Effect of dietary lipids on fatty acid composition and metabolism in juvenile green sea urchins ( Strongylocentrotus droebachiensis)

Abstract

Relatively little is known of the nutritional requirements of sea urchins, especially at the juvenile stages. One of the costly restrictions on feed formulation for marine fish species is the requirement for fish oils to supply the essential n-3 highly unsaturated fatty acids (HUFA). This study investigated the effects of different dietary lipid sources on the fatty acid composition and metabolism of juvenile green sea urchins ( Strongylocentrotus droebachiensis). Kelp ( Laminaria longicruris) served as a reference diet. Experimental diets contained 5% of: corn oil (high in 18:2n-6), linseed oil (high in 18:3n-3), menhaden oil (high in n-3 HUFAs 20:5n-3 and 22:6n-3), or 1:1 mixtures of corn oil and linseed oil, or corn oil and menhaden oil or linseed oil and menhaden oil. Unlike marine fish, the sea urchins exhibited ample ability for elongation and desaturation of shorter chain (18 carbon) polyunsaturated fatty acids (PUFA) to longer chain (20 carbon) n-3 and n-6 HUFA. Thus 18:2n-6 from corn oil in the diet resulted in high levels of 20:4n-6 in the sea urchin and similarly 18:3n-3 in the diet resulted in high levels of 20:5n-3. Even when fed kelp or diets with menhaden oil, the sea urchins did not incorporate high levels of 22:6n-3 into their lipids. In addition to the effects of dietary lipid sources on fatty acid composition of the lipids of total soft tissues of sea urchin, there were great differences in fatty acid composition among different tissues and among lipid classes of these tissues. For example, the test (shell) and associated tissue membranes, which are in constant contact with the seawater, incorporated much higher proportions of 20:4n-6 than the gonad or gut tissues. This may reflect an important role of 20:4n-6 in ion transport and osmoregulation. The sea urchins also synthesized a number of 20 and 22 carbon non-methylene-interrupted dienes (NMID) and the concentration of the major NMID was directly related to the tissue levels of 18:2n-6 and inversely proportional to the dietary and tissue levels of n-3 PUFA and HUFA. The significance of these and other interesting aspects of fatty acid metabolism in sea urchins are discussed.