Infestation of the Parasitic Isopod on Juveniles of the Black Sea Bream as an Optional Intermediate Host in Hiroshima Bay.

In Hiroshima Bay, parasitic isopods of the genus infest the black sea bream (Bleeker, 1854) and the Japanese halfbeak (Temminck and Schlegel, 1846), two fish species that are abundant and commercially important in the Seto Inland Sea of Japan. Immature and mature individuals can infect both juveniles and adults of , while immature are known to parasitize juveniles of ; i.e., no parasites are found in adult The identification of the immature parasitizing juveniles of remains uncertain, because species are morphologically identifiable only based on adult females. Also, the biological/ecological relationship between the hosts and parasites has not been studied. Here, we identified the parasites on as Bruce, 1986 by molecular sequence analyses along with other parasites obtained from , the latter being morphologically confirmed by comparison with paratype materials of as well as the similar congener Bruce, 1986. The growth rates of the infected juveniles from June to September in the years 2013-2015 and 2018 were significantly lower than those of the uninfected ones, suggesting a negative effect of the infection on the hosts. Our data on the prevalence and duration of the infection, as well as the body size gain of the hosts and parasites, corroborate a hypothesis that would utilize as an optional intermediate host before it reaches the final host, .