Abstract
A freshwater life cycle of Hysterothylacium aduncum was demonstrated experimentally for the first time. Third-stage larvae of H. aduncum from naturally infected Japanese smelt, Hypomesus transpacificus nipponensis, developed to the adult stage in experimentally infected rainbow trout, Salmo gairdneri, in fresh water. Larvae in eggs developed to the second stage and hatched even in fresh water. Although hatched larvae died quickly in fresh water, unhatched second-stage larvae remained alive there within the egg for more than 10 days and developed to the third stage in the mysid, Neomysis intermedia. The worms recovered from experimental infections were described using light and scanning electron microscopy; the characteristic features of the different stags of H. aduncum are discussed
