Neohexostoma gymnosardae n. sp. (Monogenea, Hexostomatidae), a gill parasite of Gymnosarda unicolor (Valenciennes) (Teleostei, Scombridae) in the South China Sea

Parasite biodiversity of fish in coral reefs of the South China Sea is still incompletely explored. We describe here a new species of Neohexostoma (Monogenea: Hexostomatidae) from the gill filaments of the dogtooth tuna Gymnosarda unicolor (Scombridae), collected off Yongshu Reef, South China Sea. Neohexostoma gymnosardae n. sp. is distinguished from its congeners by the following features: (i) haptor clearly marked from body proper by a strongly constricted peduncle, divided in its posterior margin into two symmetrical lobes, (ii) vagina armed with scattered small blunt spines, (iii) eggs tied by their long polar filaments, (vi) esophagus with several lateral diverticula, (v) intestinal ceca unfused and extending into the haptor. We present an analysis of the relationships of this monogenean based on partial 28S rDNA sequences. An identification key for species of Neohexostoma is provided. This is the first member of the genus Neohexostoma known to parasitize a species of Gymnosarda.