Quantitative requirements of linolenic and docosahexaenoic acid for juvenile Penaeus monodon

Abstract

The nutrient requirement levels of linolenic (18:3n - 3) and docosahexaenoic (22:6n - 3) acids for juvenile Penaeus monodon were investigated. Shrimp were fed semipurified diets containing different levels of 18:3n - 3 (0.5, 1.0, 1.5, 2.0, 2.5 and 3.0%) and 22:6n - 3 at 0.12, 0.25, 0.5, 1.0, 1.5 and 2.0% in the dry diet. The lipid mixture in the control/conditioning diet contained 5% of equal proportions of 16:0 and 18:1n - 9. In the reference diet, the dietary lipid was refined cod liver oil. Each diet was fed to five replicate groups of 20 shrimp (mean initial weight, 73 ± 12.1 mg). Shrimp were fed the conditioning diet for seven days prior to feeding the treatment diets. Using the broken line analysis, the optimal requirement levels of 18:3n - 3 and 22:6n - 3 were determined to be 2.50% and 1.44%, respectively. The estimated requirements for 18:3n - 3 and 22:6n - 3 based upon other quantitative analysis or other responses were quite similar. Growth was significantly (P 0.05) increases in weight gain. Survival responses were greater with a lower supplementation level of 22:6n - 3 relative to 18:3n - 3. Tissue fatty acid analysis showed that the content of 18:3n - 3 and 22:6n - 3 closely reflected dietary levels, particularly in the midgut gland. The level of incorporation of dietary 18:3n - 3 and 22:6n - 3 into the polar lipids of the midgut gland was shown as a good method to estimate requirement levels. No significant (P < 0.05) changes in 20:5n - 3 and 22:6n - 3 in tail muscle lipids of shrimp fed dietary 18:3n - 3 indicated no apparent bioconversion of 18:3n - 3 to higher n - 3 polyunsaturated fatty acids. However, concomitant increases in tissue content of 20:5n - 3 in shrimp fed increasing dietary levels of 22:6n - 3 suggested a retroconversion of 22:6n - 3 to 20:5n - 3. This confirmed the results of an earlier investigation concerning the relationship between levels of dietary HUFA.