Abstract: Two new species of the genus Sectonema from natural habitats of northern Vietnam are studied. This paper includes their descriptions, measurements, line illustrations, and light microscope (LM) and scanning electron microscope (SEM) pictures. Sectonema tropicum sp. nov. is characterized by a 2.56– 3.24 mm long body, 19–21 μm broad lip region, odontostyle 20–21 μm long at its ventral side, 730– 834 μm long neck, pharyngeal expansion occupying 52–59% of total neck length, uterus a simple tube-like structure 150–242 μm long or 1.2–2.5 times the body diameter, pars refringens vaginae present, V = 48–52, short (31–40 μm, c = 70–91, c’ = 0.5–0.6) and rounded tail, 91–97 μm long spicules, and only one weakly developed ventromedian supplement. Sectonema vietnamense sp. nov. is characterized by its slender (a = 33–49) and 2.71–4.25 mm long body, 14–16 μm broad lip region, odontostyle 8–9 μm long at its ventral side, 716–918 μm long neck, pharyngeal expansion occupying 63–67% of total neck length, uterus simple and 209–242 μm long or 2.5–2.9 times the corresponding body diameter, pars refringens vaginae absent, V = 54, short (34–39 μm, c = 70–115, c’ = 0.6–0.8) and rounded tail, 59–75 μm long spicules, and three or four irregularly spaced ventromedian supplements bearing hiatus. Both species are also characterized by their nearly continuous lip region, an atypical feature in this genus. Molecular analysis of S. tropicum sp. nov. confirms that Sectonema is a natural (monophyletic) taxon, very close to Metaporcelaimus.
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