Two New Pseudochromadora Species and the First Korean Record of Metachromadora itoi Kito, 1978, with SEM-Based Insights into Buccal Cavity Morphology

Three free-living marine nematode species of the family Desmodoridae are described and illustrated based on light and scanning electron microscopy. Pseudochromadora paraparva sp. nov. was collected from muddy sand sediments at Eulwangri Beach, Incheon, Korea, and P. capitata sp. nov. from sublittoral muddy sediments off Jindo Island, Korea. Metachromadora itoi Kito, 1978, is also recorded for the first time from Korean waters, based on specimens from sandy sediments off Jeju Island. Pseudochromadora paraparva sp. nov. is distinguished from P. parva Gagarin & Thanh, 2008 and related congeners by having a tripartite cephalic region consisting of a jar-shaped main capsule, an anterior transition zone with weaker cuticle bearing the cephalic setae, and a highly elevated hat-shaped labial region. It additionally shows a sexually dimorphic amphideal fovea, a unique arrangement of precloacal thorns, a gubernaculum with a dorsal apophysis, and ventral thorns lacking cuticular hillocks. Pseudochromadora capitata sp. nov. is characterized by a cephalic region composed of a rounded labial region and a thickly cuticularized main capsule, together with a sexually dimorphic amphideal fovea, arcuate spicules with a large hammer-shaped capitulum, a gubernaculum with a dorsal apophysis, and 3–8 precloacal and 3–5 postcloacal thorns arranged in a row. Molecular data (18S and 28S rRNA) were generated for both new species, and phylogenetic analyses support their placement within the genus Pseudochromadora and provide molecular evidence distinguishing them from closely related congeners. Scanning Electron Microscope (SEM) observations of M. itoi also revealed multiple configurations of the buccal cavity, providing additional morphological information useful for understanding structural variation within the genus. These findings refine the taxonomic framework within the Desmodoridae and expand current knowledge of morphological diversity in free-living marine nematodes.