Vastnema crassicutaneum gen. et sp. nov. (Nematoda, Dorylaimida, Dorylaimidae), an interesting new taxon from natural areas of Vietnam

A new dorylaimid taxon, Vastnema crassicutaneum gen. et sp. nov., collected in natural habitats of Vietnam, is characterized, including morphological description, morphometrics, SEM observations, and molecular (18S-, 28S-rDNA) analyses. Vastnema gen. nov. is distinguished and separated from its relatives by a combination of key traits: large size (body 4.79–6.35 mm long), lip region tapering and continuous with the adjoining body, very short neck ( b -ratio 6.3–8.7), presence of a peculiar uterine differentiation, pre-equatorial ( V = 38–44) vulva, and 45–58 stomata-like ventromedian supplements. The new species is characterized by its three-layered cuticle, 12.5–15.5 µm thick at the anterior region; lip region 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; uterus complex and 571–804 µm, or 3.2–4.3 body diameters long; longitudinal vulva; tail short and rounded in both sexes (39–56 µm, c = 98–140, c ’ = 0.5–0.7); and spicules 138–160 µm long. Both morphological and molecular data support a close relationship of the new taxon with some Labronematinae representatives but also reveal that internal and external relationships of Dorylaimidae might be more complex than traditionally assumed.