New species of Thoonchus (Enoplida, Enchelidiidae) from the Yellow Sea and redescription of T. giganticus Belogurova et al., 1986.

The family Enchelidiidae is recovered as monophyletic and appears to be a more recently derived taxon within the superfamily Oncholaimoidea. It combines nematodes with buccal cavity with three unequal teeth, crenate pharynx, and absence of demanian system. Genus Thoonchus was erected for nematodes characterized by the large buccal cavity with heavily cuticularized walls and several rows of denticles, short and arcuate spicules and gubernaculums with expanded corpus. One new species is described here from the Sishili Bay, located in the northern Yellow Sea. Thoonchus covidus sp. nov. is characterized by the body length 3195‒4339 µm, outer labial and cephalic setae in one circle, pocket shaped amphid above the buccal cavity base, secretory-excretory pore at the base of cephalic setae, arched spicules, gubernaculum with thin proximal extension parallel to spicules, preanal supplement absent, three pairs of preanal papillae. It differs from other congeners by the position of secretory-excretory pore, absence of supplementary organ in males and presence of three pairs of precloacal papillae. Redescription of T. giganticus is also provided. After critical evaluation of the genus we recognize 4 valid species and provide a taxonomic key to species.