Fatty acid accumulation and synthesis rates were measured in two species of daphniid Cladocera, Daphnia magna and D. pulex, to determine what fraction of deposited fatty acids accumulated by these zooplankton species are dietary and what fraction are synthesized de novo. Fatty acid accumulation rates were measured as the increment in lipid weight during the adult third instar. Lipids were extracted, separated on chromarods, and quantified by flame‐ionization detection. Fatty acid synthesis rates were measured by exposing adult third‐instar females to [3H]2O and [U‐14C]‐acetate. Synthetic rate estimates using [U‐14C]‐acetate were consistently less than estimates obtained by using [3H]2O. Lipid content (percent of dry weight) from D. magna cultures supplied with high algal food (100,000 cells/ml) increased from 16.6% at the beginning of the third instar (eggs removed from brood pouch) to 22% at the beginning of the fourth instar (with eggs). The increase in lipid of D. pulex under identical conditions ranged from 19.8% to 30.6% total lipid and under low food conditions (4,000 cells/ml) ranged from 37% to 40% lipid. Accumulated fatty acids were associated largely with triacylglycerol (60–80% of accumulated lipid) or phospholipid (10–20% of accumulated lipid). Fatty acid accumulation for high‐food animals was 1,043 ng/hr/individual and 364 ng/hr/individual for D. magna and D. pulex, respectively. The corresponding fatty acid synthesis rates were 17 ng/hr/individual and 6 ng/hr/individual, respectively. Low‐food adults of D. pulex accumulated fatty acids at a rate of 192 ng/hr/individual and only synthesized fatty acids at a rate of 0.3 ng/hr/individual. Incorporated label was present in a slow turnover pool of complex polar lipids. Fatty acid synthesis contributes less than 2% of the accumulated total fatty acids in these daphniids. Copyright © 1990 Wiley‐Liss, Inc., A Wiley Company
