Removal of two pathogenic scuticociliates Miamiensis avidus and Miamiensis sp. using cells or culture filtrates of the dinoflagellate Alexandrium andersonii

Scuticociliatosis, which is caused by parasitic protistan pathogens known as scuticociliates, is one of the most serious diseases in marine aquaculture worldwide. Thus, elimination of these ciliates is a primary concern for scientists and managers in the aquaculture industry. To date, formalin and other toxic chemicals have been used as anti-scuticociliate agents, but issues regarding their secondary effects often arise. Consequently, development of safer methods is necessary. To find out a safe method of controlling scuticociliate populations in aqua-tanks or small-scale natural environments, cultures of 14 phototrophic dinoflagellates were tested to determine whether they were able to control populations of the common scuticociliates Miamiensis avidus and Miamiensis sp. isolated from Korean waters. Among the dinoflagellates tested, both cells and culture filtrates of Alexandrium andersonii effectively killed M. avidus and Miamiensis sp. The minimal concentration of cells and equivalent culture filtrates of A. andersonii to kill all M. avidus cells within 48 h of incubation was ca. 2500 and 4500 cells ml⁻¹, respectively; whereas those needed to kill all Miamiensis sp. cells were ca. 1000 and 4500 cells ml⁻¹, respectively. It was estimated that 1 m³ of the stock culture containing 20,000 A. andersonii cells ml⁻¹ could eliminate all M. avidus cells in 7 m³ of waters within the aqua-tanks on land and all Miamiensis sp. cells in 19 m³ of waters within 48 h. None of the brine shrimp Artemia salina nauplii incubated with concentrations of 50–4500 A. andersonii cells ml⁻¹ for 24 h was dead. Furthermore, none of the flounder Paralichthys olivaceus juveniles incubated with a mean concentration of ca. 2280 A. andersonii cells ml⁻¹ for 96 h was dead. Therefore, A. andersonii cultures may be used as a safe biological method for controlling populations of scuticociliates and can replace toxic formalin. The results of this study provided the basis for developing the method to control scuticociliate populations and understanding interactions between scuticociliates and phototrophic dinoflagellates in marine ecosystems.