The lipid composition of two species of serrasalmid fish in relation to dietary polyunsaturated fatty acids

Abstract

The lipid composition of two species of Serrasalmid fish with different natural feeding habits were compared in relation to the polyunsaturated fatty acids (PUFA) supplied in their diets. Mylossoma aureum, a herbivorous piranha, was maintained on oatmeal flakes in which : 2(n‐6) and : 3(n‐3) were the only PUFA and accounted for 40–8 and 1.2%, respectively of dietary fatty acids. Serrasalmus nattereri, the carnivorous red piranha, was fed mosquito larvae containing .0‐33.4% of their total fatty acids as : 2(n‐6)+18 : 3(n‐3) and 4.9‐8.5% as 20 : 4(n‐6)+20 : 5(n‐3). The two species had similar lipid class compositions in liver, brain, viscera and carcass, except that lipids from M. aureum were generally richer in triacylglycerols. In both species, visceral and carcass lipid contained high levels of triacylglycerols whose principal PUFA was : 2(n‐6). In M. aureum the major PUFA in liver total lipid and triacylglycerols was : 2(n‐6) whilst the major PUFA in liver phospholipids were : 4(n‐6) and : 5(n‐6), with : 6(n‐3) being a minor component. The level of : 6(n‐3) in ethanolamine glycerophospholipids was significantly greater in brain than liver of M. aureum. Although absent from dietary lipid, : 6(n‐3) was the major PUFA in phosphatidylcholine and ethanolamine glycerophospholipids from both the liver and brain of S, nattereri. In both species, the ratio of (n‐6)/(n‐3)PUFA was consistently lower in tissue lipids than in dietary lipids. The results are consistent with (i) the herbivorous M. aureum converting dietary C18 PUFA to their C20 and C22 homologues, (ii) the carnivorous S, nattereri forming : 6(n‐3) from either 18:3(n‐3) or 20: 5(n‐3) and (iii) both species selectively desaturating and elongating (n‐3) rather than (n‐6) PUFA.