Difference in Serum Scuticocidal Activity Against Miamiensis avidus (Ciliophora, Scuticociliatida) Between Olive Flounder (Paralichthys olivaceus) and Starry Flounder (Platichthys stellatus).

Abstract

Miamiensis avidus is an opportunistic parasite responsible for significant mortalities in marine aquaculture. Given the rising economic losses associated with scuticociliatosis in olive flounder (Paralichthys olivaceus) cultured in Korea, alternative species such as starry flounder (Platichthys stellatus), which demonstrates resistance to certain pathogens affecting olive flounder, are being considered for aquaculture. However, the underlying biological mechanisms influencing host susceptibility to M. avidus remain unclear. In this study, we investigated differences in susceptibility to M. avidus between olive flounder and starry flounder by evaluating their serum scuticocidal activity. Our results revealed that starry flounder serum exhibited significantly higher scuticocidal activity against M. avidus compared to olive flounder serum. The observed activity was largely abolished following heat inactivation or EDTA/EGTA treatment, indicating that the alternative complement pathway plays a crucial role in the scuticocidal activity. Proteomic analysis using two-dimensional gel electrophoresis (2-DE) and mass spectrometry identified a higher abundance of complement component C3 and leucine-rich repeat (LRR) proteins in starry flounder serum. Although the scuticocidal activity of olive flounder serum was significantly lower than that of starry flounder serum, the haemolytic activity against heterologous erythrocytes was significantly higher in olive flounder. This suggests that M. avidus may possess a mechanism that suppresses the scuticocidal activity of olive flounder serum. This is further supported by the observation that the scuticocidal activity of fresh starry flounder serum was significantly reduced after M. avidus was incubated with heat-inactivated olive flounder serum prior to exposure. Further research is needed to elucidate the resistance mechanisms of M. avidus against olive flounder serum, which could contribute to the development of more effective control strategies for scuticociliatosis.