Two New Species of Desmoscolex (Nematoda: Desmoscolecidae) from Subtidal Benthic Habitats in Korea, with a Comparative Analysis of Cephalic Setae Variability

This study describes two new species of the subgenus Desmoscolex (Nematoda: Desmoscolecidae) from subtidal benthic habitats in Korea, located in the Northwestern Pacific Ocean. Both species are distinguished by the absence of cephalic setae—a rare feature previously documented in only three species within this subgenus: Desmoscolex (Desmoscolex) rostratus Timm, 1970; D. (D.) asetosus Decraemer, 1975; and D. (D.) obscurus Bussau, 1993. The new species exhibit distinct morphological features. Desmoscolex (D.) lanceosetatus sp. nov. is characterized by an asymmetrical, dorsally curved head; large amphidial fovea that extend to the posterior margin of the first main ring; and lance-shaped subdorsal setae. In females, the absence of subventral setae on the 14th ring deviates from the typical arrangement of 17 main rings. Desmoscolex (D.) rotundicephalus sp. nov. is identified by its globular head, oval amphidial fovea, and tapering subdorsal setae with an open tip. In females, the absence of subventral setae on the 14th ring further differentiates it from closely related species. This paper provides detailed morphological descriptions supported by scanning electron microscopy (SEM) and differential interference contrast (DIC) images. Additionally, it includes a comprehensive morphological comparison with previously described species, species diagnoses, and a pictorial key for identification.