Blue-Violet (405 nm) Light as a Non-Chemical Intervention for Scuticociliatosis in Large Yellow Croaker (Larimichthys crocea)

Blue-violet light has demonstrated potential in inactivating aquatic pathogens and managing associated diseases. To evaluate its efficacy against scuticociliatosis caused by parasite Metanophrys sp. in large yellow croaker ( Larimichthys crocea ), we first exposed Metanophrys sp. to 365, 405, and 465 nm LED light to identify the effective wavelengths and then conducted therapeutic trials on infected L. crocea . Results showed that the lethal effect on the parasite decreased with increasing wavelength. Both 365 nm and 405 nm irradiation caused severe morphological damage, including cilia loss, and cell rupture, leading to 100% mortality of parasite. In contrast, 465 nm light showed only a minimal effect. In the in vivo trial, infected L. crocea treated with 405 nm light exhibited a survival rate of 76.7%, which was not significantly different from the healthy control group (90.0%) but substantially higher than the infected-untreated group (26.7%) and the group treated with 365 nm group (0%). Safety assessment revealed that 405 nm light caused a temporary increase in ocular melanin, which returned to baseline within 28 days. In conclusion, 405 nm blue-violet light can safely and effectively control Metanophrys sp. infection in L. crocea , supporting its potential application as an eco-friendly method for managing parasitic diseases in aquaculture.