Phototactic responses in four monogenean oncomiracidia

Phototaxis is the common behavioral response exhibited by the oncomiracidia of various monogeneans. However, the changes in the oncomiracidial swimming behavior in response to light cues are not well understood. Here, we investigated the light responses of four monogeneans that are important pathogens in mariculture, namely Benedenia epinepheli, Benedenia seriolae, Neobenedenia girellae, and Heteraxine heterocerca. The swimming trajectory and speed of oncomiracidia of each species were assessed in a glass Petri dish with an LED light placed adjacent to it, based on three different light responses: LED light in the off position (normal swimming), LED light in the on position (phototactic behavior), and immediately (< 5 s) after switching the LED light off (photophobic behavior). The oncomiracidia of all four species exhibited positive phototactic and photophobic responses; however, the change in swimming speed between each response differed among the species. The oncomiracidia of three species (B. epinepheli, N. girellae, and H. heterocerca) exhibited high swimming speed, as a phototactic response; in contrast, the oncomiracidia of B. seriolae exhibited reduced swimming speed when moving toward the light source. Benedenia epinepheli and H. heterocerca exhibited the highest swimming speed during the phototaxis phase, whereas B. seriolae and N. girellae exhibited the highest swimming speed during the photophobic phase. These light responses are considered adaptive traits to increase the chance of encountering and infecting suitable hosts in nature, and such responses could potentially be applied to the control of parasite infections in aquaculture.