A Review of Ichthyobodo Infection in Marine Fishes

Abstract

The ectoparasitic flagellate Ichthyobodo has been recorded from 25 species of marine fishes and 3 species of anadromous salmon in the Pacific Ocean, Atlantic Ocean, and adjacent seas ranging from the subarctic to tropical zones. With an increasing number of reports on the occurrence of Ichthyobodo in the marine environment, its origin and taxonomic status have been confused. The parasite from marine fishes is morphologically comparable to the freshwater parasite I. necator, which can survive and reproduce on anadromous salmon in seawater. It was assumed that the parasite on marine fish was transmitted from freshwater hosts. However, the marine parasite from Japanese flounder (Paralichthys olivaceus) andI. necatorfrom freshwater chum salmon (Oncorhynchus keta) were differentiated by their host specificity. The parasite has been found on pure marine fishes in offshore waters. Thus, Ichthyobodo on marine fishes may be regarded as a separate species from freshwater I. necator. The parasite infects the skin and gills of host fishes, causing severe epidermal hyperplasia and necrosis. Ichthyobodo infections caused moderate deaths in yearling Japanese flounders and tiger puffer (Takifugu rubripes), while the infections induced high mortalities among larvae of marine fishes such as Japanese flounders, gilthead sea bream (Sparus aurata), and rockfish (Sebastes schlegli). The most effective treatment for Ichthyobodo infections is a formalin bath, but alternative safe control methods should be established by future studies.