Comparative Study on Growth Index and Nutritional Quality of Female Chinese Mitten Crab Eriocheir sinensis Selected at Different Growth Periods in Rice-Crab Culture Systems

Research was conducted on the growth performance and nutritional quality of Chinese mitten crabs (Eriocheir sinensis) during a 62-day growing period in a symbiotic coculture comprising rice and crab. Culture experiments were conducted in three rice fields of equal size (996 m(2)). On days 0 (July 15, D0), 15 (July 30, D15), 31 (August 15, D31), 46 (August 30, D46), and 62 (September 2, D62), tissue samples of 50 female E. sinensis were collected randomly from each rice field. The results showed that the serum growth hormone (GH) content and muscle ecdysone receptor (EcR) mRNA expression levels were higher in the D31 and D46 groups; the content of serum 20-hydroxyecdysone (20-HE) and the mRNA expression levels of retinoid X receptor (RXR), insulin-like growth factor 2 (IGF2), and chitinase (CHI) reached the maximum in the D31 group. Muscle crude protein content gradually increased; hepatopancreas crude protein and crude lipid content began to decrease after reaching the maximum value in the D0 and D15 groups, respectively; the contents of crude protein and crude lipid in the ovary significantly increased in the D46 and D62 groups (P < 0.05). The content of muscle essential amino acids (EAA) reached the maximum in the D46 group; the hepatopancreas EAA content began to decrease significantly in the D31 group (P < 0.05); and the EAA content of the ovary decreased significantly after reaching the maximum value in the D46 group (P < 0.05). The muscle contents of eicosapentaenoic acid (EPA), docosapentaenoic acid (DPA), docosahexaenoic acid (DHA), polyunsaturated fatty acids (PUFA), and n-3 polyunsaturated fatty acids (n-3PUFA) and the ratio of n-3 polyunsaturated fatty acids/n-6 polyunsaturated fatty acids (n3/n6) decreased significantly in the D31 group (P < 0.05); the hepatopancreas contents of EPA, PUFA, n-3PUFA, and n-6 polyunsaturated fatty acids (n-6PUFA) and the ratio of n3/n6 began to decrease after reaching the maximum value in the D31 group, ethyl behenate (21:0), tetracosanoic acid (24:0), DPA, and DHA contents were detected for the first time in the D31 group; the ovary PUFA, n-3PUFA contents, and n3/n6 ratio of the D46 and D62 groups were significantly lower than those of the D31 group (P < 0.05). During the experimental conditions described here, female E. sinensis raised in rice fields reached rapid growth from August 15 to August 30. Additionally, the nutritional quality of the female E. sinensis edible tissues (muscle, hepatopancreas, and ovary) began to decline after August 15, when sufficient nutrients such as protein, lipid, EAA, and PUFA should be provided to the female E. sinensis.