A novel parasite strain of Amoebophrya sp. infecting the toxic dinoflagellate Alexandrium catenella (Group I) and its effect on the host bloom in Osaka Bay, Japan.

•A novel Amoebophrya strain infecting Alexandrium catenella was isolated from Osaka Bay.•Infection was confirmed in the host genera Alexandrium, Prorocentrum, and tripos.•The maximum prevalence of 73% was recorded during the A. catenella bloom in Osaka Bay.•Infections by Amoebophrya were also confirmed in newly formed cysts of A. catenella.