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O-GlcNAc transferase gene (OGT) was considered as the sole rate-limiting enzyme in the O-GlcNAc modification. In the present study, the OGT gene of hybrid grouper (Epinephelus fuscoguttatus ♀ × E. lanceolatus ♂) was cloned and characterized, and its expression in response to dietary carbohydrate level and acute glucose treatment was investigated. The full-length of OGT (GenBank accession no. KY656469) was 4,063 bp, including a 302 bp 5′untranslated terminal region (UTR), a 3,165 bp coding region that encoded 1,054 amino acids residues and a 596 bp 3′ UTR. The highly conservation of OGT gene between fish and mammals was also observed through multiple sequences alignment and phylogenetic analysis. O-GlcNAc transferase gene was ubiquitously expressed in all detected tissues with highest expressions in brain and liver, to a lesser degree, in eye, heart, kidney and intestine. The increasing dietary carbohydrate from 8.02% to 16.08% had no significant effect on the mRNA expression of OGT. However, the expression of OGT was slightly elevated at 6 hr post-glucose injection, and the elevation became significant at 24 hr time-point. These data may enhance our understanding on the nutritional regulation of OGT and O-GlcNAc modification in fish species.

The glucose transporters (GLUTs) play vital role in mediating the glucose uptake process, the movement of glucose across plasma membranes. In this study, three GLUTs, GLUT1, GLUT2 and GLUT4 were cloned and characterized form pearl gentian grouper, a hybrid grouper, and their expressions in response to dietary carbohydrate level (8.02%, 11.89% and 16.08%) were investigated after feeding. The full-length cDNA of GLUT1, GLUT2 and GLUT4 were 2,104, 3,759 and 2,815 bp, respectively, encoding a putative protein of 491, 508 and 505 amino acids respectively. The results of sequence and phylogenetic analysis revealed that grouper GLUTs were highly conserved and clustered together with their corresponding teleost orthologues, rather than mammals. In addition, GLUT1 was ubiquitously expressed in all detected tissues with relative high expressions in heart and brain. GLUT2 is relatively abundant in some certain tissues that release glucose, such as liver and intestine, and GLUT4 was expressed primarily in muscle and eye. The elevated dietary carbohydrate level had no significant difference on the expression of GLUT1 in grouper liver. The expression of GLUT2 in grouper liver was significantly up-regulated with the increasing dietary carbohydrate from 8.02% to 11.89%, and therefore down-regulated significantly. Meanwhile, the expression of GLUT4 in grouper muscle increased significantly with increasing dietary carbohydrate. Results of this study indicate that the up-regulation of GLUTs in fish contribute to maintain glucose equilibrium to some extent when fish were fed with high carbohydrate diets.

Devil stinger is a valuable demersal scorpaenid fish while the rearing of stinger larvae still relies on live prey. This study was conducted to illustrate the development of the main digestive enzymes and digestive system during larval development of this species to provide evidence for the application of artificial feeds. Enzymatic and histological assays were conducted from 1 day post hatching (dph) to 36 dph in larvae. The result showed that the selected digestive enzyme activities increased significantly after 15 dph. Specifically, the total trypsin activities increased significantly from 18 dph to 33 dph. The total pepsin and amylase activities increased significantly first and thereafter decreased significantly. The lipase activities followed the similar pattern with trypsin. With regard to the histological study, the stinger larvae open their mouth to first feeding at 3 dph and turned into totally exogenous nutritional stage at 6 dph. In addition, mucous membrane, rich in goblet cells, was widely distributed in oesophagus epithelium at 18 dph. The height and amounts of gastric gland in cardia and main body of the stomach increased gradually with the development of stinger larvae after 15 dph. The intestine length of stinger larvae was short, and goblet cell was abundant in anterior intestine after 12 dph, not the posterior intestine. The ontogeny of liver and pancreas started from newly hatched stage, and the differentiation of liver was prior to pancreas. The above findings would provide evidence for the use of artificial feeds from the larval stage of stinger larvae (at least from 21 dph).

A 30-day feeding trial was conducted to investigate the effects of dietary lipid levels on growth performance, activities of digestive enzymes, fatty acid composition and some lipogenesis-related gene expression of half-smooth tongue sole (Cynoglossus semilaevis) larvae. Five isoproteic diets were formulated with graded lipid levels (6.68%, 9.84%, 13.47%, 17.89% and 21.88% dry weight) using fish oil as the main lipid source. Each diet was randomly allocated to triplicate groups of 150 larval tongue sole (35 DAH, 54 ± 1 mg). Fish were fed five times daily to apparent satiation during the feeding experiment. Results showed that, the survival rate (SR) of larvae increased significantly firstly, and thereafter decreased significantly. The specific growth rates (SGR) of larvae fed the diet with 13.47% lipid were significantly higher than other treatments. Larvae fed 9.84% or 13.47% dietary lipid showed higher trypsin, lipase, leucine aminopeptidase and alkaline phosphatase (AP) activities than other treatments, whereas amylase activity nearly showed reverse trend with them. The fatty acid composition of the tongue sole larvae was well correlated with dietary fatty acid profile. Expression of acetyl-CoA carboxylase alpha (ACC1) was found to be slightly negatively correlated with dietary lipid level, and high dietary lipid level depressed the expressions of acetyl-CoA carboxylase beta (ACC2) and fatty acid synthase (FAS) mRNA expression significantly, implying that larvae may cope with high dietary lipid mainly through down-regulating lipogenesis-related gene expression of FAS and ACC2. Besides, on the basis of SGR, the optimal dietary lipid level for larval tongue sole was estimated to be 13.56% using second-order polynomial curve.