POLYUNSATURATED FATTY-ACID NUTRITION IN JUVENILE FRESH-WATER PRAWN MACROBRACHIUM-ROSENBERGII

Weight gain and survival responses and analyses of fatty acid composit ion of tissue were used to investigate the nutritional value of C18 an d C greater-than-or-equal-to 20 polyunsaturated fatty acids (PUFA) pro vided to juvenile Macrobrachium rosenbergii in a purified diet. Treatm ents consisted of isonitrogenous and isoenergetic diets that contained graded levels of pure triglycerides of 18: 3n-3 (trilinolenin), 18:2n -6 (trilinolein), 22:6n-3 (tridocosahexaenoin) and 20:4n-6 (triarachid onin), alone or in combination, or pure methyl esters of C greater-tha n-or-equal-to 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 mono unsaturated ( 16:1n-7, 18: 1n-9) fatty acids as lipid sources, in prop ortions similar to that found in a 2:1 cod liver oil/corn oil mixture, served as a conditioning and control diet. Qualitative and quantitati ve fatty acid composition of freshwater prawn tissue generally reflect ed that of experimental diets. Levels of some monounsaturated and satu rated fatty acids appeared to change relative to additions of PUFA. Le vels of 18:3n-3, 20:5n-3 and 20:4n-6 were conserved in the polar lipid of the whole body tissue when they were not provided in the diet. Lev els 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 unchang ed or increased. After feeding trials of 135 and 150 days duration, a definite qualitative requirement for 18:3n-3 and/or 18:2n-6 could not be determined by evaluation of weight gain response. No significant di fferences in weight gain of prawns fed diets containing 18:3n-3 or 18: 2n-6 PUFA at comparable levels were apparent. There was no evidence of de novo synthesis of 18: 3n-3 or 18: 2n-6. A dietary source of C grea ter-than-or-equal-to 20n-3 or n-6 fatty acids is required by juvenile prawns as indicated by significant increases in weight gain of juvenil e prawns. At the termination of the feeding trials, mean weights of pr awns fed diets containing C greater-than-or-equal-to 20n-3 or n-6 fatt y acids were 30.3 to 95.3% greater than the mean weight of prawns of t he control group. Most of the juvenile groups fed the C greater-than-o r-equal-to 20 PUFA containing diets had significantly greater weight p ins than those fed the 18:3n-3 and/or 18:2n-6 diets. Diets containing either 22:6n-3 or 20:4n-6 HUFAs at levels ranging from 0.075% to 0.60% were equally effective. Juvenile M. rosenbergii have an extremely lim ited ability to convert C18 to C greater-than-or-equal-to 20 fatty aci ds in the n-3 (linolenic) and n-6 (linoleic) families. There was evide nce of the synthesis of 20:5n-3 from 22:6n-3. Examination of the combi ned percentage of C greater-than-or-equal-to 20 PUFAs of the polar lip ids 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 di fferences in the polyunsaturated fatty acid nutrition exist between th e freshwater prawn and marine shrimp species.