Virus-like particles associated with mass mortalities of the pen shell Atrina pectinata in Japan

Mass mortalities of the pen shell Atrina pectinata occurred in the fishing grounds of Ariake Bay, in southwestern Japan, during late spring and summer in 2003 and 2004. Histological examination revealed extensive necrosis in the epithelial cells of the kidney and gill, and impairment of the endothelial cells of the mantle arteria. Although cestode larvae belonging to the genus Tylocephalum were found in the mantle, adductor muscle, kidney, and digestive gland, their prevalence and the intensity of infection were low. Examinations of moribund pen shells for Haplosporidium spp. infection using PCR analysis and for Perkinsus spp. infection using Ray's fluid thioglycollate medium were negative. Unenveloped virus-like particles were detected by transmission electron microscopy in the cytoplasm of affected kidney and gill cells of moribund pen shells. They were icosahedral spherical and 50 to 55 nm in diameter. These virus-like particles found in moribund pen shells are different from those described in other marine mollusks, and may be the causative agent of the mass mortalities of pen shells.