The tongue sole, Cynoglossus semilaevis is euryhaline in nature and tolerates wide range of salinity. Salinity is one of the most important abiotic factors affecting the eco-physiological responses of aquatic organisms and has complex and wide-ranging biological effects. Many studies have been focused on the effect of constant salinity on aquatic organisms, and the idea that aquatic organisms grow better under constant salinity rather than fluctuating salinity is generally accepted and therefore most of the aquatic organisms are cultured in constant salinity water in aquaculture. However, the attempt was made to determine the optimum fluctuating salinity levels for commercial culture of tongue sole in intensive systems where the salinity can be altered and to know the effects of salinity fluctuations in the various mechanism viz, growth, biochemical composition, bioenergetics, osmoregulation, energy metabolism, digestive enzyme activities and non-specific immune responses.Effects of the amplitude (±2,±4,±6and±8ppt) and frequency (2,4and8days) of salinity fluctuations on the growth, body composition and energy budget in the juvenile tongue sole (Cynoglossus semilaevis) was investigated over64days. The results showed that the frequency and amplitude of the salinity fluctuations significantly affected the specific growth rates of the juvenile tongue sole (P<0.01) and there was significant interaction between the frequency and amplitude of salinity as well (P<0.05). The tongue sole exhibited higher specific growth rate at the amplitude of4-6ppt with the frequency of4and8days than other treatments and the control (P<0.05). Food consumption, food conversion efficiency and apparent digestion in terms of energy increased significantly at salinity regimes favored the growth of fish (P<0.05). The growth rate of juvenile tongue sole was a quadratic function of the salinity amplitude at various frequencies. Such a growth model may be described by G=β0+β1(SA)+β2(SA)2, where G represents the specific growth rate on64-day basis, SA is salinity amplitude in ppt, β0is the intercept on the G axis, β1and β2are the regression coefficients, respectively. The optimal salinity amplitude to the growth of fish at different salinity fluctuations of2,4and8days was estimated to be±3.54,±4.89and±4.74ppt respectively.In second part of study, salinity fluctuations had a strong influence on various energy components, namely energy deposited for growth, energy lost for respiration, energy lost in feces and energy lost in excretion (P<0.01). The average energy budget for juvenile tongue sole at moderate salinity fluctuations was100C=30.92G+10.30F+6.77U+52.01R. The juvenile tongue sole at fluctuating salinities favored the growth of fish partitioned more energy for growth and less for metabolism than those at constant salinity and other fluctuating salinities (P<0.05). Energetic advantage at moderate fluctuating salinities, i.e., more food consumption, better food conversion efficiency, higher apparent digestion rate, as well as reduced metabolism expenditure and more energy partitioned into growth might account for the enhancement in growth of tongue sole in the present study. Results in this study suggested that commercial farmers could rear juvenile tongue sole with moderate salinity fluctuations to achieve better growth performance in this species.The study was conducted to investigate the osmoregulatory responses in juvenile tongue sole over the influence of the amplitude (±2,±4,±6and±8ppt) and frequency (2,4and8days) of salinity fluctuations and to generate a comprehensive, multi-tissue perspective of the effects of salinity fluctuations on energy metabolism. Plasma glucose, plasma protein and plasma osmolality were significantly influenced and higher values were recorded at the amplitude of4-6ppt with the frequency of4and8days than other treatments and the control (P<0.05). Plasma ions concentration showed variation and were significantly affected by fluctuations in salinity as Na+,Ca2+and Mg2+showed similar trend to our growth studies, while K+ions concentration showed the reverse trend (P<0.05). Gills and kidney Na+-K+-ATPase activities were found to be significantly affected by the effect of salinity fluctuations (P<0.05). Gill and kidney Na+-K+-ATPase activities were found to be lower at moderate fluctuating salinities compare with the control and other treatments, which means, loss of energy in osmoregulation in gills and kidney will results in generating lower amount of energy expenditure and it subsequently resulting in lower enzymatic activities in particular treatment. Enzyme activities in carbohydrate as well as amino acid metabolism were found to be significantly influenced by fluctuations in salinities (P<0.05). Mostly, this activities were found to be increased with decreased in frequency and amplitude of salinity (up to±6ppt fluctuation levels) while dropped at higher amplitude level (±8ppt). The interactions were different among the tissues and parameters displayed more or less similar patterns.In the third part of the study, the digestive enzyme activities in juvenile tongue-sole, C. semilaevis were fully discussed. Experiments was conducted to study the effects of fluctuating salinities on the mechanism of environment on the activities of Trypsin, amylase, lipase, acid phosphate and alkaline phosphate activities in intestine and liver. Results showed that, digestive enzyme activity was significantly affected by the amplitude and frequency of fluctuating salinities and showed the same trend as growth (P<0.05). The activities of trypsin were found to be higher in intestine compare to the liver (P<0.05), whereas the activities of amylase, lipase, acid phosphate and alkaline phosphate were found to be higher in liver compare to intestine (P<0.05). In conclusion, all digestive enzyme activities in intestine and liver were recorded higher in D4S30±6, D8S30±6and D8S30±6than control and other fluctuating salinities (P<0.05). The results were similar to our growth data, which suggests that tongue sole reared at the moderate fluctuating salinities have better digestive enzymes activities than constant salinities.To understand the tolerance of tongue sole, C. semilaevis to varying salinities, the influence of the amplitude (±2,±4,±6and±8ppt) and frequency (2,4and8days) of salinity fluctuations on the activities of antioxidant enzymes was investigated in this study. The activities of immunoenzymes including acidic phosphatase (ACP), alkaline phosphatase (AKP) from hydrolytic system, and catalase (CAT), superoxide dismutase (SOD), malonyl-dialdehyde (MDA) and total antioxidant capacity (T-AOC) from antioxidant system in liver, muscle, gills and kidney were measured to evaluate the physiological responses of juvenile tongue sole to salinity fluctuations. The results showed that, the immune responses of C.semilaevis were highly tissue-specific during the varying salinity fluctuations. In all tissues, ACP and AKP activity was found to be highest at moderate salinity fluctuations than compare to control and other fluctuating treatments (P<0.05). The renal ACP as well as AKP activities was found to be higher compare to other tissues (P<0.05). SOD activities had significant effect of salinity fluctuations in all tissues, except hepatic tissue. The renal SOD activities were found to be higher than other tissues (P<0.05). The variation in branchial SOD activities proves that the salinity fluctuations had great impact on tongue sole at moderate as well as high fluctuating salinities compare to control and low fluctuating salinities, whereas the branchial CAT activities showed the reverse trend. The renal CAT activities showed non-significant differences (P>0.05) and the hepatic CAT activity showed same trend as SOD activities. This explains that the salinity fluctuation had the prominent role in the changes of SOD and CAT activity, in view of cellular antioxidants. The T-AOC and MDA activities had significant effect on all the tissues studied (P<0.05). To sum up, the results acquired from study indicate that the salinity fluctuations could effectively stimulate and enhance the antioxidant enzyme activity in the liver, kidney, gills and muscle of the juvenile tongue sole, thus effectively eliminating the excessive reactive oxygen species (ROS) and minimizing the body damage. Characterized by certain sequentiality and tissue specificity, the activation of the antioxidant enzymes could also be inhibited when the salinity fluctuates beyond the tolerance range of the body.In precise, the present study confirms the capacity of juvenile tongue sole to tolerate the changes due to salinity fluctuations and had significantly influence well over a wide range of fluctuating salinities.