Vastnema crassicutaneum, Vu & Abolafia & Nguyen & Le & Peña-Santiago, 2025

Vastnema crassicutaneum sp. nov.

Figs 1 View Figure 1 , 2 View Figure 2 , 3 View Figure 3 , 4 View Figure 4 , 5 View Figure 5 , 6 View Figure 6

Material examined.

Sixteen females and eight males from three locations, in good state of preservation, some specimens probably slightly flattened due to their large body, including wide diameter at mid-body.

Morphometrics.

See Table 1 View Table 1 .

Type population. (Nature Reserve Nam Xuan Lac, Cho Don district, Bac Can province) 11 females and seven males.

Holotype.

Female adult, Northern Vietnam, Bac Can province, Cho Don district, Nam Xuan Lac nature reserve , December 2021 (coordinates 22°17'30"N, 105°31'06"E, elevation 800 m), soil from a wild forest. GoogleMaps

Paratypes.

Eleven females and seven males with the same collection details as the holotype GoogleMaps .

Other material.

All in Northern Vietnam. i) Ha Giang province, Bac Me district, Du Gia nature reserve , October 2023 (coordinates 22°43'13"N, 105°11'32"E, elevation 300 m) GoogleMaps ; ii) Ninh Binh province, Cuc Phuong National Park , August 2019 (coordinates 20°21'06"N, 105°35'22"E, elevation 430 m), soil from a wild forest GoogleMaps .

Description.

Adult. Moderately slender to slender (a = 25–34) nematodes of large to very large size, 4.79–6.35 mm long. Body cylindrical, clearly narrowing towards the anterior end, much less towards the posterior extremity, as the tail is short and rounded. Upon fixation, habitus almost straight. Cuticle smooth (but a very fine transverse striation is appreciable with SEM), three-layered, consisting of thin outer and inner layers and much thicker intermediate layer, especially conspicuous at level of anterior region and tail, 12.5–15.5 µm thick at anterior region, 9–15 µm in midbody, and 14–20 µm on tail. Lateral chord 12–29 µm wide, occupying up to one-sixth (8–16 %) of midbody diameter, lacking any differentiation. Body pores very small, the lateral ones arranged in a single row throughout the body length, a few dorsal and ventral pores are present at cervical region. Lip region rounded, occasionally slightly asymmetrical, visibly tapering, less than one-fourth (18–24 %) of body diameter at neck base, 2.2–2.5 times as wide as high, continuous with the adjoining body or hardly offset by a very shallow depression. SEM observations: lips totally amalgamated, oral field wide as inner and outer labial papillae are quite close, perioral area not differentiated, labial papillae button-like, cephalic papillae appearing as very small pores (Fig. 6 A – C View Figure 6 ). Amphidial fovea cup-like, its aperture 10.5–13.5 µm wide, occupying less than one-half (33–48 %) of lip region diameter, appearing as a wide transverse slit visibly occluded at its middle. Cheilostom almost cylindrical, 32–40 µm long, with moderately thick walls, but lacking any other specialization. Odontostyle strong, 1.7–2.0 times longer than lip region diameter, 8.3–11.8 times as long as wide, 0.89–1.13 % of body length, its aperture 19–27 µm long or less than one-half (36–48 %) of the total length. Guiding ring, double, but very delicate and difficult to visualize in some specimens. Odontophore simple, rod-like, 1.3–1.6 times the odontostyle. A mucro 2.5–8.5 × 1.5–3 µm is present in several specimens at level of odontophore base (Fig. 2 A, B, K View Figure 2 ). Pharynx comparatively very short (b = 6.3–8.7), strongly muscular, very gradually enlarging into the basal expansion that is 3.4–6.1 times as long as wide, 1.9–3.0 times longer than body diameter at neck base, and occupies up to one-half (46–49 %) of the total neck length; pharyngeal gland nuclei located as follows: DO = 52–56, DN = 53–58, S 1 N 1 = 67–69, S 1 N 2 = 74–79, S 2 N = 83–88. Nerve ring located at 228–308 µm or 33–36 % of the total neck length from the anterior end. Pharyngo-intestinal junction consisting of a conical to somewhat cylindroid cardia 41–84 × 17–32 µm almost entirely enveloped by intestinal tissue, and a weak, ring-like structure around the junction of cardia to pharyngeal base.

Female. Genital system diovarian, with both branches equally and well-developed, the anterior 823–1250 µm long, or 16–24 % of body length, the posterior 835–1153 µm long, or 15–21 % of body length. Ovaries variable in length, 221–704 µm the anterior, 185–619 µm the posterior, often reaching and surpassing the sphincter level, with oocytes arranged first in several rows and then in a single one. Oviduct variably long too, 157–349 µm or 0.8–2.0 body diameters, consisting of two sections almost equal in length: distal part made of prismatic cells and proximal one an elongated pars dilatata always with wide lumen inside and containing abundant sperm cells, therefore functioning as spermatheca. A weak sphincter separates the oviduct from the uterus. Uterus 571–804 µm or 3.2–4.3 body diameters long, bipartite, consisting of distal, longer, and slender region with narrow lumen and a proximal, shorter, and wider region with broad lumen and often containing abundant sperm cells inside; distal region always bearing a very special differentiation, resembling the Z-like structure of some dorylaims, with a concentration of many small spindle-shaped elements visibly refringent that adopt a peculiar arrangement (Fig. 3 A, B, G, H View Figure 3 ), all together measuring 13–18 × 22–26 µm. Vagina 54–73 µm long, to ca one-third (31–36 %) of body diameter: pars proximalis 39–51 × 26–37 µm, with divergent walls surrounded by moderately developed circular musculature; pars refringens, consisting of (lateral view) two separated triangular pieces 12–16 × 6–10 µm and with a combined width of 26–32 µm; pars distalis 5–10 µm long. Vulva longitudinal, slightly elliptical, 8–11 µm long. Prerectum 2.2–3.3, rectum 0.8–1.3 body diameters long. Anus a weakly arched transverse slit ca 15 µm long. Tail short and rounded, inner core 17–22 µm long, occupying less than half (37–48 %) of tail length, caudal pores two pairs, at the middle of tail, one sublateral, another subdorsal.

Male. Prerectum 3.4–5.5, cloaca 1.3–1.5 times longer than body diameter at level of the cloacal aperture. Genital system diorchic with opposite testes. Sperm cells elliptical, 8 × 4.5 µm. In addition to the pre-cloacal pair, situated at 16–24 µm from the cloacal aperture, there is a series of 47–58, shortly spaced (7–14, exceptionally reaching 19 µm in between), very low, ventromedian supplements, the most posterior of them located at 101–154 µm from the adcloacal pair, thus with a perceptible hiatus. As seen with SEM, the ventromedian supplements are not mammiform as so usual in dorylaims but appear as short longitudinal slits that resemble the stomata of plant leaves (Fig. 6 K – M View Figure 6 ). Spicules dorylaimid, relatively slender, 6.9–9.1 times longer than wide, 1.8–2.0 times longer than body diameter at level of the cloacal aperture: head 12–25 µm long, occupying up to one-sixth (8–16 %) of spicule length, 1.2–1.8 times as long as wide, with its dorsal side visibly longer than the ventral one; median piece 6.5–8.5 µm wide, occupying 32–44 % of spicule maximum width; posterior tip 5–9 µm wide; ventral hump and hollow little marked in general, the hump situated at 49–70 µm or 33–47 % of spicule length; curvature 118–135 °. Lateral guiding pieces 16–24 µm long, 4.0–4.8 times wider than long, almost cylindrical but visibly tapering at its posterior third. Tail similar to that of female, maybe slightly more conoid.

Other populations.

(six females and one male from two locations): Morphologically identical to type material. Morphometrically very similar too, with totally coincident or widely overlapping ranges of main measurements and ratios, although a few minor differences are also noted; for instance, prerectum length is appreciably longer in Ha Giang specimens: 324–390 vs. 206–321 µm in females and 473 vs. 324–390 µm in males.

Molecular characterization.

After sequencing and editing, six (three 18 S and three 28 S) rDNA sequences were obtained of type material for phylogenetic analyses. The three 28 S sequences were 798, 679, and 651 bp long, with 511 bp in common, and a Blast search showed that the longest of them was 96.9 % similar to that of Labronema porosum Vu, Elshishka, Nguyen, Le, Mladenov & Peneva, 2024 (acc. PP 060468), 96.3 % to that of L. bidoupense Vu, Elshishka, Nguyen, Le, Mladenov & Peneva, 2024 (acc. PP 060469), 92.0 % to that of Labronema vulvapapillatum ( Meyl, 1954) Loof & Grootaert, 1981 (acc. AY 592997 View Materials ), 91.5 % to that of L. vulvapapillatum (acc. AY 592996 View Materials ), 91.3 % to that of Labronema sp. , and 90.8 % to that of Nevadanema nevadense Álvarez-Ortega & Peña-Santiago, 2012 (acc. JN 242245 View Materials ), all of them rounded-tailed representatives of Dorylaimidae de Man, 1876 , Labronematinae Peña-Santiago & Álvarez-Ortega, 2014 .

The three 18 S sequences were 1698, 1690, and 1689 bp long, with 1684 bp in common, and a Blast search showed that the longest of them was 97.2 % similar to that of Aporcelaimellus sp. (acc. JX 674034 View Materials ), 97.2 % to that of L. ferox Thorne, 1939 (acc. AY 552972 View Materials ), 97.2 % to that of Aporcelaimellus sp. (acc. PP 099686), 97.1 % to that of Prodorylaimus sp. ( EF 207246 View Materials ), and 97.0 % to that of Mesodorylaimus sp. (acc. MG 921256 View Materials ).

Diagnosis.

The new species is characterized by its 4.79–6.35 mm long body, cuticle three-layered and 12.5–15.5 µm thick at anterior region, lip region continuous with the adjoining body and 26–31 µm wide, odontostyle 49–60 µm long or 1.8–2.0 times the lip region diameter, neck 627–888 µm long, pharyngeal expansion occupying 46–49 % of the total neck length, female genital system diovarian, uterus complex and 571–804 µm or 3.2–4.3 body diameters long, pre-equatorial (V = 38–44) and longitudinal vulva, tail short and rounded in both sexes (39–56 µm, c = 98–140, c ’ = 0.5–0.7), spicules 138–160 µm long, and 45–58 small ventromedian supplements with hiatus.

Separation from some resembling species.

The new species resembles a few Labronema species. In having large size, it compares to L. magnum Altherr, 1972 , a freshwater species known to occur in Nepal, Russia, and Sweden (see taxonomic revision by Peña-Santiago, 2022), but it differs from this in its much thicker cuticle (vs. for instance, 4–5 µm at anterior region), lip region with different shape (vs. anterior region truncate, offset by weak constriction, and one-third as wide as body diameter at neck base), comparatively much shorter neck (vs. b = 4.1–6.3), more anterior vulva (vs. V = 44–53), longer spicules (vs. 118–135 µm), and more numerous (vs. 21–31) ventromedian supplements with different aspect (vs. typically mammiform).

In having continuous lip region, it also resembles several atypical Labronema representatives, including L. hyalinum ( Thorne & Swanger, 1936) , Thorne, 1939, L. neopacificum Rahman, Jairajpuri, Ahmad & Ahmad, 1987 , L. orientale Andrássy, 2011 , L. singhalense Andrássy, 2011 , L. stechlinense Altherr, 1968 , and L. thornei Ferris, 1968 , but all of them show smaller general size (body length up to 3.73 mm long, odontostyle up to 48 µm long), comparatively longer neck (b - ratio up to 5.2), more posterior vulva (V = 45–55), shorter spicules (up to 120 µm long), and much less ventromedian supplements (up to 29) with different shapes.

Type material.

Female holotype, seven female and three male paratypes, and one male paratype deposited at the Department of Nematology, Institute of Ecology and Biological Resources, VAST, Hanoi, Vietnam. Four (two female and two male) paratypes at the Nematode Collection of the University of Jaén, Spain. Four (two female and two male) paratypes at USDANC, Beltsville, Maryland, USA.

Etymology.

The species epithet derives from the Latin terms crassus = thick and cutaneum = skin, cuticle, and refers to this peculiar trait of the new taxon.