A new nemertean species of the genus Cephalothrix from intertidal calcareous red algae off the Vietnam coast is described based on histological sections, confocal laser scanning microscopy, and nucleotide sequences of three nuclear (18S rRNA, 28S rRNA, H3) and two mitochondrial (COI, 16S rRNA) DNA fragments. Cephalothrix suni sp. nov. is characterized by a unique body colour pattern consisting of transverse brown on ventral body side. A molecular phylogenetic analysis of the family Cephalotrichidae using a multigene approach has identified three clades for Cephalothrix: Cephalothrix, Procephalothrix, and interstitial cephalotrichids. The new species belongs to clade Procephalothrix sensu Chernyshev Kajihara 2019 and a subclade of species with red, orange, or dark yellow anterior tip.
Keywords: Nemertea, archinemerteans, cLSM, phylogenetic analysis, ribbon worms, Vietnam.