Three salinities (4‰, 12‰ and 25‰) were selected to determine the effects of different salinities on the growth, survival, nutrition and energy metabolism of Scylla paramamosain during indoor overwintering. Growth performance (survival rate, condition factor, hepatopancreatic index, weight gain rate and specific growth rate), nutrient composition (ash, moisture, crude fat, crude protein and total nitrogen), fatty acids, energy metabolites (glycogen, glucose, triglycerides, cholesterol and lactic acid) and energy metabolic enzymes (hexokinase, pyruvate kinase, lactate dehydrogenase and succinate dehydrogenase) were measured and calculated. The results showed that S. paramamosain exhibited the best growth and survival at 25‰ and worst at 4‰ after overwintering. In addition to consuming crude fat, excess protein was also broken down to provide energy in the 4‰ and 12‰ groups. The content of unsaturated fatty acids in the 25‰ group was higher after overwintering. Glycogen and glucose consumption and triglycerides, cholesterol and lactic acid production were lowest in the 25‰ group. The activity of hexokinase, pyruvate kinase, lactate dehydrogenase and succinate dehydrogenase was highest in the 4‰ group and lowest in the 25‰ group. The activity of hexokinase, pyruvate kinase, lactate dehydrogenase and succinate dehydrogenase in the hepatopancreas was higher than in the muscle. In conclusion, S. paramamosain uses less energy, has more unsaturated fatty acids and has a higher survival rate at a salinity of 25‰ after overwintering. The results of this study provide helpful information the indoor overwintering S. paramamosain in aquaculture production systems.

Overwintering is an important part of the conservation of Scylla paramamosain, and salinity is a primary parameter affecting this species’ metabolism at this particular time. We compared and analysed gene sequences responsible for the metabolism of S. paramamosain at 4‰ versus 25‰ salinities using transcriptome analysis, in order to find out the optimum salinity for S. paramamosain during overwintering. A total of 5895 genes were detected, of which 105 down‐regulated genes and 131 up‐regulated genes were associated with metabolism (p < 0.05). Among these genes related to metabolic pathways (117 pathways), we identified D‐aspartate oxidase, D‐amino acid oxidase, creatine kinase, sarcosine oxidase, citrate synthase, transferase, glycogen synthase, hexokinase, carbonic anhydrase, glutamine synthetase, V‐type H⁺‐transporting ATPase, 3′‐phosphoadenosine 5′‐phosphosulphate synthase, fatty acid synthase and others. Additionally, the consistency between qRT‐PCR results and transcriptome data reflected the reliability of transcriptome results in the present study. Compared with the 25‰ salinity group, the 4‰ salinity group promoted energy metabolism, amino acid metabolism, lipid metabolism and carbohydrate metabolism by regulating metabolism‐related genes, which made the overwintering S. paramamosain more active in the low‐salt environment, and hepatopancreas consumed more energy to maintain life activities. These results improve our understanding on molecular mechanisms and metabolic pathways associated with salinity changes in S. paramamosain during overwintering. The data presented in this study are also the first to reveal the molecular mechanisms of S. paramamosain during overwintering in response to different salinities at the gene level.

Artificial hybrid breeding can optimize parental traits to cultivate excellent hybrids with enhanced economic value. In this study, we investigated the growth performance and transcriptomes of Gymnocypris przewalskii (♀) and Gymnocypris eckloni (♂) and their F1 hybrid fishes. Hatched individuals of G. przewalskii (GP) and G. eckloni (GE) of the same size and their F1 hybrids (GH) were separately cultured for eight months in three cement tanks (n = 3). The growth indexes were measured, which showed that the growth rate of the groups was GE > GH > GP, while the survival rate was GH > GE > GP. The RNA-Seq data analysis of the muscles from the three Gymnocypris fish strains revealed that gene transcription has a significant impact on F1 hybrid fish and its parents. The differentially expressed genes (DEGs) in GH show less differences with GP, but more with GE. qRT-PCR was used to confirm the expression profiles of the chosen DEGs, and the results showed positive correlations with the RNA-seq data. KEGG enrichment results indicated that the DEGs were related to a variety of molecular functions, such as glycolysis/gluconeogenesis, arachidonic acid formation, citrate cycle, and the MAPK, PI3K-Akt, or mTOR signal pathways. Subsequent analysis indicated that there may be a significant correlation between the differential expression of IGF2 and a difference in the growth of GE and GP.

Overwintering is an important part of the life cycle of Scylla paramamosain. Salinity has a significant effect on the growth and metabolism of overwintering S. paramamosain. 4‰ and 25‰ are typical salinities in different regions where S. paramamosain overwinters. In order to find the optimum salinity for overwintering, the metabolites in overwintering S. paramamosain at salinities of 4‰ and 25‰ were analysed and compared using GC–MS technology. In this study, the weight gain rate of S. paramamosain was 5.51% at 4‰ salinity and 10.69% at 25‰ salinity after overwintering. A total of 1254 valid peaks and 373 metabolites were detected, including 160 significantly different metabolites (16 down‐regulated metabolites and 144 up‐regulated metabolites). In the Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis, 22 pathways were associated with metabolic changes; they have or identified L‐isoleucine, L‐lysine, urea, D‐ribose‐5‐phosphate, glucose‐6‐phosphate, fumaric acid, L‐lactic acid, docosahexaenoic acid and palmitic acid. Compared with the 25‰ salinity group, 4‰ salinity group promoted amino acid metabolism, carbohydrate metabolism, membrane transport and lipid metabolism through these differential metabolites, so that overwintering S. paramamosain needs to participate in more metabolic activities at 4‰ salinity, and hepatopancreas consumes more energy to maintain life activities, which indicated that 4‰ salinity was not conducive to the overwintering of S. paramamosain.