Fatty acid metabolism in the freshwater shrimp, Macrobrachium rosenbergii

Abstract

Freshwater shrimp, Macrobrachium rosenbergii, were fed experimental diets containing no fat, purified fatty acids (linoleate, linolenate, palmitate) and lipids of plant and animal origin. Gas chromatographic analysis of lipid extracts from the hepatopancreas, cephalothorax and abdominal tissues of animals treated for 35, 65 and 100 days revealed 23 identifiable fatty acids. Major components of the fatty acid profile were 14:0, 16:0, 18:0, 16:1n-7, 18:1n-9, 16:2n-4, 18:2n-6, 18:3n-3, 20:4n-6, 20:5n-3 and 22:6n-3. Minor components (those with individual concentrations <1% of the total fatty acid fraction) consisted of 15:0, 20:0, 20:ln-9, 22:ln-11, 16:4n-3, 18:4n-3, 20:2n-6, 20:3n-6, 20:4n-3, 22:4n-6, 22:5n-6 and 22:5n-3. All tissues contained an abundance of 16 and 18-carbon fatty acids, regardless of dietary lipid composition or length of treatment. The principal fatty acids of M. rosenbergii (in decreasing concentration) were (i) saturates 16:0 and 18:0, (ii) monounsaturates 18:1n-9 and 16:1n-7 and (iii) polyunsaturates 20:5n-3, 20:4n-6 and 22:6n-3. The saturated and monounsaturated fatty acids 14:0, 16:0, 18:0, 16:ln-7, 18;ln-9 and their n-7 and n-9 derivatives were synthesized from dietary carbohydrate as needed and comprised a relatively constant fraction (30-50%) of total tissue fatty acids, regardless of diet composition. Long chain polyunsaturates were concentrated in the abdominal tissue (perhaps in association with cell membranes) and were conserved during periods of lipid mobilization. Long chain polyunsaturates of the linoleic (n-6) and linolenic (n-3) fatty acid series may be more important growth promoters in freshwater shrimp than 18:2n-6 or 18:3n-3. Freshwater shrimp were capable of elongating linoleate (18:2n-6) to 20:2n-6, and evidence was found suggesting the desaturation of 20:2n-6 to 20:3n-6. There was no indication that M. rosenbergii was capable of synthesizing linoleate (18:2n-6) or linolenate (18:3n-3), and no evidence that linolenate was elongated or desaturated in vivo. A growth depression occurred in freshwater shrimp fed a high concentration (1%) of linolenate (18:3n-3) as the only source of dietary lipid. Linoleic acid (18:2n-6) did not result in depressed growth when fed under the same conditions. Effects of diet on n-6:n-3 fatty acid ratio in the tissues of M. rosenbergii suggest that competitive inhibition among members of the linoleic and linolenic fatty acid families may operate in this species.