Molecular and Morphological Characterizations of the Fish Parasitic Isopod (Crustacea: Cymothoidae) Parasitizing Optional Intermediate Hosts: Juveniles of the Cobaltcap Silverside and Yellowfin Seabream .

(Isopoda: Cymothoidae) is a parasitic crustacean that infests the opercular cavities of fishes. Its main final host is the Japanese halfbeak, . However, also infests the black sea bream, , 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 . We collected and examined 20 mancae and 144 juveniles of from 129 cobaltcap silversides, , and 494 yellowfin seabreams, . Molecular analysis of the cytochrome oxidase subunit I gene and 16S rRNA genes revealed that cymothoid mancae and juveniles from the two fish species were identified to be . All on and might be mancae or juveniles, with no adult parasites; thus, and juveniles were optional intermediate hosts of . In the results of morphological description, juveniles infesting the final host lacked swimming setae, while juveniles parasitizing the two optional intermediate hosts had them. 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 in the three optional intermediate hosts indicated that 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 to .