Taxonomic Study of Free-Living Marine Nematodes in the Subgenus Tricoma (Desmoscolecida: Desmoscolecidae) from the Subtidal Zone of the East Sea, Korea, with Insights into the Ultrastructure of the Lip Region

This study provides a taxonomic analysis of three newly discovered species of free-living marine nematodes in the subgenus Tricoma from the East Sea, Korea. Tricoma (Tricoma) polyringulata sp. nov. is characterized by its relatively small body size, with lengths of 280–370 µm in males and 320–390 µm in females, and 75–89 main rings. Diagnostic features include an uncovered first ring, triangular head shape, amphidial fovea with slight constriction, and a unique pattern of somatic setae, comprising 7–9 subdorsal and 8–12 subventral setae. Tricoma (Tricoma) fortiseta sp. nov. is distinguished by 65–69 main rings, 10–12 subdorsal setae, and 17–20 subventral setae, along with a hexagonal head, a well-defined labial region with six lips, and bifid-tipped cephalic setae located medially on the head. Tricoma (Tricoma) uljinensis sp. nov. exhibits 67–70 main rings, with a thick, sclerotized head that is 1.5 times as wide as its length, gently triangular in shape. Its diagnostic features include a gubernaculum bent downward near its midpoint and a conical terminal ring, with 25–39% of the desmos covered. Detailed morphological descriptions of each species are provided, along with a comparative table of morphological traits for species with 61–75 main rings and an illustrated key for identification.