Mononchus truncatus Bastian, 1865

Altash, Stela, Kostadinova, Aneta & Peneva, Vlada, 2024, Integrative taxonomic study of mononchid nematodes from riparian habitats in Bulgaria. I. Genera Mononchus Bastian, 1865 and Coomansus Jairajpuri & Khan, 1977 with the description of Mononchus pseudoaquaticus sp. nov. and a key to the species of Mononchus, ZooKeys 1206, pp. 137-180 : 137-180

Mononchus truncatus Bastian, 1865

Figs 8 , 9

Description.

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.

Voucher material.

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 S 5 – S 7.

Habitats and localities.

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).

Representative DNA sequences.

28 S rRNA gene (GenBank: PP 768890 – PP 768892); 18 S rRNA gene ( PP 768901).

Distribution.

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 (2011 a) 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.

Remarks.

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 2011 a) based on the wide ranges of morphometric variation reported in the literature (see Suppl. material 2: table S 4). Andrássy (2011 a) 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 2011 a) is applied here. Comparative morphometric data for several records deviating from this species concept are also provided in Suppl. material 2: table S 4. 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 S 4).

We are also aware of two other questionable records of M. truncatus , not included in Suppl. material 2: table S 4: 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 (2011 a) . 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 (2011 a) 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 (2011 a) 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 S 4, S 5).

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.

Figures

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.

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).