The role of the lipid during early embryonic development of Euphausia pacifica and Thysanoessa spinifera in relation to sinking mechanism

Abstract

To understand the role of lipids during early embryogenesis of Euphausia pacifica and Thysanoessa spinifera, eggs from three different embryonic development stages were examined for lipid class, fatty acid, and sterol composition. Pooled egg samples from each different embryonic stage were used for lipid measurements. Average lipid content in eggs of Euphausia pacifica and Thysanoessa spinifera from earlier stage (multiple cell stage) were 4.45 and 3.69 µg egg-1, respectively. During embryo development, lipid content decreased at the similar rates in the eggs of both species. Phospholipids were the dominant lipid class throughout all embryonic stages although concentrations dropped remarkably during the later embryonic stages. Until the gastrula stage, lipids were slowly utilized without any preferential usage of specific lipid classes. However, more than 60 % of lipids, mainly phospholipids, were lost (or consumed) between blastula or gastrula and early or late limb-bud stages for both species. Total fatty acid and sterol contents showed the same trend as lipid contents during early embryogenesis. Among fatty acids, 16:0, 16:1ω7, and 20:5ω3 were dominant with significant changes during early embryo development. Grouped fatty acid composition indicates that polyunsaturated fatty acids were most significantly metabolized throughout early embryogenesis. However, only 22:6ω3, known as one of the most essential long-chain polyunsaturated fatty acid components for egg and larval development, remained at constant levels or only decreased slightly through all stages. Cholesterols were the most dominant sterol (>82% of total sterols) found in euphausiid eggs with minor change during embryogenesis. Moreover, the appearance of algal sterols, including phytol, in T. spinifera eggs during early stages (mutiple cell and blastula) suggests significant lipid transfer from adult krill and could provide dietary history of adult krill. Although egg density calibrated us