Dynamics and distribution properties of theronts of the parasitic ciliate Cryptocaryon irritans

•We assessed swimming ability of theronts of Cryptocaryon irritans by microscopic observations and video analysis.•The highest magnitude of upward movement of theronts was only as low as 1.6 mm/s.•We examine the excystment pattern and vertical distribution of theronts in tanks.•Theronts were released during the dark period and distributed close to the bottom layer of water column.•Control strategies can be developed from these properties such as placing water outlet on the bottom of tanks and increasing the water flow in a culture tank.

Cryptocaryon irritans is a parasitic ciliate that causes major economic losses in marine fish aquaculture globally. Despite the wide array of treatment methods, control of cryptocaryoniasis is still very challenging, especially in food fish culture. Thus, additional control methods against this parasite might be required to further reduce the occurrence of this disease. In this study, the swimming ability, excystment sequence, and distribution of theronts, the infective stage of C. irritans, were investigated in our effort to develop a physical control strategy. A video analysis for assessing the swimming ability of theronts showed diminishing mobility over time. The excystment of theronts primarily occurred during the dark period. Examination of the vertical distribution of theronts showed that they were mostly distributed at the 5 cm sampling point measured from the substrate, indicating that they have low upward swimming ability. From these results, we conclude that theronts possess limited mobility. Theronts also displayed an excystment pattern that might be influenced by photoperiod, since most theronts were released during the dark period of the day. Control strategies can be developed from these properties, such as increasing the water flow in a culture tank during the release period. Further, combined treatment methods against multiple stages of C. irritans can help minimize the occurrence of cryptocaryoniasis in culture facilities.