Description of new sand-dwelling Rhynchothorax species (Pycnogonida, Rhynchothoracidae) from Korean waters with note on novel morphological traits

A new species belonging to the genus Rhynchothorax was discovered in a subtidal sandy patch off the eastern coast of Jejudo Island, Republic of Korea. The new species, Rhynchothorax arenigenus sp. nov., is morphologically close to R. nopperabo, R. philopsammum, and R. vallatus, but can be distinguished by a combination of morphological characters: an ectal tubercle at the lateral extension of cephalic segment, the lateral process intervals, the arrangement of spines and tubercles on the lateral processes, the oviger spine formula, leg tuberculation, and the relative length of auxiliary claws. We highlight three morphological traits in the present species: a pair of ventral furrows on the proboscis, variable lateral process intervals, and the structure of oviger terminal claw. Notably, the ventral furrows are described here for the first time within the genus and may have been overlooked in other Rhynchothorax species. Additionally, partial sequences of mitochondrial cytochrome c oxidase subunit I (cox1) are provided to support species delimitation and to facilitate future phylogenetic research.