The first report of phototaxis of fish ectoparasite, Argulus japonicus.

We discovered positive phototaxis in the fish ectoparasite, Argulus japonicus (Subclass Branchiura). Positive phototaxis of A. japonicus showed circadian rhythmicity. The phototaxis of A. japonicus has a unique circadian rhythm, which has been reported in no other crustacean. No phototaxis was observed at 4:00. After this time, however, A. japonicus gradually showed positive phototaxis until 16:00. There was no difference between male and female A. japonicus in this phototaxis. In addition, A. japonicus could distinguish between light of different colours, and showed positive phototaxis in the order blue light>yellow light>green light>red light.