First record of in cultured mussels in the East China Sea.

During the investigation of parasitic pathogens of , infection of a -like protozoan parasite was detected by alternative Ray's Fluid Thioglycolate Medium (ARFTM). The diameter of hypnospores or prezoosporangia was 8–27 (15.6 ± 4.0, = 111) μm. The prevalence of the -like species in was 25 and 12.5% using ARFTM and PCR, respectively. The ITS1-5.8S-ITS2 fragments amplified by PCR assay had 100% homology to that of , suggesting that the protozoan parasite was and was its new host in East China Sea (ECS). Histological analysis showed the presence of trophozoites of in gill, mantle and visceral mass, and the schizonts only found in visceral mass. infection led to inflammatory reaction of hemocyte and the destruction of digestive tubules in visceral mass, which had negative effect on health of the farmed and it deserves more attention.