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Cipangopaludina cathayensis is a snail species unique to China. Domestic research on C. cathayensis has mainly focused on aquaculture technology, especially the paddy field breeding method, where water quality plays a critical role in the cultivation of C. cathayensis. Dissolved oxygen is one of the most important factors in the aquatic environment because it impacts a series of biological activities such as the growth and development, metabolism, reproduction, and breeding of aquatic animals. When the dissolution of oxygen in water is less than 2 mg/L, the water is in a low oxygen or anoxic state. Hypoxia can slow the growth and development of aquatic animals, reduce their disease resistance and reproductive ability, and, in serious cases, can lead to their death. In recent years, research on C. cathayensis has mainly focused on the anti-tumor mechanism, immune response, nutritional value evaluation, and antibiotic resistance. However, there is currently no report on the regulation of response to hypoxia in C. cathayensis either domestically or internationally. The purpose of this study was to explore the differential expression of genes in the liver of C. cathayensis under hypoxic stress.In this study, healthy C. cathayensis without mechanical damage were cultured in a hypoxia stress group (2.5 mg/L) and a normoxia (control) group (6.9 mg/L), with 90 C. cathayensis in each group and 3 replicates. For the low oxygen stress treatment, dissolved oxygen was decreased from 6.9 mg/L to 2.5 mg/L within 1 h and maintained for 24 h. The liver tissue was taken as the experimental material in both the hypoxia stress group and normoxic group. The total RNA was extracted and an mRNA library was constructed. The liver tissue samples of C. cathayensis from both groups were sequenced and analyzed using an Illumina HiSeq-2500 technology platform, and unigenes were compared and annotated in GO, KOG, Nr, and KEGG databases. The differentially expressed genes were analyzed using DESeq. Bioinformatics analysis was performed on the function of GO and KEGG of differentially expressed genes, and the key differentially expressed genes were further validated by qPCR.Transcriptome analysis results showed that 500 584 transcripts were assembled from the original data and 23 379 unigenes were obtained by sequencing, with an average length of 686.65 bp and N50 of 1 127 bp. Among the unigenes, 26 636 were found to be homologous to genes in the Nr protein database. Additionally, 22 907 unigenes were annotated in the GO database, 13 290 in the KOG database, and at least 4 179 in the KEGG database. Compared with the control group, 176 differentially expressed genes were screened in the hypoxia stress group, among which 64 and 112 were up- and down-regulated, respectively. Further, GO functional enrichment analysis found that the differential genes were mainly enriched in chitin metabolic and glucosamine-containing compound metabolic processes in the biological process. Differential genes were also enriched in collagen trimer in the cellular component and chitin binding in the molecular function. The enrichment analysis results of the KEGG pathway mainly focused on four pathway categories, namely environmental information processing, genetic information processing, metabolism, and organismal systems. Finally, the qPCR results of six key differentially expressed genes were obtained by RT-qPCR. Among the up-regulated genes under hypoxic stress were the heat shock proteins 70B2 and beta-6, and the down-regulated genes were chitinase-like protein 4, collagen alpha-1 chain (XIV), collagen alpha-4 chain (XIV), and phosphatase-related protein type 5, which confirmed the reliability of the transcriptome sequencing results.Studies have shown that, through transcriptome sequencing, the expression information of relevant functional genes in C. cathayensis liver tissues under hypoxic stress can be obtained. Among them, the expression of heat shock protein genes were up-regulated, indicating that hypoxic stress activated the physiological activities of C. cathayensis to adapt to hypoxia and protected the body from hypoxic damage. In addition, the down-regulated expression of related genes in the metabolic pathway indicates that the growth of C. cathayensis is affected under hypoxic environments. In conclusion, the results of this study provide basic data and a theoretical basis for the in-depth study of the regulatory mechanism of C. cathayensis in response to hypoxic stress.

Procambarus clarkii Girard is an economically important cultured shrimp in China, and lipids are an important component of its raw feed materials. Studies have shown that adequate lipid content in feed can promote the health of aquatic animals, improve the utilization efficiency of feed protein, save protein, reduce feed costs, and reduce nitrogen emissions. Insufficient lipid content in feed leads to metabolic disorders in breeding animals, reduces the utilization rate of feed protein, and is accompanied by a deficiency of fat-soluble vitamins and essential fatty acids. However, a high fat content inhibits feeding and growth and leads to excessive fat deposition in the fish body, which decreases disease resistance and is not conducive to feed processing and storage. At present, studies on the dietary lipid content of P. clarkii have mainly focused on growth performance, biochemical indices, and conventional nutritional components of muscle; however, studies on muscle quality, amino acid composition, and content have not yet been reported. This study investigated the growth performance, muscle texture indices, nutritional components, and food flavor of P. clarkii by determining the effect of dietary lipid content on the growth performance and muscle quality of juveniles. The aim of this study was to provide a reference for the development of compound feed for juvenile P. clarkii and to provide a theoretical and scientific basis for rationally evaluating the effects of fat on the muscle quality of the species.Five experimental diets with lipid levels of 2.86%, 5.11%, 7.67%, 10.19%, and 13.02% were prepared using fishmeal, soybean meal, and rapeseed meal as the main protein sources, and a mixture of fish oil and soybean oil in a ratio of 1:1 as the lipid source. A total of 600 juvenile P. clarkii with an initial body weight of (4.00±1.00) g was randomly divided into five groups (L1, L2, L3, L4, and L5 groups) with three replicates per group and 40 shrimps per replicate. They were fed five groups of experimental diets at 6:00 and 19:30 each day and cultured for 60 days. The experiment was carried out in an aquaculture pond (square, 1.0 m × 1.0 m × 0.6 m) at the Guangxi University.The results showed that as the dietary lipid level increased, the specific growth rate (SGR) and survival rate (SR) of juvenile P. clarkii first increased and then decreased and were the highest when the dietary lipid level was 7.67%. The feed conversion ratio (FCR) at first decreased before increasing again and that of the L3 group was significantly lower than that of the other groups (P < 0.05). Using the specific growth rate as an evaluation index, through broken-line analysis, the optimal dietary lipid level of juvenile P. clarkii was estimated as 6.82%. Dietary lipid levels had no significant effect on the moisture, crude protein, and ash contents in the muscle (P > 0.05). With the increase in dietary lipid levels, the cooking loss (CL) and crude lipid, polyunsaturated fatty acids (∑PUFA), and total amino acid (∑TAA) contents of the muscle showed an increasing trend. The content of eicosapentaenoic acid (EPA) in the L5 group was significantly higher than that in the other groups, and the docosahexaenoic acid (DHA) content in the L5 group was significantly higher than that in the L1 and L2 groups (P < 0.05). With increasing dietary lipid levels, muscle hardness, elasticity, cohesiveness, and gumminess decreased and there were no significant differences between the L1 and L3 groups (P > 0.05), whereas dietary lipid levels showed no significant difference in muscle saturated fatty acid (SFA) and monounsaturated fatty acid (MUFA) contents (P > 0.05). The contents of umami amino acids (∑FAA), alanine (Ala), and aspartate (Asp) in the L5 group were significantly lower than those in the L1 group (P < 0.05).Under the conditions of this study, within a suitable range of fat content, growth performance increased with increased fat content; however, the addition of excessive fat cannot promote growth. According to the broken-line regression analysis of the specific growth rate, the optimal dietary lipid level was 6.82%. When the dietary lipid level was higher than 10.19%, the deposition and efficiency of muscle fat, muscle fat content, and nutrient loss increased, and the muscle texture index and Asp and Ala contents significantly decreased. The nutritional value, taste, delicious degree, and flavor of the muscle of P. clarkii were significantly decreased. As it does not affect the muscle quality of the shrimp, the dietary lipid content should not exceed 10.19%.Based on the analysis of various factors, the optimal dietary lipid content is beneficial to the healthy growth of juvenile P. clarkii and improves nutritional value and muscle quality. It is suggested that the dietary lipid supplemental level of juvenile shrimp should be 6.82%~10.19%. This study provides a theoretical basis for the scientific optimization of feed preparation for P. clarkii juvenile.