Molecular and Morphological Characterizations of the Fish Parasitic Isopod Mothocya parvostis (Crustacea: Cymothoidae) Parasitizing Optional Intermediate Hosts: Juveniles of the Cobaltcap Silverside Hypoatherina tsurugae and Yellowfin Seabream Acanthopagrus latus

Mothocya parvostis (Isopoda: Cymothoidae) is a parasitic crustacean that infests the opercular cavities of fishes. Its main final host is the Japanese halfbeak, Hyporhamphus sajori. However, M. parvostis also infests the black sea bream, Acanthopagrus schelgelii, as an optional intermediate host. Understanding the use of optional intermediate hosts is important for understanding the life history of Cymothoidae, and further information should be obtained. In this study, we aim to investigate the life cycle of M. parvostis. We collected and examined 20 mancae and 144 juveniles of M. parvostis from 129 cobaltcap silversides, Hypoatherina tsurugae, and 494 yellowfin seabreams, Acanthopagrus latus. Molecular analysis of the cytochrome c oxidase subunit I gene and 16S rRNA genes revealed that cymothoid mancae and juveniles from the two fish species were identified to be M. parvostis. All M. parvostis on H. tsurugae and A. latus might be mancae or juveniles, with no adult parasites; thus, H. tsurugae and A. latus juveniles were optional intermediate hosts of M. parvostis. In the results of morphological description, M. parvostis juveniles infesting the final host H. sajori lacked swimming setae, while juveniles parasitizing the two optional intermediate hosts had them. Mothocya parvostis mancae infested juveniles of both species just after metamorphosis, grew with the host. As the fish grows further, the parasite detached from the fish. The parasitic status of M. parvostis in the three optional intermediate hosts indicated that M. parvostis likely reproduced from June to December, and different optional intermediate hosts were used depending on the time of year in Hiroshima Bay. Therefore, a parasitic strategy involving optional intermediate hosts might increase the infestation success of M. parvostis to H. sajori.