Two New Pseudochromadora Species (Nematoda: Desmodorida) from South Korea Based on Morphological and Molecular Evidence

During a survey of the west coast of South Korea, two new Pseudochromadora species were recorded from Yeongjongdo Island. Descriptions of two new species, an updated list of valid species within the genus, a tabular key, partial sequences of mtCOI, near full-length SSU, and the D2–D3 region of LSU rDNA, together with phylogenetic analyses are provided. The two new species are classified as Pseudochromadora based on having a two-portioned cephalic capsule, unispiral amphidial fovea, lateral alae extending from the posterior end of the pharynx as far as the tail, and presence of copulatory thorns, as well as a short conical tail. The two species are distinguished from each other by their different types of labial regions of the cephalic capsule (round-shaped vs. hat-shaped). The two species, despite being found in the same locality, are morphologically and molecularly distinct from one another. Pairwise Kimura 2-parameter (K2P) distances between the two new species were 10.6% (18S) and 27.2% (28S), values consistent with interspecific divergence observed among congeners. Phylogenetic analyses showed both species as distinct lineages within Pseudochromadora. In the 28S rDNA tree, each was retrieved as a well-supported monophyletic clade with congeners, whereas in the 18S tree, all congeners including the two new species formed a single clade, except for P. plurichela, which branched outside the main group. These results highlight potential paraphyly within Pseudochromadora and suggest that overlooked morphological traits may hold phylogenetic significance, warranting further investigation.