Effects of water temperature and infection history on the severity of summer atrophy in juvenile Akoya pearl oyster Pinctada fucata martensii

Mass mortality of the juvenile Akoya pearl oyster Pinctada fucata accompanied by atrophy of the soft body was first observed in Japan in 2019, with an effect on cultured pearl production. This disease, referred to as summer atrophy, is caused by Pinctada birnavirus (PiBV), a member of the Birnaviridae family, but the occurrence and severity of this disease in pearl culture areas is unknown. In this study, to clarify the occurrence of the disease in pearl culture areas, we regularly observed juvenile P. fucata introduced in either the spring or the summer into areas where this disease occurs, and recorded mantle atrophy and mortality. Juveniles with mantle atrophy were observed in pearl culture areas in both the spring and summer groups, but mortality was significantly higher in the summer group. In laboratory infection experiments conducted at different rearing temperatures, mortality and symptoms were more severe at higher water temperatures. When uninfected naive juveniles were placed during the summer next to pearl oysters that had developed summer atrophy in the spring and recovered, mortality was significantly higher in the former group. These results suggest that if juveniles are placed in pearl culture areas during cold periods, although they may become infected with PiBV and experience mantle atrophy, mortality will likely remain low and any damage might be reduced through immune priming.