Ostracod genus Heterodesmus Brady, 1866 is known thus far to contain only three species: H. adamsii Brady, 1866; H. apriculus Hiruta, 1992; and H. naviformis (Poulsen, 1962). This genus has been recorded from the Sea of Japan, and the coastal areas of Thailand and Vietnam. The main generic character is the presence of antero-dorsal and postero-dorsal tube-like processes on the rostrum on both valves. The three species mostly differ in the shell lateral projections. Their relationship and the position of Heterodesmus within family Cypridinidae are poorly understood, partly due to the lack of publication of DNA data so far. We study Heterodesmus collected from several localities in the Northwest Pacific, namely Tsushima and Iki Islands in Japan and Maemul Island in Korea. Besides morphological characters, we also use two mitochondrial markers (16S rRNA and mtCOI) and three nuclear regions (18S rRNA, 28S rRNA, and internal transcribed spacer - ITS) in the samples to detect the biodiversity of this genus. Our phylogenetic tree based on molecular data coupled with morphology reveals the presence of two species, H. adamsii and H. apriculus. We report on their morphological variability, molecular diversity, and phylogenetic position within Cypridinidae based on 16S, 28S and 18S rRNAs, and provide a taxonomic key for all living genera of this family. For the first time, we give an overview of the intrageneric and intrafamily DNA distances of the above markers for the entire subclass Myodocopa.
Keywords: Myodocopa; Northwest Pacific; molecular distance; molecular phylogeny; taxonomic key.