Mucus pH of the tiger puffer Takifugu rubripes is an important factor for host identification by the monogenean Heterobothrium okamotoi.

We examined a host-finding factor of the monogenean Heterobothrium okamotoi oncomiracidia to develop an alternative prophylaxis. H. okamotoi oncomiracidia attached preferentially to gill filaments and skin mucus from the tiger puffer Takifugu rubripes compared with corresponding material from other tested fishes (amber jack Seriola dumerili, red sea bream Pagrus major, Japanese flounder Paralichthys olivaceus and spotted halibut Verasper variegatus). The body mucus pH of the tiger puffer was 6.40 +/- 0.09 (mean +/- S.D.), whereas that for the other tested fishes was 7.2-7.4. To find if this difference in pH could account for the specific targeting of tiger puffer by H. okamotoi oncomiracidia, the attachment response of the oncomiracidia to pieces of agar buffered at various pH between 6.0 and 7.4 was examined. The number of attaching oncomiracidia was maximal at pH 6.4. We produced gynogenetic tiger puffers from a single female. These gynogenetic individuals showed a variety of body mucus pH and they were exposed to the oncomiracidia. Thirteen days after exposure, more young H. okamotoi were found on the gills of gynogenetic tiger puffer with mucus at pH 6.3-6.6, than on gills of fish with mucus at pH 6.0-6.3 and 6.6-7.2. H. okamotoi exploits the body mucus pH to identify the host. The simplicity of pH as a lure may lead to development of a simple and economical method to control H. okamotoi outbreaks.