Docosahexaenoic acid (DHA), a long-chain polyunsaturated fatty acid, plays a critical role in animal growth, inflammatory regulation, lipid metabolism, and neurological functions. However, the optimal dietary requirement of DHA for tiger puffer remains unknown. This study systematically investigated the effects of different dietary DHA levels on the growth performance, body composition, hematological parameters and tissue physiology of tiger puffer (average initial body weight 17.78 ± 1.92 g). Six experimental diets with graded DHA concentrations (0.09%, 0.57%, 1.35%, 1.61%, 2.28%, and 3.08% dry matter) were formulated. The feeding experiment was carried out in a seawater flow-through system for eight weeks, with each diet assigned to three replicate tanks. Based on the regression analysis of weight gain and specific growth rate, the maximum values were observed at the dietary DHA level of 1.75% and 1.88%, respectively. Appropriate DHA levels also significantly improved the muscle protein synthesis and lipid metabolism, and strengthened the intestinal morphology. Furthermore, a threshold for efficient DHA deposition in muscle was identified, beyond which excess DHA (3.08%) may be β-oxidized and therefore largely wasted. In conclusion, the optimal dietary DHA level for juvenile tiger puffer should be within the range of 1.75–1.88%.

The n-3 long-chain polyunsaturated fatty acids (n-3 LC-PUFAs) have a key role in maintaining fish growth and health. However, fish oil (FO), the main source of n-3 LC-PUFAs, is in relative shortage due to the rapid development of the aquaculture industry. In this study, we investigated the efficacy of replacing fish oil with mixtures of algal oil (AO) from Schizochytrium sp. and terrestrially sourced oils (animal oil poultry oil (PO) or vegetable oil rapeseed oil (RO)) in the diets of juvenile tiger puffer (average initial body weight 23.8 ± 1.51 g). An 8-week feeding trial was conducted using three experimental diets: a control diet containing 6% added FO (control FO-C) and two diets with 3% AO + 3% PO or RO (groups AO+PO and AO+RO, respectively), replacing FO. Each diet was fed to triplicate tanks with 25 fish in each tank. The weight gain, feed conversion ratio, body composition, and serum biochemical parameters were not significantly different among the three groups, except that the AO+PO group had a significantly lower muscle lipid content than the other two groups. The AO-added diets significantly increased the DHA content in whole fish, muscle, and liver samples but significantly reduced the EPA content. The oil mixture treatments significantly increased the contents of monounsaturated fatty acid (MUFA) but significantly decreased the contents of saturated fatty acids (SFAs) in the liver and whole fish samples. However, the MUFA and SFA contents in the muscle samples were not significantly different among the dietary groups. The diets with oil mixtures did not affect the hepatic histology but tended to result in the atrophy of intestinal villi. The treatment diets downregulated the hepatic gene expression of proinflammatory cytokines (il-1β and tnf-α) and the fibrosis marker gene, acta2. However, the AO+PO diet inhibited the intestinal gene expression of the tight junction protein, claudin 18. In the muscle, the treatment diets upregulated the expression of genes related to cell differentiation and apoptosis (myod, myog, myf6, myf5, bcl-2, and bax). In conclusion, Schizochytrium sp. oil in combination with terrestrial oils (poultry oil or rapeseed oil) can be an effective alternative to fish oil in the diets of tiger puffer, but the mixing strategy may be better modified in consideration of intestinal health.

Bile acids are important for lipid digestion and metabolic regulation, but the roles of individual bile acids in crustaceans remain unclear. This study evaluated the effects of six dietary monomeric bile acids on growth and lipid metabolism in juvenile Pacific white shrimp. Juvenile shrimp (2.5 g) were fed a basal diet or the same diet supplemented with 0.04% bile acid (cholic acid, hyodeoxycholic acid, chenodeoxycholic acid, deoxycholic acid, ursodeoxycholic acid, or hyocholic acid) for 8 weeks. Each treatment was assigned to three replicate 100-L tanks, with 30 shrimp per tank. Dietary monomeric bile acids did not significantly affect growth performance, body composition, or muscle fatty acid composition. Compared with the control group, chenodeoxycholic acid, deoxycholic acid, and hyocholic acid significantly reduced the hemolymph triglyceride levels, while cholic acid, chenodeoxycholic acid, and hyocholic acid lowered the hepatopancreatic lipid levels. All bile acid treatments reduced the hepatopancreatic malondialdehyde content compared with the control group. Expression of genes related to bile acid transport, sterol metabolism, and lipid catabolism was generally upregulated by bile acids, indicating enhanced bile acid circulation and lipid turnover. In conclusion, monomeric bile acids mainly regulate lipid metabolism and oxidative status rather than directly promoting growth under the present dietary condition, with chenodeoxycholic acid, cholic acid, and deoxycholic acid showing relatively stronger effects.