Immune Gene Expression Levels Correlate with the Phenotype of Japanese Flounder Exposed to Heavy Oil

Heavy oil pollution caused by tanker accidents is one of the most important environmental issues in the world. The pollution is related to the impact on marine organisms, since heavy oil consists of toxic substances such as polycyclic aromatic hydrocarbons (PAHs), heavy metals and benzol. However, the effect of heavy oil on the fish immune system is not well understood. In the present study, therefore, we exposed Japanese flounder (Paralichthys olivaceus) to heavy oil and measured the immune response of the fish. For the exposure experiment, six Japanese flounder (mean body weight: 197 g) were acclimated under laboratory conditions at 19–20°C. Three fish were exposed to heavy oil C (bunker C) at a concentration of 3.8 g/L for 3 days, and the others were kept in seawater without heavy oil and used as control. Then, the blood, kidney and skin mucus were collected from the tested fish. Results showed the number of leukocytes in flounder blood increased after heavy oil exposure. The bacterial numbers in skin mucus of heavy oil-exposed fish were significantly higher than that from the control. Gene expression analyses displayed immune system-related genes were mostly down-regulated by heavy oil exposure, while interleukin-8 and lysozyme, involved in proinflammatory processes, were up-regulated. Considering the data on phenotypic effects and gene expression profiling, pathogen resistance in flounder may be compromised by heavy oil exposure, which can cause bacterial infection and subsequent induction of proinflammatory genes as a defensive response against the infection.