A New Species of Desmoscolex (Nematoda: Desmoscolecidae) from the Northwestern Pacific and Its Implications for Lip-Region Ultrastructure in Species Delimitation

We describe a new species of Desmoscolex collected from subtidal muddy sediments off Jindo Island, on the southern margin of Korea’s west coast. Desmoscolex (Desmoscolex) curuvus sp. nov. is distinguished by 17 main rings, a 9/8 somatic setal arrangement, subdorsal setae with a slightly swollen and hollow distal end, an oval head with laterally extended foreign material, a rounded amphidial fovea confined within the head region, and broad cephalic setae bearing a fine central canal with lateral membranes. The terminal ring is strongly bent ventrally. Females exhibit pronounced sexual dimorphism, including a pair of dorsal setae on the thirteenth main ring and a thicker terminal ring (maximum width 25–26 μm in males and 24–31 μm in females). High-resolution scanning electron microscopy (SEM) observations revealed a distinctive lip-region ultrastructure composed of a tuberculate membrane and six fragment-like membranous elements, representing a rare configuration within the genus. By documenting a new species possessing membranous structures in the cephalic setae and providing detailed ultrastructural characterization of the lip region, this study offers important new evidence for refining species delimitations in Desmoscolex. These findings underscore the diagnostic value of lip-region morphology and highlight the need for targeted sampling and further ultrastructural analyses to better resolve the hidden morphological diversity of the genus, particularly in the underexplored northwestern Pacific.