Discovery of Laophontidae (Copepoda, Harpacticoida) from marine plastic debris: Pseudonychocamptus setadefectus sp. nov. and Heterolaophonte discophora (Willey, 1929).

Two species belonging to the family Laophontidae Scott T., 1905 were identified among harpacticoid copepods collected from marine plastic debris (MPD) stranded along the Korean coastline. These species were assigned to the genera Pseudonychocamptus Lang, 1944 and Heterolaophonte Lang, 1948. Pseudonychocamptus setadefectus sp. nov. conforms to the generic diagnosis, displaying sexual dimorphism in the armature of the swimming legs. It is distinguished from its six congeners by possessing only two endites on the maxillary syncoxa, notably lacking the proximal endite bearing the seta that is present in all other known species within the genus. Although most morphologically similar to P. colomboi Ceccherelli, 1988, the new species differs in several key characteristics, including the number of setae on the maxillary endopod, the relative length ratios of setae on the female P5 exopod, the presence of an inner seta on the male P5 exopod, and the asymmetry of the male P6. The second species, Heterolaophonte discophora (Willey, 1929), was previously reported from the Atlantic coast of Canada and the Pacific coasts of the USA and Japan. The Korean specimens closely correspond to these previous records, yet exhibit intraspecific variation, particularly in the setal count on the female P3 enp-2. Additional morphological differences were also observed, including variations in the segmentation of the male P3 endopod and in the number of setae on the maxillary endopod. Based on the sexually dimorphic traits observed in males, we propose the subdivision of the genus Heterolaophonte into four distinct species groups. The discovery of P. setadefectus sp. nov. and H. discophora on anthropogenic MPD underscores the potential role of such substrates as microhabitats for benthic harpacticoid copepods.