Studies on the nutrition of marine flatfish. The effect of dietary fatty acids on the tissue fatty-acids of the plaice Pleuronectes platessa

Abstract

The ratios of polyunsaturated fatty acids in the lipids of hatchery-reared plaice [Pleuronectes platessa (L.)] maintained on a diet containing corn oil and freeze-dried cod muscle accurately reflect ratios of polyunsaturated fatty acids in the diet. Such plaice have much greater levels of triglycerides in both their livers and extrahepatic tissues than wild plaice of comparable size; the total amount of 22:6 Ω 3 acid in triglycerides of the two groups is, however, very similar. Transfer of experimental plaice to a fat-free diet caused a marked depletion of triglycerides from both livers and extrahepatic tissues with almost all fatty acids being reduced in amount. Addition of 12:0 and 14:0 ethyl esters (4% of each) to the fat-free diet caused some accumulation of triglycerides in extrahepatic tissues. Further addition of 18:2 Ω 6 and 18:3 Ω 3 acids (both at levels of 0.4%) to the diet caused an accumulation of triglycerides in the liver. These triglycerides had increased levels of saturated and monounsaturated fatty acids up to C 18 but the levels of higher polyunsaturated fatty acids were not inereased. There was relatively little change in the fatty acids of phospholipids throughout these experimental dietary regimes. The levels of phospholipids were constant and very similar to those of wild plaice. The results indicate: (1) plaice fed a diet relatively low in 22:6 Ω 3 acid may accumulate relatively large amounts of triglyceride in order to store adequate amounts of 22:6 Ω 3 acid; (2) plaice are capable of synthesising saturated and mono-unsaturated fatty acids up to C 18 in the presence of dietary 12:0 and 14:0 acids; (3) 18:2 Ω 6 and 18:3 Ω 3 acids in the diet do not undergo chain elongation and further desaturation in plaice with the dietary regimes we used.