Morphology of Ichthyophonus hoferi assessed by light and scanning electron microscopy

The morphology of Ichthyophonus hoferi in vitro at pH 3.5 and 7.0 is described using light and scanning electron microscopy. Only vegetative growth was observed. At pH 3.5, hyphal growth was seen. The hyphae of I. hoferi are characterized by evacuated hyphal walls with the cytoplasm migrating to the apex and no septation. In contrast, the growth at pH 7.0 is mainly seen as spherical bodies which vary in size and are uni- to multinucleate. Amoeboid bodies showing slow movements were observed within 3-6 h of transfer to pH 7.0. We propose a life-cycle involving the germination of thick-walled multinucleate spores in the fish stomach as a response to the low pH. The hyphae then penetrate the digestive tract and rupture when they reach a blood vessel (neutral pH), whereby uni- and binucleate bodies and/or amoeboid bodies are released. The small cells are transported in the blood vessels and spread in the organs richly supplied with blood (heart, kidney, spleen, liver and muscle tissue) where they grow to form multinucleate spores.