Effect of sex ratio on the immune system of Oreochromis mossambicus (Peters).

The social environment of fish has a crucial role to play on the immune system and hence on the overall health status. Stressors of social origin such as dominance, subordination, and fight for mate have a major impact on the immune system of fish. The present study was designed with the objective of finding the effect of sex ratio of the population on the immune system of Oreochromis mossambicus. Groups of fish were maintained for 28 days in three different sex ratios i.e., (i) all-male (ii) all-female (iii) equal male and female (mixed). The specific immune response of fish was assessed by antibody response to Aeromonas hydrophila by ELISA and bacterial agglutination assay, and to SRBC by plaque forming cell assay. Nonspecific immune mechanisms were assessed in terms of serum lysozyme activity, production of intracellular reactive oxygen species (ROS) and reactive nitrogen species (RNS) by peripheral blood leukocytes. Disease resistance against live, virulent A. hydrophila was performed to assess the overall functional immunity. The results showed that antibody responses and numbers of antibody producing cells were increased in fish in the equal male and female sex ratio group compared to fish in monosex ratio groups. Similar enhancement was also observed in nonspecific serum lysozyme level and the ROS and RNS production. The host resistance test revealed that enhanced immunity in equal male and female sex ratio group was protective against A. hydrophila infection. The study clearly reveals positive and negative effect of sex ratio on the immune system of O. mossambicus.