Response ofNeobenedenia girellae(Monogenea) oncomiracidia to brightness and black-and-white contrast

SUMMARYNeobenedenia girellae, a capsalid monogenean, is a significant pathogen due to both its ability to cause high mortality in fishes and its low host specificity. Established control methods have both advantages and disadvantages. Biological control measures with no unfavourable effects on the environment should be incorporated into the control strategy. The response ofN. girellaeoncomiracidia to brightness and black-and-white contrast was investigated to search for an alternative approach of disease prevention or control. Japanese flounder,Paralichthys olivaceus(Paralichthyidae), were exposed to oncomiracidia in an aquarium divided into areas of different brightness (∼1·3, 41·3 and 138·0 lux). The number of parasites on the fish group reared in 138·0 lux was significantly higher than on those reared in the lower brightness levels. Thus, the fish tended to be more vulnerable to infection byN. girellaeunder brighter conditions. Challenge trials using host fish mucus and whole live fish were established to detect the response by oncomiracidia to black-and-white contrast on a whiteversusa black background. Markedly moreN. girellaeoncomiracidia attached to black-painted areas and dark-coloured fish (normal spotted halibut,Verasper variegatus(Pleuronectidae) compared with white-painted areas and light-coloured fish (mal-colouredV. variegatus) on a white-coloured background. On a black-coloured background, moreN. girellaeoncomiracidia tended to attach to white-painted areas and light-coloured fish. Thus, black-and-white contrast is considered important for host finding byN. girellaeoncomiracidia. The simplicity of the positive phototactic behaviour and the response to black-and-white contrast may lead to the development of a simple, practical and inexpensive method to controlN. girellaeoutbreaks.