Polyunsaturated fatty acid nutrition in juvenile freshwater prawn Macrobrachium rosenbergii

Abstract

Weight gain and survival responses and analyses of fatty acid composition of tissue were used to investigate the nutritional value of C18 and C ≥ 20 polyunsaturated fatty acids (PUFA) provided to juvenile Macrobrachium rosenbergii in a purified diet. Treatments consisted of isonitrogenous and isoenergetic diets that contained graded levels of pure triglycerides of 18 : 3 n−3 (trilinolenin), 18 : 2 n−6 (trilinolein), 22 : 6 n−3 (tridocosahexaenoin) and 20 : 4 n−6 (triarachidonin), alone or in combination, or pure methyl esters of C≥20n−3. All diets were formulated to contain a total of 6% lipid. A diet containing only saturated (14 : 0, 16 : 0, 18 : 0) and monounsaturated (16 : 1 n−7, 18 : 1 n−9) fatty acids as lipid sources, in proportions similar to that found in a 2 : 1 cod liver oil/corn oil mixture, served as a conditioning and control diet. Qualitative and quantitative fatty acid composition of freshwater prawn tissue generally reflected that of experimental diets. Levels of some monounsaturated and saturated fatty acids appeared to change relative to additions of PUFA. Levels of 18 : 3 n−3, 20 : 5 n−3 and 20 : 4 n−6 were conserved in the polar lipid of the whole body tissue when they were not provided in the diet. Levels of n−3 PUFAs decreased in the neutral lipid of the whole body when not provided in the diet whereas levels of n−6 PUFAs remained unchanged or increased. After feeding trials of 135 and 150 days duration, a definite qualitative requirement for 18 : 3 n−3 and/or 18 : 2 n−6 could not be determined by evaluation of weight gain response. No significant differences in weight gain of prawns fed diets containing 18 : 3 n−3 or 18 : 2 n−6 PUFA at comparable levels were apparent. There was no evidence of de novo synthesis of 18 : 3 n−3 or 18 : 2 n−6. A dietary source of C≥20n−3 or n−6 fatty acids is required by juvenile prawns as indicated by significant increases in weight gain of juvenile prawns. At the termination of the feeding trials, mean weights of prawns fed diets containing C≥20n−3 or n−6 fatty acids were 30.3 to 95.3% greater than the mean weight of prawns of the control group. Most of the juvenile groups fed the C ≥ 20 PUFA containing diets had significantly greater weight gains than those fed the 18 : 3 n−3 and/or 18 : 2 n−6 diets. Diets containing either 22 : 6 n−3 or 20 : 4 n−6 HUFAs at levels ranging from 0.075% to 0.60% were equally effective. Juvenile M. rosenbergii have an extremely limited ability to convert C18 to C ≥ 20 fatty acids in the n−3 (linolenic) and n−6 (linoleic) families. There was evidence of the synthesis of 20 : 5 n−3 from 22 : 6 n−3. Examination of the combined percentage of C ≥ 20 PUFAs of the polar lipids from whole body tissue may assist in evaluating nutritional status relative to these essential fatty acids. Results suggest that n−6 and n−3 PUFA have different metabolic functions and that some distinct differences in the polyunsaturated fatty acid nutrition exist between the freshwater prawn and marine shrimp species.