Metacercariae infecting seven cnidarian species with their life cycle information including their adult stages in Japan

This study examined the metacercariae of trematodes in cnidarian jellyfish around Japan to demonstrate the importance of the jellyfish as the second intermediate or paratenic hosts. Trematodes were sampled from cnidarian jellyfish in seven coastal regions of Japan between 2024 and 2025. Trematodes (adults and metacercariae) were also obtained from marine fish and arrow worms and included for data comparisons. DNA barcoding was used for the species identification of metacercariae in the jellyfish and for elucidating their partial life cycles. Eight cnidarian species (245 individuals) were sampled, with metacercarial infection detected in seven host species. Based on morphological and molecular analyses, the metacercariae were classified into nine species belonging to three families, Accacoeliidae, Hemiuridae, and Lepocreadiidae. Six of these species were identified as the same species of adults isolated from fish hosts around Japan, although species names of two of the six remained unclear. The remaining three trematode species were assumed to belong to Lepocreadiidae, a major group of fish trematodes. These findings indicate that cnidarian jellyfish are important intermediate or paratenic hosts of fish trematodes in Japanese waters, as has been reported in other areas in previous studies. Moreover, a metacercaria occasionally detected in an arrow worm was identified as the same species as those in jellyfish, suggesting predator-prey relationships between these hosts. The study also synonymised <i>Tetrochetus hamadai</i> Fukui and Ogata, 1935, <i>T. aluterae</i> (Hanson, 1955), and <i>T. mitenevi</i> Zubchenko, 1978, with <i>T. coryphaenae</i> Yamaguti, 1934, based on molecular and morphological data.