Myo-inositol supplement helps the performance of seawater Nile tilapia, Oreochromis niloticus

Abstract

A population of seawater (SW)-acclimated Nile tilapia, Oreochromis niloticus, have been generated for six generations since 2017 and maintained under 30-ppt salinity. This fish population, however, has lower growth and survival than O. niloticus reared under freshwater (FW); this phenomenon was likely due to the osmotic stress of the animals. Two attempts were tried to improve the growth and survival of SW-acclimated O. niloticus were providing feed supplemented with lipid or myo-inositol (MI), a natural compatible osmolyte. The first attempt was based on a hypothesis that increasing lipid intake would provide more energy to deal with the osmotic stress, while the second hypothesis was that MI might support the cellular metabolism of the fish and allow the fish cells to create appropriate mechanisms against the osmotic stress. In the first trial, salmon, soybean, and palm oil were tired for one month and all failed to improve the growth and survival of the SW-acclimated fish. In the second trial, MI supplements at 250, 500, and 750 mg/kg pellets were provided to SW-acclimated O. niloticus for one month. At the end of the experiment, the fish supplemented with 500 mg MI showed significantly higher survival and biomass increase than those of the SW-acclimated fish, with significantly less FCR. At 500-mg MI supplement, the rise in plasma osmolality and Na+ observed in the SW-acclimated fish were significantly attenuated. Plasma Cl- value was decreased in all the SW-acclimated fish and further suppressed by 500-mg MI supplement. Plasma K+ value was decreased in the SW-acclimated fish but restored to normal values by MI supplement. The transcript MIPS250 of the FW- and SW-acclimated fish was comparable, but that of the SW-acclimated fish receiving MI supplement was 1.7x to 4.1x fold up-regulated, compared with the FW-acclimated fish. In contrast, the transcript MIPA1 of the SW-acclimated fish was 323x fold higher than that of the FW-acclimated one. The up-regulation was maintained by MI supplement at 250 and 750 mg; however, at 500-mg supplement, this up-regulation was attenuated significantly. The study suggests that exogenous MI at optimum dose helped to maintain cellular metabolism of the SW-acclimated O. niloticus and allow the fish cells to respond more efficiently to the osmotic stress.