Abstract
A skin ulcer disease frequently occurred in yearling Japanese flounder, Paralichthys olivaceus, cultured in artificial propagation facilities along the coasts of Hokkaido, Japan. The present study confirmed that the disease was induced by the ectoparasitic flagellate, Ichthyobodo sp., which attached to the skin and gills. The external sign was restricted to the dorsal skin, and was characterized initially by increased secretion of mucus. It then developed to extensive epidermal erosion and ulcer with increasing number of parasites. The heavy infection caused hyperplasia of the malphigian cells and depletion of mucous cells, and led to epidermal spongiosis due to the intercellular edema. The destruction of epidermal tissues was followed by remarkable changes of the dermis in the strutum spongiosum such as the infiltration of lymphocytes and expansion of scale pockets, suggesting that loss of osmotic balance caused by the integumental breakdown may be the major cause of mortality of the infected fish.
