Control of the Daily Rhythms by Photoperiods in Protomont Detachment and Theront Excystment of the Parasitic Ciliate Cryptocaryon irritans

― Cryptocaryon irritans has clear daily rhythms in protomont detachment from fish and theront excystment from tomonts. While the rhythms seem to be regulated by photoperiods, it has not been confirmed. We investigated whether the daily rhythms of the parasite would be controlled by giving different photoperiods of 12 h light and 12 h dark (06:00–18:00 L and 18:00–06:00 D; 15:00–03:00 L and 03:00–15:00 D) to infected fish and tomonts. Protomont detachment and theronts excystment mostly occurred in the last 3 h of the dark period and from 6 h to 3 h before the end of dark period, respectively, indicating that the rhythms are controlled by photoperiods.