Comparison of triacylglycerols, esterified and free fatty acids as neutral lipid sources in the diet of the prawn Penaeus monodon

Abstract

Peńaeus monodon were fed purified diets in which the sole neutral lipid source had a defined fatty acid composition supplied as either triacylglycerols (TAG), free fatty acids (FFA), ethyl esters (EE) or methyl esters (ME). Nutrient intake across treatments was kept constant through sub-satiation feeding. After 50 days, the percent weight gain of prawns fed the TAG diet (94 ± 6%) was markedly higher than those fed the FFA diet (84 ± 7%) and significantly higher ( P < 0.05) than those fed the ME (73 ± 7%) or EE (54 ± 5%) diets. Poor growth of prawns fed the EE and ME diets was attributed to the inability of prawns to either digest and/or metabolise the ethyl and methyl esters effectively. The apparent digestibility of neutral lipid in the EE diet was significantly lower (90.9 ± 0.9%) than in the other diets (96.5 ± 0.9%: TAG, 98.1 ± 0.6%: FFA, 96.1 ± 0.9%: ME). The digestive gland (DG) of prawns fed the TAG diet had the greatest amount of total lipid. DG lipids of prawns fed the TAG and FFA diets contained similar proportions of neutral and polar lipids (≈ 55% neutral, 45% polar), whereas with the EE diet, neutral lipids dominated (70%) and with the ME diet there was a higher proportion of polar lipid (68%). DG neutral lipids of both the TAG and FFA-fed prawns had high proportions of triacylglycerols and diacylglycerols with small quantities of both cholesterol and free fatty acids. However, prawns fed the ME and EE diets differed from the TAG-fed prawns in both the proportions and total quantities of DG neutral lipids. The DG polar lipids in prawns from the TAG, FFA and EE treatments had similar proportions of each polar lipid class, in contrast to those fed ME where the polar lipid was almost exclusively phosphotidylethanolamine. The fatty acid composition of the total DG lipid of prawns fed the TAG and FFA diets were similar. There was virtually no PUFA and HUFA in the DG lipid of the ME-fed prawns. These studies demonstrate that where a diet with a particular fatty profile is required, trigacylglycerols either singly or as a mixture should be used. If these do not give the required fatty acid profile, free fatty acids may be used to adjust the profile. The use of either methyl or ethyl esters as neutral lipid components in experimental diets is not recommended for examination of fatty acid requirements of P. monodon.