecosystem Research, Bulgarian Academy of Sciences, Bulgaria, under the accession numbers IBER-BAS NC 49/1, IBER-BAS NC 51/2, IBER-BAS PN 68/4 litter, IBER-BAS NC 88/4, IBER-BAS NC 88/7-9. Photovouchers for the sequenced specimens are provided in Suppl. material 1: figs S1–S3.

Soil around Ulmus sp., Populus sp., and C. betulus and litter around F. sylvatica and Acer sp. along riverbanks of the rivers Arda, Lopushnitsa, Vedena, and Devinska (see Table 1 for details).

28S rRNA gene (GenBank: PP768895–PP768898); 18S rRNA gene (GenBank: PP768899 and PP768900).

Almost cosmopolitan, except in Australia (Andrássy 2009). In Bulgaria, C. parvus has been reported with no morphological evidence supporting identification in soil samples from an oak forest in Burgas Province (Aleksiev et al. 1998), from beech forests in Strandzha Mountain (Strandzha Nature Park, protected zones “Bjalata prust” and “Propada”; Iliev and Ilieva 2014), and from arable lands in Sofia (Katalan-Gateva 1968) and Kazanlak provinces (Katalan-Gateva et al. 1981). Iliev and Ilieva (2016) described and illustrated C. parvus based on a large population of females in soil samples from one habitat in the Rhodope Mountains. The present study provides the second documented record of C. parvus in Bulgaria, the first record of this species in litter samples, and four new localities in three provinces (Tables 1, 2).

Morphologically, the present material belongs to and was identified as C. parvus. Some variation was detected in the present material with single specimens from three populations sampled in the Rhodope and Balkan Mountains showing lower values for L, a, G1, G2, the length of the genital branches, ovaries, and tail, and greater values for the distance of the amphid from anterior end compared with the population from Vitosha Mountain (Table 2). We consider these small metrical differences to represent intraspecific variation; this was confirmed by the very low levels of genetic divergence (see above).

The morphometric data for the present material fall within the range given by Andrássy (2011b), except for the slightly greater values for the width of the buccal capsule (14–15 vs 10–12 µm). Comparisons with published descriptions of C. parvus revealed an overlap with the morphometric data of the present material but also a greater variation with higher upper limits of variation for the published ranges of body length (Zullini et al. 2002; Ahmad and Jairajpuri 2010; Ishaque et al. 2022) and most of the indices, and lower ranges for the width of the buccal capsule in four populations falling below the range recorded in the present specimens (Suppl. material 2: table S1). It is worth noting that there was an overall good agreement with the descriptions and morphometric data for a population of C. parvus collected in Bulgaria by Iliev and Ilieva (2016) and especially with a population used for generating 18S rDNA and 28S rDNA sequences for this species described by Tabolin and Kolganova (2020).

However, the material described by Ishaque et al. (2022) showed little overlap with the published descriptions and the present material, with ranges for a number of characters falling outside the known ranges for C. parvus: outside the upper limits of variation (L, V, buccal capsule length and width, lip region width, and rectum length); and outside the lower limits of variation (G1, G2, and position of tooth apex) (Suppl. material 2: table S1). This material keys down to C. indicus Jairajpuri & Khan, 1982 in the key to species of Coomansus by Andrássy (1993, 2009) and to C. ulsani Choi, Khan & Lee, 1999 in the key by Vu (2021) but does not agree completely with the data for these species. Clearly, the material of Ishaque et al. (2022) does not belong to C. parvus but definite identification is not possible based on the available data and illustrations (also see comments in Suppl. material 2: table S1).

Female [Based on 4 specimens from the type-population and 8 voucher specimens from other populations; see Table 3 for measurements.] Body slender (a = 20.2–33.6), almost straight; body diameter at mid-body 44–71. Cuticle smooth under light microscope, 2–2.5 thick along body, 3–3.5 thick in post-anal region. Lip region rounded, almost continuous with adjoining body, 2.4–3.7 as wide as high; papillae small, conical; cephalic papillae somewhat larger than labial. Body at posterior end of pharynx 1.8–2.5 times as wide as body width at lip region. Amphids caliciform, with oval apertures, 4 ± 0.5 (3.5–5.0; n = 10), at 8–12 from anterior end; amphid position varying from little anterior to tooth apex to level of anterior end of buccal capsule. Buccal capsule elongate-oval, slightly flattened at base, about twice as long as wide (1.8–2.0; n = 10), 1.2–1.3 times as long as the labial diameter; its ventral wall 2–3 thick, dorsal wall posterior to dorsal tooth 3–4 thick. Dorsal tooth strong, its anterior margin 4 ± 0.5 (3–5) wide, located at 6 ± 0.4 (5–6.5) from anterior end of buccal capsule, its anterior margin perpendicular to vertical plane. Buccal capsule with short transverse ridge, small tooth-like projection visible in some specimens in sublateral position (n = 2). Ventro-sublateral transverse ribs of buccal capsule weak, situated just posterior to tooth apex. Nerve-ring at 108 ± 8 (96–125) from anterior end of body. Excretory pore small, not well visible, at level of posterior margin of nerve-ring. Reproductive system amphidelphic. Anterior genital branch 171 ± 35 (116–226) long, posterior genital branch 166 ± 32 (120–205) long. Ovaries well developed, anterior ovary 105 ± 39 (65–125; n = 11) long, posterior ovary 106 ± 26 (70–135; n = 11) long. Oviduct with well-marked pars dilatata oviductus, 20–30 wide. Uterus a short tube with thick walls, 25–35 long. Vagina slightly swollen, with straight walls, its length representing 28–38% of corresponding body width; pars refringens vaginae as two smooth rhomb-shaped sclerotised pieces 3–6 long and 2–3 wide. Two females were recovered possessing a single large, thin-shelled uterine egg measuring 86–94 × 37–46 (specimens from River Maritsa and Komluka Island). Vulva a transverse slit. Vulva-anus distance equals 2.9–4.2 tail lengths. Tail long, slender, initially conoid, then almost cylindrical (10–13 wide) and slightly swollen at the tip, slightly curved ventrally in the third part; tail length represents 10–14% of body length. Caudal glands moderately developed, arranged in group. Tail tip rounded, with terminal spinneret and one small papilla. One female with abnormal tail, very short and almost straight. Male. Not found.

Soil around Salix sp. along River Danube at Vetren, Silistra Province, North Bulgaria (44°08'24"N, 27°01'47"E; elevation 20 m a.s.l.)

Komluka Island (River Danube), rivers Veleka, Shirokoleshka, and Maritsa (see Table 1 for details).

The holotype female and one paratype female are deposited in the Nematode Collection of the Institute of Biodiversity and Ecosystem Research, Bulgarian Academy of Sciences, Bulgaria, under the accession numbers IBER-BAS NTC 105 and 106. One paratype female is deposited in the Wageningen Nematode Collection (WANECO), Wageningen, the Netherlands (WANECO accession number WT 4037), and one paratype female is deposited in the Nematode Collection of the U.S. Department of Agriculture (USDA), Beltsville, Maryland, USA (USDA accession number T-8065p).

Figure 3. 

Line drawings of Mononchus pseudoaquaticus sp. nov. Holotype female (A, C, D) and paratype specimens (B, E, F): A, B anterior region C posterior genital branch D tail region E, F tail tip. Scale bar: 25 µm.

Eight voucher specimens are deposited in the Nematode Collection of the Institute of Biodiversity and Ecosystem Research, Bulgarian Academy of Sciences, Bulgaria, under the accession numbers IBER-BAS NC 5/2, IBER-BAS NC 18/3, IBER-BAS NC 16/6, IBER-BAS NC 18/5, IBER-BAS NC 78/1, IBER-BAS NC 80/1. Photovouchers for the sequenced specimens are provided in Suppl. material 1: fig. S4.

Figure 4. 

Line drawings of Mononchus pseudoaquaticus sp. nov. Paratype females from populations collected from riverbanks of the rivers Shirokoleshka (A, H), Maritsa (B, F), Veleka (E, G) and Danube (C, D): A–E anterior region F vulval region G anterior genital branch H vulval region and posterior genital branch. Scale bar: 25 µm.

28S rRNA gene (GenBank: PP768893 and PP768894); 18S rRNA gene (PP768902).

Figure 5. 

Line drawings of the tail region in females of Mononchus pseudoaquaticus sp. nov. from populations collected in Komluka Island (A) and riverbanks of the rivers Veleka (B), Maritsa (C) and Danube (D). Scale bar: 25 µm.

The species is named Mononchus pseudoaquaticus because of its similarity with M. aquaticus, hence the prefix pseudo- meaning false.

Females of M. pseudoaquaticus sp. nov. are characterised and distinguished from the congeners by a combination of features: a medium-sized body (1.23–1.88 mm); an elongate-oval, slightly flattened at the base buccal capsule measuring 29–33 × 15–16 µm, 1.8–2.0 as long as wide and distinctly shorter than 2 labial diameters (1.2–1.3 times as long as the labial diameter); amphid openings located from slightly anterior to dorsal tooth apex to level of anterior end of buccal capsule; a strong dorsal tooth situated at 18–21% of buccal capsule length from its anterior end, its anterior margin being perpendicular to the vertical plane; subventral transverse ribs located just posterior to dorsal tooth apex; didelphic (amphidelphic) reproductive system with pars refringens vaginae distinctly sclerotised in the form of two smooth rhomb-shaped pieces; tail (171–210 µm long, c = 7.2–10.2, c’ = 4.7–5.8) slightly curved at its posterior third, spinneret terminal.

Figure 6. 

Photomicrographs of Mononchus pseudoaquaticus sp. nov. Holotype (A–C, E, F, H, I, L) and paratype (D, G, J, K) females: A body, total view B–E anterior region (transverse ridge arrowed in C; amphid opening arrowed in Е) F, G vulval region showing pars refringens vaginae and posterior genital branch (F) H–J tail (caudal glands arrowed in J) K, L tail tip showing one small papilla (arrowed in K) and terminal spinneret (L). Scale bars: 400 µm (A); 20 µm (B–E, G, J, K, L); 30 µm (F, I, H).

Morphologically, Mononchus pseudoaquaticus sp. nov. appears most similar to M. aquaticus, M. pulcher Andrássy, 1993, and M. caudatus Shah & Hussain, 2016. However, M. aquaticus likely represents a composite species (see also Baqri and Jairajpuri 1972) based on the wide ranges of morphometric variation reported in the literature (see comparative data in Suppl. material 2: table S2). However, it is not possible to revise the identification of these materials because in many cases the findings are not documented properly and important characters such as vaginal characteristics (the shape of pars refringens vaginae in particular), buccal capsule shape and length/width ratio, etc., are not described, and the voucher material is inaccessible. Therefore, the species concept for M. aquaticus (sensu stricto) used in the present comparisons is based on the original description of Coetzee (1968) and the data by Baqri and Jairajpuri (1972) who re-examined and provided metrical data for some paratypes of M. aquaticus. This concept was also applied by Andrássy (2011a) in the most recent key to the species of Mononchus (see Suppl. material 2: table S3 for details) and in the updated key to the species of Mononchus provided here.

Figure 7. 

Photomicrographs of Mononchus pseudoaquaticus sp. nov. Females from populations collected from riverbanks of the rivers Veleka (A, G, H), Danube (B, D, E, I, J), Shirokoleshka (C, L) and Maritsa (F, K): A–D anterior region (transverse ridge arrowed in D) E–G vulval region showing an egg (E) vulval opening, subventral view (F) and pars refringens vaginae (G) H, I tail J caudal glands (arrowed) K caudal pores (arrowed) L tail tip. Scale bars: 20 µm (A–G, K); 30 µm (J, L); 50 µm (H, I).

The present material differs from the type material of M. aquaticus (Coetzee 1968; Baqri and Jairajpuri 1972) by having: a smaller buccal capsule length/width ratio (1.8–2.0 vs 2.2–2.5); a different shape of the base of the buccal capsule (flattened vs tapering); a different direction of the anterior margin of dorsal tooth (perpendicular to the vertical plane vs oblique); a different shape of the vaginal sclerotised pieces (pars refringens vaginae) (rhomb-shaped vs drop-shaped); and a longer tail (171–210 vs 94–156 µm (mean 150 µm) (Suppl. material 2: table S3).

The new species differs from M. caudatus by having: a different buccal capsule length/width ratio (1.8–2.0 vs 2.0–2.5); lower a value (20.2–33.6 vs 34–38); more anteriorly situated nerve-ring (96–125 vs 125–134 µm); different arrangement of the caudal glands (in a group vs in tandem); and shorter rectum (26–31 vs 32–36 µm) and vagina (16–20 vs 27–29 µm) (Shah and Hussain 2016; Suppl. material 2: table S3).

Differentiation from M. pulcher is more complicated because the original description of Andrássy (1993) is based on two, geographically largely separated populations from Chile and Hungary. However, Andrássyʼs (1993: fig. 2) illustrations indicate that he probably dealt with different species. Unfortunately, it is impossible to separate the rather incomplete metrical data since Andrássy (1993) provided pooled data for the position of dorsal tooth apex, the length of pharynx, the width of the lip region, the body diameter at mid-body, and tail length (Suppl. material 2: table S3). Still, in addition to the morphological differences (e.g., different shape of the buccal capsule and direction of the dorsal tooth), the Hungarian population is characterised by having a smaller buccal capsule length (and hence length/width ratio) and an overall smaller body length/tail length ratio (c). The size of the buccal capsule is a feature that varies in rather narrow ranges for a given population/species and is one of the most important differentiating characters for all mononchids. These data indicate that the Hungarian population may represent another species. However, it is impossible to identify this material given the scant data provided in Andrássy (2011a). Therefore, our comparisons are based on the morphology and metrical data for the type-population of M. pulcher from Chile. The new species differs from M. pulcher (sensu stricto) by having: a shorter (29–33 vs 35–38 µm) and narrower (15–16 vs 16–18 µm) buccal capsule; lower values for a (20–34 vs 35–39); anterior margin of the dorsal tooth perpendicular to the vertical plane vs oblique, vagina not spotted in its anterior part vs spotted, rhomb-shaped pars refringens vaginae vs drop-shaped; and smaller egg length (86–94 vs 98–100 µm). Additionally, the upper ranges for body length and tail length are greater in both populations of M. pulcher (Suppl. material 2: table S3).

Female [Based on 14 specimens from 6 localities; see Table 4 for measurements.] Body of most specimens straight, with only last part of tail ventrally curved (body C-shaped upon fixation in a few specimens), comparatively slender, body diameter at mid-body 53–71. Cuticle smooth under light microscope, 2–3 thick along most of body, thicker (4–5) in post-anal region. Lip region rounded, continuous with adjoining body, papillae small, cephalic papillae very small and rounded, labial papillae somewhat larger and conical. Body at posterior end of pharynx 1.2–1.4 times as wide as body width at lip region. Amphids with oval apertures, situated at the beginning or middle of buccal capsule, at 11 ± 1 (10–13) (n = 12) from anterior end and 40 ± 3 (37–44) (n = 12) from posterior end of buccal capsule, aperture 4.5 ± 0.5 (4–5) (n = 12) wide. Buccal capsule oval, tapering at base, 2.0–2.3 as long as wide or 1.3–1.7 times as long as lip region width; its ventral wall 2–3 thick, dorsal wall posterior to dorsal tooth ~ 3–5 thick. Dorsal tooth strong, its anterior margin 5 ± 0.6 (4–6) (n = 12) wide, located at 11 ± 0.5 (10–12) from anterior end of buccal capsule. Ventral wall with short, not so well visible rib, ventro-sublateral transverse ribs located at level of tooth apex or slightly more anterior. Nerve-ring at 127 ± 8 (116–144) (n = 12) from anterior end of body. Excretory pore weakly marked, posterior to nerve-ring. Reproductive system amphidelphic. Anterior genital branch 193 ± 14 (175–223) long, posterior branch somewhat longer, 204 ± 16 (187–240) long. Ovaries well developed, not reaching uterus-oviduct junction; anterior ovary 107 ± 17 (75–142) (n = 12) long, posterior ovary 114 ± 18 (95–146) (n = 12) long. Oviduct with marked pars dilatata oviductus, 33 ± 7 (20–45) wide. Uteri thick-walled tubes, 40–60 long, length ranges for anterior and posterior uterus almost identical. Vagina with straight walls, 28 ± 3 in length representing 24–33% of corresponding body width; pars refringens vaginae as two rounded drop-shaped pieces with smooth surface, 3–5 long and 2–3 wide. Vulva transverse, not protruding. Vulva-anus distance equals 2.3–3.3 tail lengths. Tail long, slender, curved ventrally in second part, length representing 11–13% of total body length, 11–13 µm wide at cylindrical part, with rounded and slightly swollen tip. Caudal glands moderately developed, arranged in group, spinneret terminal. Male. Not found.

Ten specimens are deposited in the Nematode Collection of the Institute of Biodiversity and Ecosystem Research, Bulgarian Academy of Sciences, under the accession numbers IBER-BAS NC 5/1, IBER-BAS NC 16/1-6, IBER-BAS NC 17/1, IBER-BAS NC 18/3, IBER-BAS NC 30/13, IBER-BAS NC 311/7-9. Photovouchers for the sequenced specimens are provided in Suppl. material 1: figs S5–S7.

Soil around roots of F. excelsior, U. laevis, A. glutinosa and Salix sp. and litter around Salix sp. along banks of the rivers Shirokoleshka, Trigradska, Dyavolska, Rezovska, Veleka, Maritsa, and Danube (see Table 1 for details).

28S rRNA gene (GenBank: PP768890–PP768892); 18S rRNA gene (PP768901).

According to the abundant published data for materials reported as M. truncatus, this species appears to exhibit a worldwide distribution. However, we agree with Andrássy (2011a) who doubted that all of the records referring to M. truncatus concern in fact this species. In Bulgaria, M. truncatus has been reported from many localities but with no morphological evidence supporting identification. Andrássy (1958) recorded this species for the first time in Rila Mountains and Varna. Subsequently, M. truncatus was reported from the North Thracian Plain (Katalan-Gateva 1962) and Pazardzhik Province (Katalan-Gateva 1965) associated with cultivated plants. In aquatic habitats, the species has been reported in sediments from 24 rivers and three lakes (Stoichev 1996; Stoichev and Chernev 2011; Stoichev and Varadinova 2011). The present study is the first to provide morphological and morphometric data for M. truncatus in Bulgaria.

Figure 8. 

Line drawings of Mononchus truncatus Bastian, 1865. Females from populations collected from riverbanks of the rivers Shirokoleshka (A, C, D) and Maritsa (B): A, B anterior region C anterior genital branch D tail region. Scale bar: 25 µm.

Morphologically, the present material belongs to and was identified as M. truncatus. However, similar to the situation with M. aquaticus (sensu lato) considered above, M. truncatus also represents a composite species (Andrássy 2011a) based on the wide ranges of morphometric variation reported in the literature (see Suppl. material 2: table S4). Andrássy (2011a) summarised the data from the original description and subsequent re-descriptions of M. truncatus and provided novel data for a population from Hungary. This concept of M. truncatus (sensu stricto) (“real M. truncatus” of Andrássy 2011a) is applied here. Comparative morphometric data for several records deviating from this species concept are also provided in Suppl. material 2: table S4. Typically, these include studies providing data (sometimes pooled, e.g., Nakazawa 1999; Eisendle 2008) for nematodes from different localities (e.g., Botha and Heyns 1992; Nakazawa 1999; Eisendle 2008; Farahmand et al. 2009). Thus, the data by Botha and Heyns (1992) show upper ranges above the upper range (b, c, and V) and lower ranges below the lower range of variation in M. truncatus (sensu stricto) (buccal capsule width, position of tooth apex, anterior end to pharyngo-intestinal valve, body diameter at mid-body and tail length). Almost all of these differences were recorded in a single sample (Crocodile River) likely containing a misidentified specimen. Similarly, both samples studied by Farahmand et al. (2009) contain specimens with largely deviating morphometric data (Suppl. material 2: table S4).

Figure 9. 

Photomicrographs of Mononchus truncatus Bastian, 1865. Females from populations collected from riverbanks of the rivers Rezovska (A, D, F, G, H, K), Danube (B, E, J, L) and Shirokoleshka (C, I, M, N): A body, total view B–F anterior region (ventro-sublateral ribs arrowed in D; amphid opening arrowed in E; transverse ridge arrowed in F) G, H, L vulval region: G vulval opening, ventral view H vulva and part of posterior genital branch (posterior uterus and part of pars dilatata oviductus), lateral view L vulval region showing pars refringens vaginae I–K tail M, N tail tip showing papilla (arrowed in M) and terminal spinneret (N). Scale bars: 400 µm (A); 20 µm (B–H, L–N); 50 µm (I–K).

We are also aware of two other questionable records of M. truncatus, not included in Suppl. material 2: table S4: Koohkan et al. (2014) reported as M. truncatus nematodes of a population from Ghale Asgar, Kerman Province, Iran, that do not correspond to this species because all important morphometric characters are outside the ranges of the “true” M. truncatus sensu Andrássy (2011a). Probably this population represents a yet undescribed species as it cannot be identified using the available keys. Similarly, Rawat and Ahmad (2000) reported M. truncatus as a new geographical record for India and provided a brief description based on five females. However, the measurements of some key characters such as the length of the buccal capsule (37–38 vs 42–50 μm), tail length (172–212 vs 232–283 μm) are outside the ranges of the “true” M. truncatus and the data provided are insufficient to identify the species. We consider that the records listed above are based on composite material.

We agree with Andrássy (2011a) who considered the description by De Bruin and Heyns (1992) to represent the “real M. truncatus” and add to his list of reliable records the data by Coomans et al. (1995). The present material agrees well with the characteristics of M. truncatus (sensu stricto) based on the original description, the description and data for the neotype population by Clark (1960), Coomans and Khan (1981), and Baqri and Jairajpuri (1972), and the description by Andrássy (2011a) except for the shorter tail (205 vs 240–283 µm) in a single specimen from Vetren (Table 4), resulting in a greater value for c (9.3 vs 5.8–8.6) (Suppl. material 2: tables S4, S5).

Mononchus truncatus was first reported from Bulgaria by Andrássy (1958) who provided limited metrical data. However, the body length reported by this author is outside the range for M. truncatus; additionally, two largely differing measurements (22.3 and 43.3 μm) were given for the length of the buccal capsule in two females of similar size, suggesting that this report is based on more than one species.

Female [Based on 2 females; see Table 5 for measurements]. Body slender, straight, with strongly ventrally curved tail; body diameter 42 at posterior end of buccal capsule and 45–52 at mid-body. Cuticle smooth under light microscope, 3–3.5 thick along body, thicker (4–5) around vulva and posterior to anus. Lip region rounded, continuous with adjoining body; papillae small, cephalic papillae round and somewhat more visible than labial. Body at posterior end of pharynx twice as wide as lip region. Amphids with oval apertures (5 wide), located between dorsal tooth apex and anterior end of buccal capsule. Buccal capsule oblong, with flattened base, 2.3–2.4 as long as wide or 1.7–1.8 times as long as the labial diameter, its ventral wall around 3 thick, dorsal wall posterior to dorsal tooth 4 thick. Dorsal tooth robust, its anterior margin 4 wide, located at 10–11 from anterior end of buccal capsule. Ventral wall of buccal capsule with short, not well-visible rib, transverse ventro-sublateral ribs located at level of dorsal tooth apex. Excretory pore weakly marked, posterior to nerve-ring. Reproductive system amphidelphic, genital branches short. Ovaries well developed, not reaching uterus-oviduct junction. Oviduct with well-marked pars dilatata oviductus, 25 wide. Uteri short, anterior uterus 30 long, posterior uterus 36 long (n = 1). Oviduct-uterus junction with moderately developed muscular sphincter. Vagina with straight walls and small spots next to pars refringens vaginae, length representing 31% of corresponding body width; pars refringens vaginae as two round drop-shaped sclerotised pieces with smooth surface, 4 × 2 in size; pars distalis vaginae well visible, ~ 4 long. Vulval opening round (Fig. 11E), vulva not protruding; vulva-anus distance equals 2.1 tail lengths. Tail long, cylindrical, strongly curved ventrad, length representing 16–18% of body length; cylindrical part of tail ~ 6 wide. Caudal glands moderately developed, arranged in tandem. Tail tip rounded, somewhat asymmetrical, dorsal part better developed, with terminal spinneret and one large setiform papilla. Three pairs of caudal pores present. Male. Not found.

Two specimens are deposited in the Nematode Collection of the Institute of Biodiversity and Ecosystem Research, Bulgarian Academy of Sciences, under the accession numbers IBER-BAS NC 316/1.

Soil around roots of F. sylvatica near a waterfall (River Grafska, inflow of River Kopilovtsi; see Table 1 for details).

Morphologically, the specimens resemble most Mononchus oblongus Andrássy, 2011 regarding the shape of the buccal capsule, the actual and relative length of the tail (as percent of body length), and the position of tooth apex (Table 5; Andrássy 2011a). However, the present specimens exhibit some differences in other morphometric features and proportions such as the total body length (1.31–1.62 vs 1.60–1.88 µm), the length of the buccal capsule (45–47 vs 48–51 µm) and tail (230–263 vs 264–276 µm), the width of the lip region (25–27 vs 22–23 µm), and the ratios buccal capsule length/width (2.3–2.4 vs 2.6–2.8), buccal capsule length/lip region width (1.7–1.8 vs 2.1–2.3) and body at pharynx base/lip region width (1.9–2.0 vs 2.7–2.9).

Figure 10. 

Line drawings of Mononchus sp. female: A anterior region B vulval region and posterior genital branch C vulval region D tail. Scale bar: 25 µm.

The present specimens also show similarities with M. truncatus and M. himalayensis Rawat & Ahmad, 2000. However, Mononchus sp. differs from M. truncatus in having a shorter body (1.31–1.62 vs 1.7–2.1 mm), a more anterior position of tooth apex (22–23 vs 25–29%), longer tail in relation to body length (16–18 vs 10–13%), smaller vulva-anus length/tail length ratio (2.1 vs 2.4–3.0), a lower c value (5.7–6.1 vs 7.5–8.4) and a different shape of the vulva (round vs transverse) (Andrássy 2011a). Differences between Mononchus sp. and M. himalayensis include a shorter body (1.31–1.62 vs 1.6–1.9 mm), a more anterior position of tooth apex (22–23 vs 25–31%), lower a- and c’-values (28–31 vs 33–38 and 7.7–7.9 vs 8.8–10.4, respectively) and absence of pre-vulval papilla (vs presence) (Rawat and Ahmad 2000). Probably the two females represent a species not yet described; however, additional specimens are needed to confirm the identity of the Bulgarian population.

Figure 11. 

Photomicrographs of Mononchus sp. females: A body, total view B–D anterior region (ventro-sublateral ribs arrowed in C; amphid opening arrowed in D) E vulval opening, subventral view F, G Pars refringens vaginae (small spots next to it arrowed in F) H reproductive system I tail tip J tail K sphincter of the oviduct-uterus junction (arrow). Scale bars: 400 µm (A); 20 µm (B–G, I, K); 50 µm (H, J).