<i>Oncholaimus tripapillatus</i> sp. nov., a New Free-Living Marine Nematode of the Genus <i>Oncholaimus</i> Dujardin, 1845 (Nematoda: Enoplida: Oncholaimidae) from the Subtidal Sediment of Dokdo Island, East Sea, Korea, with a New Record of <i>O. qingdaoensis</i> Zhang & Platt, 1983

During a survey of the free-living marine nematodes of Korea, two species belonging to the genus <i>Oncholaimus</i> Dujardin, 1845 were discovered. A new species, <i>Oncholaimus tripapillatus</i> sp. nov. and a newly recorded species, <i>Oncholaimus qingdaoensis</i> Zhang & Platt, 1983, are reported. <i>Oncholaimus tripapillatus</i> sp. nov. was collected from the washing of subtidal coarse sediments around Dokdo Island of the East Sea, Korea. <i>Oncholaimus tripapillatus</i> sp. nov. is characterized by a relatively long (4071–4435 µm in males and 4514–4661 µm in females) and slender body, a slightly constricted head region, relatively long cephalic setae (10–12 µm), males having a precloacal sexual protuberance bearing two small cone-shaped supplementary spines, five pairs of long cloacal setae (three pairs of precloacal and two pairs of postcloacal setae in subventral position), and three remarkable papillae near the end of the tail, with two pairs of subventral setae. The Korean specimens of <i>Oncholaimus qingdaoensis</i> Zhang & Platt, 1983 are almost identical to the Chinese specimens of the original description from the intertidal sand of Qingdao, China. However, the Korean specimens differ from the Chinese specimens in the longer body length in males (3379–3715 µm vs. 2380–2640 µm), the larger spicule length (47–52 µm vs. 34–36 µm), and the presence of ventral tail papillae situated around 14–16 µm from the tail tip. Detailed morphological features and illustrations of two <i>Oncholaimus</i> species from Korea were obtained by differential interference contrast microscopy and scanning electron microscopy. A pictorial key to the species group with distinct tail papillae among the genus <i>Oncholaimus</i> is also provided.