Addressing the taxonomic confusion of Mesocoelium Odhner, 1910 (Trematoda: Plagiorchioidea: Mesocoeliidae) in Japanese urodelan and anuran amphibians.

Trematodes of the genus Mesocoelium Odhner, 1910 (Digenea: Plagiorchioidea: Mesocoeliidae) are globally distributed and parasitize amphibians, reptiles, or occasionally fishes. This genus is one of the most confusing taxa in trematodes because of its poor morphological features. In this study, we examined species of Mesocoelium collected from Japanese amphibians and found that they can be morphologically assigned to two species of Mesocoelium. Mesocoelium brevicaecum Ochi in Goto and Ozaki, 1929 parasitizes various both urodelan and anuran amphibians and occurred widely in Japan, while M. japonicum Goto and Ozaki, 1930 parasitizes a few hynobiid species in a limited part of Japan. We proposed ceca length as a valid key characteristic for species identification in this genus. M. elongatum Goto and Ozaki, 1929, M. lanceatum Goto and Ozaki, 1929, M. minutum Park, 1939, M. ovatum Goto and Ozaki, 1930, and M. pearsei Goto and Ozaki, 1930 are junior synonyms of M. brevicaecum, while M. japonicum can be distinguishable from them by morphologically and molecularly. Our molecular study supported the validity of both species and showed intraspecific divergence associated with geographic distance. Molecular identification suggests that the land snail Euhadra quaesita can serve as the first intermediate host for M. japonicum in Japan. This study also indicates the extremely low specificity of this genus for vertebrate hosts. Finally, we conclude that at least three species of Mesocoelium (M. brevicaecum, M. japonicum, and Mesocoelium sp. 1) are distributed in Japan. Further studies in other regions are undoubtedly required for a better understanding of the taxonomy and ecology of the genus Mesocoelium.