Temporal Change in the Swimming Ability and Infectivity of the Oncomiracidium of a Monogenean <i>Neobenedenia girellae</i>

In this study, we assessed the influence of different water temperatures on the swimming ability of the oncomiracidium of Neobenedenia girellae. ​In addition, we investigated the temporal change in their infectivity to Seriola dumerili after hatching at 25°C. ​Most oncomiracidia in the cell culture flask filled with seawater deciliated and predominantly crept on the bottom by the end of 6 h experiment. ​Approximately 9% of the oncomiracidia in seawater-containing wells of a 12-well cell culture plate were non-swimming and became trapped in the air-water surface interface. ​However, these effects were mitigated when a polyethylene sheet was placed on the seawater surface in the wells. ​When conducting the experiment using this method, the oncomiracidia remained active and were still swimming at 60, 72, and 108 h after hatching at 30°C, 25°C, and 20°C, respectively. ​The retention time of the oncomiracidia's swimming ability decreased as the water temperature increased. ​The oncomiracidial infectivity was reduced after 24–25.5 h or more after hatching. ​At 5 days after the exposure to S. dumerili, the number of attached N. girellae was significantly lower, and their body length was significantly shorter when using oncomiracidia that were 36–37.5 h after hatching compared to freshly hatched ones.