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Seawater (SW)-acclimated Nile tilapia, Oreochromis niloticus, reared under a salinity 30 environment had lower growth and survival than the freshwater (FW)-acclimated fish. It was hypothesized that cells of the SWacclimated fish had not been able to synthesize an adequate level of a compatible osmolyte, myo-inositol (MI), in adjusting to the salinity 30 environment. In this study, MI supplements, at 250, 500, and 750 mg/kg pellets, were provided to the fish through top-dressing. After the 30-day feeding trial, the following parameters were determined: final body weights; survival; biomass increase; feed conversion ratio (FCR); plasma osmolality and ions; and two transcripts in the gills mips250 and mipa1 encoding enzymes responsible for MI biosynthesis. The SW-acclimated O. niloticus receiving 500-mg MI supplement had significantly higher survival, biomass increase, and lower FCR than those of the SW-acclimated fish receiving no supplement. At 500-mg MI supplemental level, the increasing values of plasma osmolality and Na+ observed in SW-acclimated fish were significantly attenuated. The transcript mipa1, but not mips250, was markedly up-regulated in the SW-acclimated O. niloticus, compared with that of the FW-acclimated fish. Again, MI at 500-mg supplement attenuated the upregulation significantly. This study suggests that MI supplement at the optimum level enhanced the performance of SW-acclimated O. niloticus, and through yet unknown mechanisms, attenuated some of their physiological responses to the osmotic stress. [ABSTRACT FROM AUTHOR]

Seawater (SW)-acclimated Nile tilapia, Oreochromis niloticus, can grow and survive but rarely have offspring under an elevated salinity environment. To find out the cause(s) of this problem, the size and histology of the gonads of the SWacclimated Nile tilapia were determined, which revealed that the gonadosomatic index and histology of the gonads of the SWacclimated fish did not differ from those of the freshwater (FW)-acclimated ones. However, the embryos of the SW-acclimated O. niloticus could tolerate salinity from 0 to 15 ppt, with a survival rate of embryos more than 80%, but 0% under 20-ppt. However, when the embryos were incubated under 10 ppt up to the stage of eye formation, and thereafter the salinity was abruptly raised to 20 ppt, >90% of them survived. The results suggested that SW-acclimated O. niloticus have normal oogenesis and spermatogenesis, and their embryos can survive salinity above 15 ppt if the salinity is raised stepwise. [ABSTRACT FROM AUTHOR]