The Role of Lipids During Embryonic Development of the Euphausiids Euphausia Pacifica and Thysanoessa Spinifera

To understand the role of lipids during early embryogenesis, major lipid classes together with individual fattyacid and sterol composition were determined in embryos from multiple developmental stages of the euphausiids Euphausia pacifica and Thysanoessa spinifera. Average lipid content in embryos of E. pacifica and T. spinifera from the earliest stage (multicell) were 4.45 and 3.69 mg embryo21, respectively. During development, the lipid content decreased at similar rates in the embryos of both species. In contrast to many crustacean eggs, phospholipids were the dominant lipid class in all embryonic stages, with decreasing concentrations seen during development. Individual fatty acids and sterols showed selective utilization during early developmental stages. The dominant fatty acids were 16:0 and 16:1v7 and 20:5v3, with most polyunsaturated fatty acids preferentially metabolized throughout early stages. An exception was 22:6v3, which remained near constant through all stages. Cholesterol was the dominant sterol (.82% of total sterols) in embryos, with only minor changes during development. The appearance of algal sterols and fatty alcohols, including phytol, in T. spinifera embryos suggests that considerable amounts of algal lipids are directly allocated to eggs during vitellogenesis. Despite the substantial changes in lipid amount and composition during embryo development, the presence of phospholipids as the dominate lipid store acts to moderate changes in egg-sinking rate for both species until the late (early and late limb-bud) stages of development. Along the Oregon coast, two temperate euphausiid species, Euphausia pacifica Hansen and Thysanoessa spinifera Holmes, account for about 90% of euphausiid abundance and biomass (Gómez-Gutiérrez et al. 2005) and play significant roles in the marine ecosystem as links between primary producers and top predators, such as fishes, marine birds, and mammals. To understand the ecologic niche that these species occupy, we must study their life cycles and vital rates (i.e., fecundity, growth, and mortality). Yet, their reproductive strategies and recruitment mechanisms remain poorly understood. Euphausiid embryos (rather than ‘‘eggs,’’ which refers strictly to unfertilized oöcytes) are delicate and difficult to manipulate experimentally, and as a consequence, relatively few studies have examined the embryology, hatching mechanism, and vertical distribution (Marschall and Hirche 1984; Iguchi and Ikeda 1994; Gómez-Gutiérrez 2003). The embryos of most broadcast euphausiid species, including E. pacifica and T. spinifera, have a unique feature during their early embryonic development: they are released near surface waters and sink rapidly to depth, where they may hatch. After hatching, the nonfeeding larval stages (nauplii and metanauplii) ascend and arrive in the surface water at metamorphosis to the first feeding stage, calyptopis 1 (Quetin and Ross 1984; Gómez-Gutiérrez 2003). The daily migration of adults and sinking of eggs (i.e., embryos) separate the parental stock from their eggs (Brinton and Willie 1976), which makes estimations of recruitment rate (or mortality) of the embryos difficult. Embryogenesis of E. pacifica from fertilization to hatching of the first nauplius has been postulated to occur during transit of embryos to deep waters, with increased sinking rates in older stages (Ross 1981; Quetin and Ross 1984; Gómez-Gutiérrez 2003). Previous studies have 1 Corresponding author ([email protected]). Acknowledgments We thank Leah Feinberg, Tracy Shaw, Julie Keister, Jesse Lamb, Anders Røestad, and Mitch Vance for their assistance in the collection of live euphausiids. This research was partially supported by a Mamie Markham Research Award from Oregon State University, Hatfield Marine Science Center (HMSC), and the NEP-GLOBEC program to H.R.H. (OCE-0000732) and W.T.P. (NA67RJ0151). J.G.G. was also supported by an SNI-II fellowship, by COFAA–IPN, EDIIPN, and a doctoral CONACyT grant (122676) to study at Oregon State University. Contribution 3967 of The University of Maryland Center for Environmental Science and contribution 288 for U.S. GLOBEC program. Ship time and staff assistance in 1999–2000 were provided by the Office of Naval Research (National Ocean Partnership Program) and ship time in 2000– 2002 by the U.S. GLOBEC program. Limnol. Oceanogr., 51(5), 2006, 2398–2408 E 2006, by the American Society of Limnology and Oceanography, Inc.