Effects of low salinity on the composition of amino acids and 

fatty acids from juvenile Scylla paramamosain

Abstract: Salinity is an important abiotic factor affecting the survival, growth and physiology of crustaceans. The physiological response of aquatic animals to salinity involves content change of amino acids and fatty acids in some tissues such as muscle and hepatopancreas. The contents and compositions of stored amino acids and fatty acids in crustaceans can reflect the individual nutrition level, feeding status, health level and ability of physiological and biochemical regulation challenged by environmental changes. The mud crab Scylla paramamosain belongs to the genus Scylla, family Portunidae, order Decapoda, and is one of the most ubiquitous aquatic animals artificially cultivated crustacean species in China. It often distributes in the marshes with slightly less salinity than sea water, such as estuary, intertidal zone and mangrove forest. It is one of the most popular economic crabs in the southeast coast of China for its rapid growth, large adult size and rich nutrition. With the development of economy and the improvement of life quality, the demand for aquatic products increases rapidly. And aquatic products are favored due to their features of low fat and high protein. Due to the gradual lack of marine resources, aquaculture has become an important way of fishery development. At the same time, the desalination of seawater breeding is also one of the important developed aspects in aquaculture, which can reduce the pollution of mariculture in the coastal marine region, promote the development of inland aquaculture, and reduce the cost of breeding. Scylla paramamosain is a preferred species for lowsalinity aquaculture because it is a euryhaline crab with strong osmoregulatory capacity. In order to explore the effects of low salinity culture environment on amino acids and fatty acids in muscle and hepatopancreas of juvenile Scylla paramamosain, the compositions and contents of amino acids and fatty acids in muscle and hepatopancreas were studied after 30 days of culture at three different salinities. The stage II juvenile crab used for the experiment was taken from the breeding base of the laboratory in Fujian. A total of 630 healthy individuals with the same size, strong vitality and complete artus were selected, with the average body mass of (19.53 ± 0.39) mg. Three salinity gradients were set up as salinity 20, 10 and 0. Each salinity group was set up for a total of 210 juveniles, and each 70 juvenile crabs were placed in a breeding tank as a parallel. The breeding experiment time was 30 days. At the end of the breeding experiment, samples were taken after 24 h of abrosia. In order to exclude the influence of the molting cycle on nutrient composition, Scylla paramamosain in the same molting stage were transferred from the water circulation system to the laboratory, dissected, and the muscle and hepatopancreas were taken respectively. Thirty animals were sampled for each salinity group, and 10 animals were taken for each parallel group, and stored in -80 ℃. The contents of total amino acids (TAA), essential amino acids (EAA), nonessential amino acids (NEAA) and delicious amino acids (DAA) were salinity 20>10>0 group (P<0.05) in muscle. In hepatopancreas, the contents of TAA and NEAA were salinity 10>0>20 group (P<0.05). In the low salinity group, the decrease in muscle and the increase in hepatopancreas of NEAA was more than that of EAA obviously. From the composition of amino acids, NEAA ratio was higher than EAA in the muscle in all salinity groups. Comparing the two tissues, the proportion of EAA was higher in hepatopancreas than that in muscle, and the proportion of NEAA and DAA was higher in muscle than that in hepatopancreas. Challenged by low salinity, the NEAA ratio decreased in muscle but increased in hepatopancreas. The contents of fatty acids in muscle was lower than that in hepatopancreas. The contents of total fatty acids (TFA), saturated fatty acids (SFA), monounsaturated fatty acids (MUFA) and polyunsaturated fatty acids (PUFA) was salinity 0>10>20 group (P<0.05) in muscle, and was salinity 20>10>0 group (P<0.05) in hepatopancreas. The highest proportion was SFA in each tissue, followed by PUFA and the lowest was MUFA. Both SFA and PUFA ratios in muscle were higher than that of hepatopancreas, while MUFA ratio was lower than that of hepatopancreas. SFA ratio increased in muscle and decreased in hepatopancreas under low salinity. The proportion of PUFA, n3PUFA and DHA was salinity 10>20>0 group in hepatopancreas, which was the same as the sum of that in muscle and hepatopancreas. According to the sum of amino acids or fatty acids content in muscle and hepatopancreas tissues of each salinity group, TAA content increased by 0.36% and TFA content decreased by 25.61% in salinity 10 group compared with salinity 20 group; TAA content decreased by 8.14% and TFA content decreased by 42.51% in salinity 0 group compared with salinity 20 group. Under the condition of chronic low salinity, the content and composition of amino acids and fatty acids in muscle and hepatopancreas of Scylla paramamosain significantly changed. The amino acids content in muscle decreased, but increased in hepatopancreas. The accumulation of total fatty acids in two tissues of low salinity group (10 and 0) decreased significantly. In two tissues of salinity 10 group, total amino acids contents increased, but total fatty acids content decreased; in two tissues of salinity 0 group, the total amino acids and total fatty acids content decreased. Results showed that amino acids and fatty acids in muscle and hepatopancreas changed significantly under chronic low salinity stress. The content of amino acid in muscle decreased, but increased in hepatopancreas. The content of fatty acids increased a little in muscle, and decreased significantly in hepatopancreas. Based on the sum of the contents of amino acids or fatty acids in two tissues in each salinity group, the total amino acid content, n3PUFA and DHA increased in salinity 10 group, but total fatty acids content decreased. However, challenged by 0 salinity, the total amino acids content, total fatty acids content, n3PUFA and DHA in two tissues decreased. This study provides basic data for the change of nutrient composition of Scylla paramamosain under low salinity culture. Studies on the metabolic mechanism of amino acids and fatty acids should be further explored in order to research their roles in adaptation to environmental salinity changes in the future. According to the change trend of nutrients among three salinity gradients in this experiment, the nutritional physiology and nutritional demand test can be further carried out to improve the survival rate and growth property of Scylla paramamosain under low salinity stress.