Host recognition and mechanism of settlement of monogenean hatching larvae infecting fish

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Monogenean hatching larvae (oncomiracidia) settle down on the surface of fish host after they recognize some particular substance(s) present on fish body surface. However, nothing has been characterized about it. Following experiments were done to elucidate substances inducing larval settlement and. to purify them.1. A new method has been developed to assay in vitro the settlement-inducing activity in the fish body surface epithelium extracts, using low host-specific Neobenedenia girellae and Seriola-specific Benedenia seriolae. In this assay method, there were almost no differences in the activity among fish species tested, and the activity was heat tolerant. The activity against Heterobothrium okamotoi oncomiracidium was higher in mucus from tiger puffer, the natural host, than those in mucus from unnatural hosts.2. It was shown that the activity was inhibited by specific lectins : WGA against N.girellae and Con A against B.seriolae. It is suggested that substances to induce oncomira … More cidial settlement are sugar-related ones like glycoproteins or polysaccharides.3. H.okamotoi oncomiracidia started to feed blood immediately after settlement on the gills of tiger puffer, while they showed little or no ability to absorb and/or digest blood from unnatural hosts like grass puffer, Japanese flounder or red sea bream, and eventually came off from them.4. Using the in vitro assay an attempt was made to purify N.girellae oncomiracidium settlement-inducing substances extracted from body surface epithelium of Japanese flounder. They were solubilized in anion exchange resin at 0.2-0.3M NaCl, and had a molecular size above the exclusion limit by gel filtration.5. Manifestation of monogenean host specificity involves the ability to recognize host by hatching larvae upon settlement, ability to absorb nutrition from host, ability to be tolerant of host defense mechanisms, etc. It is suggested that benedeniid and polyopisthocotylean monogeneans have different levels in these abilities, and that host specificity is manifested by different factors in different groups of monogeneans. Less