THE SURVIVABILITY OF THE ECTOPARASITIC FLAGELLATE ICHTHYOBODO-NECATOR ON CHUM SALMON FRY (ONCORHYNCHUS-KETA) IN SEAWATER AND COMPARISON TO ICHTHYOBODO SP ON JAPANESE FLOUNDER (PARALICHTHYS-OLIVACEUS)

Experimental studies revealed that a freshwater ectoparasitic flagellate Ichthyobodo necator (Henneguy, 1883) could survive and reproduce in seawater after infected chum salmin fry, Oncorhynchus keta (Walbaum), were transferred directly from fresh water to 33‰ seawater. Minor morphological changes (slight reduction in body width, loss or twistlike wrinkles on body surface, and reductiom in contractile vacuoles) were observed in the attached form of I. necator following transfer to seawater. The field survey also confirmed that U. necator occurs on chum salmon fry in seawater sestuaries (salinity 17-34‰) and in freshwater habitats. It was assumed that I. necator acquired salinity tolerance as a result of adapting to the migratory behavior of its anadromous host. Two morphologically similar bodonids, I. necator from chum salmon and Ichthyobodo sp. from marine Japanese flounder, Paralichtys olivaceus (Temminck and Schlegel), were differentiated by crossinfection experiments. Thus , the parasite from marine flounder should be regarded as a seperate species from I. necator.